The Actin Cytoskeleton Controls FcgRIIB-Mediated Inhibition of B-Cell Signaling

Abstract

Abstract FcgRIIB is a major inhibitory coreceptor of B cell receptor (BCR), and its deficiency leads to B cell autoimmunity. Upon colligated with the BCR by immune complexes (ICs), this coreceptor activates SH2-containing inositol-5 phosphatase (SHIP) and inhibits BCR clustering in lipid rafts. However, the molecular mechanisms underlying FcgRIIB-mediated inhibition of BCR activation remain elusive. This study reveals a critical role of the actin cytoskeleton in FcgRIIB-mediated inhibition. Upon interacting with ICs tethered to lipid bilayers, an actomyosin ring is quickly formed surrounding the B cell membrane region interacting with the lipid bilayer (contact zone). Perturbing the actomyosin ring, by myosin inhibitors or B cell-specific gene knockout of non-muscle myosin IIA (NMIIA) or neuronal Wiskott-Aldrich syndrome protein (N-WASP), releases B cell spreading, BCR clustering and signaling from the inhibition by FcgRIIB. Using conditional gene knockout mice, we further show that NMIIA is recruited to the B cell contact zone in an SHIP-dependent manner, while N-WASP-mediated actin remodeling organizes NMIIA and actin into a ring. Surprisingly, WASP-mediated actin dynamics is required for the FcgRIIB-induced activation of SHIP but not the formation of the actomyosin ring. Our results suggest that FcgRIIB-triggered signaling induces unique actin remodeling involving NMII, WASP, and N-WASP, which limits B cell morphological changes and BCR lateral mobility and enhances the activation of inhibitory signaling. These data potentially provide a mechanistic explanation for the elevated autoantibody levels caused by WASP or N-WASP deficiency.