A B cell actomyosin arc network couples integrin co-stimulation to mechanical force-dependent immune synapse formation

Abstract

Abstract B cells that are activated by antigen-presenting cells form an immune synapse (IS), which supports B cell activation and a robust antibody response. The organization and dynamics of the actin cytoskeleton are essential for centralizing antigen and promoting the B cell receptor signaling that are central to IS formation. When antigen is limiting, B cells require integrin co-stimulation to form the IS. Given the indispensable function of actin in forming the IS, we tested the role of the actin cytoskeleton in integrin-dependent IS formation and B cell activation. Here we show, using live-cell super-resolution imaging of primary B cells, the formation of a dynamic actin arc network, which dominated the synaptic actin and appeared only when B cells bound both antigen and the integrin ligand, ICAM-1. Actin arcs were created by the formin mDia1 and were decorated with the motor protein myosin 2A, which organized actin arcs into a concentric, contractile network. Dynamic imaging of the actin in primary B cells on planar lipid bilayers showed that actin arcs moved centripetally together with antigen microclusters and that these microclusters were swept inward by individual actin arcs. Critically, integrin-dependent IS formation under limiting antigen conditions required myosin activity as blocking myosin with Blebbistatin prevented antigen centralization. Consistently, myosin inhibition also diminished B cell receptor signaling and disrupted the distribution of key signaling proteins at the IS. Together, these results argue that a contractile actomyosin arc network created downstream of integrin co-stimulation plays an important role in the mechanism by which integrin co-stimulation promotes B cell activation and IS formation.