Platelets interact with tissue factor immobilized on surfaces: effects of shear rate

Background  While procoagulant activities of Tissue Factor (TF) have been widely investigated, its possible pro-adhesive properties towards platelets have not been studied in detail. Material and methods  We explored the interaction of platelets with human Tissue Factor (hTF) firmly adsorbed on a synthetic surface of polyvinilidene difluoride (PVDF) using different shear rates. For studies at 250 and 600 s−1, TF firmly adsorbed was exposed to flowing anticoagulated blood in flat perfusion devices. Deposition of platelets and fibrin were evaluated by morphometric, immunocytochemical and ultrastructural methods. Prothrombin fragment 1 + 2 (F1 + 2) levels were also measured. Experiments at 5000 s−1, were performed on the Platelet Function Analyzer (PFA-100™) with experimental cartridges with collagen (COL) or collagen-hTF (COL + TF). Haemostatic effect of recombinant activated FVIIa (rFVIIa) was assessed in the same experimental settings. Results  Platelet deposition on hTF reached 19·8 ± 1·3% and 26·1 ± 3·4% of the total surface, at 250 and 600 s−1, respectively. Fibrin formation was significantly higher at 250 s−1 than at 600 s−1 (P < 0·05). The addition of rFVIIa did not influence platelet deposition but raised fibrin formation and thrombin generation at both shear rates (P < 0·05). At 5000 s−1, closure times (CT) in the PFA-100 were significantly shortened in the presence of hTF (154·09 ± 14·69 s vs. 191·45 ± 16·09 s COL alone; P < 0·05). Addition of rFVIIa did not cause a further reduction of CT. Conclusions  Our studies demonstrate that hTF is an adhesive substrate for platelets and suggest that the von Willebrand factor could mediate these interactions. At low and intermediate shear rates, rFVIIa enhanced the procoagulant action of hTF, but this effect was not observed at very high shear rates.