THE INTRACELLULAR-TRANSPORT OF MHC CLASS-II MOLECULES IN THE ABSENCE OF HLA-DM

The HLA-DM alpha and HLA-DM beta genes encode a nonpolymorphic, class II-like molecule that functions by an as yet undefined mechanism in th e assembly of processed antigen-HLA class II complexes. Mutant cells t hat fail to express HLA-DM are deficient in Ag processing. We previous ly isolated a subcellular compartment in mouse B cells in which functi onal processed Ag-class II complexes are first formed, referred to as the peptide-loading compartment. Here, evidence is provided that HLA-D M resides in a subcellular compartment with the characteristics of a p eptide-loading compartment in a human B lymphoblastoid cell line, but is not required for the intracellular transport of HLA-DR3 molecules t o a corresponding compartment in HLA-DM-deficient cells. Thus, the pri mary defect in HLA-DM-deficient cells does not appear to be a failure in the intracellular trafficking of class II molecules.