IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 11. Cell Interactions in the Humoral Immune Response

■ Immune activation required for antibody synthesis involves interaction between T Cells and APCs, and subsequently between these primed T cells and B cells.

■ Cell activation occurs via antigen-specific interaction involving Cell Adhesion molecules and cytokines. The strongest costimulatory signal is provided by the B7 molecule (CD80 or CD86). In the absence of appropriate costimulation, antigen recognition may lead to clonal anergy.

■ Lymphocyte proliferation occurs indirectly, i.e., it depends on the induction of growth factor receptors on lymphocytes triggered by activation. Lymphocyte growth factors (such as IL-2) are synthesized primarily by T cells.

■ There are two types of Antigens that elicit a HUMORAL Immune Response: T-dependent and T-independent. T-dependent antigens induce a secondary immune response characterized by The production of IgG and an increase in antibody affinity.

■ The primary Immune Response to T-dependent antigens is characterized by the production of low-affinity IgM Antibodies. The secondary immune response produces a greater quantity of antibodies and undergoes isotype switching to generate IgG, IgA, and IgE, accompanied by an increase in antibody affinity.

■ Isotype switching and the increase (maturation) in antibody affinity occur within germinal centers inside secondary lymphoid Tissues.

The humoral immune response (antibody production) represents the culmination of a series of CELLULAR AND MOLECULAR interactions occurring in a specific sequence:

✵ T cells recognize the antigen presented to them by antigen-presenting cells (APCs) and consequently become activated;

✵ Th cells interact with B cells that present antigen fragments to them;

✵ activated B lymphocytes proliferate and differentiate into antibody-producing cells;

✵ antibody synthesis begins, and The Nature of the subsequent immune response depends on the antibody Class.

This chapter discusses the processes of antigen presentation and T- and B-cell interaction, some of the most important molecules involved, as well as the events triggered by these interactions, including immunoglobulin class switching, enhancement (maturation) of antibody affinity, and The Development of immunological memory.

ANTIGEN PRESENTATION TO T CELLS

Antigen Processing

In the pioneering studies by Ada and Nossal, it was established that only a very small fraction of the administered antigen molecules (<1%) participate in inducing an immune response, while the vast majority is rapidly degraded and eliminated from the body. These findings suggest that antigen presentation is the rate-limiting step in the immune reaction.

Antigens that have entered the body undergo intracellular processing—Cleavage into peptide fragments that subsequently bind to class I or class II MHC molecules (see Ch. 9). These fragments determine antigen-specific T-cell activation: T-cell receptors recognize the Amino acid sequences of these fragments bound within the MHC molecular cleft (in contrast, antibodies recognize conformational determinants).

Interaction with antigen-presenting cells is crucial for T-lymphocyte activation

The interaction between T Lymphocytes and the cells comprising the heterogeneous group of so-called "antigen-presenting cells" is the most thoroughly studied example of cellular cooperation in the immune system. The interaction between T cells and APCs following antigen administration initiates the entire sequence of subsequent events and largely determines their ultimate outcome: if a sufficient number of CD4⁺ helper T cells (Th) is activated, B-cell activation or cell-mediated immune responses almost invariably ensue; conversely, if Th-cell stimulation is absent, some form of immunological tolerance may develop (see Ch. 14), in which no further immune reactions take place.

There are various types of APCs

A wide variety of cells can present antigens, depending on how and where the initial interaction between the antigen and the immune system occurs (Fig. 11.1). The initial activation of resting CD4⁺ T cells is most efficiently mediated by interdigitating dendritic cells (IDCs), which are abundant in the T-cell zones of Lymph Nodes AND the Spleen. IDCs are characterized by high expression levels of class II MHC antigens, which interact with the T-cell receptor (TCR) and the CD4 molecule On the surface of CD4⁺ Th cells. However, macrophages and B cells also express class II MHC antigens, making it impossible to attribute the superior antigen-presenting efficiency of IDCs to this property alone.

Fig. 11.1. Diagram of lymph node Structure. Shown are the afferent and efferent Lymphatic vessels, the outer cortical B-cell area, and the paracortical T-cell area. Different antigen-presenting cells predominate in these regions (although the boundaries are not absolute), selectively capturing various types of antigens that persist on The Cell surface for varying periods. For instance, many polysaccharide antigens are captured predominantly by marginal zone macrophages; these antigens can persist on macrophages for months and years. In contrast, antigens on recirculating medullary macrophages persist for only a few days or weeks. Notably, recirculating Langerhans cells, whose primary source is the Skin, alter their Morphology upon entering the lymph nodes to become interdigitating dendritic cells. Like follicular dendritic cells, they possess long processes that make close contact with lymphocytes.

