* T cell responses are initiated by signals that are generated by TCR recognition of peptide-MHC complexes on the surface of an APC and through signals provided at the same time by costimulators expressed on APCs.

* The best-defined costimulators are members of the B7 family, which are recognized by receptors of the CD28 family expressed on T cells. The expression of B7 costimulators on APCs is increased by encounter with microbes, providing a mechanism for generating optimal responses against infectious pathogens. Some members of the CD28 family inhibit T cell responses, and the outcome of T cell antigen recognition is determined by the balance between engagement of activating and inhibitory receptors of this family.

* T cell responses to antigen and costimulators include changes in the expression of surface molecules, synthesis of cytokines and cytokine receptors, cellular proliferation, and differentiation into effector and memory cells.

* The surface molecules whose expression is induced on T cell activation include proteins that are involved in retention of T cells in lymphoid organs, growth factors for cytokines, effector and regulatory molecules, and molecules that influence the migration of the T cells.

* Shortly after activation, T cells produce the cytokine IL-2 and express high levels of the functional IL-2 receptor. IL-2 drives the proliferation of the cells, which can result in marked expansion of antigen-specific clones.

* CD4+ helper T lymphocytes may differentiate into specialized effector TH1 cells that secrete IFN-y, which mediate defense against intracellular microbes, or into TH2 cells that secrete IL-4 and IL-5, which favor IgE- and eosinophil/mast cellmediated immune reactions against helminths, or into TH17 cells, which promote inflammation and mediate defense against extracellular fungi and bacteria.

* The differentiation of naive CD4+ T cells into subsets is induced by cytokines produced by APCs and by the T cells themselves. The differentiation program is governed by transcription factors that promote cytokine gene expression in the T cells and epigenetic changes in cytokine gene loci, which may be associated with stable commitment to a particular subset. Each subset produces cyto-kines that increase its own development and inhibit the development of the other subsets, thus leading to increasing polarization of the response.

* T cells of the CD8+ subset proliferate and differentiate into cytotoxic T lymphocytes (CTLs), which express cytotoxic granules and can kill infected cells.

* Some activated T cells may differentiate into memory cells, which survive for long periods and respond rapidly to antigen challenge. The maintenance of memory cells is dependent on cytokines such as IL-7, which may promote the expression of antiapoptotic proteins and stimulate low-level cycling. Memory T cells are heterogeneous and consist of populations that differ in migration properties and functional responses.

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