The Src Family Kinases Hck and Fgr Are Dispensable for Inside-Out, Chemoattractant-Induced Signaling Regulating β2 Integrin Affinity and Valency in Neutrophils, but Are Required for β2 Integrin-Mediated Outside-In Signaling Involved in Sustained Adhesion1

Neutrophil β2 integrins are activated by inside-out signaling regulating integrin affinity and valency; following ligand binding, β2 integrins trigger outside-in signals regulating cell functions. Addressing inside-out and outside-in signaling in hck−/−fgr−/− neutrophils, we found that Hck and Fgr do not regulate chemoattractant-induced activation of β2 integrin affinity. In fact, β2 integrin-mediated rapid adhesion, in static condition assays, and neutrophil adhesion to glass capillary tubes cocoated with ICAM-1, P-selectin, and a chemoattractant, under flow, were unaffected in hck−/−fgr−/− neutrophils. Additionally, examination of integrin affinity by soluble ICAM-1 binding assays and of β2 integrin clustering on the cell surface, showed that integrin activation did not require Hck and Fgr expression. However, after binding, hck−/−fgr−/− neutrophil spreading over β2 integrin ligands was reduced and they rapidly detached from the adhesive surface. Whether alterations in outside-in signaling affect sustained adhesion to the vascular endothelium in vivo was addressed by examining neutrophil adhesiveness to inflamed muscle venules. Intravital microscopy analysis allowed us to conclude that Hck and Fgr regulate neither the number of rolling cells nor rolling velocity in neutrophils. However, arrest of hck−/−fgr−/− neutrophils to >60 μm in diameter venules was reduced. Thus, Hck and Fgr play no role in chemoattractant-induced inside-out β2 integrin activation but regulate outside-in signaling-dependent sustained adhesion.