Platelet deposition inhibits tissue factor activity: in vitro clots are impermeable to factor Xa

Abstract

Upon plaque rupture or vascular injury, tissue factor (TF) protein in the vessel wall becomes exposed to flowing blood, initiating a cascade of reactions resulting in the deposition of fibrin and platelets on the injured site. Paradoxically, the growing thrombus may act as a barrier, restricting the convective and diffusive exchange of substrates and coagulation products between the blood and reactive vessel wall, thus limiting the role TF plays in thrombus growth. In this study, various in vitro, platelet-fibrin clots were prepared on TF:VIIa-coated surfaces and the rate at which factor (F) X in the well-mixed clot supernatant permeates the clot and is converted to X a was monitored over several hours. The apparent diffusion coefficients of FX (a) in fibrin and plateletfibrin clots at 37°C was 2.3 ؋ 10 ؊7 and 5.3 ؋ 10 ؊10 cm 2 /second, respectively, indicating that the mean time required for FX (a) , and likely FIX (a) , to diffuse 1 mm in a fibrin clot is 4 hours, and in the presence of platelets, 3.6 months. As complete human thrombotic occlusion has been observed within 10 minutes, an alternative source of procoagulant activity that can localize to the outer surface of growing thrombi, such as platelet factor XI or blood-borne TF, appears essential for rapid thrombus growth. IntroductionIn the generally accepted view of thrombosis, rupture or erosion of an atherosclerotic plaque exposes procoagulant regions on the vascular wall that bind platelets and initiate thrombus formation. The exposed material thought responsible for initiating coagulation is tissue factor (TF), which complexes with VII (a) to activate factor (F) IX and FX. Immediately following injury, FIX and FX are rapidly delivered to the vessel wall by flowing blood resulting in localized thrombin generation. However, as fibrin and platelets accrue to the injured surface, flow to the underlying TF region becomes restricted and substrates must percolate through an overlying platelet-fibrin mesh in order to become activated ( Figure 1). Furthermore, to achieve effective prothrombinase activity (X a :V a ) near the outer surface of a growing thrombus, the products (IX a or X a ) must diffuse a substantial distance back through the overlying mass. Through its interaction with VIIIa, factor IX (a) plays a crucial role in amplifying the amount of X a generated; However, since the only accepted source of Ixa in most coagulation models is its generation by TF:VII a , IX a would have to percolate a substantial distance through the overlying platelets and fibrin in order to generate tenase activity near the surface of the growing thrombus. (Factor XI-mediated activation of IX through the intrinsic pathway is generally not regarded as an essential step in thrombosis and is addressed later.) Hence, to obtain occlusive thrombi, the substrates (IX and X) and their respective enzymes, either individually or working in concert, must travel several millimeters through a growing thrombus without the aid of convection. As diffusion of proteins the size of fact...

Fibrin gel formation in a shear flow

“…The only mechanisms in the model to limit thrombin are inhibition by antithrombin III and removal by the flow. Other mechanisms, including chemical inhibition of other parts of the coagulation system and limitation on transport through a clot because of platelet aggregation (Kuharsky and Fogelson 2001;Hathcock and Nemerson 2004) may limit the production of thrombin. These effects could be included with a more detailed model of the coagulation reactions.…”

Pathogenesis and Pathophysiology of Pneumococcal Meningitis

“…TF is expressed primarily on cells outside the vasculature (128,149) and is exposed to coagulation factors during vascular damage. Low levels of circulating TF have been detected in healthy individuals (167), in whom the role of TF in thrombin generation remains uncertain (81,195,407). The expression of TF in blood cells is limited to monocytes and can be elevated considerably during inflammation or sepsis (370).…”

Determination of surface tissue factor thresholds that trigger coagulation at venous and arterial shear rates: amplification of 100 fM circulating tissue factor requires flow

“…1 Platelet deposition may reduce access of factor X to the TF/VIIa complex formed on the damaged wall. 2,3 Elevated TF antigen and activity are detectable in human atherosclerotic lesions and are expressed by various cell types. 4 Bonderman et al 5 determined, using ex vivo plaque disruption/scraping, that the average TF site density underneath plaques is 33 pg TF/cm 2 , corresponding to approximately 6 molecules-TF/m 2 .…”