Platelet Activation and Mechanisms of Thromboembolism Formation in Patients with Severe COVID-19. Alternative Mechanisms of Hemostasis System Activity

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The review highlights the mechanism of development of hypercoagulation and thrombosis in severe forms of COVID-19. The introduction of the SARS-CoV-2 virus into the host organism is carried out by the interaction of the spike protein S with the angiotensin-converting enzyme ACE-2, which is located in type 2 alveocytes, vascular endothelium, kidneys, liver and other organs. In the event of a serious condition in patients with COVID-19, both nonspecific and adaptive immunity are activated. Stimulation of the complement system with the appearance of C3a, C3b, C5a fragments and the membrane attack complex (MAC) creates conditions for the development of hypercoagulability. The involvement of the renin-angiotensin-aldosterone system in this process and the appearance of angiotensin 2 (Ang-2) further increase the intensity of hypercoagulability. When the SARS-CoV-2 virus enters cells, the protective reaction of the adaptive immune system can turn into a pathological one (a cytokine storm develops), characterized by a high level of pro-inflammatory cytokines IL-1α, IL-6, Il-8, TNF-α, IL-17, etc.) and chemokines (CCL-2, CCL-11, etc.), which ultimately leads to the development of thromboangiopathy or otherwise immunothrombosis in seriously ill patients with COVID-19. Patients with more severe lesions may develop a condition similar to DIC. At the same time, patients with COVID-19 have mild thrombocytopenia, elevated levels of fibrinogen, D-dimer, fibrinogen degradation products (FDP), which indicates intense thrombus formation, as well as short PT and APTT, due to a largely increased level of FVIII. In COVID-19, along with the classical one, an alternative pathway (bypassing thrombin) of regulation of the hemostasis system and thrombus formation appears, mainly associated with the influence of the spike protein S (PS, PROS1) of the SARS-CoV-2 virus and papain-like protease (PROS1). Protein S directly affects the conversion of fibrinogen to fibrin, as well as the activation of individual plasma coagulation factors. The alternative pathway of blood coagulation is also due to the activation of the complement system via the lectin pathway with the inclusion of metalloproteinases MASP-1, 2 and 3. In addition, the S protein activates tPA, which may be accompanied by hyperfibrinolysis. In seriously ill patients with COVID-19, platelets play an important role in the occurrence of thromboembolic complications. During the release reaction, platelets are released from the cytoplasm into the blood α and dense granules containing inflammatory cytokines and chemokines, which enhances the cytokine storm and, consequently, thrombus formation. By acting on the spike protein S, platelets enhance an alternative way of regulating the hemostasis system and thrombus formation.