The state of complement in covid-19

ABSTRACT Hyperactivation of the complement and coagulation systems is recognized as part of the clinical syndrome of COVID-19. Here we review systemic complement activation and local

complement activation in response to the causative virus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and their currently known relationships to hyperinflammation and

thrombosis. We also provide an update on early clinical findings and emerging clinical trial evidence that suggest potential therapeutic benefit of complement inhibition in severe COVID-19.

SIMILAR CONTENT BEING VIEWED BY OTHERS ASSOCIATION OF COVID-19 INFLAMMATION WITH ACTIVATION OF THE C5A–C5AR1 AXIS Article 29 July 2020 IMMUNITY, ENDOTHELIAL INJURY AND COMPLEMENT-INDUCED

COAGULOPATHY IN COVID-19 Article 19 October 2020 THE SIGNAL PATHWAYS AND TREATMENT OF CYTOKINE STORM IN COVID-19 Article Open access 07 July 2021 INTRODUCTION COVID-19, caused by severe

acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is the largest pandemic disease of humans in the past century. Thus far, approximately 225 million cases and more than 4.5 million

deaths have been attributed to COVID-19 globally1. The clinical manifestations of COVID-19 can be highly variable, ranging from mild upper respiratory tract symptoms in most cases to severe,

life-threatening, multi-organ disease2. Up to one-third of patients affected by COVID-19 will develop persistent symptoms3, and some will have permanent end-organ dysfunction pursuant to

tissue fibrosis4. Despite the emerging epidemiological data, the underlying pathogenesis of COVID-19 and its optimal treatment are still poorly understood. Undoubtedly, detailed mechanistic

understanding of this disease will enable more effective therapies for its treatment, save lives and preserve healthy tissues. The complement system is an ancient, evolutionarily conserved

and non-redundant component of immunity. It is classically viewed as a liver-derived and plasma-operative system constantly scanning the blood and interstitial fluids for invading pathogens

and self-derived noxious antigens. Pathogen sensing triggers one or several complement activation pathways (the lectin pathway, the classical pathway or the alternative pathway) and

cleavage-mediated activation of the key components C3 and C5 by C3 and C5 convertases, respectively, into bioactive C3a and C3b and C5a and C5b. C3a and C5a are anaphylatoxins that mediate

the general inflammatory reaction, C3b is a major opsonin that induces tagging and phagocytic uptake of pathogens and C5b seeds the formation of the membrane attack complex (MAC; C5b–9),

which directly lyses pathogens5. Owing to its central role in the detection and removal of pathogens, complement should intuitively be protective during SARS-CoV-2 infection. However,

several lines of evidence implicate this system as a key component in the pathogenesis of COVID-19, and emerging trial data suggest clinical benefits from targeting this system

therapeutically (Fig. 1). Thus, complement, like many components of the immune system, could be viewed as a ‘double-edged sword’, with its dysregulation leading to harmful effects. Of

course, how and why the normally protective complement system becomes pathogenic in COVID-19 is not yet known, but one could speculate that, consistent with the Goldilocks principle, some

complement activation is beneficial in the initial response to the virus, but too much, sustained for too long, propagates disease. Through parallels with other coronaviruses, through

systems biology approaches and through detailed pathological and clinical observations, researchers and clinicians now realize the importance of complement in pathophysiology and see the

potential therapeutic benefit of complement inhibition6,7,8,9. Here we review the emerging roles of complement in COVID-19-associated thromboinflammation, propose a key contribution from

lung intracellular complement as well as systemic complement activation, and finally provide an update on eagerly awaited clinical trial results. COMPLEMENT IN SEVERE COVID-19 Multiple lines

of evidence implicate hyperactivation of the complement system in the pathophysiology of COVID-19. Mechanistically, SARS-CoV-2 itself can activate the complement system either directly

