αIIbβ3-Dependent Platelet Procoagulant Activity Promotes Pulmonary Arterial Thrombosis Triggered By Blood-Borne Tissue Factor

Rationale. In situ pulmonary arterial thrombosis (PAT), in the absence of concomitant deep vein thrombosis, is a life-threatening complication linked to several settings, including chronic obstructive pulmonary disease, asthma, sickle cell disease, emergency and elective surgeries, and viral infections. The occurrence of thrombotic complications in these conditions is correlated with elevated concentrations and activity of blood-borne tissue factor (TF). While TF is a primary initiator of blood coagulation, activated platelets accelerate coagulation by externalizing anionic phospholipids, such as phosphatidylserine (PS), and providing a catalytic surface for the assembly of coagulation enzyme complexes. In vitro experiments demonstrate that anionic phospholipids regulate TF-initiated coagulation; however, there is a lack of in vivo data on whether or how platelets contribute to the blood-borne TF-dependent in situ PAT.

Methods. We employed state-of-the-art quantitative fluorescence intravital lung microscopy (qFILM) to investigate PAT development in C57BL/6 (WT) mice following intravascular (IV) TF administration. We examined the impact of coagulation inhibition with heparin, platelet αIIbβ3 inhibition using eptifibatide, and phosphatidylserine (PS) blocking with annexin A5 (ANX) on circulating TF-induced PAT in WT mice. Fluorochrome-conjugated anti-mouse CD49b antibody, anti-mouse fibrin antibody, and dextran were administered IV for in vivo staining and visualization of platelets, fibrin, and blood vessels, respectively. Additionally, a modified prothrombin time assay (mPTA) was used to investigate the effect of platelet procoagulant activity on mouse platelet-rich plasma (PRP) clotting.

Results. TF triggered dose-dependent PAT in mice in vivo, which involved the development of platelet-rich thrombi in the junctions of pulmonary arterioles and capillaries, resulting in transient ischemia in the arteriole and the downstream capillary tree. The PAT, triggered by IV TF, was entirely abrogated by IV administration of heparin, eptifibatide, and ANX. In addition, the mPTA study confirmed that modification of platelet procoagulant activity by αIIbβ3 inhibition significantly extended clotting times in WT mouse PRP samples.

Conclusions. Our findings strongly suggest that circulating TF may contribute to the development of in situ PAT. We demonstrate that TF's action is dependent on platelet αIIbβ3 signaling, which promotes platelet procoagulant activity, accelerates coagulation and leads to PAT. Both our models have potential applications in investigating the molecular determinants of PAT associated with various pulmonary disorders, and in evaluating the efficacy of new antithrombotic drugs.