The Rho GTPase Cdc42 Is Essential for the Activation and Function of Mature B Cells

The Rho GTPase Cdc42 coordinates regulation of the actin and the microtubule cytoskeleton by binding and activating the Wiskott–Aldrich syndrome protein. We sought to define the role of intrinsic expression of Cdc42 by mature B cells in their activation and function. Mice with inducible deletion of Cdc42 in mature B cells formed smaller germinal centers and had a reduced Ab response, mostly of low affinity to T cell–dependent Ag, compared with wild-type (WT) controls. Spreading formation of long protrusions that contain F-actin, microtubules, and Cdc42-interacting protein 4, and assumption of a dendritic cell morphology in response to anti-CD40 plus IL-4 were impaired in Cdc42-deficient B cells compared with WT B cells. Cdc42-deficient B cells had an intact migratory response to chemokine in vitro, but their homing to the B cell follicles in the spleen in vivo was significantly impaired. Cdc42-deficient B cells induced a skewed cytokine response in CD4+ T cells, compared with WT B cells. Our results demonstrate a critical role for Cdc42 in the motility of mature B cells, their cognate interaction with T cells, and their differentiation into Ab-producing cells.