through the lectin pathway, the classical pathway and/or the alternative pathway or cause endotheliopathy (endothelial cell injury and dysfunction) and thromboinflammation (inflammation

associated with coagulation and thrombosis), which in turn activate the complement system (see later and Fig. 1). Specifically, the spike and nucleocapsid proteins of SARS-CoV-2 can be

directly recognized by lectin pathway components, leading to complement activation10, and IgG and IgM antibodies directed against the receptor-binding domain of the spike protein initiate

classical pathway activation11. Furthermore, SARS-CoV-2 spike protein may directly dysregulate the alternative pathway of complement activation by binding heparan sulfate and competing with

factor H, which is a negative regulator of complement activity12,13,14. These observations are consistent with the long-known observation that complement activation is a frequent feature of

the pathophysiology of acute respiratory distress syndrome, induced by infection or other triggers15,16,17,18. For example, increased anaphylatoxin concentrations are found in serum from

patients with influenza virus H1N1 infection of the lungs19. Similarly, patients with severe COVID-19 have high circulating levels of C5a and of the terminal activation fragments, soluble

C5b–9, which correlate with clinical severity11,20,21,22, as well as high levels of processed fragments of C3, itself an independent risk factor for death23. Likewise, data from the UK

Biobank support genetic single-nucleotide variants in genes encoding the complement regulators decay accelerating factor (DAF; also known as CD55), complement factor H and complement

component 4-binding protein-α (C4BPα) as risk factors for morbidity and death from SARS-CoV-2 infection24. Significant microvascular deposition of mannan-binding lectin serine protease 2

(MASP2; the primary enzymatic initiator of the lectin pathway of complement activation), C4d and C5b–9 has been reported in the skin and lungs of patients, the last two complement components

co-localizing with SARS-CoV-2 spike protein25,26. Reports of children with vasculitic lesions and hypercoagulability in the context of COVID-19 (refs27,28) (the so-called multisystem

inflammatory syndrome in children (MIS-C)) also suggest complement involvement through co-triggering of the complement and coagulation cascades. A key study has shown that elevated plasma

levels of C5b–9 are common in MIS-C29, suggesting systemic complement activation in COVID-19. Characteristic endothelial cytomorphological changes observed in patients30,31,32 are also

pathognomonic of complement-mediated injury induced by C5b–9 (ref.33). Finally, early clinical findings and emerging trial evidence also indicate potential benefits of complement-targeting

therapies in COVID-19, as reviewed later. The findings of studies of patients align with findings in animals responding to related pandemic betacoronaviruses. For example, Middle East

respiratory syndrome coronavirus infection in mice causes extensive complement deposition in lung tissues, where the damage can be reduced by inhibition of distal complement components34. In

the same model, C5aR blockade limits the thickening of alveolar septa and prevents interstitial oedema and haemorrhage in the lungs34. Similarly, the complement components _C3_, _C1r_ and

_Cfb_ are all part of a pathogenic pulmonary gene signature associated with lethality in mouse models of SARS-CoV infection35, and deletion of _C3_ in these mice protects against disease36.

Indeed, primary infection with SARS-CoV upregulates complement genes in ferrets, including the lectin pathway components MASP1 and ficolin 1 (ref.37). SARS-CoV itself interacts with

mannose-binding lectin (MBL) to activate complement C3 proximally via the lectin pathway on virus-infected cells38 in a similar manner to SARS-CoV-2, the nucleocapsid protein of which

aggravates lung injury through MASP2-mediated complement hyperactivation10. The lectin pathway contributes to the excessive inflammatory response associated with other pandemic viral

infections, as MBL-deficient mice develop less severe disease when infected with influenza A virus strain H1N1 or strain H9N2/G1 compared with wild-type mice39. Although the detrimental

contributions of complement to COVID-19 are now recognized, the underlying molecular mechanisms driving pathology are less clear. Recent data indicate that the long-acknowledged but still

poorly understood functional connection between systemic complement and the coagulation cascade and also the intrinsic production of complement by lung cells are involved and may thus be

promising novel therapeutic targets. THROMBOINFLAMMATION IN COVID-19 SARS-CoV-2 infection of the respiratory epithelium triggers an inflammatory response driven by the host innate immune

system that in 20–30% of hospitalized patients results in endothelial injury and the activation of coagulation and thrombosis40, a phenomenon called ‘thromboinflammation’41. These events

start in the lung, but may extend to several other organs, and can clinically manifest themselves as renal, hepatic and heart dysfunction and eventually multi-organ failure42,43. Thrombosis

