Tissue Factor Firmly Immobilized on Surfaces Promotes Platelet Adhesion and Fibrin Formation in Studies with Circulating Human Blood: Importance of Shear Rate Conditions and FVIIa.

While procoagulant activity of tissue factor (TF) has been widely investigated, its possible proadhesive properties towards platelets have not been studied in detail.

We explored the interaction of platelets with human TF (hTF) firmly attached to a surface using anticoagulated blood with low molecular weight heparin (20 U/ml) at different shear rates. For studies at 250 s⁻¹ and 600 s⁻¹, TF adsorbed on a synthetic surface was exposed to circulating blood in flat perfusion devices. Deposition of platelets and fibrin formation were evaluated by morphometric, immunocytochemical and ultrastructural methods. For experiments at 5000 s⁻¹, we used the PFA-100™ with experimental cartridges with collagen or collagen-hTF. Effect of rFVIIa was assessed in all experimental settings. Prothrombin fragment F1+2 levels were also measured.

At 250 and 600 s⁻¹ platelet interaction was 19.84±1.33% and 26.12±3.42% of the total surface respectively. Our inmunocytochemical results suggest that von Willebrand factor could mediate these interactions. Fibrin formation was significantly higher at 250 s⁻¹ than at 600 s⁻¹ (p<0.05). FVIIa tended to increase platelet deposition without reaching statistical significance, and raised fibrin formation and thrombin generation (p<0.05). Our At 5000 s⁻¹, closure times in the PFA-100 were significantly shortened in the presence of hTF (154.09 ±14.69 s vs 191.45± 16.09 s with collagen alone; p<0.05). Addition of rFVIIa did not result in a further reduction of closure time.

Our studies demonstrate that hTF is reactive for platelets. von Willebrand factor could mediate these interactions. Recombinant FVIIa enhances the procoagulant action of hTF at low and intermediate shear rates, but has no impact on the hemostatic performance at very elevated shear rates.