Involvement of MIIC-like late endosomes in B cell receptor-mediated antigen processing in murine B cells

Currently, the involvement of classical vs novel endocytic compartments in the phenomenon of B cell receptor (BCR)-mediated Ag processing is a matter of considerable debate. In murine B cells, class II vesicles (CIIV) represe nt a novel endocytic compartment involved in BCR-mediated Ag processing and class LI peptide loading. Alternatively, in human B cells, the MHC class I I-enriched compartment (MIIC) represents a lysosome (L)-like endocytic comp artment that appears to be involved in this process. Presently, the relatio nship between CIIV, MIIC, and classical endosomes and L remains to be deter mined. Using density gradient centrifugation, a subcellular compartment mor phologically and immunologically similar to human MIIC has been identified, isolated, and characterized in murine B cells, These MIIC-like vesicles re present a population of class II-positive late endosomes (LE) and are disti nct from CIIV, MIIC-like LE are uniquely marked by the thiol protease cathe psin B, and along with mature L, appear to be the major repository of DM mo lecules in these cells, Importantly, both MIIC-like LE and CIIV isolated fr om Ag-pulsed B cells contain BCR-internalized Ag as well as antigenic pepti de-class II complexes.