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 β 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.

Cortactin and HS1 are actin‐binding molecules known to be overexpressed in leukemic B‐cells from adult patients with chronic lymphoid leukemia. Both proteins are required for proper actin remodeling in virtually all cell types. While cortactin overexpression is associated to invasiveness of various solid tumor cells, HS1 is correlated with poor prognosis of chronic leukemia in adult patients. However, the role of cortactin and HS1 in childhood acute lymphoblastic leukemias (ALL) has never been studied. Here, we analyzed mRNA levels of both cortactin and HS1 in model cell lines for ALL and primary bone marrow (BM) cells from B‐ALL patients, and found substantial increases for both cortactin and HS1 in leukemic cells compared to controls. Furthermore, protein levels of cortactin were highest within primary primitive Lin‐CD34+ stem and progenitor cell compartments, while lineage committed CD34‐CD19+ and non‐B cells displayed low expression levels. By contrast, HS1 expression was high but did not change during differentiation. Unraveling the importance of differential expression of cortactin and HS1 in leukemia initiating cells for the population dynamics and development of childhood acute leukemias may open novel treatment strategies to prevent progression of the disease with infiltrations into other organs.