The B cell antigen receptor (BCR) serves dual, interrelated functions in B cell activation. The first is to initiate signal cascades that result in the transcription of a variety of genes associated with B cell activation. However, signaling through the BCR alone is insufficient for full activation and for this the B cell requires the functions of antigen-specific helper T cells. The interaction of the B and T cells is mediated through the T cells' engagement of antigenic peptide-MHC class II complexes on the B cell surface by the T cell antigen receptors (TCRs). The peptide MHC complexes expressed by the B cells are assembled in an intracellular compartment from newly synthesized MHC class II molecules and peptides derived from the antigen bound to the BCR. Our previous studies showed that the BCR transports the antigen to the intracellular compartment where the antigen is degraded and the peptide class II complexes are assembled. The signaling and antigen transport functions of the BCR are interdependent in that the BCR signaling is necessary for the correct and rapid targeting of the antigen to the class II containing compartments. Indeed, for signaling defective BCR antigen is either not processed or the rate of antigen delivery is significantly reduced. In turn, the internalization of the BCR may play an important role in regulating signaling by removing activated receptors from the cell surface. At present it is not known if the antigen targeting and BCR down regulation are one in the same or separable processes. The signaling and antigen targeting functions of the BCR may be coordinated by the BCRs association with cholesterol-rich membrane microdomains termed lipid rafts. We showed that the BCR in resting cells is excluded from rafts that concentrate the Src-family kinase Lyn, required for the initiation of signaling. Following antigen binding the BCR translocates into rafts where it associates with Lyn and signaling is initiated. Subsequently, the BCR is internalized and trafficked to the class II peptide-loading compartment. We recently provided a link between the B cell signaling and internalization machinery showing that the Lyn kinase activity is required for inducible clathrin heavy chain phosphorylation, colocalization of the BCR with clathrin and regulated internalization. Uptake of the crosslinked BCR occurred only when clathrin associated with rafts and was tyrosine phosphorylated. Studies are in progress to determine how lipid rafts spatially organize signaling cascades with clathrin to regulate BCR internalization. We also learned that the internalization of the BCR is controlled during B cell development, by B cell coreceptors that regulate BCR signaling and by infection by Epstein Barr Virus (EBV). Studies in progress are focusing on the mechanism by which the BCR is internalized in order to elucidate the precise relationship between rafts and internalization and to uncover points of regulation in the process. Thus far, studies of the mechanism by which EBV regulates BCR trafficking have been most informative. We showed that the EBV latent membrane protein 2A (LMP2A) was constitutively present in rafts and blocked BCR raft association, signaling and trafficking. Genetic analyses of the cytoplasmic domain of LMP2A showed that mutations in the Lyn-binding site allowed internalization and degradation of the receptor but not trafficking to the class II assembly compartment. We are currently introducing the LMP2A gene into mouse B cells where reagents are available to more thoroughly analyze the LMP2A-induced block on antigen processing. Studies have also been initiated in collaboration with Dr. Joost Oppenheim (NCI, NIH) to elucidate the intracellular trafficking of the chemokine receptor CCR5. Dr. Oppenheim and his colleagues determined that the autoantigen histidyl-tRNA synthase (HisRS) binds to CCR5 and hypothesized that CCR5 transports HisRs to the B cell antigen processing compartments in a highly efficient manner. Subsequent presentation of the antigen to helper T cells could be an important factor in the autoimmune response to HisRS. Thus far we have learned that CCR5 appears to internalize HisRS but does not target its directly to the processing compartments.
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