Does inflammation contribute to thrombotic events?

Recent studies have focused on a myriad of mechanisms by which inflammation can potentiate blood clotting. Inflammatory mediators like endotoxin and tissue necrosis factor (TNF)-alpha can cause the expression of tissue factor on monocytes and, possibly, endothelium, thereby initiating the coagulation cascade. Activation of the complement system can lead to exposure of membrane surfaces capable of amplifying the initial tissue factor stimulus by facilitating the assembly of the factor VIIIa-factor IXa and the factor Xa-factor Va complexes. Inflammatory mediators, particularly interleukin-6, can also increase the levels of fibrinogen, an acute-phase reactant. In addition, the inflammatory mediators can elevate the levels of plasminogen activator inhibitor, thus suppressing the fibrinolytic system. These studies alone, however, do not prove that inflammation can trigger clinically relevant thrombus formation in vivo. For instance, TNF-alpha has been studied in cancer patients as a potential cure for cancer, and even though these patients are hypercoaguable, thrombosis was not commonly observed as a side effect of the near-lethal doses of TNF-alpha that were administered. Based on primate studies, inflammatory mediators like TNF-alpha can promote clot deposition effectively only if there is reduced flow and inhibition of the natural anticoagulant pathways. The requirement for multiple simultaneous injurious events probably explains why inflammation alone is not observed as a major cause of thrombosis.