Обычно в активации этого фактора участвуют высокомолекулярный кининоген и некоторые протеолитические ферменты. Но, иногда некоторые токсины тоже активируют этот про-фермент.
В случае ковида, как показали данные по содержанию и активности в крови больных этого профермента и последующих продуктов его активации, и сравнению их уровней с аналогичными показателями при ТОРС, вызванной тяжелым гриппом- имеется патологическа склонность к тому, чтобы у больных ковидом формирвоались тромбы с его участием. Более того, софрмировавшиеся с участием фактора 12 тромбы плохо поддаются растворению (фибринолизу), в несколько раз хуже, чем "гриппозные".
Авторы считают, что тромбозу при ковиде способствуют как активация FXII (осталось разобраться, почему она происходит) и его повышенная "утилизация", так и повышенные уровни фибрина в крови ( т.е. есть, чему сворачиваться), и, кроме этого- устойчивосать получившихся тромбов ( они получаются плотные, очень мелкоячеистые) к фибринолизу.
Using confocal and electron microscopy, we show that increased FXII activation rate, in conjunction with elevated fibrinogen levels, triggers formation of fibrinolysis-resistant, compact clots with thin fibers and small pores in COVID-19. Accordingly, we observed clot lysis in 30% of COVID-19 patients and 84% of ARDS influenza subjects. Analysis of lung tissue sections revealed wide-spread extra- and intra-vascular compact fibrin deposits in COVID-19. Together, our results indicate that elevated fibrinogen levels and increased FXII activation rate promote thrombosis and thrombolysis resistance via enhanced thrombus formation and stability in COVID-19.
Many patients with severe COVID-19 exhibit coagulation abnormalities that mimic other systemic coagulopathies associated with severe infections, such as disseminated intravascular coagulation (DIC) or thrombotic microangiopathy. A high incidence of venous thromboembolism, pulmonary embolism, deep vein thrombosis, and multiple organ failure with a poor prognosis and outcome appears to be causally related to dysregulation of blood coagulation in critically ill COVID-19 patients. Besides an elevated inflammatory status (e.g. increased cytokine levels) that might induce monocyte-related coagulation and suppression of anticoagulant pathways, typical laboratory findings in COVID-19 patients with coagulopathy are increased D-dimer levels and elevated fibrinogen concentrations. Moreover, inflammation-induced endothelial cell injury in different vascular beds may contribute to a hypercoagulable state and the risk of thromboembolic complications In order to provide mechanistic insights into the reported hypercoagulable state of severe COVID-19 patients, we compared changes in the contact phase system activation and fibrinolysis between COVID-19 patients, individuals suffering from ARDS-influenza, and donors. While some critical parameters such as fibrinogen, PAI- 1, and TAFI were significantly increased, FXII levels were reduced in severe COVID-19, and the process of fibrin formation and the resulting fibrin clot structure and lysis were substantially different between patient cohorts. Histological data provided evidence for widespread, compact fibrin deposition in the lungs of patients with COVID-19 as opposed to those with ARDS-influenza In particular, although the levels of FXII were significantly decreased in severe COVID-19 patients as compared to ARDS-influenza and donors, FXII-activation products were markedly altered in patients with SARS-CoV2 infection. This scenario very likely reflects FXII consumption due to its increased binding to and auto-activation on negatively charged surfaces. Decreased FXII levels in COVID-19 plasma are also in accordance with moderately elevated APPT reported in other studies. The exacerbated consumption of FXII in severe COVID-19 is further supported by our in vitro studies, in which the supplementation of COVID-19 plasma with exogenous FXII resulted in its rapid activation, presumably due to the presence of FXII auto-activation cofactors. Indeed, common pathological events observed in COVID-19 such as increased tissue cell stress together with virus-mediated necrosis, endothelial dysfunction, and excessive neutrophil activation, lead to the release/exposure of large amounts of negatively charged molecules including NETs. NETs not only bind FXII but also serve as a potent endogenous inflammation-dependent inducer of FXII auto- activation, eventually propagating thrombosis. Enhanced vascular NETosis along with impaired NET clearance were described in COVID-19 patients. In line with these findings, several studies found an increase in NET components in COVID-19 plasma including cell-free DNA, myeloperoxidase-DNA complexes, neutrophil elastase-DNA complexes, and citrullinated histone H3. In addition, active FXII was described to colocalized with NETs in the lungs of COVID-19 patients and NET positive pulmonary vessels were reported to be frequently clogged. Together with these findings, our results speak for NET-induced, accelerated, and constant activation of FXII in COVID-19 and thus for its role in immunothrombothic processes in this pathology. In fact, FXII auto-activation cofactors were found to be relevant for the initiation and progression of sepsis and DIC. Interestingly enough, low plasma levels of FXII in severe COVID-19 patients did not result in markedly prolonged kaolin clotting time (KCT) suggesting that other hemostatic abnormalities/factors compensate for low amounts of FXII in critically ill COVID-19 subjects. As previous studies reported that high plasma levels of FVIII:C may associate with a short KCT and an increased risk of thromboembolism , it is plausible to assume that the excessive amounts of FVIII:C in COVID-19, as opposed to ARDS-influenza, plasma induced shortening of KCT in our cohort of patients. These results, together with previously described high levels of fibrinogen, mild thrombocytopenia, and slightly altered plasma concentrations of coagulation factors and physiological anticoagulants argue for a specific form of intravascular coagulation in severe COVID-19 that is distinguishable from classical DIC. The prominent increase in vascular complications points to strong involvement o endothelial cells in