PI3K- and PTEN-Governed PIP<sub>2</sub> and PIP<sub>3</sub> Equilibrium Regulates the Activation and Function of B Cells Through Dock2 Controlled Growth of Antigen Receptor Microclusters

Abstract

The growth of B cell receptor (BCR) microclusters upon antigen stimulation is one of the most important events in B cell activation, however the underlying molecular mechanism unknown. Here, we systematically addressed this question through a combination of molecular imaging, genetic ablation, pharmacological inhibitor and mice model approaches. We showed that the earliest BCR signaling molecules, Lyn, Syk and PLCγ2 are required for B cells to initiate the growth of BCR microclusters. PI3K-mediated PIP3 production but not Akt and PDK1 activation was required for the sustained growth of the BCR microclusters. The PI3K counterpart signaling molecule, PTEN, potently inhibited the growth of the BCR microclusters, which was dependent on its membrane binding activities and on the lipid phosphatase but not protein phosphatase activity. Mechanistically, we showed that PIP3-dependent recruitment and activation of a unique guanine nucleotide exchange factor, Dock2, was required for the sustained growth of the BCR microclusters through the remodeling of the F-actin cytoskeleton. As a consequence, the formation of the antigen-induced B cell immunological synapse (IS) in Dock2 deficient B cells was drastically impaired. Moreover, Dock2 deficient B cells exhibited decreased Ca2 influx. Physiologically, we found that mice with reduced expression of PTEN in B cells showed enhanced antibody responses and a spontaneous lupus-like autoimmune phenotype. Clinically, we showed that primary B cells from lupus patients exhibited more prominent BCR and PI3K microclusters than B cells from healthy controls. Thus, these results demonstrated the importance of PI3K- and PTEN-governed PIP2 and PIP3 equilibrium in regulating the dynamic growth of BCR microclusters during the initiation of B cell activation.