Blood-borne tissue factor: Another view of thrombosis

Abstract

Arterial thrombosis is considered to arise from the interaction of tissue factor (TF) in the vascular wall with platelets and coagulation factors in circulating blood. According to this paradigm, coagulation is initiated after a vessel is damaged and blood is exposed to vessel-wall TF. We have examined thrombus formation on pig arterial media (which contains no stainable TF) and on collagen-coated glass slides (which are devoid of TF) exposed to f lowing native human blood. In both systems the thrombi that formed during a 5-min perfusion stained intensely for TF, much of which was not associated with cells. Antibodies against TF caused Ϸ70% reduction in the amount of thrombus formed on the pig arterial media and also reduced thrombi on the collagencoated glass slides. TF deposited on the slides was active, as there was abundant fibrin in the thrombi. Factor VII ai , a potent inhibitor of TF, essentially abolished fibrin production and markedly reduced the mass of the thrombi. Immunoelectron microscopy revealed TF-positive membrane vesicles that we frequently observed in large clusters near the surface of platelets. TF, measured by factor X a formation, was extracted from whole blood and plasma of healthy subjects. By using immunostaining, TF-containing neutrophils and monocytes were identified in peripheral blood; our data raise the possibility that leukocytes are the main source of blood TF. We suggest that blood-borne TF is inherently thrombogenic and may be involved in thrombus propagation at the site of vascular injury.Tissue factor (TF) present in the arterial wall has been considered to be responsible for the initiation of the coagulation cascade and thrombus formation (1). The role of vascular TF in acute thrombosis and atherosclerosis has been proposed based on our previous studies (2-5). To investigate the role of circulating TF in thrombogenesis, we have used a system in which pig aortic media or collagen-coated slides were mounted in a laminar flow chamber and perfused with native human blood. We noted that when stained either with derivatized factor VII a (FVII a ) or with specific anti-TF antibodies, the thrombi contained large amounts of TF staining, whereas the media and collagen-coated slides were essentially negative. Thus, we surmised that the TF came from the blood; accordingly, we examined whole blood and plasma for TF activity that we have extracted and assayed. We conclude that there is circulating, potentially active TF in normal subjects. We present evidence that this pool is thrombogenic in model flow systems. We also present evidence suggesting the TF comes from leukocytes and hypothesize that the cell-surface TF is completely encrypted (6-8) but becomes available during thrombosis. METHODSReagents. Human recombinant FVII a was a gift from NovoNordisk, Copenhagen. Factor X was purified from human plasma (9). Affigel-15 was purchased from Bio-Rad. The phospholipids used for relipidation of TF consisted of 30% 1,2-dioleoyl-sn-glycero-3-phosphatidylserine and 70% 1,2-dioleoyl-sn-gl...

Blood-born tissue factor in cardiovascular disease: where are we now?

“…Further studies suggest that platelets might also be a potential source of circulating TF. Our group has shown that immunoreactive and functionally active TF is found on platelet membrane, and its expression may be increased after stimulation of these cells by agonists such as ADP and thrombin [11]. We also demonstrate that de novo synthesis of TF by platelets may take place since TF mRNA can be found in these cells [12].…”

Microparticle‐associated tissue factor activity: a link between cancer and thrombosis?

“…TF has been demonstrated on circulating microparticles (MP), small membrane vesicles that are released from cells following activation or during apoptosis [17][18][19]. MP have been found in the circulation of healthy subjects [20].…”

Activation of platelets in whole blood by recombinant factor VIIa by a thrombin‐dependent mechanism

“…In fluid phase, FVIIa is a poor activator of FX but when bound to TF in the presence of phospholipid its enzymatic activity is increased by an order of 10 7 (Morrissey, 2001). Other workers (Key et al, 1998;Giesen et al, 1999) have suggested that TF may be present in the circulation in an 'encrypted' form, possibly on or derived from neutrophils and/or monocytes. We have shown that rFVIIa at concentrations as low as 25 nmol/l stimulated the formation of platelet aggregates with PMN, monocytes and, to a lesser extent, lymphocytes.…”