Tyrosine kinase Btk regulates E-selectin-mediated integrin activation and neutrophil recruitment by controlling phospholipase C (PLC) γ2 and PI3Kγ pathways

Selectins mediate leukocyte rolling, trigger β2-integrin activation, and promote leukocyte recruitment into inflamed tissue. E-selectin binding to P-selectin glycoprotein ligand 1 (PSGL-1) leads to activation of an immunoreceptor tyrosine-based activation motif (ITAM)-dependent pathway, which in turn activates the spleen tyrosine kinase (Syk). However, the signaling pathway linking Syk to integrin activation after E-selectin engagement is unknown. To identify the pathway, we used different gene-deficient mice in autoperfused flow chamber, intravital microscopy, peritonitis, and biochemical studies. We report here that the signaling pathway downstream of Syk divides into a phospholipase C (PLC) γ2- and phosphoinositide 3-kinase (PI3K) γ-dependent pathway. The Tec family kinase Bruton tyrosine kinase (Btk) is required for activating both pathways, generating inositol-3,4,5-trisphosphate (IP₃), and inducing E-selectin-mediated slow rolling. Inhibition of this signal-transduction pathway diminished Gα_i-independent leukocyte adhesion to and transmigration through endothelial cells in inflamed postcapillary venules of the cremaster. Gα_i-independent neutrophil recruitment into the inflamed peritoneal cavity was reduced in Btk^-/- and Plcg2^-/- mice. Our data demonstrate the functional importance of this newly identified signaling pathway mediated by E-selectin engagement.