Antigen Presentation By Dendritic Cells

For the generation of a T cell response by DC in secondary lymphoid organs peptide fragments are presented in association with molecules of the major histocompatibility complex (MHC). For activation of CD8+ cytotoxic T cells (CTL), DC have to present antigenic peptides complexed to MHC class I molecules. The majority of class I-presented peptides are generated by degradation of cytoplasmic proteins by a multicatalytic proteolytic unit, the pro-teasome [13-15]. Dedicated peptides produced in the cytosol are transported across the endoplasmic reticulum (ER) membrane in an ATP-dependent manner by TAP (transporter associated with Ag presentation). Inside the ER, the peptides bind to newly assembled MHC class I molecules and are then transported to the plasma membrane. Usually, only autologous peptides are present on MHC class I molecules, and these are ignored by the immune system. However, when a cell is infected by virus and synthesizes foreign proteins or when it expresses a mutated gene or when a normal gene is overexpressed, it is recognized by CTL. In contrast to CD8+ CTL CD4+ T-helper cells are activated by MHC class Il-bound peptides derived from extracellular antigens. These antigens enter the endocytotic pathway of the APC and are degraded in low pH endosomal compartments into peptide fragments that complex with MHC class II molecules for surface presentation. Recently, exceptions of this classic dichotomy between exogenous and endogenous antigen presentation pathways became apparent.

Additional mechanisms were identified whereby ex-ogenously acquired antigens are presented on MHC class I molecules and induce CTL responses ('cross priming') [16-19]. On the other hand, endogenous antigens can enter the MHC class II pathway and generate significant T-helper responses [20,21].

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