some microbial proteins have regions that vary considerably from one strain of microbe to another. Antibodies or cytotoxic T cells that recognize highly variable regions of microbial proteins will only be useful against a limited number of strains. A better strategy is to target regions of microbial proteins that are exposed and highly conserved (i.e., found in all strains of the microbe). Using peptide epitopes as vaccines makes it possible to program a specific immune response directed toward conserved epitopes that are exposed on the surface of the antigen. We will return to this concept in chapter 17.
Other Kinds of Macromolecular Antigens. Unfortunately, many of the bacterial surface antigens recognized by the immune system are lipid, carbohydrate, or lipid-carbohydrate combinations. Gram-negative LPS and Gram-positive LTA and PG are excellent examples. In the past, immunologists and vaccinologists have focused almost exclusively on peptide antigens because they are easier to characterize than carbohydrate or lipid-containing molecules and because peptides elicit a strong immune response. Peptide antigens consist of different amino acids linked by a single type of bond, the peptide bond, which can be readily processed into defined epitopes. Carbohydrate oligomers, by contrast, can be linked by any of 12 types of glycosidic linkages. Lipids also contain more than a single type of linkage. Because of the great diversity of carbohydrate, lipid, and lipid-carbohydrate molecules, it is not surprising that the mechanism for how these carbohydrate and lipid antigens are processed has been largely neglected until recently.
A major breakthrough in this area came with the discovery of CD1 molecules. Mycobacterium tuberculosis, the cause of tuberculosis, provides a cor nucopia of lipid-saccharide and lipid-peptide antigens not found in most other bacteria, which enabled researchers to identify the first CD1 complexes that were bound to lipid or glycolipid antigens. Since then, five forms of human CD1 have been found: CD1a through CD1e. The steps in processing and displaying lipid antigens appears to be similar to those for peptide antigens (Figure 4-6C), with CD1 taking the place of the MHC I molecule for presentation of antigens to CD1-specific CTLs (for CD1a, CD1b, and CD1c) or NK cells (for CD1d). CD1e is the only isoform that does not appear to be expressed on the surface of DCs, and it is suspected that it might play a role in antigen processing, rather than in display of the antigen. CD1 is related, at the amino acid sequence and overall domain structure level, to MHC I, but has obviously diverged during evolution. CD1 proteins have deeper and larger binding cavities to accommodate the hydrophobic alkyl chains of lipid antigens. The more hydrophilic head group of the lipid antigen faces outward and interacts with the TCR on the CTLs and NK cells.
Interaction between APCs and T Cells: The T-Cell-Dependent Response
When an APC displays a particular MHC I-epitope or MHC II-epitope complex on its surface, only a small number of the vast pool of available immature precursor T cells will have a TCR capable of recognizing that particular MHC-epitope combination and stimulating the signals needed to induce proliferation and production of cytokines (Figure 4-8). All T cells express CD3 complexes (comprised of γ, δ, and ε chains) and TCR complexes (comprised of α, β, and two ζ chains). The CD3 and ζ chains are invariant, but the large repertoire of TCRs with surface receptors that recognize a wide variety of different MHC-bound epitopes is generated through variation of the αβ chains, which undergo V(D)J recombination during T cell development in the thymus (analogous to the process mentioned earlier that leads to antibody diversity).
Figure 4-8. The T cell receptor (TCR) complex. Shown is a schematic illustration of the complex formed between the epitope-MHC II complex on the APC and the CD3-TCR complex on the CD4+ Th cell. Note: Similar complex formation occurs for MHC I recognition by CD3-CD8+ T cells. CD4 and CD8 are expressed on two different types of αβ T cells, where CD4 recognizes MHC II complexes and CD8 recognizes MHC I complexes on APCs. Activation of the T cells requires stimulation by two signals: signal 1 occurs through specific interaction of the epitope-MHC II with the CD4-TCR and CD3 complexes and signal 2 occurs upon costimulation through the CD28 binding to CD80 or CD86 on the APC.
Antigen presentation and binding to the TCR stimulates the T cell to become either a CTL or a T helper (Th) cell (Figure 4-9). Display of intracellularly derived peptide epitopes on MHC I allows the APC to activate and stimulate the proliferation of CTLs. CTLs are distinguished by the presence of CD8 molecules on their surfaces (hence the name CD8+ cytotoxic T cells). The CD8 molecules, along with the T cell receptor (TCR), are responsible for recognizing only antigen-bound MHC I complexes on the APC. CD8 binds to MHC I and stabilizes the interaction between epitope-bound MHC I and the TCR. CTLs are particularly well equipped to recognize infected cells because virtually all cells of the body produce MHC I. If a cell is infected, it displays epitopes from the invading microbe on its surface. Binding of a CTL to the surface of an infected cell causes the CTL to release cytolytic or apoptotic proteins that can kill the infected cell, as described earlier (see Figure 4-5). In addition, as described in chapter 3, NK cells also use the amount of MHC I on cell surfaces as an indicator of cell health, because infected cells generally express less MHC I than normal cells.
Figure 4-9. T-cell-mediated immunity and memory. Characteristics of the antigen (Ag) determine whether the antigen is presented via APCs through MHC I complexes or through MHC II complexes. Ag-MHCI complexes bind to TCRs on CD8+ T cells that trigger the CTL response. Ag-MHC II complexes that bind to TCRs on CD4+ Th0 cells trigger IL-2 cytokine production and maturation into CD4+ Th1 cells, leading to cellular immunity or, in the subsequent presence of IL-4 into CD4+ Th2 cells, leading to stimulation of the antibody-producing B cell response. Cytokines and the presence of Th17 cells help direct the type of cell-mediated immunity and memory that occurs. Treg cells help control the immune response, especially once the pathogen has been cleared from the system.
In contrast, if an epitope is presented by the APC in complex with MHC II rather than MHC I, a different class of T cells, T helper (Th) cells, is stimulated. Complex formation between TCRs and coreceptor CD4 molecules on the surface of precursor Th cells (called CD4+ Th0 cells) with epitope-bound MHC II molecules on the surface of APCs leads to production of IL-2 and then IL-4 by the Th0 cell and proliferation and maturation. Proliferating Th cells come in two types: Th1 and Th2 cells. These Th cells influence a variety of immune responses through release of cytokines, and how they do this is a very active area of research in the field of immunology.
Other proteins on the surface of the APC and the Th cell, called costimulatory molecules (e.g., CD54, CD11a/CD18, CD58, CD2), must also interact to make the binding between the APC and the Th cell tight enough to stimulate the APC to release cytokines that stimulate the Th cell to proliferate and become activated. Th1 cells are stimulated to proliferate by IL-12 and IL-2 released by APCs, which causes the Th1 cells to release more IL-2 (positive feedback), IFN-γ, and TGF-β. Recognition of MHC II-epitope by CD4+ Th1 cells also stimulates the production and release of IL-2 and IFN-γ, which in turn activate macrophages. Th1-cell-mediated responses lead to cellular immunity, which is most effective against intracellular pathogens. The necessity for contacts between different surface proteins of the APC and the Th cell helps ensure that only specific binding of an MHC-epitope complex to its cognate TCR will result in Th cell activation.
Th2 cells are stimulated to proliferate by IL-4, which causes the cells to release more IL-4 (positive feedback), as well as other cytokines (IL-5, IL-9, IL-10, and IL-13), which modulate other immune cells. MHC II-epitope stimulation of CD4+ Th2 cells stimulates naïve B cells to proliferate into B cells and mature into antibody-producing
