Contribution of the TF pathway (1) and thrombin-activated platelets (2) to viral acute lung injury (ALI)-associated thrombosis

Infections with influenza A virus (IAV) like the “Swine flu” are associated with increased thrombotic complications. Normally, lung tissue factor (TF) is important to maintain lung hemostasis (hemostatic TF). However, in different pathologies, there is an increase in the TF expression (pathologic TF). This pathologic TF can contribute to thrombotic complication. Indeed,  together with UNC collaborators we showed that IAV infection leads to increased TF expression in lung epithelial cells (EpC). The increased TF expression was associated with increased coagulation activation (thrombin generation) in the lung.

In the first project, we propose that during viral induced acute lung injuries (ALI) like severe flu infections, the host is exposed to viral RNA and to the host’s own extracellular (ds)RNA. This triggers RNA-dependent immune responses leading to pathologic TF expression in lung EpCs. Importantly, monocytes do not express more TF when exposed to dsRNA  (poly IC) alone. During viral ALI, the lung EpC TF comes in contact with blood component and is also released on extracellular vesicles (EVTF+). Together, this increases the local and systemic thrombotic potential of the blood. In support of this, we found that poly IC (dsRNA) stimulation can lead to increased TF expression in lung EpCs in vitro. We refined a non-infectious viral ALI model and we are now able to induce lung EpC TF in mice. We found that this was associated with increased coagulation activation and lung fibrin depositions. We found further, that the increased lung TF expression was associated with an increased distant venous thrombosis similar to “Swine flu” infected mice. Currently, we investigate new ways to reduce this pathologic TF-dependent increase in viral ALI-induced thrombosis. We propose to only inhibit the inducible pathologic TF expression and not the hemostatic TF in the lung. This would ensure normal tissue hemostasis (no spontaneous alveolar bleeding) but would reduce the thrombotic risk in viral ALI.

In a second project, we started investigating the role of platelet PAR4 in viral infections. We generated mice carrying a conditional PAR4 gene (PAR4^fl/fl). These mice allows us to investigate the cell-specific role of PAR4. First, we confirmed previous data that platelet PAR4 is essential for arterial thrombosis. In addition, we were able to show that platelet PAR4 activation enhances venous thrombosis. We are now interested in the contribution of platelet PAR4 to viral ALI-associated thrombosis. We found that severe flu infection or repeated inhalation of the dsRNA mimetic poly IC leads to comparable activation of coagulation in the lung and increased venous thrombosis. Interestingly, mice lacking platelet PAR4 exhibited reduced venous thrombosis in viral ALI. Using mice lacking the canonical activation site (PAR4^R59A), we further found that thrombin-dependent PAR4 activation contributes to viral ALI-associated thrombosis. Our findings suggest that inhibition of platelet PAR4 or canonical (thrombin-dependent) PAR4 activation could become novel treatments to reduce viral ALI-associated thrombosis.