CUTTING EDGE - SIGNALS FROM THE B-LYMPHOCYTE ANTIGEN RECEPTOR REGULATE MHC CLASS-II CONTAINING LATE ENDOSOMES

The B lymphocyte response to protein Ag is dependent upon the successf ul presentation to T cells of Ag-derived, MHC class II-restricted pept ides, The B cell Ag receptor (BCR) facilitates this process by interna lizing ligand and delivering it to specialized compartment(s) (MHC cla ss II peptide-loading compartments (MIIC)) where it is processed into peptides and loaded onto MHC class II. In addition to efficiently targ eting Ag, the BCR can provide tyrosine kinase-dependent signals that a ugment the presentation of Ag, possibly by enhancing the generation of immunogenic peptides, However, the mechanism by which this occurs is unclear. Herein, we report that the BCR signals a reorganization, fusi on, and acidification of an MIIC-like compartment into an invariant ch ain-and MHC class II-rich complex of large vesicles. This complex beco mes the primary target for endocytosed receptors, These data suggest t hat signals generated by the BCR regulate the site of Ag processing.