MHC CLASS-II INTRACELLULAR-TRANSPORT

Immune responses to protein antigens require the recognition by T lymphocytes of major histocompatibility complex (MHC) molecules associated to peptides derived from that protein. The MHC molecules can be of class I and/or II, depending on the type of immune response developed. The peptides that associate to MHC class I and II are different in structure and origin. Class I molecules generally bind 8-10 amino acid-long peptides derived from endogenous antigens. Class II molecules bind to longer peptides (14 to 20 amino acids) derived from either endogenous or exogenous antigens. The intracellular sites of generation and binding of these peptides to MHC class I and class II molecules are distinct, MHC class I-binding peptides are generated in the cytosol, translocated through the ER membrane by specific TAP transporters; the binding to MHC class I occurs in the lumen of the ER MHC class II-binding peptides are generated and bind class II molecules in the endocytic pathway. Recently, several groups identified and characterized a specialized endocytic compartment, where MHC class II encounters and binds antigenic peptides.