Interdigitating cells are thought to be the major type of antigen-presenting cell operating in the primary immune response, as they induce T-cell proliferation more efficiently than APCs of any other type. Cellular proliferation serves as a key step in the Development of the immune response, ensuring an increase in the number of antigen-specific T cells; however, this is only one aspect of effective T-lymphocyte activation. Blood monocytes (the most extensively studied APCs in humans) also possess The ability to induce both T-cell proliferation and helper function.

B cells can also act as APCs—they are capable of binding, internalizing, and cleaving a specific antigen into Peptides that form a complex with MHC class II molecules. At very low antigen concentrations, B cells with high-affinity antigen receptors (IgM or IgD) serve as the most efficient APCs, simply because Other types of APCs cannot capture enough antigenic material for presentation. During a secondary immune response (which involves A large number of antigen-specific B cells), B cells can become the primary type of APC. The Properties and functions of APCs are illustrated in Fig. 11.2 and 11.3.

Fig. 11.2. Mononuclear phagocytes (1), B cells (2), and dendritic cells (3) can present antigens to helper T cells (Th) restricted by MHC class II antigens. Macrophages engulf the antigen via non-specific receptors or in the form of immune complexes and process it, after which the resulting antigenic fragments return to the cell surface in association with class II molecules. Activated B cells can capture antigen via their surface IMMUNOGLOBULINS, subsequently internalize it, and present it in a complex with class II molecules. Dendritic cells constitutively express class II molecules and take up antigen by pinocytosis.

Fig. 11.3. Many APCs are unable to phagocytose antigens, but can take them up by other means, such as pinocytosis. Endothelial cells, which are not typically considered APCs, express class II molecules under METABOLISM/18.html">The Influence of IFNγ and can present antigens, much like certain epithelial cells. Another similar example is follicular cells of The Thyroid Gland, which function as APCs in the autoimmune process of Graves' disease.

Antigen presentation to T cells is mediated by the interaction of numerous cell-surface molecules

The T-cell receptor (TCR)—a dimer consisting of an α-chain and a β-chain—recognizes a specific peptide located within the peptide-binding groove of an MHC molecule. This binding is critical for immunological Specificity, as the peptide associated with an MHC molecule of a particular haplotype forms a unique structure recognized by the TCR. However, other molecules also participate in presentation. Evidence for this comes from experiments involving the transfection of complementary DNA (cDNA) encoding human MHC molecules into mouse fibroblasts. Mouse cells expressing human MHC molecules acquired the ability to function as human APCs, albeit less efficiently than cells that also expressed other presentation-related molecules. One such molecule is intercellular adhesion molecule 1 (ICAM-1), which interacts with lymphocyte function-associated antigen 1 (LFA-1) present on all Cells of the immune system. When both human MHC and ICAM-1 genes are transfected into mouse cells, their ability to act as human APCs increases. Other effective costimulatory molecules interacting with ligands on the T-cell surface have also been discovered (Fig. 11.4).

Fig. 11.4. Molecules involved in the interaction between T cells and APCs, along with various CYTOKINES AND THEIR directions of action.

The most efficient known costimulatory molecules are B7-1 (CD80) and B7-2 (CD86). They are constitutively expressed on IDCs, but their expression can also be stimulated on monocytes, B cells, and likely other APCs. Both molecules serve as ligands for the CD28 antigen and its homolog CTLA-4, the expression of which is induced by T-cell activation. The stimulatory effect of CD28 consists of prolonging and enhancing the production of IL-2 and other cytokines; this stimulation also appears essential for preventing the INDUCTION OF TOLERANCE. Although the CD28-B7 interaction is critically important, knockout mice (see Ch. 10) lacking the CD28 Gene still respond to antigen, albeit requiring a higher dose. Consequently, CD28 stimulation is not strictly obligatory for the activation of naive T cells. It is possible that in CD28-deficient mice, other signaling molecules fulfill The Role of the CD28/B7 costimulatory signal.

