Hart C. Anthony, Shears Paul. Color Atlas of Medical Microbiology.pdf

56 2 Basic Principles of Immunology

regions encoding the H chain segments Cl, Cd, Cc, Ca, and Ce, in consecutive order. Thus all immunoglobulin production begins with the synthesis of IgM and IgD (resulting from transcription of the VDJ and the Cl or Cd gene segments). This occurs without prior antigen stimulus and is transitional in nat-

2ure. Antigen stimulation results in a second gene rearrangement—during which the VDJ gene is relocated to the vicinity of Cc, Ca, or Ce by a process of recombination involving deletion of the intervening regions. Following this event, the B cell no longer produces H chains of the IgM or IgD classes, but is instead committed to the production of IgG, IgA, or IgE—thus allowing secretion of the entire range of immunoglobulin types (Table 2.2). This process is known as class switching, and results in a change of the Ig class of an antibody whilst allowing its antigen specificity to be retained.

Variability types. The use of different heavy or light chain constant regions results in new immunoglobulin classes known as isotypes. Individual Ig classes can also differ, with such genetically determined variations in the constant elements of the immunoglobulins (which are transmitted according to the Mendelian laws) are known as allotypes. Variation within the variable region results in the formation of determinants, known as idiotypes. The idiotype determines an immunoglobulins antigenic specificity, and is unique for each individual B-cell clone.

Functions. Each different class of antibody has a specific set of functions. IgM and IgD act as B-cell receptors in their earlier transmembrane forms, although the function of IgD is not entirely clear. The first antibodies produced in the primary immune response are IgM pentamers, the action of which is directed largely against micro-organisms. IgM pentamers are incapable of crossing the placental barrier. The immunoglobulin class which is most abundant in the serum is IgG, with particularly high titers of this isotype being found following secondary stimulation. IgG antibodies pass through the placenta and so provide the newborn with a passive form of protection against those pathogens for which the mother exhibits immunity. In certain rare circumstances such antibodies may also harm the child, for instance when they are directed against epitopes expressed by the child’s own tissues which the mother has reacted against immunologically (the most important clinical example of this is rhesus factor incompatibility). High concentrations of IgA antibodies are found in the intestinal tract and contents, saliva, bronchial and nasal secretions, and milk—where they are strategically positioned to intercept infectious pathogens (particularly commensals) (Fig. 2.5). IgE antibodies bind to high-affinity Fce receptors present on basophilic granulocytes and mast cells. Cross-linking of mast cell bound IgE antibodies by antigen results in cellular degranulation and causes the release of highly active biogenic amines (histamine, kinines). IgE antibodies are produced in large quantities following parasitic infestations of the intestine, lung or skin, and play a significant role in the local immune response raised against these pathogens.

The T-Cell System 57

The Mucosa-Associated Lymphoid Tissue

(MALT) Immune System and “Homing”

2

Fig. 2.5 Specialized APCs (M cells in the intestinal wall or pulmonary macrophages in the lung) take up antigens in mucosa and present them in the Peyer’s patches or local lymph nodes. This probably enhances T cell-dependent activation of IgA-producing B cells, which are preferentially recruited to the mucosal regions (“homing”) via local adhesion molecules and antigen depots, resulting in a type of geographic specificity within the immune response.

The T-Cell System

T-Cell Receptors (TCR) and Accessory Molecules

Like B cells, Tcells have receptors that bind specifically to their steric counterparts on antigen epitopes. The diversity of T-cell receptors is also achieved by means of genetic rearrangement of V, D, and J segments (Fig. 2.4b). However, the T-cell receptor is never secreted, and instead remains membrane-bound.

Each T-cell receptor consists of two transmembrane chains, of either the a and b forms, or the c and d forms (not to be confused with the heavy

The T-Cell System 59

immunological rejection of cell transfusions or tissue and organ transplants. Its true function as a peptide-presenting molecule was not discovered until the seventies, when its role became apparent whilst testing the specificity of virus-specific cytotoxic Tcells. During these experiments it was observed that

immune Tcells were only able to destroy infected target cells if both cell types 2 were derived from the same patient or from mice with identical MHC molecules. The resulting conclusion was that a T-cell receptor not only recognizes

the corresponding amino acid structure of the presented peptide, but additionally recognizes certain parts of the MHC structure. It is now known that this contact between MHC on the APC and the T-cell receptor is stabilized by the co-receptors CD4 and CD8.

