αIIbβ3-dependent platelet procoagulant activity promotes pulmonary arterial thrombosis triggered by circulating tissue factor.

In situ pulmonary arterial thrombosis (iPAT), occurring without concurrent deep vein thrombosis, is a life-threatening complication associated with various pathological conditions. The etiological mechanisms underlying iPAT remain poorly understood. Several studies suggest that circulating tissue factor (cTF) may contribute to the development of thrombotic complications; however, there is no direct in vivo evidence supporting the role of cTF in the pathogenesis of iPAT. Furthermore, although in vitro studies suggest that platelet anionic phospholipids enable cTF-initiated coagulation, how platelets contribute to cTF-dependent iPAT in vivo remains unknown. In the current study, we used quantitative fluorescence intravital lung microscopy to investigate the development of cTF-induced iPAT in live mice following intravascular administration of thromboplastin. To dissect the interplay between coagulation and platelet procoagulant activity, we assessed the effects of coagulation and platelet inhibition on iPAT development in vivo. Additionally, we conducted an in vitro clotting time assay using mouse plasma samples. Thromboplastin triggered iPAT in mice in a dose-dependent manner. IPAT involved the formation of platelet-rich thrombi at the bottle-neck junctions of pulmonary arterioles and capillaries, which was prevented by heparin. Notably, pretreatment of mice with annexin A5 or eptifibatide also completely abrogated thromboplastin-induced iPAT. These intravital microscopy findings were further corroborated by the in vitro clotting time assay. Our study provides in vivo evidence that cTF contributes to the development of iPAT. We demonstrate that the prothrombotic effect of cTF is dependent on platelet-αIIbβ3 signaling, which enhances platelet procoagulant activity, leading to accelerated coagulation and development of iPAT.