21

Immune system

21

Immune system

21

Immune system

21

Immune system

  21 Immune system

Lothar Thomas

21.1 Immune response

The immune system is in a constant defense against a multitude of invading pathogens e.g., bacteria, viruses, fungi, parasites and a pattern of non-living foreign substances. The pathogen initiates complex interactions between the pathogen-derived molecules and host sensors. The immune response is categorized in two components, innate immunity and adaptive immunity. The innate immune response, which is the first line and most effective defense plays a crucial role in defense against a majority of infections. The adaptive immune response generates a nearly unlimited number of antigen receptor specificities by random gene rearrangement that can detect extracellular and intracellular antigens using B cell antigen receptors and T cell antigen receptors, respectively /12/.

21.1.1 Innate immune response

The innate immune response consists of epithelial barriers, a family of soluble antimicrobial peptides, danger and pathogen associated molecular pattern-recognizing molecules and receptors of various innate immune cells. The cells, receptors and molecules, such as pentraxins, complement, innate antibodies, nucleotide binding oligomerization domain (NOD)-like and Toll-like receptors, mast cells, monocytes/macrophages, granulocytes, natural killer cells, myeloid dendritic cells, Langerhans cells and antigen presenting cells bind to the pathogen and initiate its clearance through transcription independent immunological processes (e.g., phagocytosis, degranulation, and complement fixation) /1/.

The innate immune cells include various tissue macrophages, dendritic cells and neutrophils that express a family of innate receptors and sensors known as pattern recognition receptors (PRRs). The receptors are evolutionary conserved germ-line encoded receptors that sense pathogen associated molecular patterns (PAMPs). The PAMPs are not only essential for pathogenicity to establish infection in the host but also essential for the survival of the pathogen. PRRs include several families of receptors /2/:

  • Toll-like receptors (sensing bacteria)
  • NOD-like receptors (sensing bacteria)
  • RiG-I-like receptors (sensing viruses)
  • C-type lectin receptors (sensing fungi and mycobacteria)
  • DNA-sensing molecules (sensing viruses)

This highly orchestrated defense mechanism against infectious and inflammatory insults initiates the formation of acute phase proteins driven by the endogenous cytokines interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α The cytokines are produced by macrophages and other leukocytes induced by pathogens binding to PRRs /3/.

Collectively, the responses of the innate immunity initiate either direct killing or inhibition of pathogen replication. Additionally, the responses induced by the innate immune response initiate pathogen-specific adaptive immunity through B- and T cells /2/.

Natural immune barriers

A number of mechanical factors protect the organism against pathogens. These are the skin, mucus layers, saliva and tears, in addition to a large number of chemical mechanisms such as the low pH of gastric juice.

Intestinal mucosal immunity

Mucus layers cover a surface area of several hundred square meters, which is 200 times larger than the surface area of the skin /4/. They represent the most important portals of entry for microbial pathogens that kill some 10 million children below the age of 5 years worldwide every year. Most of these deaths are due to diarrheal disease.

The mucus layers of the body defend against pathogens mainly by producing secretory IgA (sIgA). There are about 1010 IgA producing plasma cells per meter of bowel whereas the total number in all of the lymph nodes, spleen, and bone marrow together only amounts to 2.5 × 1010 plasma cells. Approximately 80% of the body’s immunoglobulin (Ig) producing plasma cells are located in the intestine and produce Ig locally in the lamina propria. In an adult, approximately 3 g of sIgA is secreted into the intestinal lumen each day.

The intestinal mucosal immune system represents the most robust Ig producing mechanism in the body and the front-line defense system against the antigens and pathogens in the more than 1,000 kg of food that passes through the intestine each year. In the intestine, the induction and regulation of mucosal immunity takes place primarily in the Peyer’s patches and mesenteric lymph nodes (gut-associated lymphoid tissue, GALT).

The mucosal immune system ensures the homeostasis of the defense system via two mechanisms:

  • By limiting epithelial contact and mucosal invasion by antigens and pathogens by binding them into immune complexes
  • By immunosuppression, also known as “oral tolerance” when induced via the intestine. Regulatory T cells (Treg) are located in the mesenteric lymph nodes. Treg are triggered by dendritic cells, which present them with dietary antigens and pathogens. The Treg induce mucosal tolerance to these substances by down regulating the immune system. The neonatal period is critical, both with regard to infections and to food allergies, because the mucosa and the immuno regulatory system is poorly developed.

Mucosal immune system of the respiratory tract

Particles with a diameter of less than 5 μm can reach the lower airways of the respiratory tract. There, they encounter the natural resistance of surfactant proteins (SP) and soluble components of the innate immune response such as lysozyme, lipopolysaccharide (LPS)-binding protein, fibronectin, lactoferrin, defensins, complement, and secretory IgA /5/.

The SPs are members of the collectin family and contain lectin-rich and collagen-rich domains. SPs protect against microorganisms by acting as opsonins and stimulating phagocytosis by alveolar macrophages. Low concentrations of SP aggregates stimulate lymphocyte proliferation whereas higher concentrations inhibit it. The SPs (SP-A and SP-D) stimulate cytokine secretion by macrophages. However, SPs do not protect against microbial pathogens only; they also bind pollen and mite allergens.

β defensins are low molecular weight cationic peptides with broad antimicrobial activity against bacteria, fungi, chlamydia, and viruses. They are produced by epithelial cells following the stimulation of toll-like receptors by microbial antigens.

21.1.2 Immune recognition

21.1.2.1 Danger-associated molecular patterns (DAMPs)

The immune system was developed before the separation of vertebrates and invertebrates. Therefore, it has highly conserved structures that can recognize the molecular antigen patterns of pathogens but are not differentiated enough to detect individual antigens. Many microorganisms express standard antigen molecular patterns (PAMPs) that are recognized by Pattern recognition receptors (PRRs) of immune cells. PRRs are localized on macrophages, dendritic cells and granulocytes /1/. Refer to:

21.1.2.2 Pathogen associated molecular patterns (PAMPs)

PAMPs include structures such as fungal β1,3-glucans, bacterial lipopolysaccharide (LPS), peptidoglycans, phosphoglycan, lipoteichoic acid, mannan, double-stranded RNA, and bacterial DNA. Certain pathogen groups are characterized by a particular type of PAMP and so are recognized globally, for example /5/:

  • Gram negative bacteria contain LPS in their cell wall. The binding of LPS to the corresponding CD14 receptor and toll-like receptor 4 (TLR 4) on a macrophage trigger a signaling cascade that causes the macrophage to secrete cytokines, which in turn stimulate the immune system.
  • Gram positive bacteria contain lipoteichoic acid
  • Fungi contain mannan.

Alarmins

Alarmins are danger signals that are produced during inflammation, infection, and stress (Tab. 21.1-1 – Danger-associated molecular patterns (DAMPs)).

Opsonins

These group of proteins attract and bind microbial pathogens. Mannose-binding protein (MBP), for example, is produced as an acute phase protein by the liver. MBP is a Ca2+-dependent lectin receptor that binds mannose-rich carbohydrates on bacteria, fungi, parasites, and, occasionally, viruses. MBP acts as an opsonin in human blood. It accelerates the phagocytosis of mannose rich proteins and activates the classical and alternative pathways of the complement system /5/.

21.1.2.3 Pattern recognition receptors (PRRs)

These proteins are expressed as receptors on dendritic cells, macrophages, and B cells or are secreted. PRR binding sites include Ca2+-dependent lectin, leucine-rich peptides, and cystine-rich peptides. PRRs stimulate endocytosis in macrophages and trigger signaling cascades that lead to inflammation. The overall result is that the immune system can capture and attack entire classes of microorganisms with a relatively small number of different receptors.

Endocytosis receptor

In the absence of opsonins, this receptor, which is a mannose binding protein, facilitates the uptake of microorganisms into macrophages, dendritic cells, and occasionally endothelial cells. This is how Pneumocystis carinii is incorporated into alveolar macrophages. It is thought that the susceptibility of AIDS patients to Pneumocystis carinii infection is due to a modification in the mannose binding receptor caused by HIV /5/.

Toll-like receptor (TLR)

TLRs are found on the cell membrane of immune cells. They recognize microbial components, activate signal transmission in the nucleus, and trigger the expression of genes involved in the inflammatory response. TLRs were discovered at the end of the twentieth century in Drosophila, where they are essential receptors for host defense against fungal infection. The mammalian TLR receptor group has 11 members; in humans, TLR 10 is functionally the most important. Like cytokine receptors, TLRs have an extracellular and a cytoplasmic domain. The cytoplasmic domain is comparable to that of the IL-1 family. The extracellular domain varies, however. In IL-1R, it is immunoglobulin-like whereas in the TLRs it consists of an accumulation of leucine residues (leucine rich repeats, LRRs) /6/.

TLRs recognize conserved pathogenic structures, so they recognize many microbial components such as lipoproteins, peptidoglycans, lipoteichoic acid in Gram-positive bacteria, lipopolysaccharide (LPS) in Gram-negative bacteria, the glycosylphosphatidylinositol anchor of Trypanosoma species, and phenol-soluble modulin in Staphylococcus epidermidis. Following contact with a microbial component, TLRs located on macrophages induce the expression of genes, which results in the production of inflammatory cytokines (IL-1, IL-8) and co stimulatory molecules. Under the influence of inflammatory cytokines and co stimulatory molecules activated by TLRs, macrophages present antigens to the T-helper cells, and in this way, couple the innate immune defense to the adaptive immune system (Fig. 21.1-1 – Pathogen recognition by dendritic cells and macrophages by means of toll-like receptor (TLR)).

TLRs do not always bind antigens directly. When TLRs are activated by LPS, the LPS must be coupled to an LPS binding protein (an acute phase protein).

Co stimulatory molecules

Macrophages and dendritic cells are stimulated by TLRs to express B7 co stimulatory molecules (CD80 and CD86) on the cell surface. Co stimulatory molecules provide additional signals that, along with the TLR signals, are necessary for T-cell activation /7/.

21.1.3 Cells expressing receptors of innate immune system

21.1.3.1 Granulocytes and macrophages

The cells of the innate immune system first travel to the area of tissue damage. The PO2 in this area is usually reduced. Polymorphonuclear neutrophils (PMNs) are the first to arrive at the diseased or injured site, followed by monocytes. Since PMNs have few mitochondria and obtain most of their energy from anaerobic glycolysis, they can perform their immune recognition and defense functions in spite of the hypoxic conditions /8/. For granulocyte functions in the innate immune response, refer to Section 19.7 – Polymorphonuclear neutrophil function.

More than 95% of macrophages in the tissues are derived from monocytes of the hematopoietic system and the remainder are derived from local tissue-resident macrophages. Circulating monocytes normally have a half-life of up to 3 days before extravasation into the tissues occurs. During inflammatory response with increased monocyte production in the bone marrow, the half-life of the circulating monocytes decreases and macrophages accumulate in the affected tissue. Macrophages are found in almost all tissues, where they often exist in tissue-specific forms (e.g., as histiocytes in connective tissue, Kupffer cells in the liver, alveolar macrophages in the lung, microglia in the central nervous system, and osteoclasts in bone).

In addition to their phagocytic capabilities, macrophages have a broad spectrum of other functions, ranging from antigen presentation, through antibacterial and antitumor activity, to secretion of regulatory substances such as enzymes, prostanoids, and cytokines.

Macrophages express a range of receptors:

  • To recognize carbohydrates such as mannose. Because mannose is not normally found on the surface of vertebrate cells but is expressed on microbial cell surfaces, recognition of these sugar enables the macrophages to differentiate between self and non self antigens.
  • To recognize phosphatidylserine. Cells that are subject to programmed cell death (apoptosis) express phosphatidylserine and are cleared by macrophages. Cells in necrotic tissue, on the other hand, release substances such as heat shock proteins, which involve macrophages in host defense as part of an inflammatory response (refer to Section 19.1 – Inflammatory response).
  • For complement factors and immunoglobulins. Complement factors and antibodies opsonize microbes by coating them and accelerating their phagocytosis. After complement activation (see Chapter 24 – The complement system), the component C3b binds to microbial cell surfaces. Macrophages have receptors for C3b and are activated by C3b binding. In the absence of C3b, macrophages are activated by microorganisms coated with IgG, IgA, or IgM. Bacteria coated with C3b or antibacterial antibodies are surrounded in the sense of a zipper mechanism through mediation by Fc receptors on the surface of the macrophage (Fig. 21.1-2 – Nonspecific phagocytosis of a microorganism by a macrophage). Within the macrophage, pathogens are attacked by a range of mechanisms including reactive oxygen species (hydroxyl anion, super oxide anion), hypochlorous acid, NO, and antimicrobial substances such as lysozyme and cationic proteins.

In simple terms, the function of macrophages is to acquire a pathogen, processes it into smaller antigenic substances, and present them to the T cells of the adaptive immune system in order to initiate an immune response (Fig. 21.1-3 – Antigen presentation to T cells). Recognition of these antigens by T cells triggers the immune response. Macrophage function is then under the control of T cells. Interferon-γ (IFN-γ) produced by T cells is the prototypical macrophage-activating cytokine.

IFN-γ-activated macrophages produce /9/:

  • IL-12 and TNF-α. IL-12 is one of the most important cytokines secreted by macrophages because it regulates the Th1 immune response. The production of IFN-γ by T cells is maintained by IL-12 and IFN-γ in turn stimulates macrophages to express co stimulatory molecules of the B7 family, which have an important role in the recognition of PAMPs by macrophage pattern recognition receptors.
  • IL-1 and IL-10. IL-1 influences the Th1 immune response while IL-10 influences the Th2 immune response (Fig. 20.1-4 – Development of the sub populations of T-helper cells under the influence of IL-4 and IL-12). IL-1 stimulates the immune response while IL-10 suppresses it. IL-10 therefore reduces the production of B7 co stimulatory molecules, TNF-α, and macrophage-inhibitory factor (MIF).

Macrophage function can be summarized as follows:

  • Macrophages amplify T-cell responses
  • The function of the macrophages themselves is then regulated by the products (cytokines) of the T-cell immune response.

Thus, macrophages play a central role in determining the extent of the immune response.

21.1.3.2 Dendritic cells (DCs)

DCs are produced in the bone marrow and are derived from myeloid or lymphoid cell lineages. They have a characteristic star-shaped structure due to their numerous cytoplasmic extensions. This means that they have a large cell surface, which enables them to establish a high degree of contact with surrounding cells. In this way, one DC can activate 100–3,000 T cells. Antigens are ingested by macro pinocytosis. Interstitial fluid is taken up into the cell and antigens are concentrated by ejecting water through special channels.

Dendritic cells occur /10/:

  • As Langerhans cells in the squamous epithelia of the epidermis and supra basal layers in the skin
  • As interstitial cells in the heart, lungs, liver, and other organs
  • In the covering of afferent lymphatics (veiled cells)
  • As inter digital cells in T cell-rich regions of lymph organs
  • As follicular cells in lymph organs. These differ from the DCs mentioned previously because they are thought to act as memory B cells.
  • DCs are strategically located in the organism to ensure that invading pathogens are recognized. They continuously absorb antigens from the extracellular milieu and search through them for pathogenic antigen patterns (e.g., microbial pathogens).

Dendritic cells express a whole range of antigen recognition receptors, for example /10/:

  • Mannose receptors, LPS receptors, and toll-like receptors to recognize fungal mannan, LPS in Gram-negative bacteria, and lipoteichoic acid in Gram-positive bacteria
  • Receptors such as FcγRII (CD32), FcγRI (CD64), FcεRI, and the C3bi complement receptor (CD11b) to make the endocytosis of immune complexes more effective.

To activate the immune response, DCs express /10/:

  • High concentrations of antigen presenting molecules such as MHC class I and MHC class II molecules and CD1a molecules. Like in macrophages, antigens are processed internally before being presented with MHC molecules as short peptides on the cell surface. MHC class II molecules present the peptides to T-cell receptors on the surface of T-helper cells, while MHC class II molecules present antigens to receptors on cytotoxic T cells (Fig. 21.1-3 – Antigen presentation to T cells). DCs are particularly effective at priming i.e., the antigen-specific activation of naive T cells (Th0) that have not had previous contact with the antigen.
  • Co stimulatory molecules of the B7 family and intercellular adhesion molecules: ICAM-1 (CD54), ICAM-3 (CD50), and lymphocyte function-associated antigens such as LFA-3 (CD58), B7-1 (CD80), and B7-2 (CD86).

The mechanisms of the innate and adaptive immune response are activated as a result of this concerted activity. DCs can also migrate into local lymph nodes and trigger an adaptive immune response there. Overall, DCs represent an important link between innate and adaptive immunity.

21.1.3.3 Natural killer (NK) cells

NK cells are an important subpopulation of lymphocytes that play a role in the innate immune response to infection and malignancy. They comprise 10–20% of circulating lymphocytes and have the morphology of large granular lymphocytes (LGL cells) in the peripheral blood smear. They are called natural killer cells because of their ability to lyse target cells without prior sensitization and without the need for MHC antigen expression by the target cell /11/.

NK cells can be distinguished from T and B lymphocytes by the lack of T-cell and B-cell receptors on their surface. They are phenotypically classified as CD56+CD3. CD56 is a neuronal cell adhesion molecule (NCAM). Other surface receptors present on subsets of NK cells include FcRγIII (CD16), IL-2 receptor, c-kit receptor, CD7, CD2, and CD8.

NK cell function (target cell recognition, killer activity) is regulated by a complex interplay of activating and inhibitory cell surface receptors. Activating receptors include β2-integrins, CD2, and receptors belonging to the immunoglobulin super family that are defined by their molecular weight (e.g., NKp46 or NKp30). The inhibitory MHC class I receptors have a molecular structure that is either lectin-like (killer cell lectin-like receptor, KLR) or immunoglobulin-like (killer cell immunoglobulin-like receptor, KIR) (Fig. 21.1-4 – Structure of inhibitory NK-cell receptors). KLR and KIR play a crucial role in determining what happens to target cells /11/.

Theories about how NK cells operate suggest that KLR and/or KIR can bind to any cell in the organism and come into play once the NK cell has bound to the target cell. If they find specific MHC structures on the target cell to which they can bind and the target cell emits an inhibitory signal, the target cell escapes lysis. If not, inhibition does not occur and the target cell is lysed (Fig. 21.1-5 – Activating and inhibitory NK cell receptors and their interaction with target cells). In virus-infected cells and tumor cells, MHC molecules are down regulated, making these cells more susceptible to lysis by NK cells.

NK cells destroy infected and malignant cells via the following mechanisms /11/:

  • Cells coated with IgG antibodies are recognized via IgG receptors (FcγR) and destroyed by the mechanism of antibody-dependent cytotoxicity
  • Killer cell activating receptors recognize structures on the target cell. In the absence of a signal from the inhibitory receptors, the contents of its cytotoxic granules in the NK cells are released. Perforin performs a hole in the target cell membrane and cytotoxic enzymes from the granules are then injected into the interior of the cell, whereby the cell contents are dissolved.

21.1.4 Complement system

The innate immune response frequently involves one of the three complement systems at an early stage (see also Chapter 24 – The complement system). The following pathways are activated:

  • The classical pathway (activated by immune complexes)
  • The alternative pathway (activated by the microbial cell wall)
  • The lectin pathway (activated by the interaction between mannose binding proteins and microbial carbohydrates).

Activation of these pathways leads to production of the following:

  • Fragment C3b of complement component C3. C3b is deposited on the surface of the pathogen, where complement activation takes place. This initiates phagocytosis of the pathogen by macrophages or dendritic cells, which have C3b receptors.
  • Fragments C3a, C4a, and C5a, which release mediators of inflammation from mast cells. C5a also acts as a chemoattractant for polymorphonuclear neutrophils.
  • The membrane attack complex, which is comprised of C5b, C6, C7, C8, and C9. This perforates the target cell membrane, leading to cell destruction.

21.1.5 Cytokines

The highly orchestrated defense mechanism against infectious and inflammatory insults initiates inflammatory cells (macrophages and other leukocytes) to produce inflammatory cytokines e.g., interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α and function as an integrated network to regulate the components of the immune system (see to Chapter 20 – Cytokines and cytokine receptors).