(venous thrombosis, pulmonary embolism and arterial thrombosis) is indeed a major complication of COVID-19. Despite routine thromboprophylaxis, almost 10% of patients hospitalized with

COVID-19 have thrombotic complications44, and clotting-associated complications are associated with death45,46,47,48,49,50. Indeed, pulmonary artery thrombosis and microangiopathy in

pulmonary tissue were observed in up to 79% of patients who died with COVID-19 as the primary cause and were associated with the presence of diffuse alveolar damage51, which supports the

role of thrombotic phenomena in the development of COVID-19-related lung damage. Patients with COVID-19-associated coagulopathy have highly elevated levels of plasma D-dimer52, a fibrin

degradation product that is present in the blood after activation of fibrinolysis. In a retrospective study of patients from Wuhan, China, D-dimer levels greater than 1 μg ml−1 at admission

to hospital predicted an 18-fold increase in the odds of dying following SARS-CoV-2 infection2. Endothelial cell injury and dysfunction (also termed ‘endotheliopathy’) play a central role in

COVID-19-associated thrombosis and coagulopathy. Endothelial cells and microvascular pericytes express entry receptors for SARS-CoV-2 (refs53,54,55), and endothelial tropism of SARS-CoV-2

has been suggested by autopsy studies and single-cell RNA sequencing atlases53,56. Here, SARS-CoV-2 binding to its host entry receptor, angiotensin-converting enzyme 2 (ACE2), blocks

ACE2-mediated production of angiotensin (1–7), thus preventing vasorelaxation and antithrombotic and anti-inflammatory effects on endothelial cells and enhancing the harmful effects of

angiotensin II, exacerbating endothelial cell activation and injury57. However, the evidence for direct SARS-CoV-2 infection of endothelial cells remains a matter of debate58,59,60,61,62.

Nevertheless, the levels of markers of endothelial cell activation and injury, including von Willebrand factor (vWF), angiopoietin 2, soluble P-selectin and soluble thrombomodulin, are

elevated in the circulation of patients with severe COVID-19 in intensive care units63,64,65; and vWF and thrombomodulin levels correlate with mortality63,66,67. Extensive endothelialitis

with inflammatory cells adhering to the endothelium and platelet-rich fibrin thrombi have been reported in large and small arterial vessels in the lungs, heart, liver, small bowel, kidneys

and other organs of patients who have succumbed to COVID-19 (refs6,30,43). Furthermore, diffuse microvascular thrombosis with endothelial cell injury and rarefaction have been observed most

often in pulmonary alveolar capillaries53, as well as in kidney glomerular and peritubular capillaries6,68,69 and in myocardial capillaries43, accompanied by deposits of complement

activation products within damaged microvasculature6,43,69. Histological examination of lungs, hearts and kidneys from patients with severe COVID-19 shows fibrin and intense C5b–9

immunostaining in the glomeruli and cardiac microthrombi, respectively6,43,69. The pattern of microvascular lesions and C5b–9 deposits, along with elevated levels of lactate dehydrogenase

and low levels of haptoglobin and schistocytes in peripheral blood, that have all been observed both in adults and children with severe COVID-19 (refs51,53,70), are similar to

complement-mediated thrombotic microangiopathy (also known as atypical haemolytic uraemic syndrome), a prototypic disease of complement C5-mediated endothelial injury71,72. The report of two

