Platelet-associated tissue factor enhances platelet reactivity and thrombin generation in experimental studies in vitro

Introduction: The thrombogenic potential of tissue factor (TF) associated to platelets is controversial. We have investigated the in vitro contribution of platelet-associated TF to thrombus formation. Materials and Methods: Platelets suspensions were exposed to human TF-rich microvesicles (TF-MV) from placental or recombinant origin. Platelet-associated TF was quantified through coagulometric assays. Adhesive and cohesive properties of platelets containing TF were assessed in perfusion models using two thrombogenic surfaces: 1) type-I collagen, or 2) damaged vascular segments. Perfusion studies were performed with heparinized blood enriched with a 30% of washed platelets exposed to TF-MV vs. washed control platelets. Thrombin generation and thromboelastometric properties of clots were also assessed using a fluorometric assay and ROTEM analysis, respectively. Inhibitory strategies with an antibody to TF were performed in some cases. Results: The addition of 30% of platelets containing TF to blood perfusates resulted in a statistically significant increase in the platelet coverage (%CS) vs. non-exposed platelets on collagen surfaces (%CS: 19.7+/-0.6 and 23.9+/-0.7 respectively, vs. 14.5+/-1.4; p<0.01) and on the vascular subendothelium (%CS: 54.0+/-1.5 and 47.2+/-6.8 respectively vs. 38.0+/-3.5, p<0.05), with a statistically significant increase in the size of large platelet aggregates (p<0.05) vs. control platelets. These effects on collagen surfaces were almost totally prevented by an antibody to TF. Platelet-associated TF significantly accelerated thrombin generation and clot formation (p<0.05), effects that were partially prevented by a neutralizing anti-TF. Conclusions: Platelet-associated TF potentiated adhesive and aggregating properties in in vitro studies with flowing blood and accelerated thrombin generation and clot formation time under steady conditions. (C) 2012 Elsevier Ltd. All rights reserved.