Innate immune response to microbial pathogens is shown in Fig. 21.1-6 – Innate immunity: responses following initial contact with microbes and microbial products.

21.1.6 Adaptive immune response

The fundamental features of the adaptive immune system are the ability for learning, gaining memory, and obtaining antigen specificity. The system provides effective response against a spectrum of environmental antigens and has the following characteristics:

  • Large variety of somatic antigen receptors
  • High degree of antigen specificity
  • The ability to generate a rapid immune response to a previously encountered antigen based on immunological memory.

In the adaptive immune response, pathogens and their products are processed primarily by components of innate immunity. Peptides are produced of a size that allows them to associate with MHC molecules in the cytoplasm of the antigen presenting cells to form a peptide MHC complex (pMHC) /12/.

The pMHC is transported to the cytoplasmic membrane, where it is recognized by T lymphocytes (T cells) of the adaptive immune system via their T-cell receptors (TCRs). Because the TCR avidity is rather low, the concentrations of TCRs and pMHCs in the micro environment must be high enough to allow T-cell activation. Interaction between the APC and T cell is facilitated by specialized structures on their membranes called micro domains, which bring the receptors and ligands together, as well as by co receptors.

The following co receptors and ligands play a supporting role:

  • CD4/CD8 molecules on the T-cell membrane, which interact with molecules of MHC II and MHC I on APCs
  • CD28 molecules on T cells, which react with CD80/CD86 ligands on APCs
  • CD40L (CD154) on activated T cells, which react with CD40 molecules on APCs.

Supported by the co receptors, TCR and pMHC interact in their micro-domains and activate the immune system.

The TCR repertoire consists of 1012–1015 possible receptors. These include receptors that bind self-peptide/MHC complexes and could theoretically destroy the body’s own tissues. In an attempt to prevent this anti-self reactivity (autoimmune reaction), bone marrow-derived lymphocytes are educated and selected in the thymus so that, in principle, autoimmune reactions cannot occur. Because this selection is not infallible, small numbers of self-reactive T cells escape and reach the peripheral tissues. In most people, however, they are harmless because the self-peptide/MHC complexes do not reach the critical concentration required to trigger an autoimmune reaction. Nevertheless, some auto aggressive T cells (known as driver cells) may trigger autoimmune reactions. Regulatory T cells (Treg) and natural killer cells (NK cells) counteract the activity of driver cells.

Antigen-mediated activation of T cells and B cells and subsequent stimulation by cytokines secreted by the T cells lead to differentiation:

  • Of naive T cells (Th0) into T helper cells (Th1), Th2 cells that suppress immune reactions, and into regulatory Treg. To ensure an effective immune response, these cells are stimulated to proliferate clonally (clonal selection theory) /7/.
  • Antibody production by B cells
  • Macrophage activation.

Activated T cells, B cells, and macrophages collaborate with the effectors of innate immunity to eliminate the pathogen.

T and B cells are derived from primordial stem cells in the fetal liver and bone marrow under the control of interactions with stromal cells, stem cell factors, and colony stimulating factors. The initial stage of lymphocyte development is not antigen dependent.

However, once the lymphocytes express a mature antigen receptor, their survival and differentiation require the presence of an antigen /7/.

T cells, B cells, Treg and NK cells are derived from a common precursor cell. From this precursor cell, the following develop:

  • Pre B cells (immature B cells)
  • Pre T cells (immature T cells).

21.1.6.1 T-cell system

T cells regulate adaptive immune responses to pathogens and tumor cells and detect antigens presented by self MHC molecules. The T cell antigen receptor on conventional αβ-T cells recognizes peptide fragments bound to MHC-I or MHC-II molecules. Each developing T cell expresses a unique T cell receptor and generation of self-MHC restricted and self-tolerant T cell repertoire results from a multistep selection process in the thymus. Naive T cells develop from stem cells in the bone marrow, migrate to the thymus and become thymocytes.

The thymic selection occurs in the following steps (Fig. 21.1-7 – Selection of T cells that migrate into the thymus from the bone marrow/13/:

  • The first stage of selection is mediated by specialized thymic cortical epithelial cells, which present self-peptides (hormones of the neurohypophyseal family, tachykinin family, and insulin super family) together with MHC proteins. The thymocyte T-cell receptor (TCR) recognizes amino acids from these self peptides and MHC antigens. Thymocytes expressing a TCR weakly reactive to the host’s self antigens receive a maturation signal to generate the functional T cell repertoire in the periphery (positive selection). In contrast, thymocytes with strongly self-reactive TCRs receive a death signal (negative selection). A failure to prevent strongly self reactive T cells from entering the peripheral T cell pool is one of the main causes of autoimmune diseases. To initiate signaling an antigen activated TCR scans multiple MHC-I and MHC-II co receptors to find one that is associated with the signal initiating kinase Lck. The kinase phosphorylates immunoreceptor tyrosine based activation motifs (ITAMs) and activating tyrosines on ZAP 70 protein. MHC-II restricted TCRs require a shorter antigen dwell time (0.2 sec.) to initiate negative selection compared to MHC-I restricted TCRs (0.9 sec.) because more CD4 coreceptors are LcK loaded compared to CD8.
  • T cells positively selected on the cortical epithelial cells undergo further selection. In this step specificity testing for antigens is carried out by dendritic cells and macrophages in the thymus medulla. In order to pass this selection step, the TCR must have a corresponding antigen specificity. However, because TCR genes rearrange haphazardly, the probability of a T cell having a corresponding TCR is low. If it does have a matching TCR, the signal that triggers automatic apoptosis of the T cell is switched off and the T cell is released into the circulation as functional CD3+CD4+T cell or CD3+CD8+T cell and usually migrates to the lymph nodes. More than 95% of T cells are not selected at this stage and therefore die in the thymus and fail to reach functionality.

A small subset of T cells that pass through the thymus possess a γ/δ receptor. These cells remain in the thymus for a short time only and develop in many locations outside the thymus (e.g., in the intestine-associated immune system). Like NK cells, they have cytotoxic activity and can lyse target cells by releasing perforin and lytic enzymes.

The ontogenetic development of the T-cell system is as follows: CD3+T cells are detectable from the 10th week of gestation while CD4+T cells and CD8+T cells are present from the 14th week of gestation.

Naive T cells

Until puberty, the thymus supplies the organism with naive T cells (Th0 cells) /14/. Even after this ceases, the size of the naive T-cell pool remains stable due to post thymic expansion of naive T cells. Naive T cells can secrete IL-2 but lack the ability to express classic effector cytokines such as IFN-γ and IL-4. Because they have not undergone clonal selection during activation with a foreign antigen, they have a highly diverse T-cell receptor repertoire.

Two populations of naive T cells exist in adults: one dormant subset from the thymus and a second subset comprising naive T cells that have proliferated in the periphery. The surface molecule CD31 (PECAM-1) can be used to distinguish CD31+thymic naive T cells from CD31–central naive CD4+ T-cells.

Individuals with reduced numbers of CD31+thymic naive T cells are potentially low responders with respect to the primary immune response. Individuals with increased numbers of CD31–central naive CD4+ T cells may be more predisposed to autoimmunity.

21.1.6.2 B cell system

B cells (B lymphocytes) are produced by hematopoietic stem cells throughout life. Mature B cells recognize pathogens and contribute to their elimination. They secrete immunoglobulins (Ig), present antigens, up regulate co stimulatory molecules, produce reactive oxygen species and cytokines, and express toll-like receptors /1516/.

B cells are produced in the bone marrow. Transcription factors such as PU.1, E2A, and paired box protein 5 (PAX5) are necessary for B-cell development. Successful rearrangement of heavy chain immunoglobulin gene segments in pro-B cells leads to their differentiation into pre-B cells that express μH, the IgM heavy chain (Fig. 21.1-8 – Antigen-independent development of B cells and their receptors).

As of this stage, clonal expansion of the B cells and rearrangement of the Ig light chain gene segments are possible. Auto reactive B cells that express IgM on their surface are selected and deleted. The surviving naive B220+IgM+ B cells leave the bone marrow and migrate to the spleen, where they undergo further maturation via transitional stages.

All B cells undergo transitional stage T1 (Fig. 21.1-8):

  • A large proportion of the T1 B cells migrate to the periarterial lymphoid follicles of the spleen, where they acquire CD23 and IgD molecules and differentiate into T2 B cells. They become long living follicular B cells that recirculate between the spleen and peripheral lymph nodes until they die (half life 4.5 months) or encounter an antigen and undergo further differentiation. These B cells, also known as F0 B cells or B2 cells, express the surface molecules IgMlow, IgDhigh, CD21high, and CD23high. B2 cells are antibody producing cells that constitute 80–90% of the cells in the spleen.
  • A small proportion of the T1 B cells migrate to the marginal zone (MZ) of the spleen and remain there as MZ B cells. They express the surface molecules IgMhigh, IgDlow, CD21high, and CD23low. These cells, also known as B1 cells, react quickly to antigens and involve macrophages and dendritic cells in an immune response. They express the activation markers CD80, CD86, CD40, and CD44 as well as the co stimulatory molecules B7-1 and B7-2 much more strongly than B2 cells. Although B1 cells do not recirculate, they migrate to the peripheral lymph nodes following contact with pathogens, where they activate immune defense. B1 cells have a half life of more than 54 weeks. They secrete IgM antibodies and express CD5 and CD11. B1 cells exhibit significantly less receptor selectivity than B2 cells and mainly produce the so called “natural antibodies”. Natural antibodies are poly reactive IgM antibodies that recognize a wide range of antigens and have a high complement binding capacity but low antigen affinity and selectivity. The natural blood group antibodies anti A and anti B are produced by B1 cells.

B cells undergo programmed development with the following stages (Fig. 21.1-9 – Class switch recombination following contact between B2 cell and antigen):

  • Rearrangement of Ig heavy chain genes at the pro B cell stage
  • Clonal expansion at the pre B cell phase
  • Arrangement of light chain genes with production of IgM at the immature B cell stage
  • Antibody production and immunoglobulin (Ig) switching following contact with an antigen.

Three mechanisms contribute to the diversification of the repertoire of the B cell system pool:

  • VDJ recombination (also known as somatic recombination), a mechanism in which variable (V), diversity (D), and joining (J) gene segments are combined in the B cell to form an antigen receptor
  • Somatic hyper mutation. This involves mutations in the VDJ sequence of the variable domains of B cell antigen receptors that lead to increased antigen specificity
  • Class switch recombination. This step enables the variable domain of the heavy chain (VH) to be expressed at the antigen binding site in association with a different constant region of the heavy chain (CH). This enables the production of different Ig isotypes (IgG, IgA, IgE) (Fig. 21.1-10 – Structure of the mature and immature B cell receptor). As a result, the innate immune system can eliminate an Ig bound antigen in a variety of ways without altering the antigen specificity.

Whereas VDJ recombination in B and T cells takes place in the thymus and bone marrow, further development of the B cell occurs in the germinal centers of the secondary lymph organs (spleen, lymph nodes).Following secondary antigen contact in the lymph nodes, mutations in the VDJ sequence occur, resulting in improved antigen specificity. This process is called somatic hyper mutation.

In addition the differentiation of subsets of activated B cells into memory cells take place in the secondary lymph organs such as Peyer’s patches. Most plasma cells have a life span of only a few days but some can survive for longer in the bone marrow.

21.1.6.3 Antigens

Antigens are recognized based on their structure. Small antigens (haptens) do not elicit an immune response. Carbohydrates are usually poorly immunogenic and have to be coupled to a carrier to elicit an immune response.

Cell membrane receptors on T cells and B cells have binding sites with a size of 600–1700 square Angstroms, which can only bind small parts of a complex antigen. These small parts are called epitopes. Complex molecules therefore have a characteristic epitope pattern.

The antigen binding site of an antibody or the peptide MHC complex of a T cell receptor is complementary to the antigen structure. The complementary antibody and receptor structure bind non covalently. Antigen recognition only occurs if the complementary molecular structures are in relatively close proximity. For small antigens, the binding site may be a pocket or cleft, but in most cases, it is an undulating surface /7/.

The α/β T cell receptors only recognizes a linear peptide. These are formed only after processing by antigen presenting cells such as macrophages and dendritic cells.

Antibodies, whether free or bound to B cells, recognize only a small part of complex antigens, referred to as the antigenic epitopes such as those present in native protein structures. Epitopes that easily fit into a B cell receptor antigen pocket or a corresponding receptor binding site are those that dominate the polyclonal immune response.

Cryptic epitopes that are normally not recognized efficiently are more easily recognized following antigen processing by macrophages or dendritic cells. The mode of antigen presentation is also important. After processing, dendritic cells present the antigen in a form such that only few epitopes are offered to the T cell, while antigen presentation by B cells results in a greater diversity in the T cell immune response /17/.

21.1.6.4 Antigen receptors

Antigen recognition in adaptive immunity involves specific interactions between antigen epitopes and receptors on B cells and T cells. B cells recognize soluble protein and non protein (bacterial polysaccharide) antigens, with and without the help of T cells. T cells only recognize antigens that are presented in combination with MHC molecules (i.e., by cells). The difference in antigen recognition is important since it ensures that both soluble and cell bound antigens are eliminated /18/.

Each T-cell and B cell receptor has an antigen recognition unit and a signaling unit. The antigen recognition unit has 1012–1015 variable regions. This remarkable diversity of the immune repertoire is achieved by random rearrangement of barely 400 genes in the early stages of lymphocyte development /7/.

B cell receptors

The genes that encode the B cell receptors (BCRs) are located on three chromosomes /7/:

  • The IGH cluster for the heavy chain is located on chromosome 3. This cluster contains gene segments for the variable (V), constant (C ), diversity (D), and joining (J) regions of the immunoglobulin.
  • The IgK cluster for the kappa light chain is located on chromosome 2
  • The IgL cluster for the lambda light chain is located on chromosome 22
  • The structure of the B cell receptor is shown in Fig. 21.1-10 – Structure of the mature and immature B cell receptor. Antigen recognition and presentation by B cells play a fundamental role in the immune response. Following BCR activation by antigen in combination with co stimulatory molecules such as CD40, B cells become potent antigen presenting cells. They process incorporated antigens in the same way as dendritic cells and macrophages and present the antigen together with an MHC-II protein. They activate CD4+ T cells and CD8+ T cells and produce cytokines.

T-cell receptors

T-cell receptor (TCR) genes are organized in a similar way to B cell receptor genes and also contain V, C, D, and J gene segments. The genes that encode the T cell recognition unit are located on three chromosomes /4/:

  • TCRA/D is located on chromosome 14 and encodes the α chain
  • TCRB is located on chromosome 7 and encodes the δ chain
  • TCRG is located on chromosome 7 and encodes the β chain.

Each locus contains multiple V, D, and J genes, but none for D segments. Each lymphocyte uses a different combination of these gene segments to create the genetic code of its antigen receptors, which results in a high degree of diversity. The structure of the T-cell receptor is shown in Fig. 21.1-11 – Structure of the mature T-cell receptor (TCR) and pre T cell TCR.

21.1.6.5 Antigen-dependent T-cell recognition

Antigen-dependent T cell receptors (TCRs) on the surface of the cell are associated with the CD3 complex of molecules that transmit signals into the cell when the TCR binds antigen /19/. This complex consists of a CD3γ molecule, a CD3δ molecule, and two molecules of CD3ε as well as a disulfide linked τ-chain homodimer. Cross-linking within the TCR when it binds to the antigen MHC complex initiates signal transmission. Aggregation of the receptor leads to phosphorylation of tyrosine residues in the cytoplasmic portion of the CD3 complex. The signal that is thus triggered initiates the transcription of various gene sequences in the cell nucleus. This results in cell proliferation and cytokine production. Refer to Fig. 21.1-12 – Activation of the T cell receptor of CD4+ T helper cells.

The B cell antigen recognition unit also associates with two signaling molecules, Igα (CD79a) and Igβ (CD79b), which transmit the activation signal into the cell in the event of antigen binding.

Co stimulatory signals

Antigen recognition by TCRs is associated with a high level of promiscuity. A second activation signal is therefore required to prevent inappropriate lymphocyte responses. This is achieved by co stimulatory signals, which are produced by contact between TCRs and ligands on the surface of neighboring cells or by TCR stimulation by cytokines. The following lymphocyte molecules react with co stimulatory ligands /20/:

  • CD28 with the antigen-presenting dendritic cell B7, or CD28 with molecule 4 (CTLA-4) that is associated with the cytotoxic T lymphocytes. The proliferation and differentiation of T cells and the synthesis of IL-2 are all activated.
  • CD154 ligands CD40 (expressed by B cells). The ligation of CD40 by CD154 of antigen stimulated CD4+T cells, stimulates B cell protein kinases that initiate antibody class switching. The switch fails when defects in the gene encoding CD154 occur. This is the case in X-linked hyper IgM syndrome with IgG, IgA, and IgE markedly decreased, but normal or elevated IgM.

Co stimulatory signals are also provided by cytokines such as TNF-α, IL-1, and IL-6. In the absence of co stimulation, antigen binding does not activate the T cell but leads instead to T cell anergy and apoptosis.

Refer to Fig. 21.1-12 – Activation of the T cell receptor of CD4+ T helper cells.

Inhibitory signals

IL-10 and TGF-β produce signals that down regulate the immune response. Binding of CTLA-4 to B7 or of IgG to B cell Fcγ receptors also exerts an inhibitory effect.

21.1.6.6 T cell independent immune recognition

Some antigens are recognized directly by B cells without the involvement of T cells. These include polysaccharides, polymerized flagellin, and microbial DNA, especially cytosine guanine dinucleotide sequences that are flanked by 5’ purines and 3’ pyrimidines. When they bind to B cell receptors, these antigens are taken up intra cellularly and processed into short peptides. The peptides, together with MHCII molecules, are expressed on the cell surface and recognized by adjacent Th2 helper cells. The helper cells become activated and express co stimulatory molecules such as the CD40 ligand (CD154). When CD154 on the helper T cell binds to CD40 on the B cell, a signal is generated that triggers an immune response in the form of antibody production.

Clonal selection

Activation of B cells, CD4+T cells, and CD8+T cells by antigens results in clonal selection. Each antigen can be recognized by only a few thousand lymphocytes. Following activation of B cells by CD4+T cells a signal is generated that prompts the B cell to begin the process of somatic hyper mutation and immunoglobulin class switching. A large number of antibodies with different specificities are produced from the lymphocytes, but every B cell expresses antibodies with only one of the many potential specificities /19/. B cells of this type are selected to participate in the immune response and proliferate to generate a family of cells (clone) that all produce the same antibody. Because a number of different clones are produced in most immune responses, microbial infections always elicits a polyclonal immune response.

Memory T and B cells

When a lymphocyte that has never been activated by an antigen (naive lymphocyte) encounters an antigen for the first time, the resulting immune response includes the production of memory T cells and B cells in addition to effector T cells and B cells. If the same antigen is encountered again, the resulting secondary immune response is faster and more effective. More lymphocytes, higher antibody concentrations and antibodies with higher avidity and specificity are produced than in the primary immune response.

21.1.6.7 Course of the immune response

The adaptive immune response is a complex process that is triggered by lymphocytes that circulate continuously through the body to detect antigens. T cells and B cells require approximately 30 minutes for each circulation. Antigen recognition and immune responses take place at various locations /19/:

  • In the spleen, for antigens circulating in the blood
  • In the local lymph nodes or the bronchial lymphatic tissues, when the antigens enter the respiratory tract or mucous membranes
  • Responses to intranasal antigens and inhaled pathogens occur in the adenoids and palatine tonsils
  • Antigens from the gut are taken up by specialized epithelial cells that transport the antigen across the epithelium to Peyer’s patches.

Registration of antigens entering the body through mucous surfaces activates lymphocytes in the mucosal associated tissues. Mainly lymphocytes in the mucous surfaces are CD8+α/β T cells with the appearance of large granular lymphocytes. An established function of these cells is to support the production of secretory IgA, while T cells with γ/δ receptors have a direct role in host defense. If, for example, an immune response is induced in the Peyer’s patches, then sensitized lymphocytes enter the blood and travel to the lamina propria of the intestinal mucous layer, where large amounts of secretory IgA are produced. However, responses induced in one mucous location (i.e., intra nasal), can also induce increased production of secretory IgA in the mucous tissues of other organs that are not exposed to the pathogen.

