The case of CD8 T cells

APCs are involved in the early activation of CD8 T cells by presenting peptides via class 1 MHC molecules. Once activated, the CD8 T cells will be able to interact and react with class I MHC-peptide complex presented in various target cells. Thus, in the antiviral response the first encounter that recruits and activates specific antiviral clones will be that of infected APCs (either macrophages or DCs) with the CD8 T cells. Once clonal expansion and activation takes place, the CD8 T cells can kill the virally infected target cells. APCs also have the ability to capture proteins from other cells (i.e. tumor antigens, alloantigens, for example) and process it into the class I MHC presentation pathway, a process that needs to be better defined in mechanistic terms. (See Antigen Presentation via MHC Class I Molecules.)

Most epithelial, vascular and mesenchymal cells do not express class II MHC molecules or costimulatory molecules; however, there are exceptions: thymic epithelium normally expresses class II MHC molecules, and some mucosal cells of the gastrointestinal tract and proximal tubular cells of the kidney also express some levels of class II MHC molecules. During inflammation, as a result of local IFNy production, many cells are made to express class II MHC molecules. Whether this aberrant expression is involved in tissue autoimmunity needs to be settled. Tissue cells express limited amounts of costimulatory molecules which makes them poor APCs for initiating T cell activation.

See also: Antigen presentation via MHC class I molecules; Antigen presentation via MHC class II molecules; B lymphocyte, antigen processing and presentation; B lymphocytes; Dendritic cells; Immunogen; Langerhans cells; Macrophage activation; Microenvironment; Monocytes; Mononuclear phagocyte system; Second signals for lymphocyte activation.

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