Recounting COVID-19 associated complement-mediated coagulopathies: Triggers and controls

Different presentations of the coagulopathies in COVID infection

The different forms of thrombotic abnormalities reported in COVID-19 are listed in Table 1. Among these, disseminated intravascular coagulation (DIC)⁵ and venous thromboembolism (VTE) were the most common,⁶ which could potentially give rise to strokes and other heart complications.⁷

DIC is marked by dysregulated systemic activation of the coagulation pathway. The excessive thrombotic and hemorrhagic complications due to intravascular fibrin formation and microangiopathic thrombosis lead to exhaustion of coagulation factors and platelets, resulting in life-threatening hemorrhage.⁸ VTE is a condition where clot formation occurs inside the veins. In pulmonary embolism, clots in veins break and reach the lungs through bloodstreams. Thrombotic microangiopathy (TMA) is a syndrome comprising a triad of microangiopathic hemolytic anemia, thrombocytopenia, and organ damage. Thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are two pro-types of thrombotic thrombocytopenia purpura (TMA). TTP is a life-threatening condition associated with the formation of blood clots in small blood vessels, thus limiting the flow of blood to vital organs such as heart, brain, and kidney. When clot formation occurs in the damaged and inflamed small vessels of the kidney and blocks the filtering system of the kidney, the condition is referred to as HUS; sepsis-induced coagulopathy (SIC) spans from precise activation of coagulation (shown by sensitive markers of the coagulation factor activation) to much sturdy activation of coagulation accompanied by a decrease in platelet count and an increase in clotting time to DIC.⁹

Pathophysiology of thrombotic events in COVID-19 patients

Angiotensin-converting enzyme 2 (ACE-2) receptors present in different organs (such as lung, kidney, and heart) facilitate entry of SARS-CoV-2 into host cells.¹⁰ This phenomenon led to viremia, causing internal injury of the blood vessels and in some cases, ultimately gave rise to severe fatal outcomes such as vascular blockage, thrombosis, and multiple organ failure.¹¹ Entry of the pathogen also induced an inflammatory immune response (cytokine storm) as a preventive measure, which expedited the clotting cascade and negatively resulted in the obstruction of blood vessels.^11,12 Upregulation of plasmin inhibitors (alpha-2 antiplasmin and plasminogen activator inhibitor-1) eventually decreased plasmin-mediated fibrinolysis, which resulted in persistent clot accumulation. Further, downregulation of thrombomodulin also induced hypercoagulopathy in COVID-19 patients.¹³ SARS-CoV-2 viruses use extracellular vesicles for their active migration to nearby or distant sites in a maintained physiological state,¹⁴ which are potent inducers of the coagulation system.^15,16 von Meijenefeldt et al. (2021) showed a link between COVID-19 and the coagulation pathway, where patients harbor elevated thrombin generation potential and diminished fibrinolytic ability.¹⁷ A probable explanation for the hypercoagulable and hypofibrinolytic states was the prolonged activation of the endothelium, which resulted in increased plasma levels of factor VIII and PAI-1.¹⁷ Markers of thrombotic events in COVID-associated coagulopathy (CAC) are summarized in Figure 1

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Figure 1:

Markers of coagulopathy during COVID-19 infection: An increase is observed in the levels of fibrin degradation products (FDP), factor VIII (F VIII), platelet count, von Willebrand factor (VWF), fibrinogen, prothrombin time, and D-dimer during SARS-CoV-2 infection.

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Aggravated thrombosis in COVID-19 infection, as demonstrated by a couple of studies, suggested the role of autoantibodies in COVID-19. The literature suggests that the antiphospholipid autoantibodies (APL) are also an important factor for the stimulation of endothelial cells and platelets,^3,18 although the exact role of these autoantibodies remains unclear.

Upon infection, the viral nucleocapsid and spike proteins trigger the mannose binding lectin (MBL)/Ficolins pathway, which eventually activate lectin pathway proteases such as mannose binding lectin (MBL)-associated serine proteases (MASP)-2, a key factor in inducing prothrombin-mediated clotting.¹⁹ COVID-19 patients showed four dysregulated coagulation factors, namely prothrombin (F2), FXI, FXII, and FXIIIa.²⁰ During infection, there is an increase in the level of FXIIIa and von Willebrand factor (VWF) glycoprotein in the renal cortex, which is associated with blood clotting in the renal cortex.²¹ Serpin Family D Member 1 (SERPIND1) and Serpin peptidase inhibitor (SERPINE1) were downregulated and upregulated, respectively, in the renal cortex, which caused microthrombi formation in COVID-19 patients.²⁰

Significantly higher levels of D-dimer²² and fibrinogen, along with mild thrombocytopenia, are biomarkers of COVID-19-associated hypercoagulopathy²³ due to varying underlying reasons.

Crosstalk between complement and coagulation system in COVID-19-associated coagulopathies?

The possible mechanisms involved in coagulopathies with known facts about SARS-CoV-2, the causative agent of contagious COVID-19, are shown in Figure 2. Its spike (S) protein cleavage by furin protease and further priming of the activated S protein by transmembrane protease serine 2 (TMPRSS2) facilitated viral attachment with the host’s ACE-2, which acts as a receptor to mediate its entry into the host cells.²⁴ Therefore, the receptor protein ACE-2 was instrumental in viral transmission and pathogenesis. SARS-CoV-2 interaction with ACE-2 also hinders its physiological role discussed in Figure 2.