Lymphocytes from the blood enter the lymph nodes via specialized post capillary venules. The passage is mediated by adhesion molecules, for example the constitutively expressed selectin on lymphocytes. L-selectin binds to endothelial adhesion molecules of the venules. This interaction induces the lymphocytes to express lymphocyte function associated antigen (LFA-1), which facilitates the adhesion of the cells. In the next step lymphocytes migrate across the endothelium into lymphoid tissue. The spleen lacks these specialized venules.

The immune response is induced in the germinal centers of secondary lymph organs such as the spleen, lymph nodes, and Peyer’s patches. These germinal centers consist of a mesh of follicular dendritic cells in which CD4+T cells present antigen, B cells proliferate, plasma cell precursors are produced, immunoglobulin class switching occurs, and memory cells are separated. The germinal center provides an environment that optimizes the antibody response by bringing all of the relevant cellular components into contact /19/.

21.1.6.8 Antigen processing and presentation

T cells recognize peptides, which are presented by antigen presenting cells by way of MHC-I and MHC-II molecules (Fig. 21.1-13 – Simplified model of antigen presentation):

  • CD8+T cells, also known as cytotoxic or killer cells, recognize peptides that are bound to MHC-I molecules. The molecules present peptides that are synthesized in the cytoplasm and are present in nearly all nucleated cells. Such antigens are mainly self peptides or peptides of viral origin in infected cells. CD8+T cells produce cytotoxic molecules such as Fas ligand, perforin, and serine esterases to destroy target cells. Because CD8+T cells are specialized in eliminating antigens produced in the cytoplasm, they are particularly efficient in attacking virus infected cells.
  • CD4+T cells, also referred as T helper cells, recognize peptides that are bound to MHC-II molecules. These molecules present peptides that are released by cellular vesicles such as endosomes. They contain exogenous peptides that are taken up from the environment and ingested by polymorphonuclear neutrophils and macrophages. Antigens are cleaved into peptides by acid hydrolases such as nucleases, proteases, lipases, and glucosidases within endocytic vesicles. In the endosomes, the peptides are bound to MHC II molecules and trans located to the cell surface.

T-helper (Th) cell paradigm

Cytokines exert an important influence on the type of immune reaction that optimally eliminates a pathogen. CD4+T cells are cytokine secreting helper cells and are differentiated into Th1 and Th2 cells. According to the T helper cell paradigm /21/:

  • Th1 cells activate the cell mediated immune response through cytotoxic CD8+T cells and macrophages. They secrete IL-2, IFN-γ and TNF-β. By producing IFN-γ and IL-2, they stimulate cytotoxic CD8+T cells to kill virus-infected cells (Fig. 21.1-14 – Immune response of the Th1 cell), activate macrophages to kill intracellular pathogens (Fig. 21.1-15 – Cell mediated immune response of the Th1 cell), and stimulate B cells to produce complement binding antibodies. IL-12, produced by macrophages, is the main stimulator of the Th1 cell response.
  • Th2 cells stimulate B cells to produce a cytokine pattern that is dominated by IL-4, IL-5, IL-6, and IL-13. Antigen is presented to the Th2 cell by dendritic cells (Fig. 21.1-16 – Cell mediated immune response by the Th2 cell). Th2 cells promote the production of IgG antibodies, which bind hardly complement, and the production IgE. In an allergic reaction, the production of IgE antibodies is promoted by an IL-4 induced shift in the Th1/Th2 equilibrium in favor of Th2 cells.
  • Cytokines secreted by Th cells modulate the immune response. For example, IFN-γ secretion by Th1 cells inhibits the immune response of Th2 cells and IL-10 secretion by Th2 cells inhibits the Th1 response by reducing macrophage function. The Th cell immune response is shown in Fig. 21.1-17 – Development and function of Th1 and Th2 cells.

Regulatory T cells (Treg)

Treg cells are CD4+CD25+T cells that are produced in the thymus and peripheral lymph organs and represent 5–10% of CD4+T cells in the peripheral blood and up to 20% in the bone marrow /22/. Treg cells have a normal α/β T cell receptor pattern and express the α-chain of the IL-2 receptor (CD25), cytotoxic T lymphocyte associated antigen 4 (CTLA-4), glucocorticoid-induced TNF receptor family related gene (GITR), and the transcriptional regulator Foxp3. This regulator acts as a master switch gene for Treg development and function. Treg cells have a high affinity for self peptides.

Treg functions are /22/:

21.1.7 Immune response in infections

Entry of a pathogen into the host initiates a multitude of interactions between soluble molecules (e. g, complement, C-reactive protein and antimicrobial peptides) and host sensors with pathogen derived molecules. The innate immune cells (e.g., neutrophils, macrophages, dendritic cells) express sensors and innate receptors known as pattern recognition receptors (PRRs). The PRRs are evolutionary conserved germ-line encoded receptors that sense pathogen derived signature molecules known as pathogen associated molecular patterns (PAMPs).

The PRRs include the following families /2/:

  • Toll-like receptors (TLRs); essential in sensing bacteria
  • NOD-like receptors (NLRs); important role in sensing bacteria
  • RIG-I-like receptors (RLRs); important role in sensing viruses
  • C-type lectin receptors (CLRs); essential for sensing mycobacteria and fungi
  • DNA-sensing molecules; important role in sensing viruses.

The responses initiated by the innate immunity result in:

  • Killing the pathogens or inhibition of their replication
  • Initiation of the pathogen specific adaptive immunity through activation of B cells and T cells.

PRRs sense PAMPs in various compartments of the cells such as cytoplasm, cell surface and endocytotic vesicles.

21.1.7.1 Immune recognition of bacterial infection

According to the cell wall structure and composition bacteria are classified as Gram-positive and Gram-negative. The cell wall of Gram-positive bacteria are characterized by a thick peptidoglycan layer, the wall of Gram-negative bacteria contains a lipopolysaccharide (LPS) also known as endotoxin.

Innate immune defense

The innate immune system is well equipped to recognize and destroy bacteria through specialized defense cells (e.g., polymorphonuclear granulocytes, monocytes/macrophages and dendritic cells). These cells express genetically inherited receptors, called pattern recognition receptors (PRRs) for recognition of conserved pathogen associated molecular patterns (PAMPs). Signalling downstream from PRRs activates cellular responses, and killing mechanisms, and the expression of cytokines. The cytokines initiate inflammation and shape the adaptive immune responses /23/.

The peptidoglycan layer of bacteria is sensed by Toll-like receptor 2 (TLR2) of the defense cells. LPS, an immunopotent PAMP and virulence factor is sensed by TLR4. Both gram positive and gram negative bacteria contain a common ligand, the flagellin protein which is recognized by TLR5 /2/. Under normal conditions TLR2 is the major PRR involved in bacterial sensing and enhances the inflammatory response. However, this is not the case at low multiplicity of infection (MOI). At low MOI, TLR9 recognizes the bacteria by sensing bacterial DNA.

Immature dendritic cells (DCs) screen for bacteria entry using conserved PRRs and enhance the inflammatory response, which recognize PAMPs in microbial cell-wall components. These PRRs include the TLRs and C-type lectins for recognition. The TLRs relay infomation about the interacting bacteria to DCs through intracellular signaling cascades, thereby eliciting appropriate cellular processes that lead to DC maturation and the induction of inflammatory cytokines. The recognition of bacteria by C-type lectins leads to internalization of bacteria in DCs. Within the phagolysosome of DCs the bacteria target the C-type lectin DC-sign (DC-specific intercellular adhesion molecule-grabbing non integrin) and the processing for presentation by MHC class I and II molecules to T cells occurs. However, misuse DC-Sign by distinct mechanisms that either circumvent antigen processing or altered TLR-mediated signalling, skewing T-cell responses (e.g., mycobacterium, legionella, toxoplasma). This implies that adaption of bacteria to target DC-signal might support survival /23/.

Adaptive immune defense

The adaptive immune defense starts after processing of the bacterial components for presentation by MHC class I and II molecules to T cells.

21.1.7.2 Immune recognition of mycobacterium infection

M. tuberculosis infects immunocompromised individuals and children. The cell wall of M. tuberculosis consists of a mixture of polysaccharides and lipids with a high content of mycolic acid. M. tuberculosis is normally controlled, yet complete eradication of the mycobacterium does not occur. When the immune response is impaired, active disease can develop, normally through reactivation of quiescent mycobacteria or in some cases through re-infection. Refer to Section 42.12 – Infection with mycobacteria.

Innate immune defense

The adaptive immune response is as follows /23/:

  • Mycobacteria are inducers of TH1-cell responses and the mycobacterial components stimulate the expression of co-stimulatory molecules and the production of IL-12 by dendritic cells (DCs) through Toll-like receptors 2 and 4 (TLR2 and TLR4). Invading M. tuberculosis is captured by macrophages/DCs and the mannose capped cell wall component lipoarabinomannan (ManLAM) is bound to the C-type lectin DC-Sign (DC-specific intercellular adhesion molecule-grabbing non integrin).
  • Recognition of M. tuberculosis by TLRs expressed by DCs results in the activation of nuclear factor kappa B leading to the activation/maturation of DCs. Activation of DCs leads to the production of inflammatory cytokines.
  • Increased secretion of ManLAM by infected macrophages/DCs targets DC-Sign and results in inhibitory signals that interfere with the TLR activating stimuli that lead to DC maturation. The ManLAM-DC- Sign interaction results in inhibition of DC maturation and induction of the immunosuppressive IL-10, thereby preventing an efficient cellular immune response against M. tuberculosis infection.

Adaptive immune defense

The adaptive immune defense starts after processing of of the bacterial components for presentation by MHC class I and II molecules to T-cells.

21.1.7.3 Immune recognition of viral infection

Viral molecules such as genomic DNA and RNA or double stranded RNA produced in viral infected cells are recognized by pattern recognition receptors (PRRs) expressed in innate immune cells of the host such as dendritic cells (DCs). PRR recognition depends on the detection viral envelope proteins and nucleic acid motifs within the DNA or RNA genomes of the virus (refer to Section 21.1.7.1 – Immune recognition of bacterial infection). Type I interferons (IFN-α and IFN-β) are under tight transcriptional regulation and are induced after recognition of viral components by various host PRRs /2425/.

21.1.7.4 Recognition of helminth infection

The surfaces of helminths as well as there excretory/secretory products are rich in glycoproteins /26/.

Innate immune defense

Recognition of these carbohydrate domains is mediated by the carbohydrate binding protein family of receptors (C-type lectins) that are expressed by cells of the innate immune system (macrophages, DCs, epithelial cells). Helminth-matured DCs have relatively immature status; they often express low levels of co-stimulatory molecules and pro-inflammatory cytokines.

Adaptive immune defense

C-type lectins, alarmins and interleukins initiate CD4+Th2 cell cytokine response. The patients have a high Th2 cell count, low Th1 cell count, and eosinophilia. The Th2 associated cytokines IL-4, IL-9, IL-13, IL-25 and IL-33 play important roles in mediating the effector mechanisms that contribute to worm expulsion such as golet cell hyperplasia and mucin production. IL-10 and IgG4 concentrations are high whereas the increase in IgE is relatively slight /27/. Formation of granulomas occurs in patients with uncontrolled inflammatory responses, for example in schistosomiasis, where there is a strong immune response to the pathogen’s eggs that are embedded in the tissues. In these cases, there is a Th1 immune response with hepato­spleno­megaly and inflammation of the lymph nodes. The IgG4 concentration is normal, while that of IgE is substantially increased. Resistant persons who remain free from helminth infections have a natural balance in the Th1/Th2 immune responses that kill invading helminths. In these cases, the ratio of IgG4/IgE elevations is not shifted to the same extent described for the previous in favor of IgE

21.1.7.5 Recognition of fungal infection

The fungal cell wall is composed of carbohydrate polymers interspersed with glycoproteins. The three major components are polymers of glucose (β-glucans), polymer of N-acetylglucosamine (chitin), and mannans. The three components are intermingled throughout the cell wall, chitin tends to predominate near the plasma membrane, whereas the mannans have a propensity for the outer cell wall /28/. β-1,3 glucan forms the main structural scaffold of the cell wall. Many receptors in the tissues recognize β-glucans and three members of the scavenger receptor family, CD36, CD5, and SCARF1. The transmembrane receptor dentin of neutrophil granulocytes has a specificity for β-1,3 glucans. Engagement of PRRs leads to activation of signaling cascades which results in phagocytosis, respiratory burst and cytokine/chemokine gene induction. Fungi are activators of the complement system, resulting in opsonization due to deposition of C3b and iC3b on the fungal surface and activation of inflammatory cells as a result of C3a and C5a generation /29/.

21.1.8 Graft-versus-host disease

In allogenic transplantation of hematopoietic cells the most common life-threatening complication is graft-versus-host disease (GVHD) which occurs when immunocompetent T cells in the donated tissue (the graft) recognize the recipient (the host) as foreign. The resulting immune response activates donor T cells to gain cytolytic capacity and to then attack the recipient to eliminate foreign antigen-bearing cells. The two main clinical presentations are acute GVHD and chronic GVHD /30/.

In the early stage of HSCT, damaged donor tissue releases cytokines, leading to the development of a cytokine storm with increased release of adhesion molecules, co stimulatory molecules, MHC antigens, and chemokines. These danger signals activate the target tissues, including the antigen-presenting cells (APCs).

In the next step, T-cell receptors and co stimulatory molecules are activated and contact with APCs occurs. This results in allo reactive T-cell proliferation and differentiation. Activated T cells migrate to the GVHD target tissues (stomach, liver, skin, lung), where they recruit effector leukocytes (monocytes/macrophages, cytotoxic T cells, NK cells, granulocytes).

This results in destruction of the target tissue. In this effector stage, the T cell induced tissue destruction triggers an intensification of existing inflammation with further tissue injury.

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21.2 Immunodeficiency

Immunodeficiencies can be described as primary or secondary. While primary immune deficiencies are due to inherent dysfunction of the immune system and are chiefly genetic in etiology, secondary immune deficiencies are consequent to other underlying causes /1/.The number of patients with immunodeficiency is constantly increasing.

Reasons for this include:

  • Demographic shift toward an aging population
  • Improved tumor survival rates
  • Expanding indications for and increased use of bone marrow transplantation
  • Improved survival following organ transplantation (5-year survival following transplantation of solid abdominal organ is greater than 80%)
  • Improved anti retroviral therapy for patients infected with HIV and greatly extended survival.

The result of all of this is an increasing number of infections with pathogens and/or courses (Tab. 21.2-1 – Typical pulmonary pathogens associated with different types of immunodeficiency).

21.2.1 Primary immunodeficiencies

Primary immunodeficiencies (PIDs) are caused by hereditary or genetic factors and more than 300 different defects have been described. The International Union of Immunological Societies Expert Committee for Primary Immunodeficiency /2/ has classified the PIDs into nine groups (Tab. 21.2-2 – Major primary immunodeficiency groups).These disorders may affect one or multiple components of the immune response, including T cells, B cells, natural killer (NK) cells, macrophages, dendritic cells, immunoglobulins and complement proteins.

PIDs can be categorized into T cell deficiencies, B cell deficiencies, phagocytosis defects, and complement deficiencies, based on the four compartments of the immune response. Most PIDs are monogenic and exhibit Mendelian inheritance. For example, Bruton’s disease affects only men since it is an X-linked disease. X-linked PID affects all men with the gene, and autosomal dominant or codominant primary immunodeficiency affects all of the progeny. However, the majority of primary immunodeficiencies are autosomal recessive and thus have a much lower penetrance /3/.

PIDs usually present in early childhood but may manifest not before adolescence and adulthood with an immune disorder that manifests as recurrent or persistent infections. The primary deficiency is often associated with autoimmune disease or the presence of autoantibodies /3/. An estimated 0.2% of the population have primary immunodeficiency and 3–5% have an autoimmune disease. The gap between the two may be explained by an underlying complete or incomplete primary or secondary immunodeficiency. PIDs with autoimmune predisposition are listed in Tab. 21.2-3 – Primary immunodeficiencies associated with a predisposition to autoimmunity and the prevalence of autoimmune diseases in selected PIDs is shown in Tab. 21.2-4 – Symptoms and clinical findings suggestive of immunodeficiency.

The overall incidence of PIDs (with the exception of secretory IgA deficiency) is estimated to be 1 in 10,000 individuals. Of the patients with PID /4/:

  • Half have an antibody deficiency
  • Some 20% have a combined T cell and B cell deficiency
  • 10% have an isolated T cell deficiency
  • 18% have phagocytosis defects
  • Some 2% have a complement deficiency.

The individual incidences vary significantly, from 1 in 330 to 1 in 700 for secretory IgA deficiency to 1 in 500,000 for severe combined immunodeficiency. In children, males are affected 5 times more as females in the same age group, while the corresponding male to female ratio in adults is 1: 1.4.

21.2.1.1 T cell deficiency

Approximately 70–90% of peripheral lymphocytes are T cells, 5–10% are B cells, and 1–10% are natural killer cells (NK cells). Because the majority of peripheral lymphocytes are T cells, lymphopenia is most commonly due to a reduced number of T cells and is likely to be associated with immunodeficiency.

The antigen CD3 is carried by all T cells and is associated with the antigen receptor. Refer to Fig. 21.1-12 – Activation of the T cell receptor of CD4+ T helper cells.

Functional CD3 cells are:

  • CD3+CD4+T cells, which produce cytokines following contact with an antigen. The cytokines are important for macrophage and B cell activation and antibody production.
  • CD3+CD8+T cells, which are responsible for killing abnormal host cells (virus infected cells, malignant cells, cell mediated allogeneic graft).

T cell deficiencies are genetically heterogeneous, affect various components of adaptive immunity, and are due to a disorder in the development of the T cell repertoire.

These disorders can include:

  • Lack of development of thymocytes or the environment required for their activation
  • Abnormal peripheral T cells
  • Disorder of signaling between T cells or between T cells and their environment
  • Lack of co stimulation.

Approximately 10% of primary immunodeficiencies are caused by specific T cell immunodeficiencies, which are associated with increased susceptibility to infection by intracellular microorganisms such as Mycobacterium, Salmonella, Listeria, Toxoplasma, and viruses as well as fungal and protozoal infections /5/. On the other hand, microorganisms that are not usually pathogenic, such as the Mycobacterium vaccine strain (BCG) and infection by opportunistic pathogens such as Pneumocystis jirovecii can trigger a severe T cell immunodeficiency.

Patients with T cell deficiencies have an increased incidence of malignant tumors. This is due to several factors /6/:

  • Reduced detection and destruction of free DNA due to a compromised immune response. The latter also promotes the development of malignant lymphoproliferative diseases.
  • Reduced clearance of viruses such as the Epstein-Barr virus, Hepatitis B virus, Hepatitis C virus, Human T cell lymphotropic virus, Kaposi sarcoma associated virus, and Human papilloma virus. These viruses contribute to the immortalization and transformation of infected lymphocytes and are responsible for 10–15% of cancers worldwide.
  • The inability to eliminate viruses also causes chronic inflammation with increased cell proliferation. This results in an increased risk that rapidly dividing cells will sustain oncogenic mutations.

A spectrum of mainly primary T cell deficiencies is shown in Tab. 21.2-5 – Primary (mainly) T cell immunodeficiencies.

21.2.1.2 B cell deficiency

Plasma cells that are capable of producing immunoglobulin develop during the final stage of B cell differentiation. They develop from B lymphocytes, which are derived from hematopoietic stem cells. The B cells undergo a series of differentiations with reassignment of the B cell receptor genes. This results in expression of the μ chain and the light chain (kappa or lambda) on the B cell surface to produce a naive B cell. The naive B cell leaves the bone marrow and migrates to the B cell pool. Contact with an antigen triggers further differentiation and leads ultimately to the secretion of immunoglobulins /7/. Following interaction with antigen-specific T cells, the B cells initially secrete IgM and then undergo a class switch to produce high-affinity IgG, IgA, or IgE antibodies (Fig. 21.1-9 – Class switch recombination following contact between B2 cell and antigen). The role of the initially produced IgM antibody in the circulation is to bind to invading pathogens and activate complement. High-affinity IgG, IgA, and IgE antibodies are produced through class switch. These protect the organism from further spread and reinfection by the pathogen.

