DR CLIP (CLASS II-ASSOCIATED INVARIANT CHAIN PEPTIDES) AND DR/PEPTIDECOMPLEXES COLOCALIZE IN PRELYSOSOMES IN HUMAN B-LYMPHOBLASTOID-CELLS/

In APCs, MHC class II molecules (MHC class II) bind antigenic peptides after HLA-DM mediated removal of CLIP, To characterize intracellular sites of peptide loading in human B lymphoblastoid cell lines, we cond ucted immunoelectron microscopy studies with Abs recognizing MHC class II associated with CLIP or bound peptide, respectively, together with Abs to HLA-DM and endocytic markers, The distribution of these molecu les indicates that peptide binding occurs in compartments with charact eristics of normal late endosomes, and in compartments that show chara cteristics of late endosomes, but are not detectably accessed by endoc ytosed BSA-gold, The latter compartments may represent or give rise to recycling vesicles that deliver peptide-loaded class II molecules to the cell surface. In addition, we have compared cells in which HLA-DM and HLA-DR interaction is defective with cells in which this interacti on is intact, and find that DM/DR interaction is not required for the proper localization of either molecule to peptide-loading compartments .