Dysfunctional Endothelium Drives a Pre-DIC State in Endotoxemia

Uncontrolled pro-coagulant responses in sepsis can lead to disseminated intravascular coagulation (DIC), a complication associated with markedly increased mortality. Abnormalities of coagulation, fibrinolysis, and platelet function can lead to both microvascular thrombosis contributing to multi-organ dysfunction syndrome or hemorrhagic complications. To understand the inciting causes of DIC in sepsis, we first evaluated the time course of platelet function, coagulation parameters and markers of endothelial activation following LPS exposure. These studies demonstrated that endothelial dysfunction preceded derangement of platelet function or coagulation parameters. To evaluate the thrombotic response of the endothelium early in endotoxemia, we injured the microvascular endothelium of cremaster arterioles of mice using a laser 1-3 hours following LPS exposure. Platelet and fibrin formation at sites of injury were significantly increased following LPS exposure to respectively 190% (p=0.026) and 195% (p<0.001) of control values. No significant differences were observed in platelet counts, platelet function (aggregation and activation) or coagulation parameters (PT) in mice treated with LPS for 3 hours compared to vehicle controls. Plasma levels of the endothelial markers VWF, soluble VCAM and E-selectin, however, were significantly increased following LPS exposure, demonstrating early endothelial activation. Furthermore, even when platelet accumulation was inhibited using the anti-platelet drug eptifibatide, fibrin generation at sites of laser injury was still significantly (p<0.01) increased in LPS-treated mice. Together, these data show that endothelial activation precedes disruption of platelets and coagulation in endotoxemia. Endothelial dysfunction is associated with perturbation of the endothelial Ang1/Tie2 pathway, characterized by significantly reduced Tie2 function and Ang1 levels. Therefore, we assessed thrombus formation in Tie2^+/- mice in the absence of LPS. Fibrin generation at sites of laser injury was significantly increased in Tie2^+/- mice to 192% of littermate (Tie2^+/+) controls (p<0.01). As loss of Tie2 mimics the LPS-induced phenotype, we next determined whether activation of the Tie2 pathway could reduce fibrin clot formation. Ang1 stimulates phosphorylation of Tie2, promoting protective signaling in endothelium. To determine whether the pro-thrombotic consequences of endotoxemia on Tie2 signaling could be reversed using Ang1, mice were injected with adenovirus expressing Ang1 or control adenovirus prior to LPS exposure. Dephoshorylation of Tie2 associated due to LPS exposure was reduced with Ang1-treatment. In our in vivo thrombosis model, Ang1 inhibited the increased fibrin accumulation at sites of laser injury in endotoxemic mice to baseline levels. Tail snip assays showed that even though elevated Ang1 levels normalized LPS-induced augmentation of thrombus formation, increased Ang1 did not affect bleeding times. These data indicate that Ang1 stimulation of Tie2 signaling can regulate pathologic clot formation in the setting of inflammation without bleeding risk. Together, our studies show that endothelial dysfunction drives a pre-DIC state in endotoxemia. Targeting these early endothelial responses might represent an novel approach for reducing thrombosis in sepsis without enhancing bleeding risks. Future studies will evaluate the mechanism involved.