21.2.2 Immunodeficiency categories

Some of the primary immunodeficiencies fit the criteria for more than one category.

21.2.2.1 Primary antibody deficiency

Primary antibody deficiency includes deficiencies in the production and function of individual Ig classes, Ig subclasses, and antibody specificities. They occur either in isolation or in combination. A deficiency is diagnosed based on the respective age specific reference intervals /9/. Primary antibody deficiency accounts for approximately 55% of primary immunodeficiencies. Although it can occur at any age, it is most prevalent in childhood and in the third decade of life.

Primary antibody deficiencies are a heterogeneous group of diseases with heterogeneous etiologies. They are classified in Tab. 21.2-3 – Primary immunodeficiencies associated with a predisposition to autoimmunity.

Patients with humoral immune deficiencies have increased susceptibility to infections with encapsulated bacteria such as Hemophilus influenzae type B and Streptococcus pneumoniae. Patients with B cell deficiencies usually begin having infections at the age of 7–9 months, when placental antibodies no longer provide immune protection. Viral and fungal infections are not usually a significant problem in patients with antibody deficiency, apart from patients with X-linked agammaglobulinemia (XLA), who are susceptible to infection with the Enterovirus, which can cause chronic encephalomyelitis. In addition, if antibody deficiency is diagnosed before the occurrence of organ injury (bronchiectasis, pneumonic foci), developmental disorders are not a feature /9/. Immunoglobulin substitution enables many patients to lead normal lives.

There are four distinct clinical entities that account for the majority of primary immunodeficiencies and three rare entities /1011/:

  • Selective IgA deficiency
  • IgG subclass deficiency
  • Transient hypogammaglobulinemia in infancy
  • Specific polysaccharide antibody deficiency
  • Common variable immunodeficiency (CVID)
  • X-linked agammaglobulinemia
  • Hyper-IgM syndrome

Refer to Tab. 21.2-6 – Clinics and laboratory findings in primary antibody deficiency.

Algorithms for the differentiation of antibody deficiencies are shown in:

Primary antibody disorders usually present 3–4 months after birth, once maternal immunoglobulin from placental transfer is gone. The babies present with recurrent or severe bacterial infections with encapsulated bacteria e.g., Streptococcus pneumoniae, and Haemophilus influenzae.

Antibody disorders are characterized by the presence or absence of B cells. When B cells are present, disorders are further characterized by whether B cells are of normal quality or quantity /11/. Refer to Tab. 21.2-7– B cell phenotyping in the diagnosis of B cell deficiency.

Agammaglobulinemia accounts for 13% of antibody disorders and for 84% with Bruton’s disease. Infants who have agammaglobulinemia are born with a complete absence of B cells and may have no tonsils or lymph nodes /11/.

Hypogammaglobulinemia is characterized of by low or deficient serum concentrations of any of the Immunoglobulin classes and subclasses. Common variable immunodeficiency (CVID) accounts for 46% of hypogammaglobulinemias and 82% of cases of primary antibody disorders involve a hypogammaglobulinemia /11/. IgA deficiency, IgG subclass deficiency, transient hypogammaglobulinemia and CVID are the most common types of primary hypogammaglobulinemia.

21.2.2.2 Combined immunodeficieny

Immunodeficiencies that affect both the T-cell and B cell systems are known as combined immunodeficiencies. They can occur as mild or as severe combined immunodeficiency syndrome (SCID). SCID is a heterogeneous group of congenital disorders associated with markedly reduction of T cells and variable amounts of B cells. Typically, patients presents early in life with failure to thrive, recurrent diarrhea, rashes. and serious bacterial, fungal and viral infections /112/. Newborns are screened using an assay for determination of direct T cell receptor excisions circles (TRECs). TRECs are small circles of DNA created in T cells during their passage through the thymus as they rearrange their TCR genes.Their presence indicates maturation of T cells; TRECs are reduced in SCID /13/. Refer to Tab. 21.2-8– Frequency and inheritance of SCID.

21.2.2.3 Defects of the phagocytic system

Defects of the phagocytic system may be due to a defect in the function of macrophages and dendritic cells (e.g., defective migration and adhesion, or a lack of antimicrobial activity) /14/. Chronic granulomatous disease is the most common disorder /11/. It is characterized by pneumonia, abscesses, suppurative adnexitis, and gastrointestinal infections. Infections are related to the inability of the phagocytic system to kill catalase positive organisms including S. aureus, Burkholderia cepacia, Nocardia, Aspergillus, Serratia and Candida species. Patients are usually young and in some cases, the pathogen indicates the cause of the disease.

Refer to Tab. 21.2-9 – Primary phagocyte disorders.

21.2.2.4 Complement deficiencies

The actions of the complement system (opsonization, chemotaxis, destruction of bacteria via the classical, alternative, and lectin pathways) are mediated by the products of sequential complement activation and complement proteins. Complement deficiencies may be associated with severe bacterial infections. For more about complement deficiencies, refer to Chapter 24 – The complement system.

21.2.2.5 Diseases of immune deregulation

Primary immune deficiencies are often associated with autoimmune disease due to the deregulation of the immune system as a whole. In many immune deficiencies, lymphocytes may be present but dysfunctional, allowing for the development of excessive auto reactivity. Immune deregulation is commonly manifested as autoimmunity, cytopenias and inflammatory bowel disease. Clinical features not directly associated with immunodeficiency are prominent /15/. A wide range of organs may be affected. Autoimmunity is a hallmark of the clinical disease presentation. Cytokine/interleukin pathway defects involve mutations that can result in gain or loss of function.

Primary immune deficiencies with immune deregulation and/or autoimmunity /116/:

  • Autoimmune poly endocrinopathy candidiasis and ectodermal dystrophy (APECED)
  • Autoimmune lyphoproliferative syndrome (ALPS)
  • Familial hemophagocytic lymphohistiocytosis (FHL)
  • Lymphoproliferative disorders associated with EBV /17/
  • Immunodysregulation, polyendocrinopathy, enteropathy, and X-linked (IPEX)
  • IPEX-like diseases
  • IL-10/L-10 receptor deficiencies
  • PLCG2-associted antibody deficiency and immune dysregulation

21.2.2.6 Defects in innate immunity

Diseases of innate immunity are disorders with a predispositin to viral and fungal infections. Included are Toll like receptor defects and natural killer cell defects. Disorders of innate immunity are /1/:

  • NF-kappa B pathway defects, X-linked NEMO (online Mendelian inheritance in man; OMIM 300248 and autosomal gain of function mutations)
  • Toll-like receptor signalling pathway deficiency
  • Nk cell deficiency

Refer to Section 21.1.6.4– Antigen receptors.

21.2.2.7 Auto inflammatory disorders

Refer to Section 19.1.10 – Auto inflammation.

21.2.3 Secondary immunodeficiency

Secondary (acquired) immunodeficiencies occur when a healthy immune system of an individual is compromised by harmful influences. Acquired immunodeficiencies are secondary to diseases or therapy and play a far greater role than primary immunodeficiencies in numeral terms /18/. Viral infections (especially HIV), immunosuppressive therapies, malignancies, metabolic disorders, protein loss syndromes and poly trauma are the most prominent.

In transplant recipients receiving immunosuppressive medications, Cytomegalovirus infection or lymphoproliferative disease caused by the Epstein-Barr virus are indicators of severe immunosuppression. Fungal infections are associated with high morbidity and mortality in immunocompromised patients /19/.

Infections in association with low immunoglobulin levels in serum are relatively uncommon in secondary immunodeficiencies, with the exception of hypogammaglobulinemias, which occur in malignant disease, rarely due to medication, or in nephrotic syndrome. Reduced IgG levels occur in lymphoproliferative diseases and deficiencies of IgA and IgG occur in treatment with immunosuppressive, antirheumatic, or anticonvulsive medications. The dose and duration of treatment are important influence factors in treatment related antibody deficiencies /8/.

Refer to Tab. 21.2-10 – Secondary immunodeficiencies.

21.2.4 Evaluation of immunodeficiency

A congenital immunodeficiency is suggested by the symptoms listed in Tab. 21.2-4 – Symptoms and clinical findings of immunodeficiency /14/. The symptoms should prompt a thorough medical history and a step-by-step diagnostic strategy (Fig. 21.2-5 – Stepwise procedure for diagnosis of immunodeficiency).

21.2.4.1 Medical history

Regardless of the age of the patient, the medical history and clinical findings indicate possible immunodeficiency (Tab. 21.2-11 –Tests for suspected immunodeficiency/12/:

  • Increased susceptibility to infection or failure to thrive in neonates and infants. The spectrum of pathogens involved provides important clues. Infection with agents that are normally apathogenic or repeated severe infections with certain pathogens are particularly significant. Frequent viral and fungal infections may be due to an isolated T cell deficiency or combined T cell and B cell deficiency. Recurrent infections with bacteria may be due to a B cell, granulocyte, or complement deficiency or to common variable immunodeficiency syndrome (CVID).
  • Signs of dysregulation of the immune system (e.g., granuloma, autoimmunity, recurring fever, lymphoproliferation, inflammatory bowel disease, unusual eczema). Refer to Tab. 21.2-12 – Prevalence of autoimmunity in primary autoimmune disorders.
  • Conspicuous family history (e.g., infectious disease susceptibility, immunodeficiency, atopic disease, unknown causes of death)
  • Malformations of the urinary and respiratory tracts, cystic fibrosis, ciliary dyskinesia, cerebrospinal fluid fistula, neurospora infection /18/
  • HIV infection
  • Malignant disease, particularly in older individuals. The B cell system is involved. Noticeably low immunoglobulin values are observed as a result of chronic lymphocytic leukemia, Hodgkin’s disease, and multiple myeloma.

21.2.4.2 Laboratory findings

Diagnosis is based on /2021/:

  • Basic screening investigations
  • Immunophenotyping of peripheral blood cells to prove the integrity of the immune system
  • Functional in vitro assays to test the functionality
  • Immunization with recall antigens to test the functionality of the immune system
  • Molecular analysis.

Refer to Fig. 21.2-5 – Stepwise procedure for diagnosis of Immunodeficiency.

Basic screening analysis

  • Complete blood cell count and differential
  • Quantitative determination of IgG, IgA, IgM, IgE
  • Possibly, determination of IgG subclasses
  • Immunofixation electrophoresis in cases of suspected plasma cell dyscrasia in adults.

Immunophenotyping analysis

To discriminate among major lymphocyte populations, T cells, B cells, NK cells, and for the evaluation of the activation status of the immune system CD4+T cells and CD8+T cells the following cell surface markers are used /20/: CD45, CD3, CD4, CD8, CD19, CD16, CD56, and HLADR.

B cell deficiencies are more common than T cell deficiencies.

In decrease of the B cell count the differentiation of B cells is recommended Tab. 21.2-7 – B cell phenotyping in the diagnosis of B cell deficiency is recommended for further differentiation. The profile captures important B cell sub populations.

Immunophenotyping analysis assesses the count and percentage distribution of lymphocytes.

Functional in-vitro assays with antigens

After basic screening assays further workup in the diagnosis of primary immunodeficiency may need functional in-vivo assays or for testing the T cell dependent antibody response. Immunization with the recall antigen tetanus toxoid is used to test antibody response against proteins and pneumococcal vaccine is used to test the antibody response against polysaccarides (T cell independent antigen).

Genetic defects and presumed pathogenesis

For primary immune deficiency diseases refer to Reference /2/.

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Table 21.1-1 Danger associated molecular patterns (DAMPs) /1/

Source

Category

Examples

Exo-
genous

Microbial (microbes and their products)

Gram negative and gram-positive bacteria, lipopolysaccharides, lipopeptides, lipoteichoic acid, bacterial DNA, single-stranded RNA

Non microbial

Allergens, foreign substances, toxic substances

Endo-
genous

Stress molecules (alarmins)

Products of cell death, high-mobility group box 1 proteins, heat shock proteins, S100 proteins, DNA, RNA, adenosine, high ADP/ATP ratio, fibrinogen, uric acid crystals

Table 21.1-2 Pattern recognition receptors that mediate the intracellular signal transduction required to initiate inflammation /1/

Receptor family

Localization

Toll like receptors

Transmembrane
proteins

Receptors for advanced glycation end products

Transmembrane
proteins

Nucleotide binding domain, leucine-rich repeat containing proteins

Cytosolic

Retinoic-acid-inducible-gene-I-like receptors (retinoic-acid-inducible gene I, RIG-I; melanoma differentiation-’associated gene 5, MDA5)

Cytosolic

Table 21.2-1 Typical pulmonary pathogens associated with different types of immunodeficiency /2/

 

Neutrophil
deficiency

T-cell
deficiency

Antibody deficiency
syndrome

Example

Aplastic anemia (post-chemotherapy nadir)

AIDS, organ transplantation, malignant lymphoma

Agammaglobulinemia, multiple myeloma, B-CLL

Pathogen

Staphylococcus aureus

Pseudomonas aeruginosa

Aspergillus spp.

Other gram-negative pathogens

Pneumocystis jiroveci

Mycobacterium tuberculosis

Cryptococcus neoformans

Legionella spp.

Cytomegalovirus

Streptococcus pneumoniae and other Gram positive cocci, Hemophilus influenzae, Neisseria

AIDS, acquired immunodeficiency syndrome; B-CLL, chronic lymphocytic leukemia

Table 21.2-2 Major primary immunodeficiency groups /2/

Combined immunodeficiencies (CIDS)

Combined immunodeficiency with associated or syndromic features

Predominantly antibody deficiency

Diseases of immune dysregulation

Congenital defects of phagocyte number, function or both

Defects in innate immunity

Autoinflammatory disorder

Complement deficiency

Phenocopies of primary immunodeficiency

Table 21.2-3 Primary immunodeficiencies associated with a predisposition to autoimmunity /3/

Monogenic immunodeficiencies

Defects of T cells and/or thymic or extra thymic tolerance induction

  • Immune deregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome
  • Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED)

Complement deficiencies (refer also to Chapter 24 – The complement system)

  • C1q, C1r/s, C4, C2, MBL, ficolins
  • Partial deficiencies of C4A and/or C4B

B cell and immunoglobulin deficiencies

  • Mutations in CD40, CD40L, activation induced cytidine deaminase (AID)

Gene defects involving multiple cellular subgroups

  • Wiskott-Aldrich syndrome (WAS), X-linked agammaglobulinemia (XLA), nuclear factor kappa essential modulator (NEMO), purine nucleoside phosphorylase (PNP)

Polygenic immunodeficiencies

Defects of T cells and/or thymic or extra thymic tolerance induction

  • Omenn syndrome, autoimmune lymphoproliferative syndrome (ALPS)

Primary hypogammaglobulinemia

  • Selective IgA deficiency, selective IgG subclass deficiency
  • Common variable immunodeficiency syndrome (CVID)

Table 21.2-4 Symptoms and clinical findings suggestive of immunodeficiency /19/

  • Positive family history of immunodeficiency
  • 8 or more episodes of suppurative otitis per year
  • 2 or more severe episodes of sinusitis per year
  • 2 or more episodes of pneumonia per year
  • Indicated antibiotic therapy for more than 2 months without effect
  • Complications due to live vaccines (BCG, polio)
  • Failure to thrive in infancy (not related to diarrhea)
  • Recurrent deep skin or organ abscesses
  • 2 or more visceral infections (meningitis, osteomyelitis, septic arthritis, empyema, sepsis)
  • Persistent cutaneous and mucosal candida infections beyond the first year of life
  • Chronic graft-versus- host disease (e.g., erythema of unclear etiology in early infancy)
  • Recurrent systemic infections with atypical Mycobacterium (more than an isolated incidence of cervical lymphadenitis)

Table 21.2-5 Primary (mainly T-cell) immunodeficiencies

Clinical and laboratory findings

DiGeorge anomaly (DGA) /2223/

The DiGeorge anomaly (OMIM 188400), originally considered to be a manifestation of isolated thymic aplasia, is now defined as a member of a group of diseases that share a 22q11 chromosome deletion and are known as CATCH 22 syndrome (CATCH, cardiac anomalies, abnormal facies, thymic hypoplasia, cleft palate, and hypocalcemia). It is caused by micro deletions of specific DNA sequences in the DiGeorge chromosomal region. DGA is rarely familial, although some familial cases have been reported. It affects both sexes, with an incidence of 5 in 100,000 in Germany and 1 in 66,000 in Australia. However, because increasing numbers of cases are being detected using molecular biological methods, the estimated incidence of DGA/velocardiofacial syndrome could be as high as 1 in 3000 /23/.

Clinical symptoms: abnormal development of the third and fourth pharyngeal pouches in the early stages of embryonic development leads to aplasia or hypoplasia of the thymus and parathyroid glands. Other structures that develop around the same time may also be affected, resulting in vessel anomalies (right aortic arch), esophageal atresia, congenital heart defects, cleft uvula, hypertelorism, mandibular hypoplasia, low-set ears with a notched ear fold, short philtrum, and upper limb anomalies. Children with DGA typically present with hypoparathyroidism in the perinatal period rather than an increased susceptibility to infection. Usually, the cardiac anomalies associated with the syndrome are so severe that they also become clinically evident at the same time. Many patients exhibit mental retardation. The thymus should be measured using magnetic resonance tomography. Some patients first present later in life with persistent viral or fungal infections or recurrent hypocalcemic tetany. The severity of DGA varies significantly. In many patients, the immunodeficiency resolves in the first few years of life. Older patients may present with autoimmune diseases.

Laboratory findings: mild lymphopenia; CD3+T cells are reduced to a variable degree or are completely absent; CD4+T cells are completely absent or present at concentrations of less than 0.4 × 109/L; a compensatory increase in B cells occurs; NK cells are normal. The proliferative response to phytohemagglutinin is weak (index below 10) and there is a lack of a proliferative response to antigens. Hypocalcemia usually develops in the first two weeks of life.

Definitive diagnosis /24/: conotruncal cardial defect (truncus arteriosus, tetralogy of Fallot, interrupted aortic arc, or aberrant right subclavia) hypocalcemia of more than 3 weeks duration requiring treatment, deletion on chromosome 22q11.2.

Probable diagnosis /24/: CD3+T cells below 1.5 × 109/L and deletion on chromosome 22q11.2.

Possible diagnosis /24/: CD3+T cells below 1.5 × 109/L and one of the following symptoms: cardiac defect, hypocalcemia of more than 3 weeks duration requiring treatment, dysmorphic facies, palatal abnormalities.

Severe combined immunodeficiency syndrome (SCID) /5/

SCID comprise a group of rare monogenic disorders of early onset and a profound block in the development of T cells, with or without abnormal B cell differentiation. It can be the result of reticular dysgenesis, in which both myeloid and lymphoid cell lineages are affected, or of a disorder that affects only the lymphocytes (Tab. 21.2-8 –Frequency and inheritance of SCID). SCID has an incidence of 1 in 100,000 live births. Children with the disease are usually diagnosed within the first year of life, as soon as maternal antibodies cease to protect against infection. Approximately 25% of patients have a mild graft-versus-host reaction due to maternal chimerism. This is thought to occur due to a lack of alloreactive T cells /25/. One difficulty associated with clarifying the pathogenesis of SCID is that different clinical pictures may be produced by different mutations in the same gene and a similar phenotype may be caused by mutations in different genes.

Clinical findings: SCID should be considered in all children with recurrent infections. It is associated with tonsillar atrophy and non palpable peripheral lymph nodes. Secondary findings due to infection include hepatosplenomegaly, skin infiltration, and fever. Radiographic findings include an absent thymic shadow and, not infrequently, interstitial pneumonia. Other genetic syndromes commonly occur together with SCID, such as chromosomal anomalies (Down’s syndrome, Fanconi anemia), multiple organ dysfunctions (short limbed dwarfism, partial albinism, cartilage hair hypoplasia), or metabolic disorders.

