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From the * Department of Cell Biology, Yale University School of Medicine,
New Haven, Connecticut 06520-8002; and the 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 antigen processing and class II peptide loading may occur within one or more
endocytic compartments, the functions of these compartments and their relationships to endosomes and lysosomes remain uncertain. In murine B cells, at least one population of class II-
containing endocytic vesicles (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 the time frame during which BCR-mediated antigen processing and formation of peptide-class II complexes occurs. Only a fraction of
the BCR-internalized antigen was delivered to CIIV, with the majority of internalized antigen
being delivered to lysosomes that are largely class II negative. The extensive colocalization of
BCR-internalized antigen and newly synthesized class II molecules in CIIV suggests that CIIV
may represent a specialized subcellular compartment for BCR-mediated antigen processing.
Additionally, we have identified a putative CIIV-marker protein, immunologically related to the
Ig
Department of Pediatrics,
National Jewish Center for Immunology, Denver, Colorado 80206
subunit of the BCR, which further illustrates the unique nature of these endocytic vesicles.
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