SUMMARY Neutrophils are recruited from the blood to sites of sterile inflammation, where they are involved in wound healing but can also cause tissue damage. During sterile inflammation, necrotic cells release pro-inflammatory molecules including formylated peptides. However, the signaling pathway triggered by formylated peptides to integrin activation and leukocyte recruitment is unknown. By using spinning-disk confocal intravital microscopy, we examined the molecular mechanisms of leukocyte recruitment to sites of focal hepatic necrosis in vivo. We demonstrated that the Bruton’s tyrosine kinase (Btk) was required for multiple Mac-1 activation events involved in neutrophil recruitment and functions during sterile inflammation triggered by fMLF. The Src family kinase Hck, Wiskott-Aldrich-syndrome protein, and phospholipase Cγ2 were also involved in this pathway required for fMLF-triggered Mac-1 activation and neutrophil recruitment. Thus, we have identified a neutrophil Btk signalosome that is involved in a signaling pathway triggered by formylated peptides leading to the selective activation of Mac-1 and neutrophil recruitment during sterile inflammation.

A subset of PSGL-1 is constitutively associated with L-selectin and signals through Src family kinases to activate LFA-1, which regulates neutrophil slow rolling and recruitment.

Boras et al. demonstrate that Skap2, via interaction with WASp, regulates actin polymerization and binding of talin-1 and kindlin-3 to the β2 integrin, thereby being indispensable for β2 integrin activation and neutrophil recruitment.

Neutrophil recruitment to site of inflammation plays a pivotal role in host defense. However, an overwhelming activation and accumulation of neutrophils in the tissue may cause tissue damage and autoimmunity due to release of cytokines, oxidants, and proteases. Neutrophil adhesion in acute inflammation is initiated by activation of αLβ2 (LFA-1), which can be induced by rolling on E-selectin (slowly) or by exposure to the chemokine CXCL1 (rapidly). Despite the clinical importance, cell-intrinsic molecular mechanisms of negative regulation of integrin adhesiveness and neutrophil recruitment are poorly understood. Mice deficient in the tyrosine phosphatase Shp1 show increased leukocyte adhesion, but interpretation of these data is limited by the severe global phenotype of these mice. Here, we used mice with global and myeloid-restricted deletion of Shp1 to study neutrophil arrest, adhesion, crawling and transendothelial migration in vitro and in vivo. Shp1 deficiency results in an increased neutrophil adhesion in vivo. However, neutrophil crawling, transmigration and chemotaxis were reduced in these mice. Mechanistically, Shp1 binds and controls PIPKIγ-activity and thereby modulates PtdIns(4,5)P2 levels and adhesion. Thus, Shp1 is involved in the deactivation of integrins and regulation of neutrophil recruitment into inflamed tissue.

No abstract