Regulation of MHC class II antigen presentation by sorting of recycling HLA-DM/DO and class II within the multivesicular body

MHC class II molecules bind antigenic peptides in the late endosomal/lysosomal MHC class II compartments (MIIC) before cell surface presentation. The class II modulatory molecules HLA-DM and HLA-DO mainly localize to the MIICs. Here we show that DM/DO complexes continuously recycle between the plasma membrane and the lysosomal MIICs. Like DM beta and the class II-associated invariant chain, the DO beta cytoplasmic tail contains potential lysosomal targeting signals. The DO beta signals, however, are not essential for internalization of the DM/DO complex from the plasma membrane or targeting to the MIICs. Instead, the DO beta tail determines the distribution of both DM/DO and class Il within the multivesicular MIIC by preferentially localizing them to the limiting membrane and, in lesser amounts, to the internal membranes. This distribution augments the efficiency of class II antigenic peptide loading by affecting the efficacy of lateral interaction between DM/DO and class II molecules. Sorting of DM/DO and class If molecules to specific localizations within the MIIC represents a novel way of regulating MHC class II Ag presentation.