ANTIGEN-PROCESSING AND CLASS-II MHC PEPTIDE-LOADING COMPARTMENTS IN HUMAN B-LYMPHOBLASTOID CELLS

THE peptide/class II major histocompatibility complex (MHC) complexes recognized by CD4(+) T cells have been characterized at the structural (1) and biochemical(2-4) levels and studies on the transport and matur ation of class II MHC indicate that specialized sites may be involved in peptide acquisition(5-11). Here we report the characterization of t he compartments involved in antigen processing and class II MHC loadin g relative to distinct functional domains of the endocytic pathway in antigen-specific human B lymphocytes(12-14). Peroxidase-mediated cross linking analysis(15) in intact cells demonstrates that peptide loading of class II MHC takes place in a compartment accessible to membrane i mmunoglobulin but not to transferrin receptors, although processing ma y be initiated within the latter domain. The density of membrane vesic les carrying newly assembled class II MHC complexes was distinct from early and late endosomes and dense lysosomes. Endocytosed antigen-gold complexes enter a class II MHC-rich compartment morphologically very similar to that described previously(9) and within the time frame of b iochemically detectable peptide loading.