hemostatic abnormalities seen in COVID-19. Injured endothelial cells may provide a scaffold for thrombus generation and elevated levels of von Willebrand factor multimers (recently described in COVID-19 plasma ) may facilitate 388 platelet-vessel wall interactions ultimately leading to the formation of platelet-rich thrombotic deposits in microvasculature. Such platelet-rich thrombotic aggregates have been observed in alveolar capillaries of critically ill COVID-19 patients. occlusive thrombotic microangiopathy with destruction of alveoli that supports persistence of microthrombi. Elevated levels of fibrinogen were reported to contribute to the faster fibrin formation and increased fibrin network density, strength, and stability . In line with this assumption, clots generated from COVID-19 plasma exhibited much higher packing density as compared to those formed from ARDS-influenza plasma. Indeed, higher levels of fibrinogen and increased rate of FXII activation were associated with denser fibrin clots with smaller pores. The compact architecture of clots generated from COVID-19 plasma correlated with their resistance to lysis consolidating the notion of hyperfibrinogenemia and FXII consumption coagulopathy as driving causes of an increased risk of thrombosis in critically ill COVID-19 patients. Our findings are consistent with the studies demonstrating the role of fibrinogen and FXIIa in organization of clot architecture and the reports linking abnormal fibrin network structure/function with thrombotic events seen in patients with diabetes , ischemic stroke , pulmonary hypertension , myocardial infraction , or venous thromboembolism . Although, increased fibrinogen levels independently promote thrombus formation and stability, the role of FXII in these processes seems to be more complex and dependent on environment conditions. Those include, the presence of NETs (or any other molecule being able to activate FXII) which orchestrate not only FXII but also platelets activation, activated platelets may perpetuate FXIIa generation by the release of polyphosphates and the availability of haemostatic factors. Coagulation proteases ensure FXIIa-dependent thrombin formation and a direct binding of FXII/FXIIa to fibrinogen may define aggregation of fibrin fibers. Whether the interaction of FXII/FXIIa with fibrinogen can interfere with the binding of t-PA to fibrin and thereby inhibits fibrinolysis warrants further investigation. Clots generated from COVID-19 plasma exhibited higher packing density, small pores and were built of thin fibers. Interestingly enough, previous studies suggested that thrombi made of thin and numerous fibers organized in tight network are resistant to fibrinolysis .Persistent vessel occlusion seen in critically ill COVID-19 patients is reinforced by markedly increased plasma levels of TAFI and moderately elevated amounts of PAI-1 . Thus, persistent occlusion of microvessels in the lungs of COVID-19 patients appears to be a result of unfortunate circumstances, starting from sustained activation/presence of thrombosis-promoting factors, going through the formation of lysis resistant thrombi, and finishing on the accumulation of fibrinolytic inhibitors57. Based on current and previous findings, the scenario of defense mechanisms, including the immune and coagulation system, running out of control emerges as an underlying mechanism for severe SARS-CoV2 infection. Multiple hits from abnormalities in plasma composition, vascular cell function, and blood immune cell landscape through virus-mediated cell damage and release of intracellular debris create a milieu favoring activation of FXII. In combination with high levels of fibrinogen, FXIIa contributes to pathologic thrombus formation not only via thrombin generation but also through the formation of compact and lysis resistant clots. Our study thus establishes a model for future investigations on the role of altered fibrin clot structure in thrombosis and thrombolysis in severe COVID-19. The high incidence of thrombotic events, in particular deep vein thrombosis and pulmonary embolism, in conjunction with mildly prolonged activated partial thromboplastin time (APTT) , suggests a possible role of coagulation factor XII (FXII) in COVID-19 coagulopathy. FXII is a serine protease of the contact-phase system of blood coagulation and circulates in plasma as a single-chain zymogen . Following contact with anionic surfaces such as kaolin, but also extracellular RNA (eRNA) released from damaged cells , neutrophil extracellular traps (NETs)22, or polyphosphates secreted from activated platelets , FXII undergoes autoactivation to 96 αFXIIa (herein referred to as FXIIa) . FXIIa cleaves plasma prekallikrein (PK) to kallikrein (PKa), which in turn reciprocally activates FXII and amplifies FXIIa generation .As a consequence, the plasma kallikrein-kinin system is activated, leading to the release of the vasodilatory and vascular barrier disrupting peptide bradykinin (BK) from high molecular weight kininogen (HK . Overall, activation of the contact-phase system contributes to an increased production of thrombin and fibrin, although FXIIa/PKa-mediated conversion of plasminogen to plasmin may have a minor effect on fibrinolysis To demonstrate the in vivo relevance of our findings, we stained autopsy lung tissue sections from SARS-CoV2- and influenza-infected ARDS patients as well as subjects who died due to no respiratory causes for fibrin. Notably, time from death to autopsy was matched for all groups examined. As demonstrated in figure 5A, intra- and extra- vascular fibrin aggregates were observed in both severe COVID-19 and ARDS- influenza patients. However, in contrast to ARDS-influenza subjects, in the lungs of COVID-19 patients the deposits of fibrin appeared to be more widespread and evenly present not only in alveolar spaces but also around alveolar septae over the whole lung examined. In ARDS-influenza patients, fibrin deposit were predominantly observed in alveolar spaces and present in selected regions of the lung (Figure 5A). Overall, in COVID-19 lungs fibrin clots were more compact and homogeneous whereas in ARDS-influenza lungs they were widespread and characterized by regions of high and low fibrin fiber density .