An alternative Ligand for B7 is CTLA-4, an inhibitory receptor that restricts T-cell activation. Thus, initially, the constitutively expressed CD28 molecule interacts with B7, leading to T-cell activation; however, activation-Induced Expression of the higher-affinity CTLA-4 molecule limits the degree of activation as the available B7 antigen now binds to CTLA-4 (Fig. 11.5). The Importance of this molecule is highlighted by the fact that CTLA-4-deficient mice develop an aggressive lymphoproliferative disorder caused by the lack of effective inactivation of dividing T cells.

Fig. 11.5. Resting T cells express the CD28 molecule, which binds to B7-1 and B7-2 on antigen-presenting cells (e.g., B lymphocytes). Upon activation, T cells begin to express an alternative, high-affinity ligand for B7—CTLA-4. This ligand binds B7, after which T cells no longer receive an activation signal.

Alongside the TCR, the T-cell surface molecule CD2—the receptor for lymphocyte function-associated antigen 3 (LFA-3), carried by many cells and all APCs—participates in T-cell activation. The presence of LFA-3 on sheep erythrocytes is responsible for the rosetting reaction (E-rosettes), which was widely used to obtain purified T-cell fractions before Monoclonal Antibodies were introduced into research practice.

The signal induced by B7-like molecules, which enhances the TCR signal and promotes positive T-cell activation, is termed the "second signal." Without such a signal, resting T cells cannot respond optimally and, if they recognize a specific antigen in the absence of the second signal, they become inactivated, entering a state of immunological tolerance. This tolerance is specific, as only the function of those Th cells that respond to the given antigen is lost. Tolerance not associated with cell death is known as "clonal anergy."

In addition to cell-surface molecules, locally acting cytokines are involved in T-cell activation. Of particular interest are IL-1 and IL-6, cytokines produced by APCs, including macrophages. Stimulation of T cells by these agents is not always necessary, such as when T cells are already dividing. Acting on resting T lymphocytes, IL-1 and IL-6 induce the expression of receptors for the T-cell growth factor, IL-2. IL-12 is also of great importance for T-cell activation, as it promotes increased IFNγ production, thereby steering the development of uncommitted T cells into Th1 lymphocytes. Antigen-presenting cells produce IL-15, which is also capable of inducing T-cell proliferation and is crucial during the period before IL-2 synthesis has begun.

The interaction between CD4⁺ T cells and APCs has been studied in considerable detail, whereas information regarding the analogous process involving CD8⁺ T lymphocytes and APCs remains sparse. It is known that CD4⁺ cells function as helpers during the activation of CD8⁺ T cells. Since a single interdigitating dendritic cell can bind to many T lymphocytes, it has been suggested that activation occurs within clusters formed by CD4⁺ and CD8⁺ cells on the IDC surface.

Antigen presentation influences the subsequent development of the immune response

Antigen-presenting cells can be rapidly activated at the very onset of an immune response, for example, by microbial products or certain Viruses that are immunogenic on their own, or under the influence of an adjuvant component in a vaccine. The antigen presentation they carry out is not a unidirectional process. As soon as T cells are activated, they secrete cytokines, notably IFNγ and GM-CSF, which, alongside cell-surface signaling molecules such as the CD40 ligand, enhance antigen-presenting function. Activated APCs exhibit upregulation of MHC class I and II molecules, Fc receptors, and costimulatory adhesion molecules, including B7-1, B7-2, CD11a/b/c, ICAM-1, and ICAM-3. They also produce numerous cytokines (e.g., IL-1, IL-6, TNFα), Enzymes, and other mediators.

Lymphocyte activation leads to two partially competing processes: cellular proliferation and differentiation into effector cells. At the terminal stage of differentiation, cells (such as plasma cells) become so specialized that they lose surface molecules, such as MHC class II, as well as the ability to respond to regulatory signals or proliferate.

The Fate of antigen-responding lymphocytes can vary. Some persist for long periods as immunological memory cells. Their potential lifespan in humans exceeds 40 years, as established by chromosomal abnormalities in Blood Cells (e.g., the presence of DNA cross-links that prevent mitosis) in survivors of the Hiroshima atomic bombing. The lifespan of other lymphocytes is short, which explains why moderate antigenic stimulation does not lead to lymphoid tissue hypertrophy. Nevertheless, this lifespan of antigen-responding lymphocytes is sufficient for the development of effective cellular and humoral immune responses. Accumulating evidence indicates that the elimination of cells no longer needed after the completion of the immune response occurs via apoptosis.