Laboratory findings: Lymphopenia below 1 × 109/L. Determination of lymphocyte subsets using flow cytometry provides important information about the underlying defect. Tests of T cell function (lymphocyte proliferation assay, lymphocyte transformation assay, and cytokine production) correlate broadly with the lymphocyte count. The screening of SCID in neonatal diagnostics is carried out by T-cell receptor cycles (TRECs) using PCR. TRECs are formed during T-cell receptor rearrangements in the thymus. These are small stable circular DNA elements that are formed exclusively during the maturation of T cells in the thymus and are the indicator for maturation of T cells. In cases with SCID the formation of T cells is limited and the number of TRECs is reduced.

Definitive diagnosis /24/: male or female patient below the age of 2 years with:

  • A reaction to trans placentally acquired maternal T cells
  • Lymphocyte count below 3 × 109/L, less than 20% CD3+T cells and at least one of the following: mutation in the γ chain of the IL-2 receptor, mutation in JAK3, mutation in the recombination-activating genes (RAG1 or RAG2) of the immunoglobulin receptors, mutation in IL-7Rα, adenosine deaminase (ADA) activity less than 2% of that seen in healthy controls or a mutation in both ADA genes (Fig. 21.2-6 – Purine degradation and recovery).

Probable diagnosis /24/: male or female patient below the age of 2 years with:

  • Lymphocyte count below 3 × 109/L, less than 20% CD3+T cells and a proliferative response to mitogens that is below 10% of that seen in healthy controls
  • Presence of maternal T cells in the blood.

X-linked SCID (XSCID), γ-chain defect /2526/

XSCID (OMIM 300400) is the most common variant of SCID, accounting for approximately 50% of cases. It is caused by a mutation of a gene on chromosome Xq13 that encodes the γ chain (γc) of the IL-2 receptor on T lymphocytes. γ chain is also a component of the cytokine receptors IL-4R, IL-7R, IL-9R, and IL-15R (Fig. 21.2-7 – Activated CD4+T cell with antigen and cytokine receptor). Because the cytoplasmic domain of the γ chain cannot activate the JAK-STAT (signal transducer and activator of transcription) signaling pathway, cellular communication is disrupted. Although the B cells are normal, they do not undergo isotype class switching from IgM to IgG. Few T cells are produced, since the effectiveness of IL-7 as a thymocyte-stimulating cytokine and of IL-2 as a stimulator of mature T cells are dramatically reduced by the receptor defect. Without successful bone marrow transplantation, the condition is fatal.

Laboratory findings: XSCID is also known as T NK B+ SICD, due to the behavior of the lymphocyte subpopulations. Lymphocyte count below 1 × 109/L, less than 1% T cells, 60–80% B cells, less than 1% NK cells. Hypogammaglobulinemia is present.

Clinical findings: between the 2nd and 7th months of life, failure to thrive and persistent diarrhea, respiratory symptoms, and oral thrush. Infection with bacteria, Pneumocystis carinii, and Mycobacterium.

Definitive diagnosis /24/: male or female patient with:

  • A reaction to trans placentally acquired maternal T cells; less than 10% CD3+T cells; less than 2% CD16/56 NK cells, and more than 75% CD19 B cells
  • In addition, one of the following: mutation in the gene that encodes the γ chain; absence of γ chain mRNA on Northern blot analysis of lymphocytes; lack of γ chain expression in lymphocyte cell lineage; maternal cousins, uncles, or nephews with SCID.

Probable diagnosis /24/: male or female patient with:

  • Less than 10% CD3+T cells; less than 2% CD16/56 NK cells; and more than 75% CD19 B cells
  • In addition, one of the following: failure to thrive in the first year of life; serum IgG and IgA at least 2 SD below normal for age; persistent diarrhea, oral thrush, or urinary tract infections.

Possible diagnosis /14/: male or female patient with:

  • More than 40% circulating CD19 B cells
  • In addition, one of the following: a reaction to trans placentally acquired maternal T cells; maternal cousins, uncles, or nephews with SCID.

Interleukin 2 receptor α-(IL-2) deficiency /27/

The IL-2 receptor is a multimeric complex composed of the two constitutively expressed subunits IL-2β (CD122) and IL-2γ (CD132) and a variably expressed subunit IL-2Rα (CD45) (Fig. 21.2-7 – Activated CD4+T cell with antigen and cytokine receptor). Depending on which of the three chains is expressed, IL-2 binds with lower or higher affinity. Of the three subunits, IL-2Rα is the most highly regulated.

An important function in the thymus is the regulation of triple negative T cells (CD3, CD4, and CD5). These cells express the bcl-2 protein, which protects them against programmed cell death (apoptosis). The presence of the IL-2Rα chain is important for cell selection. Cells that lack IL-2Rα express high levels of bcl, which enables them to evade apoptosis. Unlike CD25+CD4+T cells, they do not suppress self-tolerance. The phenotype of CD25 deficiency differs significantly from other forms of SCID, with extensive lymphocytic infiltration of the tissues.

Clinical findings: the incidence of IL-2Rα deficiency is very low. It is associated with susceptibility to bacterial, viral, and fungal infections. Diarrhea, splenomegaly, lymphadenopathy, pneumonia, and gingivitis develop by the third year of life.

Laboratory findings: CD3+T cells and CD4+T cells decreased, reduced T cell function. IgG slightly elevated, IgM normal, IgA decreased.

CD3 deficiency /28/

Mature T lymphocytes recognize antigens by means of the T-cell receptor (TCR) a multimeric complex containing the CD3 molecule (TCR)/CD3 complex) (refer to Fig. 21.1-12 – Activation of the T cell receptor (TCR) of CD4+ T helper cells). The TCR is first expressed during T-cell development in the thymus. CD3ε and CD3τ are important signaling molecules of the TCR. A genetic deficiency in these molecules therefore leads to a signaling defect and strongly influences early T cell differentiation events.

Clinical findings: the incidence of CD3 deficiency is very low. Clinical symptoms develop before the third year of life and are highly variable, ranging from severe SCID to mild immunodeficiency. CD3γ deficiency has a high mortality rate.

Laboratory findings: mild lymphocytopenia, lack of CD3+T cells, CD4+ T cells and CD8+ T cells are normal or slightly decreased. IgG, IgA, and IgM are normal.

Interferon-γ receptor deficiency /29/

IFN-γ is a pleiotropic cytokine secreted by NK cells and T cells and transmits its action via IFN-γ receptors. The IFN-γ receptor 1 (IFNGr-1) is encoded by four genes. Mutations in IFNGR-1 gene are recessive and can be homozygous or compound heterozygous. IFN-γ is one of the most important activators of macrophages, which play an important role in the activation of immune cells and in the pathogenesis of mycobacterial infections.

Clinical findings: patients with this immunodeficiency experience recurrent infections with M. tuberculosis and atypical mycobacteria that are difficult to treat. Similar, but milder symptoms are seen in patients with a defect of the β1 molecule of the IL-12 receptor.

Laboratory findings: attenuated (but still present) stimulation of the cellular immune response may indicate IFNGR-1 deficiency. Gene sequencing and gene transfer provide the definitive diagnosis.

JAK3-deficient SCID /30/

JAK3 is a member of the Janus kinase family of non receptor protein intracellular tyrosine kinases. Together with the STAT (signal transducer and activator of transcription) proteins, the JAKs are part of a system that cytokine receptors use to transmit signals into cells (Fig. 20.1-1 – The receptor complex for the transduction of the IL-6 signal consists of the receptor protein IL-6Rα and the signal transducer protein gp130). SCID due to JAK-3 deficiency is caused by mutations in the JAK gene on chromosome 19p13.1. The mode of inheritance is autosomal recessive.

Clinical findings: the clinical picture associated with JAK3 deficiency SCID is similar to that of XSCID. Symptoms emerge at the age of 3 months, most commonly recurrent severe respiratory infections, failure to thrive, and diarrhea.

Laboratory findings: absent T cells and NK cells, normal B cell count; other findings as in XSCID.

ZAP-70 deficiency /3132/

ZAP-70 is a member of the cytoplasmic tyrosine kinase family and plays a role in the transmission of signals from antigen receptors into cells (Fig. 21.2-7 – Activated CD4+T cell with antigen and cytokine receptor). This family includes ZAP-70, which is located in T cells and NK cells, and Syk, which is expressed by B lymphocytes and thrombocytes. Mutations in the ZAP gene lead to the production of functionally inactive and unstable ZAP proteins.

Clinical findings: ZAP deficiencies are very rare. Affected children present from the age of 3 months with bacterial, viral, and fungal infections. Also common are opportunistic infections caused by Pneumocystis carinii. These infections are as severe as those that occur in XSCID. The thymus is of normal size and the lymph nodes are palpable.

Laboratory findings: normal lymphocyte count. Lymphocyte subsets: CD4+T cells, NK cells (CD56), and B cells normal; CD8+T cells decreased. The CD8+T cell count is lowest in early childhood and then increases. Lymphocyte proliferation in response to both mitogens and antigens is decreased. Specific antibody production (e.g. following tetanus vaccination) fails to occur. Isohemagglutinin titers are substantially reduced (0 to less than 1 : 1). Concentrations of IgG, IgA, and IgM are normal.

Wiskott-Aldrich syndrome

WAS (OMIM 301000) is an X-linked recessive syndrome caused by a defect of the Wiskott-Aldrich syndrome protein (WASP). The mutated WASP gene is located on chromosome Xp11.22–11.2384 and is found in lymphocytes and megakaryocytes. The defective WAS protein, a peptide of 501 amino acids, lacks the hydrophobic transmembrane domain (Fig. 21.2-7). The WAS protein regulates signaling and the reorganization of hematopoietic cells.

Clinical findings: eczema, thrombocytopenic purpura with morphologically normal megakaryocytes. Patients present in childhood with hemorrhage (e.g., following circumcision, or spontaneous epistaxis). During the first year of life, patients develop atopic dermatitis and are subject to recurrent infections due to encapsulated bacteria such as S. pneumoniae, which cause pneumonia, otitis media, meningitis, and sepsis. They subsequently develop autoimmune diseases such as vasculitis and autoimmune thrombocytopenia. Approximately 40–72% of Caucasian WAS patients and 22% of Japanese WAS patients develop autoimmune disorders. The tumor incidence is 13–22%.

Laboratory findings: thrombocytopenia (microthrombocytes); low isohemagglutinin titers; IgM often low; IgA and IgE elevated; IgG normal or slightly decreased. The T cell counts are slightly decreased, the mitogen-stimulated lymphocyte proliferation is moderately reduced.

Definitive diagnosis /24/: male patient with congenital micro thrombocytopenia and a thrombocyte count of less than 70 × 109/L in addition to at least one of the following: mutation in the WASP gene; absence of WASP mRNA in Northern blot analysis of lymphocytes; absence of WASP in lymphocytes; maternal cousins, uncles, or nephews with micro thrombocytopenia.

Probable diagnosis /24/: male patient with congenital micro thrombocytopenia and a thrombocyte count of less than 70 × 109/L in addition to at least one of the following: eczema, abnormal immune response to polysaccharide antigens, recurrent bacterial or viral infections, autoimmune disease, leukemia, lymphoma, or brain tumor.

Possible diagnosis /24/: male patient with congenital micro thrombocytopenia and a thrombocyte count of less than 70 × 109/L or a male patient who has undergone splenectomy due to thrombocytopenia in addition to at least one of the following: eczema, abnormal immune response to polysaccharide antigens, recurrent bacterial or viral infections, autoimmune disease, leukemia, lymphoma, or brain tumor.

The antigen receptors of B cells and T cells are encoded by genes that are assembled as a sresult of lymphocyte-receptor-specific V(D)J recombinations. DNA repair proteins are involved in the recombination process. Natural mutations in the lymphocyte-specific V(D)J recombination genes, Recombinase-activating genes 1 and 2 (RAG1 and RAG2), lead to a primary immunodeficiency. In Omenn syndrome, the deficiency is partial.

Clinical findings: opportunistic and fungal infections from the age of 3–6 months, refractory candidiasis, bacterial infections as described for SCID. Non-infectious complications due to a graft-versus-host reaction (e.g., following blood transfusion). Immunization with live viral or bacterial vaccines can cause fatal reactions. Tonsils and lymph nodes are absent.

Laboratory findings: absence of mature B cells and T cells in peripheral blood but normal NK cell count. Immunoglobulins not detectable, no serum antibodies.

RAG mutation

In contrast to SCID, patients with the RAG mutation have at least one missense mutation, which enables them to retain some residual RAG activity. In genetic and biochemical terms, Omenn syndrome is a form of SCID caused by mutation of one of the RAG genes in which partial V(D)J recombination activity is maintained.

Clinical findings: Omenn syndrome presents in early infancy with a severe exudative erythroderma similar to that seen in a graft-versus-host reaction, hepatosplenomegaly, lymphadenopathy, and chronic diarrhea.

Laboratory findings: hypoproteinemia due to protein loss through the skin and intestine, elevated serum IgE, eosinophilia. Variable T-cell count with coexpression of activation markers such as CD45R0, HLA-DR, CD25, CD30, CD70, and CD95.

Artemis gene mutation

Artemis is a DNA V(D)J recombination repair factor of the metallo-beta-lactamase superfamily that repairs double-stranded DNA that has been cleaved by RAG1 and RAG2. A deficiency in Artemis results in inability to repair DNA after double stranded DNA has cut by RAG1 and RAG2 gene products in rearranging antigen receptor genes from their germline T configuration. The defect results in a TBNK+ SCID, also called Athabascan SCID.

Purine nucleoside phosphorylase deficiency

PNP and adenosine deaminase (ADA) are important enzymes in purine metabolism. A deficiency in one of these enzymes leads to the development of SCID. PNP catalyzes the phosphorylation of inosine, deoxyinosine, guanosine, and deoxyguanosine with the formation of guanine or hypoxanthine and ribose-1-phosphate or 2’-deoxyribose-1-phosphate (Fig. 21.2-6 – Purine degradation and recovery). In this way, a balance is established between the formation of dephosphorylated toxic purines and their disposal as uric acid. The lack of products in PNP deficiency does not contribute to the immunodeficiency because deficiency in the enzyme of the of the purine salvage pathway hypoxanthine-guanine phosphoribosyl transferase (HGPRT) causes Lesh-Nyhan syndome with normal immune function. Of the four substrates of the PNP reaction (inosine, deoxyinosine, guanosine and deoxyguanosine) only deoxyguanosine has an alternative fate, because it is phosphorylated by deoxyguanosine kinase. Therefore deoxyguanosine is the only candidate to mediate toxic effects and causes the lymphoid abnormalities in PNP deficiency.

Clinical findings: PNP deficiency is a rare autosomal recessive defect that accounts for 4% of all SCID cases. Various mutations exist in the gene product, which lead to different degrees of reduction in PNP activity. Patients usually have a triad of symptoms that includes recurrent infections (with unusual pathogens), neurological abnormalities, and autoimmune disorders. Children with PNP deficiency are clinically remarkable during the first year of life because for failure to thrive, respiratory infections, oral thrush, and chronic diarrhea.

Laboratory findings: serum uric acid below 2 mg/dL (119 μmol/L), although in some families levels are only slightly reduced. However, significant decreases in uric acid are also seen in diseases of the proximal renal tubule (Fanconi syndrome) and xanthine oxidase deficiency. Increased concentrations of inosine and deoxyinosine in the blood and urine, decreased PNP activity in the hemolysate of washed erythrocytes, guanosine triphosphate concentrations in the erythrocytes around 10% of normal. Patients are anemic, CD3+T cell count and in vitro lymphocyte functions are decreased.

Adenosine deaminase (ADA) deficiency

ADA is a monomeric, zinc-containing enzyme with a molecular weight of 41 kDa. It is encoded by a 32 kb gene on chromosome 20 that consists of 12 exons. ADA converts adenosine and 2’-deoxyadenosine into inosine and deoxyinosine. Although it is not essential for cell survival, ADA reduces the cytotoxic DNA degradation products of cellular turnover that accumulate in the thymus, bone marrow, and lymphocytes in particular during an immune response. A further detrimental process is due to the fact that unconverted adenosine and 2’-deoxyadenosine inhibit the enzyme S-adenosyl homocystein hydrolase, and the accumulated S-adenosyl homocysteine in turn inhibits S-adenosyl methionine dependent cellular methylation reactions (Fig. 21.2-6 – Purine degradation and recovery). These and other mechanisms lead to disorders of the purine metabolism in lymph organs and to impaired formation of immune cells. ADA deficiency is genetically heterogeneous, and more than 60 gene mutations are known. Most patients are heteroallelic.

Clinical findings: ADA deficiency has an incidence of 1 in 200,000 to 1 in 1 million births. Approximately 15–20% of patients are diagnosed between the ages of 1 and 8 years. Other patients present with recurrent respiratory tract infections between the ages of 8 and 16 months. These infections are not caused by opportunistic pathogens and are not usually severe enough to require admission to hospital. Most patients with suspected SCID who have ADA are older children or adults. Thymus, lymph nodes, and tonsils are absent. Often, adults have already suffered for a decade, frequently from chronic pulmonary insufficiency. Patients with ADA are at higher risk of CNS impairments than other SCID patients.

Laboratory findings: lymphocytopenia since birth, often below 0.5 × 109/L, decrease in or absence of circulating T cells, B cells, and NK cells. IgG, IgA, and IgM deficiency, lack of specific antibodies. In vitro: reduced lymphocyte proliferation in response to mitogenic stimulation, decreased lymphocytic response to stimulation with recall antigens. Aminotransferase elevation is not uncommon. ADA activity in the lysate of washed erythrocytes is less than 1% of normal. A 5 mL sample of EDTA blood is required.

Ataxia teleangiectasia (AT)

AT is an autosomal recessive disorder caused by mutations in the ATM gene on chromosome 11q22. The ATM gene encodes a serine/threonine kinase of the same name. The ATM protein phosphorylates p53 and other proteins that are involved in repairing damaged DNA before the start of the next replication cycle. It is required for V(D)J rearrangement in the synthesis of antigen receptors for CD4+T cells, CD8+T cells, and B cells as well as for the normal maturation of these cells. AT is associated with a general functional defect in the absence of significant changes in the lymphocyte count. Patients have a normal innate immune response and no significant NK cell abnormalities. Absence of the thymus is a characteristic feature; if the thymus is present, the Hassall corpuscles are absent. The disorder of DNA repair is not limited to the immune cells but affects all body cells to varying degrees, especially the parenchymal cells of the cerebellum, brain stem, spinal cord, and peripheral nerves.

Clinical findings: AT is a congenital disorder associated with a range of congenital anomalies, of which immunodeficiency is one of the most prominent. Affected children become remarkable because of ataxia and frequent falls once they start walking. Many are confined to a wheelchair by the age of 10 years. The immunodeficiency symptoms include upper respiratory tract infections such as retropharyngeal rhinitis, otitis media, and retropharyngeal accumulation of mucus. Other congenital anomalies include telangiectasia of the conjunctiva, nose, or ears that develops at the age of 4–8 years. AT patients have a 40% lifetime risk of malignancy (primarily hematological), most commonly malignant B cell lymphomas. A small proportion of patients have hypogonadism. Patients have an increased sensitivity to ionizing radiation. Some patients are not diagnosed until the second decade of life.

Laboratory findings: the serum concentration of α1-fetoprotein, which in children above the age of 2 years is less than 10 μg/L, is persistently elevated. This is thought to be of hepatic origin. Aminotransferases may also be elevated in the absence of liver disease. CEA elevation also occurs.

Immunoglobulins: often very low IgA and IgE; decreased IgG, in particular IgG2.

Lymphocyte count: B cells normal, CD3+T cells decreased, CD4+T cells decreased, CD8+T cells normal, NK cells variable.

Lymphocyte function: decreased proliferation and transformation in response to mitogenic stimulation. The response (cytokine production, transformation) to antigens such as tetanus toxin and EBV infected autologous B cells is decreased or absent.