MHC classes. Molecules encoded by the MHC can be classified into three groups according to their distribution on somatic cells, and the types of cells by which they are recognized:

&MHC class I molecules. These molecules consist of a heavy a chain with three Ig-like polymorphic domains (these are encoded by 100–1000 alleles, with the a1 and a2 domains being much more polymorphic than the a3 do-

main) and a nonmembrane-bound (soluble) single-domain b2 microglobulin (b2M, which is encoded by a relatively small number of alleles). The a chain forms a groove that functions to present antigenic peptides (Fig. 2.7). Human HLA-A, HLA-B, and HLA-C molecules are expressed in varying densities on all somatic cells (the relative HLA densities for fibroblasts and hepatic cells, lymphocytes, or neurons are 1x, 100x and 0.1!, respectively). Additional, nonclassical, class I antigens which exhibit a low degree of polymorphism are also present on lymphohematopoietic cells and play a role in cellular differentiation.

&MHC class II molecules. These are made up by two different polymorphic

transmembrane chains that consist of two domains each (a1 is highly polymorphic, whilst b1 is moderately polymorphic, and b2 is fairly constant). These chains combine to form the antigen-presenting groove. Class II molecules are largely restricted to lymphohematopoetic cells, antigen-presenting cells (APC), macrophages, and so on. (see Fig. 2.9a, p. 65) In humans, but not in mice, they are also found on some epithelial cells, neuroendocrine cells, and T cells. The products of the three human gene regions HLA-DP, HLA-DQ and HLA-DR can additionally form molecules representing combinations of two loci—thus providing additional diversity for peptide presentation.

&MHC class III molecules. These molecules are not MHC antigens in the classical sense, but are encoded within the MHC locus. These include complement (C) components C4 and C2, cytokines (IL, TNF), heat shock protein 70 (hsp70), and other products important for peptide presentation.

The T-Cell System 61

Presentation of Endogenous and Exogenous Antigens

2

Fig. 2.8 Intracellularly synthesized endogenous antigen peptides (left side) are bound to MHC class I molecules within the endoplasmic reticulum, fixed into the groove by b2M, and presented on the cell surface. Antigens taken up from exogenous sources (right) are cleaved into peptides within phagosomes. The phagosome then merges with endosomes containing MHC class II molecules, the binding site of which had been protected by the so-called CLIP fragment. These two presentation pathways functionally separate MHC class I restricted CD8+ Tcells from MHC class II restricted CD4+ T cells.

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length. The groove of MHC class II molecules is open-ended, and can contain peptides of 9–15 (usually 10–12) amino acids in length.

T cells can only recognize antigenic peptides in combination with either MHC class I (which presents endogenous linear peptides, such as those de-

2rived from viruses) or MHC class II (which present exogenous linear peptides, such as those derived from bacterial toxins) (Table 2.3). In contrast to antibodies that recognize soluble, complex, nonlinear, three-dimensional struc- tures—T-cell recognition is restricted to changes on the surfaces of cells signaled via MHC plus peptide.

T-cell specificity. T-cell recognition therefore involves two levels of specificity: first, MHC presentation molecules bind peptides with a certain degree of specificity as determined by the shape of the groove and the peptide anchoring loci. Second, the MHC-peptide complex will only be recognized by specific T-cell receptors (TCR) once a minimum degree of binding strength has been obtained. For this reason diseases associated with the HLA complex are determined largely by the quality of peptide presentation, but can also be influenced by the available TCR repertoire.

The structure of the MHC groove therefore determines which, of all the potentially recognizable, peptides will actually be presented as T-cell epitopes. Thus, the same peptides cannot function as T-cell epitopes in all individuals. Nonetheless, certain combinations of peptides and MHC are frequently observed. For example, approximately 50% of Caucasians carry the HLA-A2 antigen, although this is sometimes found in a variant form.

Antigen-presenting cells (APC). APCs belong to the lymphohematopoietic system. They attach peptides to MHC class II molecules for presentation to T cells, and induce T-cell responses. The complex mechanisms involved in this process have not yet been fully delineated. Stromal cells present in the thymus and bone marrow (i.e., connective tissue cells, dendritic cells and nurse cells in both thymus and bone marrow, plus epithelial cells in the thymus) can also function as APCs. The following cell types function as APCs in peripheral secondary lymphoid organs:

&Circulating monocytes.

&Sessile macrophages in tissues, microglia in the central nervous system.

&Bone marrow derived dendritic cells with migratory potential—these occur as cutaneous Langerhans cells, as veiled cells during antigen transfer into the afferent lymph vessels, as interdigitating cells in the spleen and lymph nodes, and as interstitial dendritic cells or as M cells within MALT.

&Follicular dendritic cells (FDC)—these are found within the germinal centers of the secondary lymph organs, do not originate in the bone marrow, and do not process antigens but rather bind antigen-antibody complexes via Fc receptors and complement (C3) receptors.

The T-Cell System 63

& B lymphocytes—these serve as a type of APC for T helper cells during T-B collaborations.

