ASSEMBLY OF AN ABUNDANT ENDOGENOUS MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II PEPTIDE COMPLEX IN CLASS-II COMPARTMENTS

To identify the intracellular site(s) of formation of an endogenous cl ass II/peptide complex in a human B cell line, we employed kinetic pul se-chase labeling experiments followed by subcellular fractionation by Percoll density gradient centrifugation and immunogold labeling on ul trathin cryosections. For direct demonstration of assembly of such com plexes, we used the monoclonal antibody YAe, which detects an endogeno us complex of the mouse class II molecule I-A(b) with a 17-amino acid peptide derived from the alpha chain of HLA-DR (DR alpha 52-68). We sh ow that in human B lymphocytes, these class II/peptide complexes assem ble and transiently accumulate in major histocompatibility complex cla ss II-enriched compartments before reaching the cell surface.