Inflammation and endothelium-platelet interaction in pulmonary thrombosis

The process of hemostasis and thrombosis is a tightly regulated process as a result of the interplay between the endothelium and blood components. The endothelium covers the inner layer of the vasculature and form a physical barrier between blood and organs. Circulating cells such as platelets, leukocytes and erythrocytes are flowing by and contribute to blood coagulation upon activation. Endothelium-platelet interaction is the key initiator that mediates blood clotting in primary hemostasis. Alterations in endothelial cells may enhance this process, thereby contributing to various thromboembolic diseases. Understanding what these adaptations are and how they contribute to the pathophysiology of disease are essential for the identification of novel treatment strategies and the improvement of disease management. In this thesis, we investigated the role of the endothelium in pulmonary thrombosis. In the first part, we provide an overview of the role of VWF in thrombosis and how its expression is regulated on a transcriptional and translational level in inflamm¬¬atory conditions such as atherosclerosis, chronic thromboembolic pulmonary hypertension (CTEPH) and corona virus disease 2019 (COVID-19). To investigate the role of inflammation in endothelium-platelet interaction we developed a preclinical model to study in situ pulmonary thrombosis. We show that the thrombotic response is not only different in diseased conditions such as in CTEPH, but also among different vascular beds. In the second part of this thesis, we investigated how the pulmonary artery endothelium contributes to thrombus formation and platelet adhesion in CTEPH. It is acknowledged that VWF levels are upregulated in CTEPH, and we present evidence of an inflammation-driven mechanism of in situ thrombosis in which VWF plays a central role. We showed that the VWF promoter was epigenetically regulated in the endothelium of CTEPH patients, which supports NFkB2 binding to initiate VWF expression. We followed up on this by performing transcriptomics and proteomics of CTEPH-PAEC and evaluated whether differential expression was due to epigenetic modifications. Subsequently, those expression profiles were linked to cell-cell and cell-matrix pathways that may induce thrombosis. The current treatment options for CTEPH are limited to pulmonary endarterectomy surgery, balloon pulmonary angioplasty or medical treatment with vasodilators. In addition, patients receive lifelong anticoagulation to prevent recurrent thrombi. However, the combined use of vasodilators with anticoagulative drugs can be associated with an increased risk of bleeding. In the final part of this thesis, we identified that riociguat affects endothelial function by inhibiting ultra large VWF string formation, which destabilizes platelet adhesion and may lead to increased risk of bleeding. Finally, we investigated why severe COVID-19 patients experience vascular leakage by pulmonary edema and thrombosis, and identified that these patients have altered antibodies that induces an excessive inflammatory response, that play a key role in the progression of the disease. In conclusion, we propose that inflammation plays a critical role in in situ thrombosis by interfering with VWF-mediated platelet adhesion. Our findings may provide interesting therapeutic treatment options for inflammatory induced VWF mediated thrombosis, interfering between inflammation, coagulation and homeostasis.