Pyk2 and Syk participate in functional activation of granulocytic HL-60 cells in a different manner

β2 Integrin Signaling Cascade in Neutrophils: More Than a Single Function

Neutrophils are the most prevalent leukocytes in the human body. They have a pivotal role in the innate immune response against invading bacterial and fungal pathogens, while recent emerging evidence also demonstrates their role in cancer progression and anti-tumor responses. The efficient execution of many neutrophil effector responses requires the presence of β2 integrins, in particular CD11a/CD18 or CD11b/CD18 heterodimers. Although extensively studied at the molecular level, the exact signaling cascades downstream of β2 integrins still remain to be fully elucidated. In this review, we focus mainly on inside-out and outside-in signaling of these two β2 integrin members expressed on neutrophils and describe differences between various neutrophil stimuli with respect to integrin activation, integrin ligand binding, and the pertinent differences between mouse and human studies. Last, we discuss how integrin signaling studies could be used to explore the therapeutic potential of targeting β2 integrins and the intracellular signaling cascade in neutrophils in several, among other, inflammatory conditions in which neutrophil activity should be dampened to mitigate disease.

Src family kinases mediate neutrophil adhesion to adherent platelets

Polymorphonuclear leukocyte (PMN)-platelet interactions at sites of vascular damage contribute to local and systemic inflammation. We sought to determine the role of "outside-in" signaling by Src-family tyrosine kinases (SFKs) in the regulation of alphaMbeta2-integrin-dependent PMN recruitment by activated platelets under (patho)physiologic conditions. Activation-dependent epitopes in beta2 integrin were exposed at the contact sites between PMNs and platelets and were abolished by SFK inhibitors. PMNs from alphaMbeta2(-/-), hck(-/-)fgr(-/-), and hck(-/-)fgr(-/-)lyn(-/-) mice had an impaired capacity to adhere with activated platelets in suspension. Phosphorylation of Pyk2 accompanied PMN adhesion to platelets and was blocked by inhibition as well as by genetic deletion of alphaMbeta2 integrin and SFKs. A Pyk2 inhibitor reduced platelet-PMN adhesion, indicating that Pyk2 may be a downstream effector of SFKs. Analysis of PMN-platelet interactions under flow revealed that SFK signaling was required for alphaMbeta2-mediated shear-resistant adhesion of PMNs to adherent platelets, but was dispensable for P-selectin-PSGL-1-mediated recruitment and rolling. Finally, SFK activity was required to support PMN accumulation along adherent platelets at the site of vascular injury, in vivo. These results definitely establish a role for SFKs in PMN recruitment by activated platelets and suggest novel targets to disrupt the pathophysiologic consequences of platelet-leukocyte interactions in vascular disease.