The Lyn tyrosine kinase negatively regulates neutrophil integrin signaling.

The Src family kinase Lyn has been shown to play both stimulatory and inhibitory roles within several hemopoietic cell types. In this study, we investigated the role played by Lyn in neutrophil integrin signaling. Loss of Lyn resulted in a hyperresponsive phenotype on engagement of surface integrins at low valency. Lyn(-/-) neutrophils displayed enhanced respiratory burst, secondary granule release, and a hyperadhesive phenotype when adherent to surfaces coated with either cellular counterreceptors or extracellular matrix proteins. In contrast, Lyn-deficient and wild-type cells expressed similar levels of surface integrins and responded equivalently to activating agents in suspension, indicating that the enhanced responses of lyn(-/-) cells was specific to the integrin signaling pathways. Lyn-deficient macrophages also displayed a hyperadhesive phenotype. Biochemical analysis of macrophages from lyn(-/-) mice revealed that Lyn plays an essential role in the adhesion-dependent phosphorylation of the immunoreceptor tyrosine-based inhibitory motif of the inhibitory receptors SIRP1alpha and PIR-B, which in turn recruit the phosphatase SHP-1. These observations suggest that reduced mobilization of SHP-1 to the membrane in lyn(-/-) neutrophils results in a hyperadhesive and hyperactive phenotype. This hypothesis is further supported by the fact that neutrophils from me(v)/me(v) mice, which have significantly reduced SHP-1 activity, are also hyperresponsive following integrin engagement. This is the first direct evidence using primary leukocytes from lyn(-/-) mice that this kinase functions as a negative regulator in integrin signaling.