MULTIPLE SIGNALS REGULATE THE INTRACELLULAR TRAFFICKING OF HLA-DM IN B-LYMPHOBLASTOID CELLS

Peptide loading by major histocompatibility complex (MHC) class II mol ecules occurs in the endocytic pathway and is critically dependent upo n the function of the class II-related molecule human leucocyte antige n-DM (HLA-DM). We have previously shown that a tyrosine-based lysosoma l targeting signal present in the cytoplasmic tail of DMB has the capa city to target HLA-DM to peptide-loading compartments in HeLa cells. H ere we investigate the importance of this signal in directing HLA-DM t o processing compartments in professional antigen-presenting cells. We reconstituted a DMB-negative B-lymphoblastoid cell line with native o r targeting-deficient DMB and show that in the absence of its tyrosine signal, DMB-Y230A is as efficient as the wild-type molecule in induci ng MHC class II SDS stable dimer formation; restoring expression of th e conformation-dependent DR3 epitope 16:23; the removal of CLIP; and a ccessing lysosomal peptide-loading compartments. By transient transfec tion in HeLa cells we show that Ii is able to compensate for loss of D MB-encoded targeting information. These data imply that in cells expre ssing physiological levels of class II, Ii and DM, there is sufficient association with Ii to direct the majority of DM into the endocytic p athway. Thus MHC class II and HLA-DM may follow similar intracellular trafficking pathways on route to antigen-processing compartments.