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5-22 MHC class I and class II molecules are also first found in the cartilaginous fishes.
Hashimoto, K., Okamura, K., Yamaguchi, H., Ototake, M., Nakanishi, T., and Kurosawa, Y.: Conservation and diversification of MHC class I and its related molecules in vertebrates. Immunol. Rev. 1999, 167:81–100.
Kurosawa, Y., and Hashimoto, K.: How did the primordial T cell receptor and MHC molecules function initially? Immunol. Cell Biol. 1997, 75:193–196.
Ohta, Y., Okamura, K., McKinney, E.C., Bartl, S., Hashimoto, K., and Flajnik, M.F.: Primitive synteny of vertebrate major histocompatibility complex class I and class II genes. Proc. Natl Acad. Sci. USA 2000, 97:4712–4717.
Okamura, K., Ototake, M., Nakanishi, T., Kurosawa, Y., and Hashimoto, K.: The most primitive vertebrates with jaws possess highly polymorphic MHC class I genes comparable to those of humans. Immunity 1997, 7:777–790.
Antigen Presentation to T Lymphocytes
Vertebrate adaptive immune cells possess two types of antigen receptors: the immunoglobulins that serve as antigen receptors on B cells, and the T-cell receptors. While immunoglobulins can recognize native antigens, T cells recognize only antigens that are displayed by MHC complexes on cell surfaces. The conventional α:β T cells recognize antigens as peptide:MHC complexes (see Section 4-13). The peptides recognized by α:β T cells can be derived from the normal turnover of self proteins, from intracellular pathogens, such as viruses, or from products of pathogens taken up from the extracellular fluid. Various tolerance mechanisms normally prevent self peptides from initiating an immune response; when these mechanisms fail, self peptides can become the target of autoimmune responses, as discussed in Chapter 15. Other classes of T cells, such as MAIT cells and γ:δ T cells (see Sections 4-18 and 4-20), recognize different types of surface molecules whose expression may indicate infection or cellular stress.
The first part of this chapter describes the cellular pathways used by various types of cells to generate peptide:MHC complexes recognized by α:β T cells. This process participates in adaptive immunity in at least two different ways. In somatic cells, peptide:MHC complexes can signal the presence of an intracellular pathogen for elimination by armed effector T cells. In dendritic cells, which may not themselves be infected, peptide:MHC complexes serve to activate antigen-specific effector T cells. We will also introduce mechanisms by which certain pathogens defeat adaptive immunity by blocking the production of peptide:MHC complexes.