BACKGROUND: ADP-induced platelet activation leads to cell surface expression of several proteins, including TF (tissue factor). The role of ADP receptors in platelet TF modulation is still unknown. We aimed to assess the (1) involvement of P2Y ₁ and P2Y ₁₂ receptors in ADP-induced TF exposure; (2) modulation of TF ^pos -platelets in anti-P2Y ₁₂ –treated patients with coronary artery disease. Based on the obtained results, we revisited the intracellular localization of TF in platelets. METHODS: The effects of P2Y ₁ or P2Y ₁₂ antagonists on ADP-induced TF expression and activity were analyzed in vitro by flow cytometry and thrombin generation assay in blood from healthy subjects, P2Y ₁₂ ^−/− , and patients with gray platelet syndrome. Ex vivo, P2Y ₁₂ inhibition of TF expression by clopidogrel/prasugrel/ticagrelor, assessed by VASP (vasodilator-stimulated phosphoprotein) platelet reactivity index, was investigated in coronary artery disease (n=238). Inhibition of open canalicular system externalization and electron microscopy (TEM) were used for TF localization. RESULTS: In blood from healthy subjects, stimulated in vitro by ADP, the percentage of TF ^pos -platelets (17.3±5.5%) was significantly reduced in a concentration-dependent manner by P2Y ₁₂ inhibition only (−81.7±9.5% with 100 nM AR-C69931MX). In coronary artery disease, inhibition of P2Y ₁₂ is paralleled by reduction of ADP-induced platelet TF expression (VASP platelet reactivity index: 17.9±11%, 20.9±11.3%, 40.3±13%; TF ^pos -platelets: 10.5±4.8%, 9.8±5.9%, 13.6±6.3%, in prasugrel/ticagrelor/clopidogrel-treated patients, respectively). Despite this, 15% of clopidogrel good responders had a level of TF ^pos -platelets similar to the poor-responder group. Indeed, a stronger P2Y ₁₂ inhibition (130-fold) is required to inhibit TF than VASP. Thus, a VASP platelet reactivity index <20% (as in prasugrel/ticagrelor-treated patients) identifies patients with TF ^pos -platelets <20% (92% sensitivity). Finally, colchicine impaired in vitro ADP-induced TF expression but not α-granule release, suggesting that TF is open canalicular system stored as confirmed by TEM and platelet analysis of patients with gray platelet syndrome. CONCLUSIONS: Data show that TF expression is regulated by P2Y ₁₂ and not P2Y ₁ ; P2Y ₁₂ antagonists downregulate the percentage of TF ^pos -platelets. In clopidogrel good-responder patients, assessment of TF ^pos -platelets highlights those with residual platelet reactivity. TF is stored in open canalicular system, and its membrane exposure upon activation is prevented by colchicine.