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Figure 2:

Schematic representation showing the pathophysiology of COVID-associated coagulopathy. SARS-CoV-2 activates three different pathways. (a) The antithrombotic physiological role of ACE-2 is impaired due to interaction with SARS-CoV-2, resulting in hypofibrinolysis and vasoconstriction. (b) Virus-induced endothelium damage led to simultaneous activation of the classical coagulation cascade and immunothrombosis. (c) Complement activation in response to the recognition of pathogen-associated molecular patterns (PAMPs) contributes to inflammatory thrombosis and nonclassical (MASP protease mediated) fibrin clot generation. ACE: angiotensin-converting enzyme 2, Ang: Angiotensin, MASP: MBL-associated serine proteases, tPA: tissue plasminogen activator, TNF: tumor necrosis factor, CRP: C-reactive protein, IL: interleukin, PAI-1: Plasminogen activator inhibitor-1.

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The complement system plays a critical role in augmenting the coagulation process during COVID infection. Activation of proteins of the complement system and coagulation pathway, namely thrombin, trypsin, and plasmin, significantly contributes to COVID-associated thrombotic events.^25,26 Previously, an in vivo study with C3 null mice indicated the involvement of the complement pathway in COVID-19 infection.²⁷ Bhagwat et al. (2022) demonstrated that mannan-binding lectin-associated serine proteases (MASP-1 and MASP-2) of the lectin complement pathway also have substrates in the coagulation pathway.^28,29 Importantly MASP-1, which has thrombin-like activity, may have played a key role in COVID-19-associated coagulopathies.³⁰ Furthermore, the lectin pathway was also found to be activated in SARS-CoV-2 infection, particularly through the recognition of the nucleocapsid and spike proteins (S protein) of the virus.³¹ It induced lectin pathway via downstream activation of MASP-1 and 2 leading to thrombotic events.³¹ Gao et al. (2020) also suggested that in response to SARS-CoV-2, autoactivation of MASP-2 occurred, which eventually activated the lectin pathway via a series of enzymatic cascades.³² In addition, hypercoagulation in COVID-19, marked by VWF,³³ was observed consistently, and the literature suggested that it could be the outcome of crosstalk between the complement and coagulation systems.^34,35

Furthermore, S protein of SARS-CoV-2 played a key role by binding with heparan sulfate and factor H, which dysregulated the alternative pathway of the complement system.^36,37 Moreover, antibodies such as immunoglobin G (IgG) and immunoglobin M (IgM) produced against specific domains of viral spike proteins were capable of activating the classical complement pathway.³⁸ Therefore, SARS-CoV-2 potentially dysregulated or hyperactivated the different components of the complement system, which, in turn, led to endothethilopathy and thromboinflammation, the classical pathophysiology associated with COVID-19.³⁷

Although the exact reasons why other people were predisposed to severe illness while some remained asymptomatic remain undeciphered; the hypercoagulable state and COVID-19 are interconnected [Figure 2].

Complications of complement-mediated coagulopathies in COVID-19: Different forms of thrombotic complications have been reported in clinical patients, which originally initiated in the lungs³⁹ and eventually led to multiple organ failure.⁴⁰ Pulmonary arterial and venous thrombosis, microangiopathy, and pulmonary embolism were the major clotting-associated complications in COVID-19 patients due to the altered complement system.^41,42 Furthermore, diffuse microvascular thrombosis in the capillaries of different vital organs, such as the lung, kidney, and myocardium, along with deposition of complement activation products, were also reported in patients with COVID-19 infection.^40,43,44 Niederreiter et al. (2022) showed the deposits of complement proteins, namely C1q, MASP-2, complement factor D, C3c, C3d, and C5b-9, in lungs and kidney samples of COVID-19 patients using immunohistochemical studies, which clearly explains the involvement of the lectin pathway in COVID-19.⁴⁵ Another study by Carvelli et al. (2020) reported that MASP-2, C4, MAC, and macrophages with overexpressed C5aR1 are associated with endothelium damage and microthrombi formation.⁴⁶ Clinical studies with critically ill COVID-19 patients demonstrated significantly elevated levels of circulating markers of complement activation. In addition, upregulated alternative pathways along with markers of endothelial injury (i.e., angiopoietin-2) and hypercoagulability (i.e., thrombomodulin and VWF) were reported as the most common features of COVID-19 patients.³³

Systemic activation of the complement and coagulation cascade in response to the viral pathogen also promotes localized complement protein production by lung cells.³⁷ Therefore, complement-targeting therapies have been developed as an attractive therapeutic strategy for COVID-19. Although the pernicious effects of hyperactivated complement pathways in COVID-19 are quite evident, an in-depth understanding of the driving molecular mechanisms is not there. Several studies have demonstrated that the complement system, especially the lectin pathway, is indeed instrumental in viral illness and plays a crucial role in COVID-19 pathogenesis.

Authors’ contributions

PL, MM and AA: conceptualization, writing the original draft, BK and MT: writing-review, and editing, AK: coordinated revisions, KH and DB: conceptualization, writing-review, and editing. SS: conceptualization, writing the original draft, writing-review, and final editing. All authors approved the submitted version.