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

Immune Responses and Effector Mechanisms 75

compartmentalized organs. The cytokines IFNc, IL-1, IL-2, IL-4, and IL-12 play an important role in this process—as do various other factors.

During the second phase (Fig. 2.13), activated T helper cells recognize the same MHC class II peptide complex, but on the surface of a B cell. Prior to this

event, the B cell must have responded to the same antigen (by virtue of its Ig 2 surface receptor recognizing a conformational antigenic epitope), then internalized the antigen, processed it, and finally presented parts of it in the form

of linear peptides bound to MHC class II molecules on the cell surface for recognition by the T helper cell. The resulting B-T cell contact results in further interactions mediated by CD4, CD40, and CD28 (see Fig. 2.9, p. 65)—and sends a signal to the B cell which initiates the switch from IgM to IgG or other Ig classes. It also allows induction of a process of somatic mutation, and probably enhances the survival of the B cell in the form of a memory B cell.

Subpopulations of T Helper Cells

Soluble signaling substances, cytokines (interleukins), released from T helper cells can also provide an inductive stimulus for B cells. Two subpopulations of T helper cells can be differentiated based on the patterns of cytokines produced (Fig. 2.14). Infections in general, but especially those by intracellular parasites, induce cytokine production by natural killer (NK) cells in addition to a strong T helper 1 (TH1) response. The response by these cells is characterized by early gamma interferon (IFNc) production, increased levels of phagocyte activity, elimination of the antigen by IFNc-activated macrophages, production of IgG2a and other complement-binding (opsonizing) antibodies (see the complement system, pp. 86ff.), and induction of cytotoxic T-cell responses. IL-12 functions as the most important promoter of TH1 cell function and additionally acts as an inhibitor of TH2 cells.

In contrast, worm infections or other parasitic diseases induce the early production of IL-4, and result in the development of a TH2 response. TH2 cells, in turn, recruit eosinophils and induce production of IgG1 and IgE antibodies. Persons suffering from allergies and atopic conditions show a pathologically excessive TH2 response potential. IL-4 not only promotes the TH2 response but also inhibits TH1 cells.

Cytotoxic T Cells (CD8+ T Cells)

Mature CD8+ T cells perform the biologically important function of lysing target cells. Target cell recognition involves the association of MHC class I structures with peptides normally derived from endogenous sources, i.e., originating in the cells themselves or synthesized within them by intracellular parasites. Induction of cytotoxic CD8+ Tcell response often does not require helper

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Lymphocytes and APCs in the Primary Immune Response

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cells—oronlyrequiresthesecells indirectly. However, should the antigenstimulus and the accompanying inflammation be of a low-level nature, the quantityof cytokines secreted by the cytotoxic Tcells themselves may not suffice, in which case the induction of a CD8+ T cell response will be reduced unless additional cytokines are provided by helper T cells. The cytotoxic activity of CD8+ T cells is mediated via contact and perforin release (perforin renders the membrane of the target cell permeable resulting in cellular death). CD8+ T cells also function in interleukin release (mainly of IFNc) by which they mediate non-cytotoxic effector functions (Fig. 2.15). The role of perforin in con- tact-dependent direct cytolysis by natural killer (NK) cells and cytotoxic T cells (see also Fig. 2.17, p. 88) has been investigated in gene knockout mice. In these animals the perforin gene has been switched off by means of homologous recombination, and as a result they can no longer produce perforin. Per- forin-dependent cytolysisis important for the controlof noncytopathic viruses, tumors, and transformed cells, but also plays a large role in the control of highly virulent viruses that produce syncytia (e.g., the smallpox virus). Release of noncytolytic effector molecules by CD8+ cells, mostly IFNc, plays a major role in control of cytopathic viruses and intracellular bacteria. Cytolytic effector mechanisms may also contribute to release of intracellular micro-organisms and parasites (e.g., tuberculosis) from cells that only express MHC class I.

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Cytokines (Interleukins) and Adhesion

Cytokines are bioactive hormones, normally glycoproteins, which exercise a wide variety of biological effects on those cells which express the appropriate receptors (Table 2.6). Cytokines are designated by their cellular origin such that monokines include those interleukins produced by macrophages/ monocytes, whilst lymphokines include those interleukins produced by lymphocytes. The term interleukins is used for cytokines which mostly influence cellular interactions. All cytokines are cyto-regulatory proteins with molecular weights under 60 kDa (in most cases under 25 kDa). They are produced locally, have very short half-lives (a matter of seconds to minutes), and are effective at picomolar concentrations. The effects of cytokines may be paracrine (acting on cells near the production locus), or autocrine (the same cell both produces, and reacts to, the cytokine). By way of interaction with highly specific cell surface receptors, cytokines can induce cell-specific or more general effects (including mediator release, expression of differen-

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Fig. 2.14 TH1 and TH2 cells are derived from a TH0 cell, and undergo differentiation in the presence of help derived from cytokines, DC, macrophages, and other cell types. TH1 cells are activated by IL-12 and IFNc and inhibited by IL-4; whilst for TH2 cells the reverse is true. Viruses and bacteria (particularly intracellular bacteria) can induce a TH1 response by activating natural killer cells. In contrast, allergens and parasites induce a TH2 response via the release of IL-4. However, the strong invitro differentiation of CD4+ Tcells intoTH1–TH2 subsets is likely to be less sharply defined in vivo.

tiation molecules and regulation of cell surface molecule expression). The functions of cytokines are usually pleiotropic, in that they display a number of effects of the same, or of a different, nature on one or more cell types. Below is a summary of cytokine functions:

&Promotion of inflammation: IL-1, IL-6, TNFa, chemokines (e.g., IL-8).

