As already noted, the immune response involves several different types of cells (T and B lym- phocytes), each of which has various forms as noted both by morphology and surface antigens. However, it is clear that the immune response involves many more different types of cells, their subtle interactions, and numerous protein signals termed cytokines or lymphokines, which regu- late both the expression of specific genes as well as cell proliferation in the target cell. The rela- tions of some of these cell types may be seen in Figure 19.12. In the figure, the developmental origin of T and B cells as well as macrophages from the bone marrow is depicted. The macro- phage or antigen-presenting cell—which takes on various forms in tissues such as “dendritic” cells, monocytes, and sinusoidal-lining cells—plays a key role in cellular interactions in the im-
aDuring the recombination events leading to the ultimate immunoglobulin molecule, there occurs loss or addition of nucleotides at the recombination junctions. Additions of nucleotides at this junction fall into two categories: template-dependent (P nucleotides) and template-independent (N regions) (cf. Komori et al., 1993). N region addition may be effected by the enzyme, termi- nal deoxytransferase, in B cells (Desiderio et al., 1984).
From Janeway and Travers, 1996, with permission of the authors and publisher.
mune response. The mechanism of antigen processing and presentation will be considered be- low. Although antigens may interact directly with B cells to evoke or accelerate subsequent immune responses (see below), most foreign antigens, including tumor antigens, are first pro- cessed by one of several pathways. In the sequence of events that follow the presentation of the processed antigen by the macrophage to unstimulated T and B cells, the T cells play a major role. Some T cells, termed helper cells, interact with B cells in the presence of the antigen to stimulate the further differentiation of the B cell to the effector plasma cell that produces anti- bodies specifically reactive with the antigen (cf. Janeway and Travers, 1996). Such an inter-
action results in the elaboration by the helper T cell of a number of lymphokines which in turn stimulate the B or T recipient cell to proliferate, thus enhancing and accelerating the clonal ex- pansion of the effector T or B cells. The effector or killer T cell has on its surface, along with the T-cell receptor, the CD8 coreceptor, as noted in Figure 19.13. The helper T cell has a similar structure but with the CD4 coreceptor in place of the CD8 molecule.
In addition to the coreceptors seen in Figure 19.13, it is now apparent that there are several different costimulatory receptors that interact between the antigen-presenting cell and the T lym- phocyte. CD28 is a major costimulatory receptor on T cells, and B7 its costimulatory ligand on antigen-presenting cells (cf. Robey and Allison, 1995). Another regulatory molecule, CTLA-4, also occurring on the surface of T cells when interacting with B7, causes a block in the produc- tion of cytokines, cell cycle progression, and cell differentiation (cf. Lee et al., 1998).
Subgroups of T-Helper Cells
As we have noted above, a class of T cells appears to exist, the T suppressor cell population, whose function is to inhibit specific immune responses. In opposition to this, helper T cells en- hance the immune response, both cell-mediated and humoral. However, it is now apparent that there are at least two different types of T-helper cells, known as Th1 and Th2, which are commit- ted before antigenic stimulation (cf. Abbas et al., 1996). But the most potent differentiation- inducing stimuli are specific cytokines or lymphoid growth factors. Interleukin-12 (IL-12), pro- duced by activated macrophages and dendritic cells, is the principal Th1-inducing cytokine, while IL-4 induces the development of Th2 cells. In turn, each of these classes of T-helper cells produces a variety of cytokines and exhibits functions as shown in Table 19.3.
Figure 19.13 Diagram of the interaction of helper and killer (effector) T cells with presenting cells. The interaction involves the association of the T cell receptor and its coreceptor (CD4 or CD8) with the presen- tation of antigen in association with appropriate MHC (major histocompatibility complex) molecules (see below). (After von Boehmer and Kisielow, 1991, with permission of the authors and publisher.)
T Cell–Independent B-Cell Response
As noted above, there is a subset of B lymphocytes, termed B1, that do not interact with T cells in order to produce antibodies. Such cells respond directly to antigens by their interaction with the B-cell receptor. B1 cells are usually not found in the bone marrow but rather home predomi- nantly in the peritoneal and pleural cavities. Such cells recognize common bacterial antigens as well as self-antigens (cf. Fagarasan and Honjo, 2000). They appear to contribute to the mucosal immune response (Figure 19.3) and also in the production of autoantibodies.
