Antithrombotic activity of TNF-α

1589 Introduction Inflammation is often considered a contributing factor to thrombotic and hemostatic disorders associated with various disease states. These diseases include the coagulopathy of septicemia (1), the prothrombotic state associated with atherosclerotic vessels (2) or cancer (3), and the veno-occlusive disease of the liver after bone marrow transplantation (4). Previous studies have shown that the proinflammatory mediators, especially TNF-α, can induce a procoagulant state by eliciting tissue factor production on the surface of vascular endothelium and monocytes, downregulating the protein C anticoagulant pathway and stimulating thrombin and fibrin formation (5). Neither these studies nor those testing the local administration of high doses of TNF-α as a potential cure for cancer patients provided evidence that TNF-α triggers a thrombotic response in vivo (6), however. Although TNF-α is considered to be a key factor involved in the pathogenesis of diseases affecting the cardiovascular system, its role in platelet thrombus formation still remains to be addressed. TNF receptor-1 and -2 (TNF-R1 and -R2) are found on almost all nucleated cell types but are poorly detectable on platelets. Although it appears that platelets bind TNF-α (7) and that murine megakaryocytes express TNF-R1 (8), at present it is not clear whether normal human platelets express TNF-Rs. In addition, it was recently shown that CD40 ligand (CD40L), a member of the TNF family of ligands, plays a role in the stability of arterial thrombi through the specific binding of its amino acids Lys-Gly-Asp (KGD) KGD sequence to the platelet β3 integrin (9). Interestingly, TNF-α also contains a KGD sequence, but its involvement in platelet thrombus formation has not been investigated. In this study, we used intravital microscopy to examine the role played by TNF-α in the dynamic highshear environment of an in vivo model of arterial injury. We show that the systemic administration of TNF-α at a dose described as procoagulant led to a marked defect in thrombus formation and delayed or prevented vessel occlusion. We show that the inhibitory effect of TNF-α is mediated through TNF-Rs and the rapid production of NO.