Severe coronavirus disease-2019 (COVID-19) is frequently associated with microvascular
thrombosis, especially in the lung, or macrovascular
thrombosis, mainly
venous thromboembolism, which significantly contributes to the disease mortality burden.
COVID-19 patients also exhibit distinctive laboratory abnormalities that are compatible with a prothrombotic state. The key event underlying COVID-19-associated thrombotic complications is an excessive host inflammatory response to severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2)
infection generating multiple inflammatory mediators, mainly
cytokines and complement activation products. The latter, along with the virus itself, the increased levels of
angiotensin II and
hypoxia, drive the major cellular changes promoting
thrombosis, which include: (1) aberrant expression of
tissue factor by activated alveolar epithelial cells, monocytes-macrophages and neutrophils, and production of other prothrombotic factors by activated endothelial cells (ECs) and platelets; (2) reduced expression of physiological
anticoagulants by dysfunctional ECs, and (3) suppression of fibrinolysis by the endothelial overproduction of
plasminogen activator inhibitor-1 and, likely, by heightened
thrombin-mediated activation of
thrombin-activatable fibrinolysis inhibitor. Moreover, upon activation or death, neutrophils and other cells release nuclear materials that are endowed with potent prothrombotic properties. The ensuing
thrombosis significantly contributes to
lung injury and, in most severe
COVID-19 patients, to multiple organ dysfunction. Insights into the pathogenesis of COVID-19-associated
thrombosis may have implications for the development of new diagnostic and therapeutic tools.