Radiosensitivity testing: this test has a diagnostic sensitivity and specificity of greater than 95% in young children with suspected AT who have not yet developed the full clinical picture. It measures radiation sensitivity by irradiating a lymphoblast cell line from the patient in vitro with one Gy. In AT patients, this causes increased DNA strand breakage.

Definitive diagnosis /24/: progressive cerebellar ataxia, increased radiation induced DNA strand breakage in radiosensitivity testing, mutation in both alleles of the ATM gene.

Probable diagnosis /24/: progressive cerebellar ataxia and three of the following: ocular or facial telangiectasias, serum IgA at least 2 SD below normal for age, α1-fetoprotein at least 2 SD above the expected concentration, increased radiation-induced DNA strand breakage in radiosensitivity testing.

Possible diagnosis /24/: progressive cerebellar ataxia and one of the findings mentioned above.

Nijmegen breakage syndrome (NBS)

NBS (OMIM 2511260) is an autosomal recessive disorder caused by mutations in the NBS1 gene on chromosome 8q21. NBS is similar to ataxia telangiectasia insofar as it also involves rearrangements of chromosomes 7 and 14, hypersensitivity to ionizing radiation, and immunodeficiency. However, ataxia and telangiectasia are absent.

Clinical findings: patients have short stature, microcephaly, and bird-like facies. Immunodeficiency manifests as bronchopneumonia, urinary tract infections, recurrent mastoiditis, sinusitis, and otitis media. Some 40% of patients develop malignant disease.

Laboratory findings: α1-fetoprotein is normal; IgA, IgG2, and IgG4 decreased; CD3+T cells and CD4+ T cells decreased; CD8+ T cells normal; lymphocyte proliferation in response to mitogenic stimulation decreased.

MHC class I deficiency (MHC I)

MHC I antigen deficiencies are rare and the immunodeficiency they cause is not as severe as SCID. Serum from these patients contains a normal amount of MHC I protein and a normal concentration of β2-microglobulin. There is a deficiency of CD8+T cells while the CD4+T cell count is normal. Mutations exist in two genes in the MHC locus on chromosome 6. The genes TAP1 and TAP2 both encode molecules that transport peptide antigens. The peptide antigens are transported from the cytoplasm through the membrane of the Golgi apparatus, where they are bound to the α-chain of the MHC I molecule and to β2-microglobulin (Fig. 18.12-1 – HLA antigens on the cell membrane of nucleated cells. MHC class I antigens are composed of two chains, a heavy chain and β2-microglobulin). The complex then moves through the cytoplasm to the cell membrane, where it is presented to the immune cells. If the MHC I molecule is defective, the complex disintegrates in the cytoplasm before reaching the cell surface.

MHC class II deficiency (MHC II)

Four different gene defects leading to decreased expression of MHC II molecules have been identified:

  • Mutation in the gene on chromosome 1q that encodes the protein RFX5, a subunit of the RFX complex, which binds the X-box motif of the MHC II promoter
  • Mutation in the gene on chromosome 13q that encodes a 36 kDa subunit of the RFX complex (RFXAP), which binds the X-box motif of the MHC II promoter
  • Mutation in the gene that encodes a third subunit of the RFX complex (RFXANK)
  • Mutation in the gene on chromosome 16p13 that encodes an MHC II transactivator. This molecule controls the cell specificity and inducibility of the MHC II molecules as a non-DNA-binding coactivator.

A deficiency of MHC II molecules results in abnormal T cell selection in the thymus, and is characterized by the presence of normal amounts of dysfunctional T cells and B cells (T+ B+ SCID). All bone marrow derived cells fail to express MHC II molecules (DR, DP, DQ) and HLA-DM.

Clinical findings: this deficiency most commonly affects individuals of North African and Mediterranean origin. Affected children usually develop severe infections (Pseudomonas spp., Cytomegalovirus, and Cryptosporidium spp.) in the first 6 months of life. Severe diarrhea, pneumonia, and sepsis occur frequently. Overall, symptoms tend to be less severe than in SCID. Mycobacterial infections do not typically occur and patients do not develop graft-versus-host disease if they receive non irradiated blood products.

Laboratory findings: Low CD4+T cell count, CD8+T cells normal or increased, MHC II antigens HLA-DP, DQ, and DR are not detectable on B lymphocytes and monocytes.

Definitive diagnosis /24/: male or female with reduced expression of HLA-DR and HLA-DP (below 5% of normal) on B lymphocytes and monocytes. Mutations in one of the genes CIITA, RFX-B, RFX-5, or RFX-AP.

Probable diagnosis /24/: male or female with reduced expression of HLA-DR and HLA-DP (below 5% of normal) on B lymphocytes and monocytes in addition to all of the following: failure to thrive, opportunistic infections, persistent viral infections; normal T-cell count and B cell count; normal proliferative response to mitogens.

Possible diagnosis /24/: male or female with reduced expression of HLA-DR and HLA-DP (below 5% of normal) on B lymphocytes and monocytes in addition to one of the following: hypogammaglobulinemia; normal proliferative response to mitogens but lack of T cell proliferation in response to specific antigens; decreased CD4+T cell count; defective monocyte stimulation of T cells in mixed lymphocyte culture.

X-linked proliferative disease

X-linked proliferative disease (XLP, Duncan disease) is a rare immunodeficiency characterized by the triad of fulminant infectious mononucleosis, dysgammaglobulinemia, and lymphoma. Affected patients also have an inappropriate immune response to EBV infection that is characterized by the uncontrolled expansion of EBV infected B cells as well as expansion of CD8+T cells and macrophages. This is often associated with hemophagocytic lymphohistiocytosis. On a molecular basis, XLP is caused by mutations in the Src homology 2 domain containing gene 1A (SH2DIA), which encodes the signaling activation molecule (SLAM) associated protein SAP (SLAM associated protein).

Laboratory findings: DNA sequencing is the gold standard for diagnosing XLP.

IL-2 inducible T cell kinase

IL-2 inducible T cell kinase (ITK) deficiency is characterized by severe EBV induced immune dysregulation. Patients exhibit uncontrolled EBV infection and features consistent with hemophagocytic lymphohistiocytosis. ITK is a cytoplasmic non-receptor associated tyrosine kinase that is expressed by thymocytes, mature T cells, NK cells, and mast cells. ITK is expressed in response to the activation of antigen receptors on these cells and is required for phospholipase C induction. The ITK deficiency is due to missense or nonsense germline mutations.

Laboratory findings: patients with ITK deficiency have significantly decreased numbers of CD45RA+ T cells.

Epidermodysplasia verruciformis

Epidermodysplasia verruciformis (EV) is a rare genodermatosis that is characterized by increased susceptibility to specific HPV (Human papilloma virus) types. It begins in childhood with polymorphic skin lesions including pityriasis versicolor like maculas and flat wart-like papules. Approximately 75% of patients have mutations in the EVER1 and EVER2 genes. These genes encode proteins in the endoplasmic reticulum and keratinocytes. The EVER proteins modify ion channels.

Laboratory findings: decreased T lymphocyte count and CD4+T cell count. Reduced T cell responsiveness to mitogens; cutaneous anergy to common skin antigens; defective cell mediated immune response to EV specific HPV types.

WHIM syndrome

WHIM syndrome is characterized by warts, hypogammaglobulinemia, infections, and myelokathexis (retention of mature granulocytes in the bone marrow). It has an autosomal dominant inheritance pattern. Patients develop warts and condylomata acuminata of the anogenital tract as a result of HPV infection. Mutations are identified in the CXCR4 gene, which encodes the chemokine receptor CXCR4. The receptor is a member of the G protein coupled receptor super family, which selectively binds the stromal cell derived factor CXCL12. The identified CXCR4 mutations include 3 nonsense mutations and a frame shift mutation.

Laboratory findings: neutropenia with hyper cellular granulopoiesis in the bone marrow; B cell lymphopenia, particularly of CD27+ memory cells; T cell lymphopenia with a normal CD4/CD8 ratio; preserved proliferative response to mitogens; hypogammaglobulinemia.

DOCK 8 deficiency

DOCK 8 (dedicator of cytokinesis 8) belongs to the DOCK 180 super family and is an exchange factor for Rho family GTPases that functions downstream of cell surface receptors in signal transduction pathways. Mutations in the DOCK 8 gene reduce the expression of the DOCK 8 protein. Patients are prone to cutaneous viral infections, caused by Herpes viruses in particular, and have increased susceptibility to malignancy.

Laboratory findings: decreased lymphocyte count due to reduction in CD4+T cells and CD8+T cells with normal CD4/CD8 ratio; mild to moderate eosinophilia.

Chronic mucocutaneous candidiasis

CMC refers to a rare group of disorders characterized by chronic cutaneous and mucosal infections with Candida spp. The autosomal dominant form (OMIM 114580) is more common than the autosomal recessive form (OMIM 21250). CMC can be associated with autoimmune poly endocrinopathy-candidiasis-ectodermal dysplasia. Patients have a normal lymphocyte count /5/.

Hyper immunoglobulin E syndrome

Hyper immunoglobulin E syndrome, also known as Job syndrome, is caused by mutations in the STAT3 gene, which encodes the protein of the same name. STAT proteins are phosphorylated by Janus kinases (JAKs) during IL-6 receptor signal transduction. The prevalence of hyper IgE syndrome is around 1 in 1 million. Patients present with recurrent bacterial infections (especially of the respiratory tract) caused by S. aureus, S. pneumoniae, and H. influenzae.

Laboratory findings: elevated IgE, limited IFN-γ responsiveness to antigen stimulated monocytes.

Table 21.2-6 Primary defects in antibody response: clinical and laboratory findings

Clinical and laboratory findings

X-linked agammaglobulinemia (XLA) /3839/

The primary deficiency in XLA (also known as Bruton’s agammaglobulinemia) is the absence of B cells in the peripheral blood and organs. The lymph nodes and tonsils are rudimentary and lymph nodes lack germinal centers and follicles. There are no plasma cells in the lamina propria of mucus layers. In patients with XLA there is a block in B cell maturation: pro B cell to pre B cell; pre B cell to immature B cell; and immature B cell to mature B cell. Refer to Fig. 21.1-8 – Antigen-independent development of B cells and their receptors. These blocks are variable and partial. The molecular basis of XLA is a genetic defect of the gene BTK which is located on chromosome Xq21.3. The gene encodes the BTK protein, a member of the cytoplasmic tyrosine kinase family that transmits signals from antigen receptors into the cell. Cross linking of surface receptors of B cells such as IgM, IL-5R, IL-6R, CD38, and FcRε induces activation of the BTK protein by tyrosine phosphorylation. BTX protein expression is limited to hematopoietic cells and B cells from the CD34 pro B cell stage through to mature B cells. More than 500 mutations of the BTX gene have been described, of which one third are missense mutations.

Clinical findings: recurrent bacterial infections from the age of 6 months. Respiratory tract infections are most common (60%), followed by gastroenteritis (35%), pyoderma (25%), arthritis (20%), meningoencephalitis (16%), septicemia (10%), chronic conjunctivitis (8%), and osteomyelitis (3%). H. influenzae and S. pneumoniae are commonly involved. Because the cellular immune response is intact, the risk of mycobacterial, viral, and fungal infections is not increased. However, meningoencephalitis and dermatomyositis may develop from chronic infection with Enterovirus, Echovirus, or Coxsackievirus and joint infections may be caused by Enterovirus and Ureaplasma urealyticum. Chronic gastrointestinal and pulmonary infections are the main problems faced by these patients.

Laboratory findings: decreased serum concentration of IgG, IgA, and IgM; absence of isoagglutinins; absence of B cells.

Definitive diagnosis /24/: male with peripheral CD19 cells below 2% in addition to the following: mutation in the BTK gene; BTK mRNA undetectable in neutrophils and monocytes on Northern blot analysis; BTK protein undetectable in monocytes and thrombocytes; maternal cousins, uncles, and nephews with B lymphocytes below 2%.

Probable diagnosis and possible diagnosis Refer to Ref. /24/.

Common variable immunodeficiency syndrome (CVID) /4041/

CVID, also known as acquired hypogammaglobulinemia, adult-onset hypogammaglobulinemia, or dysgammaglobulinemia, comprises a heterogeneous group of disorders. Although both the B cell and T cell immune responses are affected, these disorders manifest predominantly as hypogammaglobulinemia. The term “variable” describes both the age of presentation (early childhood, adolescence, early adulthood) and the nature and severity of the hypogammaglobulinemia. CVID is the most common clinically relevant primary immunodeficiency (prevalence 1 in 25,000). Immunopathology of CVID:

  • B cell system: although the number of circulating B cells is normal, immunoglobulin secretion is impaired. In vitro stimulation of B cells from these patients reveals sub fractions that secrete only IgM, or only IgM and IgG. Others have defects in the CD40 signaling pathway and some show restricted hyper mutation of variable immunoglobulin genes, which leads to reduced antibody diversity.
  • Many patients are lymphopenic due to a decreased CD4+T cell count with a normal CD8+ T-cell count. The observed T cell abnormalities may be epiphenomena caused by cytokine deregulation.

Patients may have mutations in the following genes:

  • TNFRSF13B (TACI), which encodes protein 13B of the TNF receptor super family
  • TNFRSF13C (BAFF-R), which encodes protein 13C of the TNF receptor super family
  • CD19, which encodes the inducible co stimulator protein ICOS.

In addition to hypogammaglobulinemia, patients with these mutations have a reduced functional antibody response, increased susceptibility to bacterial infection, and an increased predisposition to malignancy, in particular lymphomas and gastric carcinoma.

B cells from patients with a TNFRSF13B (TACI) mutation do not produce IgG and IgA in response to contact between the TACI protein and its ligand proliferation inducing ligand (APRIL), which means that immunoglobulin class switching does not occur. DNA rearrangement, also known as class switch recombination (CSR) (refer to section 21.1.4.2), normally takes place in immunoglobulin-producing B cells. CSR changes the immunoglobulin isotype from IgM to IgG, IgE, or IgA while maintaining antigen specificity. Two signals are required for CSR (Fig. 21.2-8 – Collaboration of B cell and T cell to synthesize antibodies):

  • The first signal is delivered by cytokines, which target specific Ig heavy chain genes for transcription. If there is a mutation in the TNF receptor family, this step does not take place or does not proceed optimally and the clinical picture of CVID may emerge.
  • The second signal is delivered in the case of T cell dependent antigens through the interaction of CD40 on B cells with the CD40 ligand on activated T cells.

Clinical findings: the mean age of onset of symptoms is 25 years and the mean age at presentation to a doctor is 28 years. The clinical symptoms are recurrent respiratory tract infections, otitis media, chronic sinusitis, and pneumonia. The pathogens involved are the same as those described for XLA. Around 50% of patients have gastrointestinal involvement with chronic diarrhea, malabsorption, lactose intolerance, and exudative gastroenteropathy. Superinfection with Yersinia spp. or Campylobacter spp. is not uncommon, and Giardia lamblia can also cause gastrointestinal symptoms. Nodular lymphoid hyperplasia of the gastrointestinal tract is also common and up to 30% of patients have hepatosplenomegaly. Some 10% of patients develop pulmonary fibrosis with granulomas and another 10% develop an autoimmune disorder, most commonly autoimmune thrombocytopenia with autoimmune hemolytic anemia and neutropenia. Granulomatous diseases are also relatively common, in particular perisinusoidal granulomas in the liver, which can lead to elevated alkaline phosphatase. Patients are also at increased risk of malignant disease, in particular lymphomas. These patients have a 20 year survival rate of 65%, compared with 93% in the general age matched population. Children with CVID present with similar symptoms and findings to adults /44/. Causative bacteria for infection include Clostridium difficile, Gardia, Salmonella, Campylobacter, and Yersinia.

Laboratory findings: CVID patients characteristically are unable to produce specific antibodies in response to vaccination and have decreased levels of IgG, IgA, and often also IgM.

Probable diagnosis and possible diagnosis: Refer to Ref. /24/.

Selective IgA deficiency /42/

Selective IgA deficiency is the most common immunodeficiency, with an incidence of 1 in 500. In this disorder, B cell class switching from IgA to IgG is inhibited for unknown reasons and the peripheral B cell count is normal. The B cells co express IgA, IgM, and IgD but do not transform into IgA secreting plasma cells. IgA deficiency can occur:

  • In isolation: both subclasses IgA1 and IgA2 are significantly decreased (although some cases have also been described in which only one subclass is decreased)
  • In combination with a deficiency in subclasses IgG4 or IgG2 and IgG4
  • In ataxia-telangiectasia
  • In chromosomal anomalies (e.g., of chromosome 18; 18q syndrome, ring 18)
  • During the course of drug therapy (e.g., with D-penicillamine, phenytoin, sulfasalazine, or hydroxychloroquine).

Clinical findings: most individuals with selective IgA deficiency are asymptomatic. Those who do have symptoms present with sinopulmonary infections and gastrointestinal involvement in the form of nodular lymphoid hyperplasia and giardiasis. There is a clear relationship between selective IgA deficiency and atopy. There is also an increased predisposition to autoimmune diseases such as systemic lupus erythematosus, endocrinopathies, chronic hepatitis, Crohn’s disease, ulcerative colitis, hemolytic disease, and arthritis. Recurrent sinopulmonary infections are more severe in combined IgA and IgG deficiency than in isolated IgA deficiency. Patients with selective IgA deficiency who receive blood products are at risk of developing anti IgA antibodies; this is not the case in partial IgA deficiency.

Laboratory findings: in complete IgA deficiency, the serum concentration is below 7 mg/L; in partial IgA deficiency, it is 2 SD below the normal age adjusted mean but ≥ 7 mg/L.

Definitive diagnosis /14/: male or female above the age of 4 years with a serum IgA concentration of less than 7 mg/L and normal serum IgM and IgG levels in whom other causes of hypogammaglobulinemia have been ruled out. These individuals have a normal IgG vaccine response.

Possible diagnosis /14/: male or female above the age of 4 years with a serum IgA concentration of more than 2 SD below the normal age adjusted mean and normal serum IgM and IgG levels in whom other causes of hypogammaglobulinemia have been ruled out. These individuals have a normal IgG vaccine response.

Hyper-IgM immunodeficiency (HIM) /3843/

HIM is caused by a group of molecular defects characterized by impaired immunoglobulin class switch recombination (CSR) and/or somatic hyper mutation (SHM). Three forms of HIM have been described: an X-linked form (XHIM), an autosomal recessive form (ARHIM), and an X-linked form with hypo hidrotic ectodermal dysplasia.

XHIM /44/: this is the most common form and is caused by mutations in the CD40L gene, which encodes the CD40 ligand. The CD40 ligand is a type 2 cell membrane glycoprotein from the TNF super family. CD40L (CD154) is expressed transiently by CD4+T cells and interacts with B cells, macrophages, and dendritic cells that express CD40. CD40L regulates immunoglobulin production and class switching in B cells. CD40 and CD154 are members of the cytokine/cytokine receptor family. During an immune response, the CD40 receptor on a resting B cell receives a signal via the CD154 ligand (Fig. 21.2-8 – Collaboration of B cells and T cells to synthesize antibodies). This signal has a number of possible effects. If the B-cell antigen receptor is occupied and cross-linked, activated T cells bearing the Fas ligand (CD95) stimulate the B cell to proliferate and produce antibodies. If the antigen receptor is unoccupied, these interactions lead to apoptosis of the B cell. The switch from immunoglobulin class IgM to IgE in the humoral immune response (Fig. 21.2-8) is triggered by signals from the T cell to the B cell. The T cell triggers the B cell via CD40 to produce switch factors. The molecular basis of HIM is deficient expression of CD40L, which means that the required contact between CD40 and CD40L (CD154) does not occur. Consequently, class switching does not take place and IgM production predominates in the plasma cells.

XHIM affects males only, while ARHIM affects both sexes. ARHIM is associated with a mutation in the gene that encodes activation induced cytidine deaminase. The X-linked form with hypohidrotic ectodermal dysplasia is associated with a mutation in the NFκB essential modifier gene (NEMO) in CD4+T cells, which attenuates B cell switching.

