As a consequence of tissue injury or infection, neutrophils are recruited in a stepwise recruitment process from the bloodstream into the surrounding tissue. Selectins are a family of adhesion molecules comprised of L-, E-, and P-selectin. Differences in expression patterns, protein structure, and ligand binding characteristics mediate distinct functions of each selectin. Interactions of selectins and their counter-receptors mediate the first contact of neutrophils with the endothelium, as well as subsequent neutrophil rolling along the endothelial surface. For efficient neutrophil recruitment, activation of β₂-integrins on the cell surface is essential. Integrin activation can be elicited via selectin- as well as chemokine-mediated inside-out signaling resulting in integrin conformational changes and clustering. Dysregulation of selectin-induced integrin activation on neutrophils is involved in the development of severe pathological disease conditions including leukocyte adhesion deficiency (LAD) syndromes in humans. Here, we review molecular mechanisms involved in selectin-mediated signaling pathways in neutrophils and their impact on integrin activation, neutrophil recruitment, and inflammatory diseases.

Neutrophils play a key role in autoimmune diseases like rheumatoid arthritis, contributing to tissue damage through rapid recruitment and activation. In this study, we investigated the regulatory properties of two receptor-like tyrosine phosphatases (RPTPs), CD45 and CD148, in inflammatory arthritis. Using an in vivo mouse model of K/BxN serum transfer-induced arthritis, we found that CD45 and CD148 feature distinct regulatory properties during inflammatory arthritis. CD45 is required for neutrophil infiltration, cytokine release, and reactive oxygen species production, whereas CD148 deficiency leads to a delayed onset of arthritis but unaltered overall neutrophil infiltration and reduced ROS production. Furthermore, we could demonstrate that activation of Src family kinases in neutrophils is differentially regulated by CD45 and CD148 in a stimulus-dependent manner. Summarizing, our results suggest that CD45 is positively involved, while CD148 is positively and negatively involved in neutrophil recruitment and function during inflammatory arthritis.

An acute inflammatory response to infection or sterile injury involves an adequate activation and recruitment of leukocytes. Activation of β2-integrins is required for neutrophil recruitment and is also mandatory for various neutrophil cell-intrinsic functions. Guanosine triphosphatases (GTPases) are key regulators of the actin cytoskeleton and are required for β2-integrin activation. Myosin-IXb (Myo9b), a Rho GTPase-activating protein, is essential for regulating Rho activity in neutrophils. Yet, the exact molecular mechanism through which Myo9b regulates β2-integrin activity and neutrophil recruitment into inflamed tissue is unknown. We demonstrate that Myo9b deficiency causes RhoA overactivation, increases actin cytoskeleton rearrangement in neutrophils, decreases neutrophil recruitment into the kidney, and improves kidney function in murine models of acute kidney injury. Loss of Myo9b also affects neutrophil effector functions and causes increased rolling velocity, decreased adhesion, impaired crawling, and strongly reduced transmigration of neutrophils in vivo. Mechanistically, Myo9b regulates RhoA activity, which is required for chemokine- and selectin-induced talin-1 recruitment to β2-integrins. Thus, Myo9b is a crucial regulator of important signaling pathways in neutrophils and is required for an adequate immune response triggered by chemokines and selectins.

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.