Neutrophil Akt2 Plays a Critical Role In Heterotypic Neutrophil-Platelet Interactions During Vascular Inflammation

Platelet-leukocyte interactions on activated endothelial cells play important roles in mediating pathological thrombosis and inflammation. Heterotypic platelet-leukocyte aggregation is mediated by the interaction of two crucial receptors and counter receptors; P-selectin-P-selectin glycoprotein ligand-1 and glycoprotein Ibalpha-alphaMbeta2 integrin. In spite of extensive understanding of receptor-counter receptor interactions, it remains unclear how heterotypic cell-cell interactions are regulated under thrombo-inflammatory conditions. Using real-time fluorescence intravital microscopic analysis of Akt isoform-specific knockout (KO) mice, we have demonstrated that Akt2, but not Akt1 or Akt3, plays an important role in neutrophil adhesion to the site of TNF-alpha-induced vascular inflammation. Further, heterotypic platelet-neutrophil interactions on the activated endothelium were markedly reduced in Akt2 but not Akt1 or Akt3 KO mice. Studies with chimeric mice generated from bone marrow transplants on wild-type and Akt2 KO mice revealed that hematopoietic but not endothelial cell Akt2 regulates neutrophil recruitment and platelet-neutrophil interactions during vascular inflammation. Using in vitro reconstituted systems in which platelets and neutrophils were treated with an Akt2 specific inhibitor or cells were isolated from WT and Akt KO mice, we observed that both platelet and neutrophil Akt2 play an important role in platelet-neutrophil aggregation under shear conditions. In particular, neutrophil Akt2 was critical for membrane translocation, activation, and adhesive function of alphaMbeta2 integrin. We found that the basal phosphorylation levels of Akt isoforms are significantly increased in neutrophils and platelets of patients with sickle cell disease (SCD), which is an inherited hematological disorder with vascular inflammation and occlusion. Also, SCD patients’ neutrophils show increased alphaMbeta2 integrin activation in the absence of an agonist, in comparison with healthy donors’ cells. Inhibition of Akt2 dose-dependently reduced heterotypic aggregation of patients’ neutrophils and platelets in vitro and inhibited neutrophil adhesion and neutrophil-platelet aggregation in SCD mice, thereby improving blood flow. Our results provide important genetic and pharmacologic evidence that neutrophil Akt2 regulates alphaMbeta2 integrin function and thus plays a critical role during neutrophil recruitment and neutrophil-platelet interactions under thrombo-inflammatory conditions such as SCD.