DYNAMIC INTERACTIONS AT THE BCR SYNAPSE
Just as TCRs form immunological synapses during contact with specific peptide–MHC, B‐cell receptors have also been found to exhibit similar behavior, particularly when antigen is presented on a membrane surface. Although B‐cells can be stimulated by soluble antigen, it is now widely accepted that the primary form of antigen that triggers B‐cell activation in vivo is localized to membrane surfaces. The most likely culprits here are the follicular DCs that are resident within lymph nodes, as well as macrophages and DCs that migrate there bearing gifts of antigen. Antigens can be immobilized on cell surfaces by complement or Fc receptors as immunocomplexes, or through direct binding to various scavenger receptors. An encounter between a B‐cell and membrane‐associated antigen provides the opportunity for the B‐cell membrane to spread along the opposing membrane, gathering sufficient antigen to trigger B‐cell activation, as well as providing an opportunity for other contacts to be made, such as those that can be provided by membrane integrins. This spreading response is driven by BCR engagement of antigen at the leading edge of the B‐cell and, apart from increasing the number of BCR–antigen contacts that are then available to trigger B‐cell activation, the spreading response also increases the amount of antigen that is ultimately concentrated and internalized by the B‐cell, leading to more efficient antigen presentation to activated T‐cells when the B‐cell subsequently goes looking for T‐cell help (Figure 7.30).
Cell spreading in response to engagement of the BCR with specific antigen is triggered in response to signals propagated via the BCR, with Lyn and Syk playing especially important roles in this process. Clearly, spreading along an antigen‐bearing surface requires extensive reorganization of the cytoskeleton. Although this is not fully understood at present, activation of Vav, which as discussed earlier is involved in the regulation of the cytoskeleton via Rac and Rho, is essential here.
There is evidence that BCRs within resting B‐cells are not scattered randomly within the plasma membrane but are confined to certain zones, with free diffusion restricted by contacts with the underlying actin‐based cytoskeleton. In line with this, disruption of the actin network in B‐cells has been shown to lead to spontaneous BCR‐dependent calcium signaling, possibly owing to the spontaneous formation of BCR microclusters. Thus, the cytoskeleton appears to play an important role in restricting the surface distribution and behavior of BCRs in a resting B‐cell. Binding of multivalent antigen to the BCR can disrupt the arrangement of BCRs in the resting B‐cell, resulting in the formation of BCR microclusters containing 50–500 BCRs, the formation of which also depends on an intact cytoskeleton. Indeed, the actin network within activated B‐cells has been found to encircle or corral BCR microclusters within the plasma membrane.
Figure 7.31 The B‐cell receptor (BCR) immunological synapse. (a) Imaging of the BCR immunological synapse. Real‐time quantification of antigen and ICAM‐1 recruitment to the B‐cell synapse. naive B‐cells were settled onto planar lipid bilayers containing glycosylphosphatidylinositol (GPI)‐linked ICAM‐1 (red) and p31 antigen (green). Central panels show the accumulation of the antigen p31 (green) and ICAM‐1 (red) in the pattern of a mature synapse at the specified time points. Top and bottom panels show differential interference contrast and interference reflection microscopy images of the same time points. Reproduced with permission of Elsevier.) (b) Schematic representation of the BCR immunological synapse, depicting the central supramolecular activation complex (cSMAC) that is enriched in BCR–Ag microclusters, and the surrounding peripheral supramolecular activation complex (pSMAC) that is enriched in integrins such as LFA‐1/ICAM‐1.
Spreading of the B‐cell across the antigen‐bearing surface increases the number of BCR microclusters and eventually engages sufficient numbers of BCRs to permit crossing of the threshold for B‐cell activation. Similar to T‐cells, mature B‐cells also express high levels of the LFA‐1 and VLA‐4 integrins. Interaction of these adhesion molecules with their cognate ligands, ICAM‐1 and VCAM‐1/fibronectin, on the cell that is displaying the immobilized antigen also promote B‐cell adhesion and facilitate cell spreading along the target surface. Following spreading across an antigen‐bearing surface, B‐cells undergo a prolonged contraction phase that culminates in a major rearrangement of the BCR microclusters within the membrane that coalesce to form an immunological synapse, similar to that seen with T‐cells (Figure 7.21). The mature BCR immunological synapse contains a central ring (cSMAC) enriched in BCR–antigen complexes, with an outer ring (pSMAC) enriched in integrins (Figure 7.31). Not only do the integrin contacts promote spreading and adhesion between the interacting cell pairs, but recent evidence also suggests that such contacts lower the threshold for B‐cell activation by lowering the concentration of antigen required to form a stable synapse and trigger the B‐cell.