&Inhibition of inflammation: IL-10, TGFb.

&Promotion of hematopoiesis: GM-CSF, IL-3, G-CSF, M-CSF, IL-5, IL-7.

&Activating B cells: CD40L, IL-6, IL-3, IL-4.

&Activating T cells: IL-2, IL-4, IL-10, IL-13, IL-15.

&Anti-infectious: IFNa, IFNb, IFNc, TNFa.

&Anti-proliferative: IFNa, IFNb, TNFa, TGFb.

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Antiviral Protection by T Cells

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Lysis caused by virus

Diffusion, protection of many other cells

Fig. 2.15 Certain viruses destroy the infected host cells (right), others do not (left). Cytotoxic T cells can destroy freshly infected cells by direct contact (with the help of perforin), thus inhibiting viral replication (middle). Whether the result of this lysis is clinically desirable depends on the balance between protection from viral proliferation, and the damage caused by immunologically mediated cell destruction. In perforin knockout mice (perforino/o), T cells are unable to produce perforin and therefore do not destroy the infected host cells. Replication of non-cytopathic viruses thus continues unabated in these mice. Soluble anti-viral interleukins (especially IFNc and TNFa), and neutralizing antibodies, combat cytopathic viruses (which replicate comparatively rapidly) more efficiently than do cytolytic Tcells; this is because interleukin and antibody molecules can readily diffuse through tissues and reach a greater number of cells, more rapidly, than can killer T cells.

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Table 2.6 The Most Important Immunological Cytokines and Costimulators plus Their Receptors and Functions

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Table 2.6 Continued: The Most Important Immunological Cytokines. . .

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Table 2.6 Continued: The Most Important Immunological Cytokines. . .

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Cell adhesion molecules often play an essential role in cell-to-cell interactions. Two lympho-hematopoietic cells can only establish contact if one of them expresses surface molecules that interact with ligands expressed on the surface of the other cell. As for APC and T cell interactions, the result

2of such contact may be that a signal capable of inducing differentiation and functional changes will be induced. Adhesion proteins are usually comprised of several chains which can induce different effects when present in various combinations. Interaction of several cascades is often required for the final differentiation of a cell. Cell adhesion molecules normally form part of the Ig superfamily (e.g., ICAM, VCAM, CD2), integrin family (lymphocyte function antigen, LFA-1), selectin family, cadherin family, or various other families. Selectins and integrins also play an important role in interactions between leukocytes and the vascular wall, and thus mediate the migration of leukocytes from the bloodstream into inflamed tissues, or the entry of recirculating lymphocytes into the lymph node parenchyma through high endothelial venules (HEV).

Chemokines (chemoattractant cytokines) comprise a family of over 30 small (8–12 kDa) secreted proteins. These contribute to the recruitment of “inflammatory cells” (e.g., monocytes) into inflamed tissues, and influence the recirculation of all classes of leukocytes (Table 2.6). Some chemokines result in the activation of their target cell in addition to exerting chemotatic properties. Chemokines can be classified into three families based on their N terminus structure: CC chemokines feature two contiguous cysteine residues at the terminus; CXC chemokines have an amino acid between the two residues; and CX3C and C chemokines thus far comprise only one member each (fractalkine and lymphotactin, respectively). Although the N terminus carries bioactive determinants, using a chemokines amino acid sequence to predict its biological function is not reliable. The chemokine system forms a redundant network, or in other words, a single chemokine can often act upon a number of receptors, and the same receptor may recognize a number of different chemokines. Many of the chemokines also overlap in terms of biological function.

Chemokines can be grouped in two functional classes: inflammatory chemokines which are secreted by inflamed or infected tissues as mediators of the nonspecific immune response; and constitutive chemokines which are produced in primary or secondary lymphoid organs. Together with endothelial adhesion molecules, inflammatory chemokines determine the cellular composition of the immigrating infiltrate. In contrast, the function of constitutive chemokines is to direct lymphocytes to precise locations within lymphoid compartments. Thus, chemokines play a major role in the establishment of inflammatory and lymphoid microenvironments. Chemokine receptors are G protein-coupled membrane receptors with seven transmembrane sequences. In keeping with the above nomenclature, they are designated as CCR, CXCR, or CX3CR plus consecutive numbering. Some viruses, for instance