Sorting of MHC Class II Molecules into Exosomes through a Ubiquitin-Independent Pathway

Major histocompatibility complex class II (MHC-II) molecules accumulate in exocytic vesicles, called exosomes, which are secreted by antigen presenting cells. These vesicles are released following the fusion of multivesicular bodies (MVBs) with the plasma membrane. The molecular mechanisms regulating cargo selection remain to be fully characterized. As ubiquitination of the MHC-II β-chain cytoplasmic tail has recently been demonstrated in various cell types, we sought to determine if this post-translational modification is required for the incorporation of MHC-II molecules into exosomes. First, we stably transfected HeLa cells with a chimeric HLA-DR molecule in which the β-chain cytoplasmic tail is replaced by ubiquitin. Western blot analysis did not indicate preferential shedding of these chimeric molecules into exosomes. Next, we forced the ubiquitination of MHC-II in class II transactivator (CIITA)-expressing HeLa and HEK293 cells by transfecting the MARCH8 E3 ubiquitin ligase. Despite the almost complete downregulation of MHC-II from the plasma membrane, these molecules were not enriched in exosomes. Finally, site-directed mutagenesis of all cytoplasmic lysine residues on HLA-DR did not prevent inclusion into these vesicles. Taken together, these results demonstrate that ubiquitination of MHC-II is not a prerequisite for incorporation into exosomes.