The consequences of MHC variation. Because every individual differs with regard to the set of polymorphic MHC molecules and self antigens expressed

(with the exception of monozygotic twins and inbred mice of the same strain), 2 the differences between two given individuals are considerable. The high de-

gree of variability in MHC molecules—essential for the presentation of a large proportion of possible antigenic peptides for T-cell recognition—results in these molecules becoming targets for T cell recognition following cellular or organ transplantation resulting in transplant rejection. The term “transplantation antigens” is therefore a misnomer, and is only used because their real function was not discovered until a later time. Normally antigens are only recognized by T cells if they are associated with MHC-encoded self-structures. Transplant recognition, which apparently involves the imitation of the combination of a non-self antigen plus a self-MHC molecule, can therefore be considered an exception. The process probably arises from T-cell receptor crossreactivity between host self-MHC antigens plus foreign peptides on the one hand, and non-self transplantation antigens associated with self-peptides from the donor on the other hand (for example, the T-cell receptor for HLA-A2 peptide X cross-reacts with HLA-A13 peptide Y). Transplant rejection is therefore a consequence of the enormous variety of combinations of antigenic peptide plus MHC, which is exhibited by each individual organism.

T-Cell Maturation: Positive and Negative Selection

Maturation of Tcells occurs largely within the thymus. Fig. 2.2 (p. 47) shows a schematic presentation of this process. Because the MHC-encoded presentation molecules are highly polymorphic, and are also subject to mutation, the repertoire of TCRs is not genetically pre-determined. One prerequisite for an optimal repertoire of T-cells is therefore the positive selection of T cells such that these preferentially recognize peptides associated only with self transplantation (MHC) antigens. A second prerequisite is negative selection, which involves the deletion of T cells that react too strongly against self MHC plus self peptide. The random processes governing the genetic generation of an array of T-cell receptors results ab or cd receptor chain combinations which are in the majority of cases are non-functional. Those T cells preserved through to maturity represent cells carrying receptors capable of effectively recognizing self-MHC molecules (positive selection). However, the T cells within this group which express too high an affinity for self-MHC plus self-peptides are deleted (negative selection).

The process of positive selection was demonstrated in experimental mice expressing MHC class I molecules of type b (MHC classIb) from which the

64 2 Basic Principles of Immunology

thymus had been removed (and which therefore had no T cells). Implantation of a new thymus with MHC class I molecules of type a (MHC class Ia) into the MHC class Ib mice resulted in the maturation of T cells which only recognized peptides presented by MHC class Ia molecules, and not peptides presented by

2MHC class Ib molecules. However, recent experiments have shown that this is probable an experimental artefact and that it is not (or not solely) the thymic epithelial cells that determine the selection process, but that this process is driven by cells formed in the bone marrow. Positive selection is generally achieved by weak levels of binding affinity between the T-cell receptor and the self-MHC molecules, whereas negative selection eliminates those T cells exhibiting the highest levels of affinity (namely the self-or auto-reac- tive T cells) and absence of binding causes death by neglect. Thus, only T cells with moderate binding affinities are allowed to mature and exit the thymus. These T cells can potentially react to non-self (foreign) peptides presented by self MHC molecules. The enormous proliferation of immature thymocytes is paralleled by continuous cell death of large numbers of thymocytes (apoptosis, see summary in Fig. 2.17, p. 88). In general, the maturation and survival of lymphocytes is considered to be dependent on a continuous, repetitive, signaling via transmembrane molecules, and cessation of these signals is usually taken as a reliable indicator of cell death.

T-Cell Subpopulations

In order to recognize the presented antigen, T cells require the specific T-cell receptor and a molecule which functions to recognize the appropriate MHC molecules (i.e. CD4 or CD8 which recognize MHC class II and MHC class I, respectively). Thus T cells are classified into different subpopulations based on the CD4 or CD8 surface molecules:

CD4+ T cells. These T cells recognize only MHC class II-associated antigens. They are also called T helper cells due to their important role in T-B cell collaboration (Fig. 2.9a), although they exhibit many other additional functions. CD4+ cells can produce, or induce, the production of cytokines by which means they can activate macrophages and exercise a regulatory effect on other lymphocytes (see p. 75f.). Although these cells sometimes demonstrate an ability to cause cytotoxic destruction in vitro, this does not hold true in vivo.

CD8+ T cells. Only MHC class I-associated antigens are recognized by the CD8+ molecule. These cells are also known as cytotoxic T cells due to their ability to destroy histocompatible virus-infected, or otherwise altered, target cells as well as allogeneic cells. This effect can be observed both in vitro and in vivo (Fig. 2.9b). Costimulatory molecules are not required for this lytic