Suppressor T Cells
Another population of T cells that is central to the regulation of the immune response is that of suppressor T cells. These cells modulate both T and B cell–effected immunity by suppressing the replication, maturation, and/or function of potential effector cells (Kapp et al., 1984). There has been some controversy with respect to their actual existence (Arnon and Teitelbaum, 1993). A number of suppressor T cells are antigen-specific, while others may suppress pathways by nonspecific cellular interactions (Kapp et al., 1984). Earlier studies have suggested that sup- pressor T cells are restricted—i.e., require an interaction by a specific gene product, that of the I-J gene in the mouse, which is involved in macrophage–T cell interactions (Dorf and Benacer- raf, 1985; Hayes and Klyczek, 1985; Asano and Tada, 1989). The development of suppressor T cells involves many fewer antigen-presenting cells (macrophages) for their induction than T-helper cells (Dorf et al., 1992). In addition, there is substantial evidence that T-suppressor cells mediate the suppressor action via factors comparable to lymphokines (Dorf et al., 1992). These factors may include transforming growth factor-β (Chapter 16) and interleukin-10 (Mason and Powrie, 1998).
γδ T Cells
While the majority of T-cell receptors are made up of the α and β chains (greater than 90%), a small percentage (1% to 10%) of T cells express the heterodimer T-cell receptor consisting of γ and δ chains (Figure 19.11). While the function of these cells is not absolutely clear, it is noted that they do have distinct properties. They are relatively small cells that express almost no CD4 and considerably less CD8 than αβ T cells (cf. Kabelitz et al., 1999). Furthermore, the interac- tion of the γδ T cell with an antigen-presenting cell does not require the MHC molecules as do the αβ T cells, as noted in Figure 19.13. While their function is not clear at present, they do appear to express reactivity towards a variety of foreign, self, and neoplastic antigens.
Natural Killer Cells
Another subset of lymphocytes is the natural killer (NK) cells, which are shown in Figure 19.12 as derived from T cells. Other evidence suggests that these lymphocytes are unique and different in their origin from either T or B cells and thus likely derived in the bone marrow, like all other major players in the immune response (Lotzová, 1993). NK cells appear to function in graft re- jection, tumor immunity, and the regulation of hematopoiesis (Robertson and Ritz, 1990). NK cells spontaneously react with cells exhibiting foreign antigens on their surface, but may also interact with specific carbohydrates on the cell surface (Brennan et al., 1995) as well as a variety of other surface targets (Storkus and Dawson, 1991). However, in contrast to cytotoxic T cells, NK cells not only do not require the presence of proteins of the MHC locus (see below) in order to exert their lytic effect but such proteins actually inhibit NK cells from interacting with cells containing such proteins. Such inhibition appears to be mediated by specific membrane recep- tors expressed on NK cells (cf. Lanier, 1998). In this way, NK cells complement the T- and B- cell repertoire of the cell-mediated and humoral responses that are restricted by the presence of such histocompatibility antigens (Gumperz and Parham, 1995; Reyburn et al., 1997). The pro- duction of NK cells in the host may also be enhanced by immunostimulatory agents such as interferon and lymphokines, especially interleukin-2 (Robertson and Ritz, 1990). Newborn hu- mans are deficient in NK cell activity—a factor that may be involved in the peculiar immunolog- ical reactivity of neonates (Kaplan et al., 1982).
REGULATION OF THE IMMUNE RESPONSE
As noted in Figure 19.12, the development of the immune response is extremely complex but also must be highly regulated if the host is to mount an appropriate response to one or more specific antigens. The mechanisms of the regulation of the immune response involve the pres- ence of specific surface gene products as well as the production of stimulatory and inhibitory signals both within the immunocyte and via signals secreted and received by cells in the immune response. These regulatory factors are considered below under several different headings.
Genetics in the Regulation of the Immune Response
While it is obvious that specific genes control the amino acid sequence of the immunoglobulins and T cell–receptor components involved in the immune response, the elucidation of the genet- ics involved in cellular interactions that mediate the immune response has also been a major topic of immunobiology. Many of the genes coding for products involved in the regulation of the immune response of the organism are located in regions of the genome designated as the major histocompatibility complex (MHC), located in the human on chromosome 6 (the HLA complex)
and in the mouse on chromosome 17 (H-2 complex). A diagram of the major components of these two MHC regions is seen in Figure 19.14, along with a diagram of the protein structures of the major histocompatibility complex antigens. Class I antigens are ubiquitous and found on most cells within the organism. Class I molecules have a single integral membrane unit consist- ing of three domains—α1, α2, and α3—as shown in Figure 19.14. This molecule is in associa- tion noncovalently with β2 microglobulin, a 12 kDa soluble protein. This latter protein is, in the mouse, found on the H-3 minor histocompatibility locus present on chromosome 2 (Rammensee and Klein, 1983). The class II MHC antigen is a heterodimer of two transmembrane units, α (33 kDa) and β (29 kDa), both encoded in the MHC. Class II antigens are expressed primarily on B cells, macrophages, and all antigen-presenting cells as well as endothelium (cf. Germain, 1994). Recently, another set of MHC-like genes, termed the CD1 family, have been described. These genes appear to be involved in antigen presentation to a subset of phenotypically identifiable CD1-restricted T cells (cf. Park et al., 1998). In general, the antigens presented by this system involve glycolipid structures and related molecules (e.g., Prigozy et al., 2001).
