Although exposure of tissue factor (TF) at sites of disrupted atherosclerotic plaques plays a critical role in platelet-triggered ischemic complications, a direct interaction of TF with platelets has not been yet demonstrated. We have investigated the effects of TF on platelets under different shear rate conditions. Exposure of isolated washed platelets to TF derived from human placenta resulted in reversible aggregation (35-50% max at 1 min) using standard platelet aggregometry. This response was αIIb3 dependent and occurred in the presence of residual concentrations of calcium. TF-induced reversible aggregation was enhanced and made irreversible to 90–100% by the addition of FVIIa (5μg/ml). This second phase of activation was blocked by the presence of serine-protease inhibitors. A combination of ultrastructural and flow cytometry studies revealed that although TF induced shape change and moderate expression of P-selectin an ANV positive phospholipids, internal contraction and full platelet degranulation only occurred after addition of FVIIa. All these transformations occurred together with tyrosine phosphorylation of proteins. Platelets adhered, spread and aggregated onto synthetic membranes coated exclusively with TF that had been exposed to circulating blood. Addition of FVIIa caused a 50% enhancement on platelet interactions and a two-fold increase in fibrin deposition. Serotonin enhanced overall TF induced platelet interactions under the different experimental approaches. A selective serotonin reuptake inhibitor (citalopram) reduced the intensity and extent of the effects of TF in the two experimental settings used in our investigations. Our results are consistent with a direct role of TF on platelet-triggered ischemic complications. We demonstrate that a lipidated from of TF activates isolated platelets in suspension. In studies conducted with flowing whole blood we show that TF firmly attached to a surface acts itself as an adhesive substrate, and promotes formation of aggregates. These TF-platelet interactions are enhanced through serotoninergic mechanism and markedly potentiated in the presence of FVIIa. The possible synergism between FVIIa and serotoninergic mechanisms may well contribute to the development of severe ischemic complications triggered on ruptured atherosclerotic plaques.

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