Csk-mediated Src family kinase regulation dampens neutrophil infiltration during pulmonary infection.

Neutrophil recruitment is crucial for pathogen elimination. However, precise control of the inflammatory response prevents overshooting reactions. Neutrophil activation initiates signaling, resulting in integrin β2 (Itgb2) activation and neutrophil arrest. Src family kinases are involved in multiple cellular processes and are negatively regulated by the C-terminal Src kinase (Csk). During this study, we investigated the mechanism by which Csk regulates integrin activation and neutrophil recruitment. Here, we demonstrated that Csk deficiency in murine neutrophils resulted in increased neutrophil adhesion to the endothelium along with decreased neutrophil transmigration into inflamed tissues compared with their littermate controls. In bacterial pneumonia, infected Csk-deficient mice showed higher bacterial burdens and decreased neutrophil recruitment, while other immune cell counts and cytokine levels were not significantly different compared to control. Analyses of Csk-deficient neutrophils revealed an increased Itgb2 affinity, leading to reduced migration and intravascular crawling. Mechanistically, elevated cAMP levels increased protein kinase A activity, which subsequently enhanced Csk activation. Csk, in turn, suppressed Src family kinase activation through phosphorylation (Y529). Hence, Csk-mediated regulation of neutrophil infiltration contributes to maintain a balanced immune response during bacterial pneumonia.