MHC CLASS-II INTRACELLULAR-TRANSPORT

Immune responses to protein antigens require the recognition by T lymp hocytes of major histocompatibility complex (MHC) molecules associated to peptides derived from that protein. The MHC molecules can be of cl ass I and/or II, depending on the type of immune response developed. T he peptides that associate to MHC class I and II are different in stru cture and origin. Class I molecules generally bind 8-10 amino acid-lon g peptides derived from endogenous antigens. Class II molecules bind t o longer peptides (14 to 20 amino acids) derived from either endogenou s or exogenous antigens. The intracellular sites of generation and bin ding of these peptides to MHC class I and class II molecules are disti nct, MHC class I-binding peptides are generated in the cytosol, transl ocated through the ER membrane by specific TAP transporters; the bindi ng to MHC class I occurs in the lumen of the ER MHC class II-binding p eptides are generated and bind class II molecules in the endocytic pat hway. Recently, several groups identified and characterized a speciali zed endocytic compartment, where MHC class II encounters and binds ant igenic peptides.