B-CELL ANTIGEN RECEPTOR SIGNALING LINKS BIOCHEMICAL-CHANGES IN THE CLASS-II PEPTIDE-LOADING COMPARTMENT TO ENHANCED PROCESSING

In B cells, processing of antigens in the context of MHC class II mole cules is initiated by the binding of antigen to the B cell antigen rec eptor (BCR), BCR-mediated processing is highly efficient, as a consequ ence of the BCR's linked roles of delivering antigen to the class II p eptide-loading compartment and of signaling for increased antigen-proc essing activity, Evidence is emerging that receptor signaling regulate s intracellular transport through the activities of kinases, These in turn have been implicated in the regulation of small mel. wt GTPases w hich govern membrane transport, Therefore, we investigated the changes in the phosphoprotein and GTPase profiles associated with the class I I peptide-loading compartment following BCR cross-linking. We first sh ow that protein kinase inhibitors, known to block BCR signal transduct ion, inhibit BCR-enhanced antigen processing, demonstrating the critic al dependence of enhanced processing on the signaling activity of the BCR, Consistent with this observation, the phosphoprotein profile of t he class II peptide-loading compartment underwent rapid and transient changes following BCR cross-linking, We also observed a marked increas e in the low mel. wt GTPases associated with the class II peptide-load ing compartment within 5 min of BCR cross-linking, The observed change s in both the phosphoprotein and GTPase profiles associated with the p eptide-loading compartment were blocked by kinase inhibitors and were not accompanied by overall gross changes in the protein composition of the subcellular compartments, Thus, signal cascades initiated by BCR cross-linking at the plasma membrane are translated into changes in sp ecific subsets of regulatory proteins associated with the peptide-load ing compartment.