DELIVERY OF B-CELL RECEPTOR-INTERNALIZED ANTIGEN TO ENDOSOMES AND CLASS-II VESICLES

B cell receptor (BCR)-mediated antigen processing is a mechanism that allows class II-restricted presentation of specific antigen by B cells at relatively low antigen concentrations. Although BCR-mediated antig en processing and class II peptide loading may occur within one or mor e endocytic compartments, the functions of these compartments and thei r relationships to endosomes and lysosomes remain uncertain. In murine B cells, at least one population of class II-containing endocytic ves icles (i.e., CIIV) has been identified and demonstrated to be distinct both physically and functionally from endosomes and lysosomes. We now demonstrate the delivery of BCR-internalized antigen to CIIV within t he time frame during which BCR-mediated antigen processing and formati on of peptide-class II complexes occurs. Only a fraction of the BCR-in ternalized antigen was delivered to CIIV, with the majority of interna lized antigen being delivered to lysosomes that are largely class II n egative. The extensive colocalization of BCR-internalized antigen and newly synthesized class II molecules in CIIV suggests that CIIV may re present a specialized subcellular compartment for BCR-mediated antigen processing. Additionally, we have identified a putative GIN-marker pr otein, immunologically related to the Ig alpha subunit of the BCR, whi ch further illustrates the unique nature of these endocytic vesicles.