the NADPH Oxidase Catalytic Subunit Nox2 Displays Differential Roles in Arterial Vs. Venous Thrombosis

NADPH oxidase is a major superoxide generating enzyme in vascular tissues. Deficiency of the Nox2 (gp91phox) catalytic subunit of NADPH oxidase is a genetic cause of X-linked chronic granulomatous disease. These patients are prone to infection due to loss of oxidant production by neutrophils, and recent evidence suggests they also have a defect in platelet adhesion to collagen. However, the role of Nox2 in thrombosis is controversial, with one study demonstrating no effect of murine Nox2 deficiency on platelet activation and another study implicating Nox2 in platelet aggregation as well as thrombosis in small vessels. Given that Nox2 may be important for both platelet activation as well as release of neutrophil extracellular traps (NETs, a mediator of venous thrombosis), we hypothesized that genetic deficiency in murine Nox2 leads to decreased susceptibility to experimental thrombosis in both arterial and venous systems. We studied male Nox2 deficient (Cybb-/y) mice and wild type (Cybb +/y) littermates at 10-14 weeks of age. There were no differences in platelet count (921.9±33.6 x10³/µl vs. 978±85.2 x10³/µl), hematocrit (36.5±0.6% vs. 38.7±0.8% ) or white blood cell count (5.5±0.3 x10³/µl vs. 6.1±0.6 x10³/µl) between Cybb+/y and Cybb-/y mice. We next, examined platelet activation in response to thrombin (0.05 and 0.2 U/ml) or collagen (80 and 320 ng/ml) using flow cytometry. We observed similar levels of integrin α2bβ3 activation, fibrinogen binding, and intra-platelet levels of H₂O₂ in platelets from Cybb+/y and Cybb-/y mice after activation with either agonist, which suggests no alteration in platelet inside-out signaling due to loss of Nox2. No significant difference in susceptibility to carotid artery thrombosis in a photochemical injury model was observed between Cybb+/y and Cybb-/- mice (time to stable occlusion 21.97±4.7 vs 27.6±4.2, P>0.4). In contrast, Cybb-/y mice demonstrated significant decreases in the weight and length of venous thrombi in the inferior vena cava after 48 hours of stenosis (P<0.05). Our findings suggest that Nox2 is not a major mediator of platelet activation or arterial thrombosis but contributes to the development of venous thrombi. Future studies are warranted to examine the role of NETs and the prothrombotic effects of Nox2 in association with other cardiovascular risk factors.