Pathophysiological mechanisms of thrombosis in acute and long COVID-19.

COVID-19 patients have a high incidence of thrombosis, and thromboembolic complications are associated with severe COVID-19 and high mortality. COVID-19 disease is associated with a hyper-inflammatory response (cytokine storm) mediated by the immune system. However, the role of the inflammatory response in thrombosis remains incompletely understood. In this review, we investigate the crosstalk between inflammation and thrombosis in the context of COVID-19, focusing on the contributions of inflammation to the pathogenesis of thrombosis, and propose combined use of anti-inflammatory and anticoagulant therapeutics. Under inflammatory conditions, the interactions between neutrophils and platelets, platelet activation, monocyte tissue factor expression, microparticle release, and phosphatidylserine (PS) externalization as well as complement activation are collectively involved in immune-thrombosis. Inflammation results in the activation and apoptosis of blood cells, leading to microparticle release and PS externalization on blood cells and microparticles, which significantly enhances the catalytic efficiency of the tenase and prothrombinase complexes, and promotes thrombin-mediated fibrin generation and local blood clot formation. Given the risk of thrombosis in the COVID-19, the importance of antithrombotic therapies has been generally recognized, but certain deficiencies and treatment gaps in remain. Antiplatelet drugs are not in combination with anticoagulant treatments, thus fail to dampen platelet procoagulant activity. Current treatments also do not propose an optimal time for anticoagulation. The efficacy of anticoagulant treatments depends on the time of therapy initiation. The best time for antithrombotic therapy is as early as possible after diagnosis, ideally in the early stage of the disease. We also elaborate on the possible mechanisms of long COVID thromboembolic complications, including persistent inflammation, endothelial injury and dysfunction, and coagulation abnormalities. The above-mentioned contents provide therapeutic strategies for COVID-19 patients and further improve patient outcomes.

The impact of platelets on pulmonary microcirculation throughout COVID-19 and its persistent activating factors.

Patients with COVID-19 often have hypoxemia, impaired lung function, and abnormal imaging manifestations in acute and convalescent stages. Alveolar inflammation, pulmonary vasculitis, and thromboembolism synergistically damage the blood-air barrier, resulting in increased pulmonary permeability and gas exchange disorders. The incidence of low platelet counts correlates with disease severity. Platelets are also involved in the impairment of pulmonary microcirculation leading to abnormal lung function at different phases of COVID-19. Activated platelets lose the ability to protect the integrity of blood vessel walls, increasing the permeability of pulmonary microvasculature. High levels of platelet activation markers are observed in both mild and severe cases, short and long term. Therefore, the risk of thrombotic events may always be present. Vascular endothelial injury, immune cells, inflammatory mediators, and hypoxia participate in the high reactivity and aggregation of platelets in various ways. Microvesicles, phosphatidylserine (PS), platelets, and coagulation factors are closely related. The release of various cell-derived microvesicles can be detected in COVID-19 patients. In addition to providing a phospholipid surface for the synthesis of intrinsic factor Xase complex and prothrombinase complex, exposed PS also promotes the decryption of tissue factor (TF) which then promotes coagulant activity by complexing with factor VIIa to activate factor X. The treatment of COVID-19 hypercoagulability and thrombosis still focuses on early intervention. Antiplatelet therapy plays a role in relieving the disease, inhibiting the formation of the hypercoagulable state, reducing thrombotic events and mortality, and improving sequelae. PS can be another potential target for the inhibition of hypercoagulable states.

The cross-talk of lung and heart complications in COVID-19: Endothelial cells dysfunction, thrombosis, and treatment.

The pandemic respiratory illness SARS-CoV-2 has increasingly been shown to be a systemic disease that can also have profound impacts on the cardiovascular system. Although associated cardiopulmonary sequelae can persist after infection, the link between viral infection and these complications remains unclear. There is now a recognized link between endothelial cell dysfunction and thrombosis. Its role in stimulating platelet activation and thrombotic inflammation has been widely reported. However, the procoagulant role of microparticles (MPs) in COVID-19 seems to have been neglected. As membrane vesicles released after cell injury or apoptosis, MPs exert procoagulant activity mainly by exposing phosphatidylserine (PS) on their lipid membranes. It can provide a catalytic surface for the assembly of the prothrombinase complex. Therefore, inhibiting PS externalization is a potential therapeutic strategy. In this paper, we describe the pathophysiological mechanism by which SARS-CoV-2 induces lung and heart complications through injury of endothelial cells, emphasizing the procoagulant effect of MPs and PS, and demonstrate the importance of early antithrombotic therapy. In addition, we will detail the mechanisms underlying hypoxia, another serious pulmonary complication related to SARS-CoV-2-induced endothelial cells injury and discuss the use of oxygen therapy. In the case of SARS-CoV-2 infection, virus invades endothelial cells through direct infection, hypoxia, imbalance of the RAAS, and cytokine storm. These factors cause endothelial cells to release MPs, form MPs storm, and eventually lead to thrombosis. This, in turn, accelerates hypoxia and cytokine storms, forming a positive feedback loop. Given the important role of thrombosis in the disease, early antithrombotic therapy is an important tool for COVID-19. It may maintain normal blood circulation, accelerating the clearance of viruses, waning the formation of MPs storm, and avoiding disease progression.

