HS1 deficiency impairs neutrophil recruitment in vivo and activation of the small GTPases Rac1 and Rap1

Neutrophil extravasation is a critical step of the innate immune system's response to inflammation. This multistep process is tightly regulated by adhesion and signaling molecules in the endothelium and neutrophils. Activation of the β2 integrin LFA‐1 is critical for adhesion of leukocytes to postcapillary venules. This step requires coordinated activation of signaling pathways in chemokine‐stimulated neutrophils, including GTPase activation and cytoskeletal remodeling, leading to conformational changes in LFA‐1. Hematopoietic cell‐specific lyn substrate 1 (HS1) is a cortactin‐related and leukocyte‐specific actin‐binding protein (ABP) that regulates several processes in various immune cells. It has been shown in vitro that HS1 is important for neutrophil chemotaxis and transendothelial migration of NK cells, but its role in neutrophil extravasation in vivo has not been investigated yet. Intravital microscopy of CXCL1‐stimulated cremaster venules revealed an increased rolling velocity and reduced neutrophil adhesion and transmigration in HS1 knockout (KO) mice. CXCL1‐induced rapid neutrophil arrest in vivo and adhesion under flow conditions in vitro were also reduced significantly. Whereas random motility of neutrophils was unaffected, chemotaxis toward a CXCL1 gradient was reduced in the absence of HS1. Further analysis of the underlying mechanisms demonstrated that HS1 controls CXCL1‐induced activation of the small GTPases Ras‐related C3 botulinum toxin substrate 1 (Rac1) and Ras‐related protein 1 (Rap1), thus supporting LFA‐1‐mediated neutrophil adhesion. Importantly, with the use of Rac1 KO neutrophils, we could show that Rac1 acts upstream of Rap1. Our results establish HS1 as an important regulator of proper Rac1 and Rap1 activation and neutrophil extravasation.