CLASS-II ANTIGEN-PROCESSING - ANALYSIS OF COMPARTMENTS AND FUNCTIONS

The endocytic and phagocytic pathways are composed of a series of vesi cular or vacuolar organelles within the cytoplasm in which exogenous p roteins or particles are processed. This processing includes catabolis m of protein antigens by proteases that exist within endosomes, lysoso mes, phagosomes, and phagolysosomes. Antigen processing produces pepti des that bind to class II MHC (MHC-II) molecules, which also target to vesicular compartments. Multiple different vesicular compartments may contribute to antigen processing, although some compartments appear t o be specialized in order to perform certain antigen processing functi ons. The relative levels of MHC-II molecules, processing enzymes, and other specialized molecules (e.g., HLA-DM) are among the variables tha t determine the relative contribution of different compartments. Recen t studies have begun to identify specific compartments that mediate ce rtain antigen processing functions, including the binding of immunogen ic peptides to MHC-II molecules.