Clinical findings: bacterial and viral infections of the upper and lower respiratory tract and interstitial lung disease, most commonly caused by Pneumocystis carinii. Clinical symptoms usually appear in the 8th month of life in the form of infections and failure to thrive. 75% of patients develop liver disease by the age of 20 years. 80% of patients die by the age of 25 years. In XHIM, the lymph nodes lack germinal centers. The clinical symptoms of ARHIM are similar to those of XHIM. However, ARHIM patients exhibit hypertrophy of the lymphoid tissue and tonsils but no splenomegaly.

Laboratory findings: chronic, episodic, or cyclic neutropenia; 50% of patients have elevated aminotransferases; 5–30% have anemia caused by Parvovirus B19. Immunoglobulin concentrations in XHIM (mean and range, in g/L): IgG 1.58 (0–6.5); IgA 0.17 (0–1.23); IgM 3.88 (0.2–20). 53% of patients have an IgM concentration of < 3 g/L, this concentration tends to increase with age. Following antigen exposure, patients produce IgM antibodies but IgE antibodies are low. IgM concentrations are usually higher in ARHIM than in XHIM.

Definitive diagnosis /24/: male or female patient with an IgG concentration at least 2 SD below normal for age and one of the following: mutation in the CD40L gene; maternal cousins, uncles, or nephews with confirmed XHIM.

Probable diagnosis and possible diagnosis refer to Ref. /24/.

IgG subclass deficiency

Refer to Section 18.10 – IgG subclasses.

Specific antibody deficiency (SAD) /45/

SAD is a primary immunodeficiency disease characterized by normal immunoglobulins IgA, IgM, total IgG, and IgG subclass levels, but with recurrent infections and diminished antibody responses following vaccination. The infections encountered in SAD are similar to those of other antibody deficiencies. The phenotype of SAD can be similar to that of CVID, however CVID patients exhibit low IgG and usually low IgA levels. Response to pneumococcal vaccines is usually determined by assessing levels of IgG specific serotypes included in the vaccine by multiplex bead immunoassay or enzyme-linked immunosorbent assay The Centers for Disease Control and Prevention recommend 13-valent pneumococcal conjugate vaccine (PCV13) and 23-valent pneumococcal polysaccharide vaccine (PPSV23) for adults and a series of PVC 23 vaccinations for children under 2 years of age. A serotype-specific level of 1.3 μg/l has been considered protective with respect to invasive disease following polysaccharide immunization.

Transient hypogammaglobulinemia /46/

Serum immunoglobulin levels typically reach their nadir in infants aged 2–4 months as diaplacentally transferred maternal immunoglobulins are catabolized. In some children, this physiologic hypogammaglobulinemia can be particularly marked and prolonged. This condition, known as transient hypogammaglobulinemia is a self limiting condition that resolves by the age of 18–36 months. Children with the disorder must be followed up to rule out the existence of a primary immunodeficiency such as CVID.

Clinical findings: children with delayed immunoglobulin synthesis typically present with upper respiratory tract infections and otitis media. Pulmonary involvement is unusual.

Laboratory findings: the serum IgM concentration is normals or elevated while IgG and IgA concentrations are decreased. The peripheral B cell count and B cell receptors are normal. The immune response to protein antigens is normal, while the response to viral antigens may be attenuated.

Table 21.2-7 B cell phenotyping in the diagnosis of B cell functional deficiency /47/

Marker

Cell type

Comment

IgM+ IgD+ CD27

Naive B cells

 

IgM+ IgD+ CD27+

Marginal zone-like B cells

Decreased in CVID with granulomatous disorder

IgD CD27+

Class- switched memory B cells

Usually decreased in CVID, often closely correlated with IgG concentration

CD38hi IgMhi

Transitional B cells

Often slightly decreased in CVID, often elevated in CVID with lymphadenopathy

CD21low CD38low, (CD19+++)

Activated B cells

Often elevated in CVID with autoimmune disorder/splenomegaly

CD38+++IgM

Class-switched plasmablasts

Usually decreased in CVID, completely absent in CVID with cytopenia

IgDCD27+

Class switched memory B cells and CD38+++IgM class switched plasmablasts show close correlation.

CVID, common variable immunodeficiency

Table 21.2-8 Frequency and inheritance of severe combined immunodeficiency (SCID) forms /5/

Disease

Inci-
dence
(%)

Deficient
cells
Inheritance

Molecular
damage/
deficiency

Reticular
dysgenesis

< 1

Leukocytes, thrombocytes T, B, and NK cells

Autosomal recessive (AR)

Unknown

Alympho-
cytosis

20

T and B cells

AR

RAG1/RAG2 or DCLRE1C (Artemis)

T-cell
deficiency

50

T and NK cells

X-linked

IL-R2-γ chain

10

T and NK cells

AR

Janus kinase 3

1

T cells

AR

IL-7Rα

< 1

T cells

AR

CD45

< 1

T cells

AR

CD3δCD3ε

Adenosine
deaminase
deficiency

10–20

T cells, B cells,

NK cells

AR

Adenosine deaminase deficiency

Purine
nucleoside
phospho-
rylase
deficiency

< 1

Progressive loss of T cells

AR

Purine nucleoside phosphorylase deficiency

Table 21.2-9 Primary phagocytosis defects

Clinical and laboratory findings

Congenital neutropenia – Generally

The congenital neutropenias are a group of disorders characterized by abnormalities in the life cycle of polymorphonuclear neutrophils (PMNs). In severe neutropenia, the PMN count is below 0.5 × 109/L, which can lead to serious bacterial infections.

– Cyclic neutropenia /48/

Cyclic neutropenia is an autosomal dominant disorder in which cyclic hematopoiesis leads to recurrent episodes of neutropenia. These neutropenic intervals typically last for 3–6 days out of every 21-day period. In about 30% of patients, however, the cycles range from 14 to 36 days. Mutations in the ELA2 gene, which encodes elastase in PMNs, have been identified in patients with cyclic neutropenia. The mutations affect the catalytic site of the enzyme, which leads to the failure of inhibitors to bind to and thus inactivate elastase. The ability of these PMNs to kill pathogens is impaired as a result; however, the neutrophil count is normal.

Clinical findings: patients are usually asymptomatic but may develop aphthous ulcers, gingivitis, stomatitis, and cellulitis during neutropenic periods. Abdominal pain must be investigated aggressively because of the high risk of Clostridium infection.

Laboratory findings: very severe neutropenia with PMN counts reaching a nadir of 0.2 × 109/L for 3–6 days during each cycle.

– Severe congenital neutropenia /48/

A distinction is made between X-linked neutropenia and Kostmann syndrome:

  • Patients with X-linked neutropenia have a defect in the WAS gene, which encodes the WAS protein. The WAS protein is involved in membrane receptor signal transduction.
  • Patients with Kostmann syndrome have a defect in the HAX1 gene, which encodes the protein of the same name. The HAX1 protein plays a role in the regulation of apoptosis.

Severe congenital neutropenia begins during the first year of life and results from the failure of promyelocytes to mature into myelocytes.

Clinical findings: bacterial infections with S. aureus and Baumholderia aeruginosa lead to complications such as perirectal abscess, stomatitis, peritonitis, and meningitis. The disorder is also associated with an increased risk of myelodysplastic syndrome.

Laboratory findings: PMN count below 0.5 × 109/L; the absolute monocyte and lymphocyte counts are often increased.

Schwachman-Diamond syndrome /48/

The Schwachman-Diamond syndrome is an autosomal recessive disorder that is characterized by neutropenia (cyclic or intermittent), bone marrow dysfunction, exocrine pancreatic insufficiency, skeletal abnormalities, and recurrent infections. Infections begin during the first year of life and involve the para nasal sinuses, lungs, bones, urinary tract, and skin. 10–25% of patients have pancytopenia.

Leukocyte adhesion deficiency (LAD)

LAD is divided into two subtypes: LAD I and LAD II. LAD I results from a lack of β2 integrin adhesion molecules on PMNs. LAD II is a defect of fucosylation that affects the interaction of PMNs with E-selectins on vascular endothelial cells. Refer to

LAD I is an autosomal recessive disorder that results from a lack of the β2 integrin molecule on PMNs. There are three β2 integrins with different α chains but a common β chain (CD18). Mutations in the gene that encodes CD18 account for the loss of β2 integrin and the clinical symptoms. PMNs that lack β2 integrin are unable to aggregate and do not adhere to the vascular endothelium. Clinical features include oral and genital mucosal infections as well as infections of the respiratory and gastrointestinal tracts. Infecting pathogens include Enterobacteriaceae, S. aureus, and Candida.

LAD II is a defect of carbohydrate fucosylation. The loss of α-1,2-fucose, α-1,3 fucose, and α-1,6 fucose groups in a variety of carbohydrates leads to decreased PMN adhesion. Clinical features include growth retardation, dysmorphic features, and neurologic deficits.

Definitive diagnosis /24/: male or female patient with reduced neutrophil CD18 (below 5% of normal) and at least one of the following:

  • Mutation in β2 integrin
  • Absence of β2 integrin mRNA in leukocytes
  • Evidence of a fucosylated surface molecule; blood group antigen sialyl-Lewis X (CD15).

Probable diagnosis /24/: male or female patient with reduced neutrophil CD18 (below 5% of normal) and all of the following:

  • Recurrent or persistent bacterial or fungal infections
  • Leukocytosis of greater than 25 × 109/L
  • Delayed separation of the umbilical cord and/or defective wound healing.

Possible diagnosis /24/: child with leukocytosis of greater than 25 × 109/L and one of the following:

  • Recurrent bacterial infections
  • Severe, deep-seated infections
  • Absence of pus at sites of infection.

Defects of leukocyte signaling /48/

The presence of a pathogen stimulates the production of IL-12 by macrophages and dendritic cells, which in turn triggers the secretion of IFN-γ by T cells and NK cells (Fig. 21.2-9 – Interleukin-12 binds to the corresponding receptor on T cells and NK cells, stimulating them to secrete IFN-γ). The IFN-γ – IL-12 axis is particularly important for resistance to intracellular pathogens such as Mycobacterium, Salmonella, and Listeria. Gene mutations that cause changes in ligand binding or IFN-γ receptor signaling or mutations in the gene that encodes the IL-12 receptor result in increased susceptibility and decreased resistance to pathogens.

Clinical findings: defects in leukocyte signaling are characterized by severe infections that begin in early childhood. The main features are disseminated infections with atypical Mycobacterium, fatal BCG infections after vaccination, and an inability to form granulomas.

Chronic granulomatous disease) /48/

Chronic granulomatous disease (CGD) is characterized by a defect in the intracellular bacterial killing. The normal granulocyte response to pathogens is phagocytosis followed by intracellular destruction induced by reactive oxygen species (refer also to Section 19.7 – Polymorphonuclear neutrophil function). This takes place through the generation of hydroxyl radicals and H2O2 by NADPH reductase (refer to Section 19.2 – Oxidative stress). CGD is caused by mutations in the genes for membrane bound phagocyte oxidase (phox) components. The membrane bound molecules gp91phox and p22phox interact with the cytoplasmic molecules p47phox and p67phox in signal transmission (Fig. 21.2-10 – Production of chloride for bacterial killing by phagocytes). Glucose-6-phosphate dehydrogenase is activated as a result and generates NADPH (refer to Section 19.7).

Chronic granulomatous disease is characterized by recurrent infections with catalase positive bacteria such as Burkholderia (pseudomonas) cepacia, S. aureus, Nocardia, and Serratia marcescens and fungi such as Aspergillus. Catalase destroys the H2O2 produced by PMNs.

Clinical findings: recurrent or persistent infections of the soft tissues, lungs, and other organs despite aggressive antibiotic therapy. Children with the X-linked form of CGD (60–70% of patients) are more severely affected than those with autosomal recessive mutations.

Definitive diagnosis /25/: male or female patient with an abnormal nitro blue tetrazolium (NBT) or respiratory burst test (less than 5% of normal) and one of the following (refer also to Section 19.7):

  • Mutation in gp91, p22, p47, or p67phox
  • Absence of mRNA for one of these genes on Northern blot analysis
  • Maternal cousins, uncles, or nephews with an abnormal NBT or respiratory burst test.

Probable diagnosis /25/: male or female patient with abnormal NBT or respiratory burst test (less than 5% of normal) (refer to Section 19.7) and one of the following:

  • Deep-seated infection (hepatic, perirectal, lung abscess, adnexitis, osteomyelitis) due to Staphylococcus, Serratia marcescens, Candida, or Aspergillus
  • Diffuse granulomas in the respiratory, gastrointestinal, and urogenital tracts
  • Developmental delay, hepatosplenomegaly, and lymphadenopathy.

Differential diagnosis /25/: LAD, sarcoidosis, hyper-IgE syndrome

Myeloperoxidase (MPO) deficiency /49/

PMNs have different sub populations of granules. Azurophilic granules contain lysosomal enzymes, including MPO. This enzyme catalyzes the formation of hypochlorous acid (HOCl) from H2O2 and chloride ion. HOCl kills pathogens that have been taken into the cell.

Despite this, patients with MPO deficiency generally do not have an increased frequency of infections. Cutaneous, mucosal, and pulmonary infections caused by S. aureus, Enterobacteriaceae, and Candida are predominantly observed in MPO-deficient patients who also have diabetes mellitus. Refer also to Section 19.7 – Polymorphonuclear neutrophil function.

Chediak-Higashi syndrome (CHS) /48/

Chediak-Higashi syndrome is an autosomal recessive disorder of all granule containing cells. LYST, the mutated gene in this syndrome, encodes a protein that is involved in vacuolar formation, and function, and transport of proteins in granulocytes. The mutated LYST encodes a defective protein and granulocytes lack the proteins elastase and cathepsin G. Although leukocytosis still occurs during an infection, granulocyte diapedesis is slower than normal and granulocyte function is diminished.

Clinical findings: recurrent infections with S. aureus and β-hemolytic streptococci, peripheral nerve defects (nystagmus and neuropathy), mild mental retardation.

Laboratory findings: mild neutropenia; granule containing cells with giant granules due to the abnormal fusion of azurophilic granules with specific granules; normal immunoglobulin levels. Refer also to Section 19.7.

Table 21.2-10 Secondary (acquired) immunodeficiencies

Clinical and laboratory findings

HIV infection /50/

The HI virus, which can also infect macrophages and dendritic cells, enters T helper cells (CD4+T cells) via the CD4 molecule. Like in other viral infections, this induces the proliferation of cytotoxic CD8+T cells and the production of antibodies. The primary infection is concentrated in the lymph nodes, which are usually unable to prevent its further spread due to the high viral replication and mutation rate, which complicates immune recognition and produces a high viral load. As destruction of the CD4+T cells increases, the immune response diminishes. The disease has a cyclic or continuous progressive course.

Laboratory findings: the numbers of CD3+T cells, CD4+T cells, CD8+T cells, B cells (CD19), and NK cells (CD19, CD56) are determined and the expression of CD38 on CD8+T cells, Th1 cells, and Th2 cells is determined to assess the course of the disease. Results of laboratory findings at various stages:

  • Acute infection: negative HIV antibody assay; high viral load; CD4+T cells above 0.5 × 109/L; CD8+T cells above 0.8 × 109/L; normogammaglobulinemia in adults; hypogammaglobulinemia in children (especially premature infants)
  • Latent stage: positive HIV antibody assay; low viral load; CD4+T cells above 0.5 × 109/L; CD8+T cells above 0.8 × 109/L; hypergammaglobulinemia
  • Lymphadenopathy and progression: positive HIV antibody assay; low viral load; CD4+ T cells below 0.5 × 109/L; CD8+T cells above 0.8 × 109/L; hypergammaglobulinemia
  • AIDS-related complex: positive HIV antibody assay; high viral load; CD4+T cells (0.2–0.4) × 109/L; CD8+T cells below 0.8 × 109/L; despite hypergammaglobulinemia, reduced immune response to encapsulated bacteria due to B cell defect.
  • AIDS: positive HIV antibody assay; very high viral load; CD4+T cells below 0.2 × 109/L; CD8+T cells below 0.8 × 109/L; despite hypergammaglobulinemia, diminished immune response to encapsulated bacteria due to B cell defect.

Epstein-Barr virus (EBV) infection /51/

In immunocompetent individuals, EBV infection may be asymptomatic or may result in infectious mononucleosis, with elevated NK cells and CD8+T cells. It is associated with hypogammaglobulinemia, which is transient in most cases but occasionally permanent. Approximately half of patients with X-linked lymphoproliferative syndrome (XLP) develop a severe form of the disease, one third develop severe hypogammaglobulinemia, and one quarter develop malignant lymphoma. Immunological findings in XLP demonstrate a reversed CD4/CD8 ratio; elevated IgA or IgM; decreased IgG1 or IgG3; high virus specific antibody titers against the capsid antigen and early antigen; and low antibody titers against EBNA (Epstein-Barr nuclear antigen) in men. In women with EBV infection, all EBV antibody titers are low.

Cytomegalovirus infection

Transient decrease in CD4+T cells and B cells; possible hypogammaglobulinemia.

Glucocorticoids (GCs)

GCs have an immunosuppressive effect on T cells, B cells, and monocytes/macrophages by inhibiting the transcription of cytokine producing genes in these cells. As a result the production of IL-1β, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, RANTES (regulated on activation, normal T-cell expressed and secreted; a member of the IL-8 super family), IL-11, TNF-α, macrophage chemotactic protein (MCP-1, MCP-3, MCP-4), macrophage inflammatory protein 1α, and eotaxin is inhibited /52/. GCs exert their effects via cytoplasmic glucocorticoid receptors in lymphocytes and monocytes/macrophages. Their mode of action is shown in Fig. 19.1-14 – Actions of glucocorticoids (GCs).

Treatment with GCs causes hypogammaglobulinemia. In asthmatics, for example, short-term GC treatment (mean duration 8 days) with a prednisone dose of 20–250 mg/day leads to a 20% reduction in IgG, a 17% reduction in IgA, and unaltered levels of IgM within 2–4 weeks of starting treatment /53/. Long-term glucocorticoid treatment increases the risk of hypogammaglobulinemia. For instance, 12% of patients who received a prednisone dose of greater than 5 mg daily for longer than 2 years developed hypogammaglobulinemia, with IgG levels of only 3.2–6.0 g/L. However, no increase in the number of infections was seen in these patients /54/.

Penicillamine /51/

Following 5 years of treatment with this antirheumatic drug, approximately 5% of patients exhibit a 5–30% decrease in IgA and IgG concentrations.

Sulfasalazine /51/

Sulfasalazine is used to treat inflammatory bowel and joint diseases and has a mild inhibitory effect on T cell and B cell function. When used to treat arthritis over a period of 1–10 years, it has been shown to cause IgA, IgG, and IgM deficiency in 2.9%, 1.7%, and 4.9% of patients, respectively /55/. However, patients did not exhibit increased susceptibility to infection.

Antiepileptic drug /51/

Phenytoin: this causes a selective decrease in IgA of approximately 10% after 3–4 months of treatment. Approximately 5% of patients develop a complete, but reversible, IgA deficiency. Pan hypogammaglobulinemia with decreased B cell count and infections occasionally occurs.

Carbamazepine: decreased count of B cells, T cells, and IgG.

Nephrotic syndrome (NS)

Congenital NS: newborns with this condition have serum IgG of less than 1 g/L, barely detectable IgA, and normal or elevated IgM (1.5 g/L or higher). In one infant with congenital nephrotic syndrome, urinary IgG excretion of 0.01–0.065 g/L was measured /56/. The hypogammaglobulinemia was caused by loss of protein into the amniotic fluid. However, the decline in Ig levels over the subsequent weeks during intravenous Ig replacement therapy cannot be explained by renal losses alone. Other mechanisms must also play a role. It is hypothesized, for example, that a T cell disorder may lead to a reduction in the production of IgG secreting B cells. The normal or increased IgM levels suggest a compensatory increase in synthesis. Children with nephrotic syndrome are at increased risk of bacterial infection, especially due to S. pneumoniae, β-hemolytic streptococci, and Enterobacteriaceae /51/.