Neutrophil extracellular traps induced by pro-inflammatory cytokines enhance procoagulant activity in NASH patients.

BACKGROUND: Nonalcoholic steatohepatitis (NASH) patients are at a high risk of developing venous thromboembolism, with a high rate of morbidity and mortality. The role of neutrophil extracellular traps (NETs) in procoagulant activity (PCA) in patients with NASH remains unclear. Our study aimed to investigate the formation of NETs in NASH patients stimulated by specific pro-inflammatory factors. Moreover, we evaluated the pivotal role of NETs in the induction of hypercoagulability in NASH and the interaction between NETs and endothelial injury. METHOD: The levels of the NETs biomarkers were evaluated in the plasma samples of 27 NASH patients and 18 healthy subjects. The formation of NETs was visualized using immunofluorescence microscopy. The PCA of the NETs was assessed using coagulation time, purified coagulation complex, and fibrin formation assays. Confocal microscopy was further used to evaluate the interactions between the NETs and HUVECs. RESULTS: The levels of NETs markers in the plasma of NASH patients were significantly higher than healthy controls. NETs derived from NASH enhanced thrombin and fibrin formation and significantly reduced CT (p<0.05). The mixture of IL-6 and TNF-α triggered the NETs release in the plasma rather than them alone. Additionally, the NETs exerted cytotoxic effects on the endothelial cells, converting them to a procoagulant and pro-inflammatory phenotype, and DNase I could reverse these effects. CONCLUSION: Our results revealed the primary role of NETs in promoting the hypercoagulable state in NASH patients. Methods that prevent the formation of NETs may be a novel approach for the prevention and treatment of NASH.

Neutrophil extracellular traps (NETs): the role of inflammation and coagulation in COVID-19.

COVID-19 has swept quickly across the world with a worrisome death toll. SARS-CoV-2 infection induces cytokine storm, acute respiratory distress syndrome with progressive lung damage, multiple organ failure, and even death. In this review, we summarize the pathophysiologic mechanism of neutrophil extracellular traps (NETs) and hypoxia in three main phases, focused on lung inflammation and thrombosis. Furthermore, microparticle storm resulted from apoptotic blood cells are central contributors to the generation and propagation of thrombosis. We focus on microthrombi in the early stage and describe in detail combined antithrombotic with fibrinolytic therapies to suppress microthrombi evolving into clinical events of thrombosis. We further discuss pulmonary hypertension causing plasmin, fibrinogen and albumin, globulin extruding into alveolar lumens, which impedes gas exchange and induces severe hypoxia. Hypoxia in turn induces pulmonary hypertension, and amplifies ECs damage in this pathophysiologic process, which forms a positive feedback loop, aggravating disease progression. Understanding the mechanisms paves the way for current treatment of COVID-19 patients.

Neutrophil extracellular traps contribute to tissue plasminogen activator resistance in acute ischemic stroke.

Circulating neutrophil extracellular traps (NETs) resistant to t-PA have not been studied completely although NETs in thrombi may contribute to tissue plasminogen activator (t-PA) resistance. This research intended to elucidate whether circulating NETs are associated with t-PA resistance and the underlying mechanism. The levels of NETs were detected in the circulating neutrophils, ischemic brain tissue of acute ischemic stroke (AIS) patients, and transient middle cerebral artery occlusion (tMCAO) models. NET formation in blood, thrombi, and ischemic brain tissue of mice were analyzed by immunofluorescence. Exposed phosphatidylserine (PS) was assessed using flow cytometry and confocal microscopy. Procoagulant activity (PCA) was evaluated using fibrin formation assays, thrombin, and purified coagulation complex. The plasma levels of NETs in AIS patients were significantly higher than those in healthy individuals. After thrombolysis, a significant increase was noted in NET markers in no-improvement patients, while the changes in improvement patients were not significant. Importantly, NETs were decorated with von Willebrand factor (vWF) and plasminogen activator inhibitor-1 (PAI-1) in the blood and thrombi, which could reverse the fibrinolytic effects. In addition, NETs activated platelets (PLTs) and endothelial cells (ECs), stimulating a procoagulant phenotype and facilitating vWF and PAI-1 release. DNase I, activated protein C (APC), and sivelestat markedly inhibited these effects. Furthermore, targeting NETs protected mice from tMCAO-induced cerebral ischemia, possibly by regulating vWF and PAI-1. In summary, NETs may contribute to t-PA resistance in AIS through activation of PLTs and ECs. Strategies against NETs may present a promising therapeutic approach to improve the thrombolysis efficiency of t-PA in AIS patients.