Abstract Number: OC 11.1
Meeting: ISTH 2021 Congress
Background: The functional relevance of platelet TF in thrombosis and hemostasis is controversial.
Aims: Here, we delineate the mechanism of human platelet-TF activation.
Methods: We studied platelet TF-induced pro-coagulant activity (PCA) in platelet suspensions and under flow over VWF-coated surfaces.
Results: Ristocetin-induced binding of VWF (VWF-R) to GPIbα in leukocyte-free platelet suspensions promotes TF-FVIIa activation of FX (tenase assay) and thrombin generation (prothrombinase assay) without inducing significant platelet granule secretion. No exogenous FVII and FV were required, which were contributed directly by the platelets. Peak levels of FXa (within 3-5 min), were 146 (17-786) nmoles/2×10-7 platelets (median and 10%-90% percentiles). Endogenous thrombin potential induced by ristocetin and TRAP stimulation were 671nM (386-1172) vs 407nM (220-737), respectively (p<0.0001). FXa generation was blocked by specific inhibitors of GPIbα, TF, FVIIa and by selective inhibitors of PI3 kinase, Src kinases, and Akt signaling pathways. TFPI suppressed FXa induced by VWF-ristocetin. In contrast, TRAP-induced PCA occurred only after platelet pre-incubation with α-TFPI/α-Protein S. In this condition, the amount of FXa generated was similar to that induced by VWF-R, highlighting the role of platelet-derived TFPI and Protein S in platelet PCA inhibition (Fig 1).
Experiments under flow expanded these observations. Adhered platelets to VWF at arterial shear rate (1.500sec-1) were exposed to a flowing reaction solution with FVIIa, FX, prothrombin, fibrinogen and Ca++, which led to fibrin strand deposition and fibrin labelling of adherent platelets. Prothrombin exclusion from the clotting solution annulled fibrin formation. Moreover, the platelets from a Glanzmann thrombasthenia patient revealed normal fibrin strand deposition, but no fibrin over the platelets, demonstrating that fibrin bound to αIIbβ3 integrin independent of clotting dependent fibrin strand formation (Figure 2).
Conclusions: Taken together, these observations disclose a novel mechanism of fibrin formation at arterial flow rates initiated by GPIbα engagement leading to the activation of platelet TF-FVII.
To cite this abstract in AMA style:Panes O, Becerra MF, Valle R, Sáez CG, Hidalgo P, Pereira J, Royce J, Pedrosa D, Ruf W, Mezzano D. VWF-bound GPIbα under Flow Activates Human Platelet TF and Surface-bound FVII, Sparking Clotting Reactions, Thrombin Generation and Fibrin Deposition [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/vwf-bound-gpib%ce%b1-under-flow-activates-human-platelet-tf-and-surface-bound-fvii-sparking-clotting-reactions-thrombin-generation-and-fibrin-deposition/. Accessed December 3, 2021.
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