Cellular and molecular aspects of antigen processing and the function of class II MHC molecules.

Antigen processing is the conversion of native antigen molecules into short peptides that can then bind to major histocompatibility complex (MHC) molecules. Class II MHC (MHC-II) molecules target to endocytic compartments, where they bind peptides that are produced by internalization of extracellular antigens and subsequent antigen catabolism. The resulting peptide-MHC complexes are displayed on the surface of antigen-presenting cells for recognition by T cells. Thus, MHC-II molecules first serve as peptide receptors that rescue peptides from total lysosomal degradation and transport them to the plasma membrane; the MHC-II molecules then form a composite peptide-MHC-II determinant that is recognized by the T cell receptor. Recent work has begun to clarify the molecular events and transport mechanisms that govern antigen processing.