B-cell signalling: Synapse formation wRAPped up with help from RAC2

signaling update home

SOURCE:

The B-cell receptor signals through the small GTPases Rap and Rac2 to induce the cytoskeletal and morphological changes that lead to the formation of an immunological synapse.

When a B cell encounters membrane-bound antigen, signalling through the B-cell receptor (BCR) induces cytoskeletal reorganization and morphological changes that lead to the formation of an immunological synapse. Two papers published in Immunity describe a crucial role for the RAP and RAC2 GTPases in this process.

The first step in immune-synapse formation is F-actin-dependent spreading of the B cell to allow maximal contacts between BCRs and antigens, followed by concentration of the antigen-bound BCRs in a central supramolecular activation cluster (cSMAC). Lin et al. showed that the F-actin-dependent spreading of B cells plated on surfaces coated with antibodies specific for the BCR or lymphocyte function-associated antigen 1 (LFA1) could be inhibited by blocking the activation of RAP1 and RAP2 (referred to collectively here as RAP). Consistent with this, B-cell spreading was associated with increased amounts of RAP1–GTP (the active form of RAP1).

Formation of the cSMAC is accompanied by the clustering of LFA1 molecules, which after binding ligands on the antigen-presenting cell (APC) are organized into a peripheral SMAC (pSMAC). B cells in which RAP activation was blocked could not form pSMACs in contact with lipid bilayers that contain BCR-specific antibodies and the LFA1 ligand intercellular adhesion molecule 1 (ICAM1). Moreover, the contact area between the B cell and the bilayer was markedly decreased as was the amount of antigen concentrated in the cSMAC.

Arana et al. looked at the formation of the pSMAC in more detail and showed that SRC-family tyrosine kinases, VAV1, VAV2 and RAC2 but not RAC1 are required for LFA1-dependent B-cell adhesion and pSMAC formation after signalling through the BCR. RAC2 activation in response to BCR stimulation was shown to depend on VAV1 and VAV2, as well as phosphoinositide 3-kinase (PI3K). Finally, Arana et al. showed that the activity of SRC-family tyrosine kinases, VAV1, VAV2, PI3K and RAC2 was required for sufficient RAP1–GTP generation for LFA1-mediated B-cell adhesion.

Formation of the immunological synapse increases the area and length of the interaction between the B cell and the APC, thereby enhancing the duration and strength of BCR signalling. In accordance with the role shown for RAP in the formation of the synapse, Lin et al. showed that blocking RAP activation in B cells impaired the ability of beads coated with a BCR-specific antibody to induce sustained phosphorylation of extracellular-signal regulated kinase (ERK) and protein kinase B (PKB; also known as AKT), which are involved in signalling downstream of the BCR. RAP activation, by promoting synapse formation, is therefore likely to have an important role in the activation of B cells by antigens that are present at low concentrations on the surface of APCs.

Kirsty Minton

References

Lin, K. B. L. et al. The Rap GTPases regulate B cell morphology, immune-synapse formation, and signaling by particulate B cell receptor ligands. Immunity 28, 75–87 (2008) | Article | PubMed |
Arana, E. et al. Activation of the small GTPase Rac2 via the B cell receptor regulates B cell adhesion and immunological-synapse formation. Immunity 28, 88–99 (2008) | Article | PubMed |