Adult NS: approximately 20% of adults with NS develop severe bacterial infections, mainly due to Gram negative bacteria. Risk factors for recurrent infections include an IgG concentration of less than 6 g/L and a serum creatinine concentration of greater than 2 mg/dL (176 μmol/L) /57/. Patients with minimal change lesions and focal glomerulosclerosis are thought to have more severe decreases in IgG than patients with other forms of NS /57/.

Hemodialysis

Hemodialysis patients have normal immunoglobulin levels. There are conflicting reports in the literature concerning the antibody response in these patients following immune stimulation by the tetanus or diphtheria toxin or the pneumococcal vaccine /51/.

Immunodeficiency associated with gastrointestinal disease

In protein losing gastrointestinal disease, all proteins are lost to the same degree. Protein losing enteropathy can be caused by primary or secondary lymphangiectasia, intestinal malrotation, cavernous hemangioma, Crohn’s disease, celiac disease, or chronic intestinal pseudo obstruction. In many cases, it does not result in hypogammaglobulinemia or increased susceptibility to bacterial infections /51/.

Intestinal lymphangiectasia: this can occur as a primary disorder or be secondary to lymphoma, radiation induced vasculitis, or constrictive pericarditis. Patients with this disorder are lymphopenic and hypogammaglobulinemic since cells and Ig are lost into the intestine. They may exhibit a diminished lymphocyte proliferation response to mitogenic and antigenic stimulation. They may also be subject to recurrent bacterial infections /51/.

Crohn’s disease: hypogammaglobulinemia and increased susceptibility to infection are rare.

Malignant disease – Generally

Malignant disease, of the lymphatic system in particular, is associated with immunodeficiency due to an impaired T cell and B cell immune response.

– Chronic lymphocytic leukemia (CLL)

CLL is characterized by the clonal expansion of CD5+B cells. This B cell proliferation results in secondary hypogammaglobulinemia since the Ig secreted by the CD5+B cells is abnormal. CLL patients have an increased incidence of infection, especially infections due to encapsulated bacteria. Intravenous administration of immunoglobulin is recommended in CLL patients with serum IgG below 3 g/L since the risk of infection is highest in these patients /58/.

– Multiple myeloma (MM)

Polyclonal synthesis of Ig is reduced due to the increasing replacement of plasma cells by myeloma cells. The resulting immunodeficiency is associated with an increase in CD8, CD11b, and Leu8-positive T cells /59/.

Table 21.2-11 Tests for suspected immunodeficiency and the evaluation

Test

Complete blood count (CBC) /12/

The CBC including absolute counts for individual leukocyte fractions, cell morphology in the blood smear, as well as the thrombocyte count and platelet count are relevant. It is important to assess the cell count in comparison to age-adjusted reference intervals for infants, children, and adults. If persistent and associated with disease, leukopenia, lymphopenia, neutropenia, and thrombocytopenia can be the first indication of immunodeficiency disease. Low numbers of cells may result from a developmental block in the case of SCID, congenital neutropenia or Wiskott-Aldrich syndrome /61/.

Lymphopenia: the lowest thresholds for the lymphocyte count are 4.5 × 109/L for infants up to the age of 18 months, 3.0 × 109/L for children from 19–36 months, and 1.5 × 109/L for children over the age of 3 years. In adults, the lowest threshold is 1.0 × 109/L /6061/. If lymphopenia is present, further tests should be carried out to investigate T cell and B cell deficiencies. Lymphopenia occurs in up to 90% of children with severe combined immunodeficiency (SCID) /19/. Lymphopenia is also commonly found in common variable immunodeficiency (CVID). The determination of the lymphocyte sub populations is important in this case.

Lymphocytosis: lymphocytosis may indicate Omenn syndrome or other primary immunodeficiencies such as leukocyte adhesion deficiency (LAD) and hyper-IgE syndrome.

Eosinophilia: in association with increased susceptibility to infections, eosinophilia might be indicative of an underlying immune defect (e.g., hyper-IgE syndrome, Omenn syndrome, immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome) /20/.

Thrombocytes: small thrombocytes are typically seen in Wiskott-Aldrich syndrome.

Howell-Jolly bodies in the case of congenital asplenia.

Immunoglobulins (Ig) /21/

In cases of suspected B cell deficiency or combined immunodeficiency, IgG, IgA, IgM, and isohemagglutinins should be determined. For assessment of the findings, refer to

IgG, IgA, IgM

General information about antibody production can be obtained by quantitatively determining IgM, IgG, and IgA. The results must be assessed based on age-adjusted Ig values (Refer to Section 18.9 – Immunoglobulins). In general, a normal IgA level indicates a normal humoral immune response and undetectable levels of IgA indicate the presence of a primary immunodeficiency. Hypogammaglobulinemia does not prove the presence of an immunodeficiency; it is only present if specific antibodies are not produced. In patients with diarrhea, it is important to determine the serum albumin level to check for gastrointestinal loss of protein. With the exception of selective immunodeficiencies, an effective humoral immune response can be maintained with IgG levels as low as 4 g/L.

Reduction of all Ig classes are findings in X-linked agammaglobulinemia, transient hypogammaglobulinemia, common variable immunodeficiency, autosomal recessive B cell deficiency, and combined T and B cell deficiency /10/.

Elevated concentrations of Ig may be also suggestive of immunodeficiency (e.g., hyper-IgE syndrome, Omenn syndrome, IPEX syndrome).

Transient hypogammaglobulinemia: low IgG concentrations in infancy are usually due to transient hypogammaglobulinemia. This is confirmed by normal age-adjusted IgA levels and decreased IgG subclass levels /62/.

X-linked agammaglobulinemia: children over the age of 6 months with X-linked agammaglobulinemia have IgG levels of less than 1 g/L, undetectable levels of IgM and IgA, and absent specific antibody response.

Hyper IgM syndrome: this syndrome is indicated by elevated (or occasionally, normal) IgM levels and the absence of other Ig classes.

IgG subclasses

An IgG subclass deficiency may exist in spite of a normal IgG concentration since IgG is a combination of IgG1 (61%), IgG2 (30%), IgG3 (5%), and IgG4 (4%) (refer also to Section 18.10 – Immunoglobulin G subclasses). The immune response to protein antigens generally involves IgG1 antibodies while the response to polysaccharide antigens involves the IgG2 subclass. In view of the broad reference intervals for IgG subclasses, the variability associated with the different methods of determination used, and the transient mild decreases in IgG seen in children, the value of IgG subclass determination is controversial /63/. Although there is a correlation between IgA deficiency and IgG2 deficiency and an impaired immune response to polysaccharide antigens (S. pneumoniae, H. influenzae) in IgG2 deficiency, this deficiency is less significant than originally thought /62/. According to some authors, the determination of IgG subclasses is only relevant in the case of selective IgA deficiency, since this is associated with a deficiency in IgG2 and IgG4 /20/.

Specific antibodies /21/

The specific humoral immune response can be assessed by determining specific antibodies against tetanus, diphtheria toxin, pertussis, or polysaccharide antigens of H. influenzae in vaccinated individuals. If titers are low, the patient should be immunized with the antigens mentioned. Blood samples are obtained prior to and 2–3 weeks after immunization to determine the antibody titers. A ≥ 4-fold increase in the antibody titer indicates a normal immune response and successful immunization of the patient /64/.

Isohemagglutinins anti-A and anti-B

Natural IgM antibodies against AB0 blood group antigens are produced by newborn infants with a normal humoral response. Depending on the blood group type, 70% of children have positive antibody titers by the end of the first year of life. In adults, the titer is usually greater than 1 : 8. Note, however, that individuals with blood group AB do not have any natural antibodies.

Skin test with recall antigens

This test should only be performed if flow cytometry has shown a normal T cell count and T cell distribution. Skin testing is used to check the functional competency of T cells by assessing delayed hypersensitivity. Testing is carried out using either the Multi test Merieux or intradermal injection of antigens such as tetanus toxin, Candida albicans extract, diphtheria toxin, Mumpsvirus extract, and tuberculin. Functional competency in response to an antigen is indicated by induration of at least 5 mm by 48 h after the injection at the latest /65/. Non responders have either a cellular immunodeficiency, anergy, or are taking corticosteroids.

Phenotyping of lymphocytes – Generally

Lymphocyte subsets are determined quantitatively using flow cytometry. Pan T cells (CD3+), MHC I-restricted cytotoxic T cells (CD8+), MHCII restricted T helper cells (CD4+), B cells (CD19, CD20), and NK cells (CD56) are determined. T-cell counts in adults are: CD3+ (0.5–1.8) × 109/L; CD4+ (0.3–1.3) × 109/L; CD8+ (0.1–1.0) × 109/L; CD19 (0.06–0.4) × 109/L /14/. In children, the CD4+ and CD19 cell count decreases continuously from approximately twice the adult value in the first month of life to the age of three, when it reaches the adult value. The 5th percentile for CD4+T cells in children under the age of 1 year is 1.5 × 109/L. CD8+T cells show the opposite pattern, with a low level at birth, which increases continuously until the age of 3 years, when it reaches the adult level. The CD4/CD8 ratio is therefore high in early childhood and decreases from an average of 2.2 at birth to approximately 1.6 at the age of 3 years /60/. NK cells (CD56) are high at birth, decline continuously, and reach adult values by the age of 7 years /60/. The percent distribution of lymphocytes is: T cells 60–80%, B cells 15–20%, NK cells 5–10%.

– B cells

A deficiency of B cells excludes transient hypogammaglobulinemia of infancy and other causes of primary hypogammaglobulinemia. The absence of B cells supports the diagnosis of XLA in a male individual.

If the B cell fraction is less than 2%, tests for a BTK (Bruton tyrosine kinase) mutation, B cell receptor abnormalities, or abnormal intracellular signal transmission must be performed.

X-linked agammaglobulinemia: this deficiency is indicated by absent B cells and normal Ig concentrations in the first months of life in males.

Thymoma: no mature B cells, decreased Ig.

– T cells /64/

Methods such as the lymphocyte proliferation test and lymphocyte transformation test are used to determine T cell function. Lymphocytes from the patient are stimulated by mitogens such as concanavalin A (ConA), phytohemagglutinin (PHA), and pokeweed antigen (PWA) to proliferate and synthesize DNA. The degree of stimulation is monitored either by the incorporation of radio labeled thymidine (proliferation test) or the transformation of the lymphocytes into blast cells (transformation test). These tests only assess the stimulation potential of the cells, but not their effector functions or dysregulation. The lymphocyte proliferation test and lymphocyte transformation test can also be used to a limited degree to evaluate antigen recognition functions. The T cell dependent B cell response can be tested using PWA and T cell independent B cell function can be tested using the Epstein-Barr virus antigen /7/. Cytokine determinations are also useful for assessing T-cell function. These can be performed intracellularly using flow cytometry or in culture supernatants using immunoassays (refer also to Chapter 20 – Cytokines and cytokine receptors). An absent or limited proliferation response indicates a cellular or combined immunodeficiency.

Phagocytic and complement dysfunction

Refer to Section 19.7 – Polymorphonuclear neutrophil function for phagocytic dysfunction and Chapter 24 – The complement system for complement dysfunction.

Table 21.2-12 Prevalence (%) of autoimmune disorders in primary immunodeficiency /3/

Autoimmune
disorder

IPEX

APECED

ALPS

CD40*
def.

AID
def.

CVID

IgA def.

WAS

NEMO
def.

XLA

SLE

 

 

 

 

 

1–3

0–1

 

 

 

Vasculitis

 

2–3

 

 

 

1–2

 

20–29

 

 

Sjögren’s
syndrome

 

12

 

 

 

1–10

0–1

 

 

 

Rheumatoid
arthritis

 

 

4

11

3

1–4

3–5

20–29

3

15–20

Psoriasis

 

 

 

 

 

 

3–4

 

 

 

Sarcoidosis

 

 

 

 

 

 

2–4

 

 

 

AIH

30

2

29–39

2–3

3

2–3

2–3

15–36

1

 

AIT

14

 

23–34

 

3

3–8

0.5

 

 

 

AIN

 

 

18–

19

> 60

 

1

 

25

 

 

Type 1 diabetes

73

2–20

 

 

3

 

 

 

 

 

Autoimmune
thyroid disease

16–28

2–11

 

 

 

1–3

3–7

 

 

 

Autoimmune
hypopara-
thyroidism

 

76–93

 

 

 

 

 

 

 

 

Addison’s
disease

 

70–100

 

 

 

 

 

 

 

 

Glomerulo-
nephritis

 

 

3–14

 

 

 

1

4

 

 

Autoimmune
hepatitis

 

20

 

6–20

7

1

1

 

 

 

Inflam. bowel
disease

 

 

 

2

 

6–10

0–1

9

21

 

Celiac disease

 

 

 

 

 

0–1

1–4

 

 

 

Autoimmune
enteropathy

97

10

 

 

3

 

 

 

 

 

Pernicious
anemia

 

13–15

 

 

 

1–9

1–2

 

 

 

Alopecia

 

32–37

 

 

 

2–4

 

 

 

 

Vitiligo

 

8–13

 

 

 

0–13

1–4

 

 

 

Other
autoimmune
disorders

62

 

 

 

 

 

 

 

 

 

Guillain-Barré
syndrome

 

 

1–3

 

 

 

 

 

 

 

AIH, autoimmune hemolytic anemia; AIT, autoimmune thrombocytopenia; AIN, autoimmune neutropenia; def., deficiency; IPEX, immune dysregulation, polyendokrinopathy, enteropathy, X-linked syndrome; APECED, autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy; ALPS, autoimmune lymphoproliferative syndrome; AID, acquired immune deficiency syndrome; CVID, common variable immunodeficiency syndrome; WAS, Wiskott-Aldrich syndrome; NEMO, nuclear factor kappa essential modulator; XLA, X-linked agammaglobulinemia; * X-linked deficiency

Figure 21.1-1 Pathogen recognition by dendritic cells and macrophages by means of toll-like receptor (TLR). The TLR recognizes a pathogen and induces the expression of co stimulatory molecules and inflammatory cytokines. Antigen is then presented together with the co stimulatory molecules to naive Th cells (Th0). In this way, the innate immune response includes the adaptive immune system in host defense. Modified from Ref. /6/.

Phagocytosis Pathogen InflammatoryCytokines Th1 Innate Immunity Acquired Immunity Dendritic cell/Macrophage IL-4 IFN-γ Costimulatorymolecules Antigenpresentation TLR Th0 Th2

Figure 21.1-2 Non specific phagocytosis of a microorganism by a macrophage. The microorganism is coated with C3b or immunoglobulin and engulfed by the macrophage surface complement or Fc receptors in the zipper mechanism principle.

Zipper mechanism Clearance of immune complexes 1. Nonspecific phagocytosis (Foreign object, microorganisms) 2. Phagocytosis on the Fc receptor (Opsonization, immune complexes) Phagolysosom Fc-Receptor Fc receptor Complement-receptor (C3)

Figure 21.1-3 Antigen presentation to T cells.

Top: presentation of an antigen (peptide) by an antigen-presenting cell (dendritic cell/macrophage). The antigen is presented to the T helper cell by an MHC class II molecule.

Bottom: presentation of an antigen (peptide) by an antigen presenting cell (dendritic cell/macrophage). The antigen is presented to the cytotoxic T cell by an MHC class I molecule.

T-cell antigen receptor Antigen-presenting cell T-helper cell MHC Class IIprotein MHC Class IIprotein Antigen receptor Viral antigen Virus infected cell Cytotoxic T cell MHC Class Iprotein MHC Class Iprotein Antigen

Figure 21.1-4 Structure of inhibitory NK cell receptors. Lectin like receptors are shown under A; immunoglobulin like receptors are shown under B. The molecules contain intracellular immune receptor based inhibitory motifs (ITIMs) with phosphorylation sites for signal transmission. With kind permission from Ref. /15/.

B A

Figure 21.1-5 Activating and inhibitory NK cell receptors and their interaction with target cells. The killer activating receptor binds to a surface molecule on the target cell and the killer inhibitory receptor binds to an MHC class I molecule on the target cell. If a signal is received from the target cell via the inhibitory receptor, the target cell escapes lysis. If no signal is received or if the MHC class I molecules are down regulated (e.g., in tumor cells or virus-infected cells), the target cell is lysed.

Target cell Inhibitory receptor MHC class I molecule Any molecule Activating receptor Killer cell

Figure 21.1-6 Innate immunity: responses following initial contact with microbes and microbial products. Modified from Ref. /5/. AP, alternative complement pathway; IL-1, interleukin-1; MAC, membrane attack complex; MBLP, mannose binding lectin pathway; Mφ, macrophage; TNF-α, tumor necrosis factor α; TLR, toll like receptor.

Virus Viral RNA, DNA, protein Bacteria, fungi + products Virus infected cell NKcell Bacteria,fungi Bacteria, fungi,chlamydia,envelope viruses Gram negativebacteria,enveloped viruses Antimicrobialpeptides Circulatingphagocytes Epithelialcell Complement(AP, MBLP) target organism Effector Activation signals Initialcontact Microbes +products Chemokines Endothelial cell TLR TLR IL-12 TNFαIL-1 C3b C5a MAC (C5b-C9) Virus infected cell

Figure 21.1-7 Selection of T cells that migrate into the thymus from the bone marrow. The T cell receptors of naive T cells consist of the MHC molecules CD3, CD4 as well as of α/β chains (CD4 cells) or CD3, CD8 and α/β chains (CD8 cells). T-cell selection takes place in the cortex (left side of figure) and medulla (right side of figure) of the thymus.

– Negative selection (lower left): T cells, whose receptors have variable binding affinity for self (MHC)-antigen, are selected by cortical thymic epithelial cells. Many of these cells have a high affinity for self peptides and self MHC molecules and are autoreactive. After interacting with macrophages or dendritic cells in the thymic medulla, these autoreactive cells are apoptosed.

– Positive selection: T cells with weak affinity for self peptides and self MHC molecules are tested for reactivity to foreign antigen in the thymic medulla and, if they possess the required specificity, they escape apoptosis.

MHC CD8 T cell receptor T cell T cell CD4 Cortical epithelial cell CD8 or CD4 T cell Negative selectionand apoptosis Cortical epithelial cell CD8 or CD4 MHC Antigen T cell receptor Apoptosis CD8 or CD4 Positive selection Positive selection,but apoptosis Surviving anddelivery at thecirculation Thymus cortex with cortical epithelial cells Thymus medulla with dendritic cellsand macrophages T cell T cell

Figure 21.1-8 Antigen-independent development of B cells and their receptors. The cell lineage depicted at the top develops in the bone marrow. The naive, immature B cells then undergo further maturation through transitional stages T1 and T2 in the spleen. A minority of B cells migrate to the marginal zone of the spleen, where they become naive marginal zone B cells (B1 cells). The majority of B cells migrate to the splenic follicles, where they become long living naive follicular B cells (B2 cells). Modified according to Ref. /17/.

Bone Marrow Stem cell Pro B cell Pro B cell Naive B cell (immature) IgM IgD low CD21 hi CD23 IgM hi IgD hi CD21 hi CD23 + IgM low IgD hi CD21 hi CD23 + IgM hi IgD low CD21 low CD23 Naive marginal-zone B cell (B1 cell) Spleen Naive long-lasting follicular B cell(B2 cell) T2-B cell T1-B cell

Figure 21.1-9 Class switch recombination following contact between B2 cell and antigen.

Stem cell Pro B cell Pre B cell Immature B cell Mature B cell Activated B cell Plasma cell Class-switch Antigen independent Antigen dependent IgM IgG IgA IgE

Figure 21.1-10 Structure of the mature and immature B cell receptor (BCR). Pre B cells express a primitive version of the recognition unit. This consists of two heavy chains (H), each with a constant region (Cμ), variable region (VH), and surrogate light chains (Vs). In the mature B cell, the surrogate light chains are replaced by the final kappa or lambda light chains, which have a constant region (Ck) and a variable region (Vk). The variable regions of the H and L chain contain three hyper variable complementarity determining regions (CDRs), which bind to the antigen. The mature IgM molecule acts as a B cell receptor, either alone or in combination with an IgD receptor with the same specificity. Modified with kind permission from Ref. /7/.