HLA-DM is localized to conventional and unconventional MHC class II-containing endocytic compartments

HLA-DM molecules remove invariant (Ii) chain peptides from newly synthesized MHC class II complexes. Their localization may thus delineate compartments, e.g., MIIC, specialized for loading peptides onto class II molecules. In murine A20 B cells, however, DM is not restricted to specialized endosomal class II-containing vesicles (CIIV). Although DM was found in CIIV, it was also found throughout the endocytic pathway, principally in lysosomes devoid of class II molecules. In human lymphoblasts, HLA-DM was found in structures indistinguishable from late endosomes or lysosomes, although in these cells the lysosomes contained MHC class II molecules. Thus, the distribution of HLA-DM does not necessarily identify specialized class II compartments. Many "MIIC" may represent conventional lysosomes that accumulate MHC class II and HLA-DM in a number of cell types.