HLA-DM IS LOCALIZED TO CONVENTIONAL AND UNCONVENTIONAL MHC CLASS II-CONTAINING ENDOCYTIC COMPARTMENTS

HLA-DM molecules remove invariant (ii) chain peptides from newly synth esized MHC class II complexes. Their localization may thus delineate c ompartments, e.g., MIIC, specialized for loading peptides onto class I I molecules. In murine A20 B cells, however, DM is not restricted to s pecialized 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 lymph oblasts, HLA-DM was found in structures indistinguishable from late en dosomes or lysosomes, although in these cells the lysosomes contained MHC class II molecules. Thus, the distribution of HLA-DM does not nece ssarily 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.