B cell antibody responses are triggered by the binding of antigen to the clonally distributed B cell antigen receptors (BCRs). The BCR serves to initiate signal cascades and to transport antigens into the cell for presentation, both essential processes in B cell activation. Over the last several years a great deal has been learned about the biochemistry of the complex signal cascades triggered by BCR antigen engagement and the intracellular pathway by which antigen is transported for processing. Both signaling and antigen targeting appear to be initiated by phosphorylation of the BCR by a membrane associated member of the Src family kinase, Lyn. However, the initiating event in B cell activation that brings the antigen bound BCR into contact with Lyn is not known. It now appears that cholesterol- and sphingolipid-rich membrane microdomains, termed lipid rafts, serve as platforms for both BCR signaling and antigen trafficking. In resting cells the BCR is excluded from rafts that concentrate Lyn but upon multivalent antigen binding, the BCR oligomerizes and associates with rafts where it is phosphorylated by Lyn and signaling is initiated. The translocation of the BCR into rafts does not require two of the earliest events in BCR signaling, namely the phosphorylation of the BCR by Lyn or association of the BCR with the actin cytoskeleton. Thus, the association of the BCR with rafts and the resulting initiation of signaling appear to be dependent only on the oligomerization of the BCR. Following raft association antigen bound to the BCR is internalized for processing. The initiation of signaling in the rafts is followed by raft clustering and ultimately by the formation of a highly organized structure termed an immunological synapse from which BCR signaling may be prolonged. An exciting theme that emerged from our studies of the relationship of the BCR with rafts is one in which BCR raft association is regulated by a variety of factors that control the outcome of the B cell?s encounter with antigen including the developmental state of the B cell, the engagement of coreceptors and viral infection. We have learned that the ability of the BCR to stably associate with lipid rafts changes during development and that the changes correlate with the outcome of antigen engagement by the B cell. Significantly, the BCR does not associate with rafts in immature B cells and BCR antigen binding results in the death of the B cell rather than activation. We also learned that coreceptors that function to enhance BCR response prolong the residency of the BCR in rafts. Conversely, B cell receptors that attenuate BCR signaling destabilize the BCR in rafts. Lastly, Epstein Barr Virus that established a latent infection in human B cells blocks the BCR?s access to rafts and thus blocks the ability of antigen to activate infected cells. Over the last year we have made progress in defining the mechanisms by which coreceptors function to regulate the association of the BCR with lipid rafts and as a consequence regulate signaling. We have focused on two important B cell coreceptors namely, CD19/CD21, an essential positive regulator of BCR signaling, and FcgammaRIIB, a potent negative regulator of BCR signaling. CD19/CD21 when coligated to the BCR through the binding of complement tagged antigens prolongs BCR residency in and signaling from rafts. We determined that the ability of the CD19/CD21 complex to function in rafts was dependent on a tetraspanin CD81 that is a component of the CD19/CD21 complex. Thus, in B cells from CD81-deficient mice and B cells expressing chimeric CD19 receptors that fail to associate with CD81, the CD19/CD21 complex when coligated to the BCR fails to stabilize the BCR in rafts. We are investigating whether the ability to stabilize signaling complexes in microdomains is a common feature of tetraspanins. The FcgammaRIIB is a potent negative regulator of BCR signaling in mature B cells when coligated to the BCR through the binding of immune complexes and in immature B cells induces apoptosis. Our results over the last year provide evidence that the Fcgamma RIIB mediate these functions from within lipid rafts. Studies are in progress to determine which structural features of the FcgammaRIIB controls its ability to partition into microdomains in mature and immature B cells.
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