Although tissue factor (TF), the principial initiator of physiological coagulation and pathological thrombosis, has recently been proposed to be present in human blood, the functional significance and location of the intravascular TF is unknown. In the plasma portion of blood, we found TF to be mainly associated with circulating microvesicles. By cell sorting with the specific marker CD42b, platelet-derived microvesicles were identified as a major location of the plasma TF. This was confirmed by the presence of full-length TF in microvesicles acutely shedded from the activated platelets. TF was observed to be stored in the α-granules and the open canalicular system of resting platelets and to be exposed on the cell surface after platelet activation. Functional competence of the blood-based TF was enabled when the microvesicles and platelets adhered to neutrophils, as mediated by P-selectin and neutrophil counterreceptor (PSGL-1, CD18 integrins) interactions. Moreover, neutrophil-secreted oxygen radical species supported the intravascular TF activity. The pools of platelet and microvesicle TF contributed additively and to a comparable extent to the overall blood TF activity, indicating a substantial participation of the microvesicle TF. Our results introduce a new concept of TF-mediated coagulation crucially dependent on TF associated with microvesicles and activated platelets, which principally enables the entire coagulation system to proceed on a restricted cell surface.Key words: lipopolysaccharide • platelet rich plasma • superoxide dismutase • catalase • microparticles T wo principal events that are initiated after disrupture of the endothelial barrier are thought to mark the initiation of hemostasis. Blood platelets adhere to subendothelial collagen providing a provisional, mechanically unstable closure of the vessel perforation. Concomitantly, the coagulation process is started. This is mainly due to the formation of an initiator complex between tissue factor (TF), an integral cell membrane protein predominantly present in the adventitial layer of the vessel wall, and the blood-based factor VII/VIIa (1). The TF/factor VIIa complex proteolytically activates factor X, which, in turn, elicits the formation of thrombin. The TF/factor VIIa complex is likely to play a central role in the genesis of arterial and venous thrombosis (2, 3), leading causes of mortality in many countries. TF is present in the lipid rich core of unstable atherosclerotic plaques and may be a major determinant of the thrombogenicity of the plaques (4-6). Indeed, on rupture of the plaque, the interaction of TF with factor VIIa substantially contributes to the rapid formation of the occluding thrombus, the principal final step in the genesis of coronary ischemic disease.Apart from its presence in the vascular wall, TF has also been detected in the blood (intravascular TF). So far, no clear conclusion has been reached about the localization and the functional meaning of the blood-based TF. In the plasma compartment, TF is present un...
While the adenosine 5-diphosphate (ADP) pathway is known to enhance thrombus formation by recruiting platelets and leukocytes to the primary layer of collagenadhering platelets, its role for the initiation of coagulation has not been revealed. Ex vivo inhibition of the P2Y 12 ADP receptor by clopidogrel administration diminished the rapid exposure of tissue factor (TF), the major initiator of coagulation, in conjugates of platelets with leukocytes established by the contact of whole blood with fibrillar collagen. Under in vitro conditions, the P2Y 12 and P2Y 1 ADP receptors were both found to be implicated in the exposure of TF in collagen-activated whole blood. Immunoelectronmicroscopy revealed that collagen elicited the release of TF from its storage pools within the platelets. Functional activation of the intravascular TF was reduced by inhibition of the ADP receptors, partially due to the disruption of the platelet-neutrophil adhesions. Injection of collagen into the venous system of mice increased the number of thrombin-antithrombin complexes, indicative for the formation of thrombin in vivo. In P2Y 1 -deficient mice, the ability of collagen to enhance the generation of thrombin was impaired. In conclusion, the platelet ADP pathway supports the initiation of intravascular coagulation, which is likely to contribute to the concomitant formation of fibrin at the site of the growing thrombus. (Blood.
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