Thromboinflammatory Conditions Research Articles

NADPH Oxidase 2 Is Critical for Heterotypic Neutrophil-Platelet Interactions during Vascular Inflammation

Intravital microscopic studies have shown that neutrophil-platelet interactions on activated endothelial cells are the major determinant of vascular occlusion under thromboinflammatory conditions. Heterotypic neutrophil-platelet interactions are mainly mediated by two crucial receptors and counter receptors, P-selectin-P-selectin glycoprotein ligand-1 and glycoprotein Ibalpha-alphaMbeta2 integrin. Although the major receptors and counter receptors are well identified, it remains unclear how heterotypic cell-cell interactions are regulated during vascular disease. Recently, we demonstrated that neutrophil AKT2 is a key regulator for neutrophil-platelet interactions during vascular inflammation. Since it is known that AKT2 is important for NADPH oxidase 2 (NOX2) activity and reactive oxygen species (ROS) generation during neutrophil activation, we investigated whether NOX2 regulates the heterotypic neutrophil-platelet interaction during vascular inflammation. Using real-time fluorescence intravital microscopy, we found that platelet-neutrophil interactions are nearly completely inhibited during TNF-alpha-induced cremaster venular inflammation in NOX2 KO mice, compared with WT mice. Studies with bone marrow chimera further demonstrated that both hematopoietic and endothelial cell NOX2 are crucial for neutrophil-platelet interactions on the inflamed venules. In vitro studies with neutrophils and platelets isolated from WT and NOX2-deficient mice indicated that both neutrophil and platelet NOX2 are required for neutrophil-platelet aggregation under shear conditions. Using ROS scavengers and NOX2 KO cells, we observed that neutrophil NOX2-generated ROS regulate the activation and ligand-binding activity of alphaMbeta2 integrin during neutrophil activation and that platelet NOX2-produced ROS are important for P-selectin exposure upon agonist stimulation and the ligand-binding function of glycoprotein Ibalpha. Interestingly, neutrophil NOX2 was crucial for store-operated calcium entry (SOCE), but not calcium release from the endoplasmic reticulum, whereas platelet NOX2 modulated intracellular calcium release, but not SOCE. We further demonstrated that the differential regulation of platelet and neutrophil NOX2 in calcium signaling correlates with the differences in the phosphorylation of AKT, ERK, and p38MAPK, suggesting the importance of AKT and its downstream molecules for NOX2-mediated calcium signaling. Our results indicate that neutrophil and platelet NOX2-produced ROS play critical roles in regulating the function of surface receptors required for heterotypic neutrophil-platelet interactions during vascular inflammation. DisclosuresNo relevant conflicts of interest to declare.

Abstract 25: Neutrophil Akt2 Regulates Heterotypic Cell-Cell Interactions During Vascular Inflammation

Introduction: Interactions between platelets, leukocytes and endothelial cells are important during microvascular occlusion. Although the major receptors and counter receptors have been well identified, it remains unclear how heterotypic cell-cell interactions are regulated under inflammatory conditions. Hypothesis: We assessed the hypothesis that AKT, a Ser/Thr protein kinase, plays a regulatory role in platelet-neutrophil-endothelial cell interactions during vascular inflammation. Methods and Results: Using real-time fluorescence intravital microscopy to evaluate mice lacking specific Akt isoform, we demonstrated that Akt2, but not Akt1 and Akt3, regulates neutrophil adhesion and crawling, and platelet-neutrophil interactions on the activated endothelium during TNF-α-induced cremaster venular inflammation. Further, using chimeric mice generated from bone marrow transplants on wild-type (WT) and Akt2 knockout (KO) mice, we found that hematopoietic cell-associated AKT2 is important for the regulatory effect in vivo. Studies with an AKT2-specific inhibitor and cells isolated from WT and Akt KO mice revealed that platelet- and neutrophil-associated AKT2 regulates heterotypic neutrophil-platelet aggregation under conditions of shear. In particular, neutrophil AKT2 was critical for intracellular calcium mobilization, β2-talin1 interaction, and membrane translocation of αMβ2 integrin - a crucial receptor for neutrophil-platelet interactions. We observed that the basal phosphorylation levels of AKT isoforms are markedly increased in neutrophils and platelets from patients with sickle cell disease (SCD), an inherited hematological disorder associated with vascular inflammation and occlusion. AKT2 inhibition significantly reduced heterotypic aggregation of neutrophils and platelets isolated from SCD patients and diminished neutrophil adhesion and neutrophil-platelet aggregation in venules of Berkeley (SCD) mice, thereby improving blood flow rates. Conclusion: Our results provide genetic and pharmacologic evidence that neutrophil AKT2 regulates αMβ2 integrin function and is important for neutrophil recruitment and neutrophil-platelet interactions under thrombo-inflammatory conditions such as SCD.

Neutrophil AKT2 regulates heterotypic cell-cell interactions during vascular inflammation

Interactions between platelets, leukocytes, and activated endothelial cells are important during microvascular occlusion; however, the regulatory mechanisms of these heterotypic cell-cell interactions remain unclear. Here, using intravital microscopy to evaluate mice lacking specific isoforms of the serine/threonine kinase AKT and bone marrow chimeras, we found that hematopoietic cell-associated AKT2 is important for neutrophil adhesion and crawling and neutrophil-platelet interactions on activated endothelial cells during TNF-α-induced venular inflammation. Studies with an AKT2-specific inhibitor and cells isolated from WT and Akt KO mice revealed that platelet- and neutrophil-associated AKT2 regulates heterotypic neutrophil-platelet aggregation under shear conditions. In particular, neutrophil AKT2 was critical for membrane translocation of αMβ2 integrin, β2-talin1 interaction, and intracellular Ca2+ mobilization. We found that the basal phosphorylation levels of AKT isoforms were markedly increased in neutrophils and platelets isolated from patients with sickle cell disease (SCD), an inherited hematological disorder associated with vascular inflammation and occlusion. AKT2 inhibition reduced heterotypic aggregation of neutrophils and platelets isolated from SCD patients and diminished neutrophil adhesion and neutrophil-platelet aggregation in SCD mice, thereby improving blood flow rates. Our results provide evidence that neutrophil AKT2 regulates αMβ2 integrin function and suggest that AKT2 is important for neutrophil recruitment and neutrophil-platelet interactions under thromboinflammatory conditions such as SCD.

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. Disclosures:No relevant conflicts of interest to declare.