Actin Reorganization Is Required for the Formation of Polarized B Cell Receptor Signalosomes in Response to Both Soluble and Membrane-Associated Antigens

Abstract

B-cells encounter both soluble (sAg) and membrane-associated antigens (mAg) in the secondary lymphoid tissue, yet how the physical form of Ag modulates B-cell activation remains unclear. This study compares actin reorganization and its role in BCR signalosome formation in mAg- and sAg-stimulated B-cells. Both mAg and sAg induce F-actin accumulation and actin polymerization at BCR microclusters and at the outer rim of BCR central clusters, but the kinetics and magnitude of F-actin accumulation in mAg-stimulated B-cells are greater than those in sAg-stimulated B-cells. Accordingly, the actin regulatory factors, cofilin and gelsolin, are recruited to BCR clusters in both mAg- and sAg-stimulated B-cells but with different kinetics and patterns of cellular redistribution. Inhibition of actin reorganization by stabilizing F-actin inhibits BCR clustering and tyrosine phosphorylation induced by both forms of Ag. Depolymerization of F-actin leads to unpolarized microclustering of BCRs and tyrosine phosphorylation in BCR microclusters without mAg and sAg, but in much slower kinetics than those induced by Ag. Therefore, actin reorganization, mediated via both polymerization and depolymerization, is required for the formation of BCR signalosomes in response to both mAg and sAg.

Utilization of a photoactivatable antigen system to examine B-cell probing termination and the B-cell receptor sorting mechanisms during B-cell activation

“…Moreover, both our studies and those of others showed that these accumulation events are sensitive to the biochemical and biophysical features of the antigens that B cells likely encounter in vivo (4,5). These features include but are not limited to antigen density (6,7), antigen affinity (6,7), antigen valency (8)(9)(10)(11)(12)(13), the mobility of the antigen (14)(15)(16)(17), the stiffness of the substrates presenting the antigen (18,19) and the mechanical forces delivered to the BCRs by the antigens (20,21). These facts highlight a long-standing question in immunology: how can the initiation of B-cell activation process the information of antigen specificity, density, affinity, valency, mobility, substrate stiffness, and mechanical forces in such an efficient way?…”

“…Interestingly, the termination of the probing behavior seems to be dependent on the interactions between the BCR and antigen molecules, as neither the interaction between MHC-I and anti-MHC-I antibodies, nor that between integrin and the adhesion molecule ICAM-1, terminated the B-cell probing behaviors. This highly selective and strict dependence on the interactions between the BCR and antigen molecules may be linked to the welldocumented actin remodeling events in response to BCR and antigen recognition (15,16,28,29). All these in vitro data support the early intravital imaging studies, which showed that B cells terminated the migratory behavior on recognition of cognate antigens on the surface of antigen-presenting cells (APCs) and subsequently developed an enlarged contact interface for the acquisition of the antigenic information (37,38).…”

Molecular Control of B Cell Activation and Immunological Synapse Formation

“…As such, microclusters seem to function both as hotspots for signaling and units for antigen transportation to facilitate their subsequent endocytosis. The actual mechanism of microcluster movement to the synapse center is not well defined but has been suggested to involve passive movements along centripetal, rearward actin flow (59). TCR microcluster mobility has been associated, in motile T cells, with actin depolymerization that increases toward the filamentous actin (F-actin) poor cSMAC (60).…”

“…The cSMAC exhibits very low levels of F-actin, however, the formation of the central antigen cluster is dependent on active actin dynamics (10). Indeed, actin polymerization is sustained in the pSMAC and can continue gathering new antigen from the APC surface (59). Further pointing toward synapse plasticity and ability to collect more antigen over time, it has also been reported that B cells are able to move along the surface of APC while maintaining the IS structure (19).…”

Actin-binding protein 1 links B-cell antigen receptors to negative signaling pathways

“…Actin reassembly expands the contact of B cells with Ag-presenting surfaces and induces polarized movement of surface BCRs, enhancing BCR clustering and signaling (8-10, 12, 13). Upon maximal cell spread, F-actin decreases in the B-cell region contacting Ag-presenting surfaces, and the cells contract, facilitating coalescence of BCR microclusters and signal attenuation (1,8,9,13). Persistent actin accumulation at the B-cell contact zone and delayed cell contraction, caused by B-cell-specific neuronal Wiskott-Aldrich syndrome protein (N-WASP) knockout, lead to enhanced BCR signaling and elevated levels of autoantibody (autoAb) (14).…”