Antigen Processing and Presentation in the Immune Response
As indicated earlier (Figure 19.5), a specific antibody reacts with a specific epitope or confirma- tion of a relatively small peptide unit or comparable structure within an antigen. While the struc-
Figure 19.14 Schematic representations of the gene organization of the human HLA and mouse H-2 major histocompatibility complexes as well as cartoons of the class I and class II MHC antigen molecules with their domain organizations. Details of the class III regions of the MHC are not shown. It is in this region that other genes, especially those for complement, occur (cf. Claman, 1992). (After Germain, 1994, with permission of the author and publisher.)
ture of the antibody-combining region dictated a relatively small molecular size for the epitope, the mechanism for selection of the epitope by the antibody-producing cell as well as the T-cell receptor of the T lymphocyte was not clear. It is now apparent that this is dictated by the process- ing of antigens by antigen-presenting cells. In addition, antigen processing is required for the T- cell receptor to engage molecules of the MHC that are associated with a small polypeptide chain bearing the epitope. The receptor does not engage the antigen unless the antigen forms part of the complex involving the MHC molecule. This requirement of the interaction of the T-cell re- ceptor with MHC molecules is termed MHC restriction. Such restriction then requires that for a T cell to become activated by processed antigens, there must be an interaction between the T-cell receptor and the peptide carried in the cleft of the MHC antigen. As noted earlier, exceptions to this restriction are δγ T cells and NK cells.
Since at least two sets of MHC molecules, class I and class II, are required for the recogni- tion of protein antigens, other factors in such association and recognition are the CD8 and CD4 molecules on the surface of cytolytic and helper T lymphocytes respectively. In addition to CD8 and CD4 molecules, several other “costimulatory” molecules have been described that enhance the stimulation of the T cell interacting through its receptor with the polypeptide fragment car- ried by the MHC class 1 or class 2 antigen (cf. Jenkins, 1994; Yoshinaga et al., 1999).
While the processing and presentation of antigens are clearly effective in adult and devel- oped tissues, there is evidence that neonatal macrophages are deficient in their ability to present antigen and express cell-surface MHC antigens (Lu and Unanue, 1985). Although T-cell differenti- ation in the thymus is presumably complete by 18 to 20 weeks of fetal life in the human, T cells of the newborn show a decreased potential to provide help and suppression for antibody production by B lymphocytes, and they do not develop into cytotoxic T lymphocytes (Hanson et al., 1997).
Mechanisms of Antigen Processing and Presentation
During the past several years it has become apparent that antigens from different locations vis-à- vis the cell are processed by somewhat different routes. A diagram of this is seen in Figure 19.15. As noted in the figure, peptides produced from endogenous cytosolic proteins that enter
Figure 19.15 Pathways of antigen presentation by MHC class I and class II molecules. Antigens taken up into the cell are internalized into endosomes (E), where they are partially degraded and subsequently bind to class II MHC molecules and are thence transported to the cell surface. Proteins that either occur in the cytosol or migrate there from external sources (X) are processed by the proteasome, transported into the cisternae of the endoplasmic reticulum through the Golgi and subsequently to the surface. (Adapted from Unanue and Cerottini, 1989, with permission of the authors and publisher.)
the cytosol either endogenously or from exogenous sources, as noted, are processed probably through a structure known as the proteasome, which results in relatively short peptides of 10 to
20 amino acids length. These are transported through the membrane of the endoplasmic reticu- lum by a “transporter associated with antigen processing” (TAP). From thence the peptide asso- ciates with an MHC class I molecule occurring in the endoplasmic reticulum, the complex that is subsequently passed through the Golgi and ultimately expressed on the surface of the plasma membrane (Germain, 1994; Unanue and Cerottini, 1989). Within the proteosome are compo- nents, LMP proteins, which appear to alter the substrate specificity of the protein degradation occurring in the proteosome, favoring cleavage near residues suitable for binding of the peptide within the “pocket” or “cleft” of class I MHC molecules. Proteins to be processed for presenta- tion in MHC class II molecules are those internalized from the exterior into endosomes, where they are partially degraded and subsequently bound to the “cleft” of class II MHC molecules and then exteriorized by fusion of the endosome to the plasma membrane. A diagram of the peptide- binding clefts of class I and class II MHC molecules is seen in Figure 19.16. The overall struc- ture of the MHC class I and class II molecules as shown in Figure 19.16 is strikingly similar to the antibody-reactive site of immunoglobulins (Figures 19.2 and 19.5). However, the molecular diversity of the MHC molecules is much less than that of the immunoglobulins, numbering only in the hundreds (Le Bouteiller, 1994).