cases of atypical haemolytic uraemic syndrome relapse triggered by COVID-19, one of which was in a patient with a heterozygous missense variant in the _C3_ gene (an R139W gain-of-function

point mutation resulting in formation of a hyperactive C3 convertase), and cured by the anti-C5 antibody eculizumab73,74, indirectly supports a pathogenetic role of the complement terminal

pathway in COVID-19-associated thromboinflammation. The terminal complement components C5a and the MAC can promote endothelial injury and dysfunction through multiple processes75. C5a

recruits and activates neutrophils, monocytes and macrophages76. Furthermore, both C5a and the MAC induce chemokine release and upregulation of adhesion molecules on endothelial cells,

events that support transmigration of neutrophils and macrophages75,77,78. C5a and the MAC also promote platelet adhesion by stimulating the exocytosis of P-selectin and vWF multimers from

endothelial cells, tissue factor expression and shedding of thrombomodulin from cell surfaces76,79,80, which trigger the coagulation cascade. Activated platelets and C3 subsequently activate

the recruited neutrophils to release neutrophil extracellular traps (NETs), generating extracellular DNA and histones that are highly toxic to endothelial cells and accumulate in

microthrombi of patients with COVID-19 (refs8,81). The release of NETs (NETosis) is a unique form of neutrophil cell death that is intimately linked with the complement and coagulation

cascades82. C5a receptor 1 (C5aR1) and C5aR2 activation generates extracellular histones, which are major components of NETs and activate the intrinsic coagulation pathway83,84,85. NETs act

as a scaffold for thrombus formation by inducing platelet aggregation, while platelets in turn induce NET formation86,87,88. The cumulative results of these events are vascular injury and

loss of endothelial antithrombogenic properties, with massive formation of blood clots. Consistent with this, exposure of cultured microvascular endothelial cells to SARS-CoV-2 spike protein

induced leukocyte recruitment, C3 and C5b–9 deposition and platelet adhesion and thrombus formation on cell surfaces. Thrombus formation was halted by complement inhibitors89. The

observation that the signature of complement deposition across different organs in patients with severe COVID-19 mimics the signatures found in complement-driven thrombotic disease90

supports the notion that complement is a driving pathogenic force in severe COVID-19. Furthermore, it is possible that complement also contributes to SARS-CoV-2 vaccine-induced thrombotic

complications (Box 1). Thus, collectively, the available evidence places the consequences of C5 activation among the drivers of COVID-19-associated thromboinflammation and highlights the

potential of inhibitors targeting C5, C5a or C5aR1 to prevent and control thromboembolic complications in this debilitating infectious disease. BOX 1 SARS-COV-2 VACCINE-INDUCED THROMBOTIC

COMPLICATIONS The development of vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has occurred with unprecedented speed, with at least three vaccines currently

approved in most countries123,124 and more than six billion doses administered worldwide so far125. Overall, the risk of serious adverse effects has been remarkably low. However, several

cases of unexpected thrombotic events and thrombocytopenia have been observed in individuals vaccinated with the adenovector-based vaccines ChAdOx1 nCoV-19 (AstraZeneca, University of Oxford

and Serum Institute of India)126,127 and Ad26.COV2.S (Janssen; Johnson & Johnson)128,129,130. Most affected patients developed acute disseminated intravascular coagulation, which can

evolve into life-threatening cerebral venous thrombosis and thrombocytopenia following the first vaccination dose. The syndrome has been termed ‘vaccine-induced immune thrombotic

thrombocytopenia’ (VITT) and is caused by high titres of circulating IgG autoantibodies that recognize platelet-bound platelet factor 4 (PF4; also known as CXCL4) and induce uncontrolled,

FcγIIa-dependent platelet activation and uninhibited (local) coagulation126. Treatment of VITT includes use of non-heparin anticoagulants in combination with high-dose intravenous

immunoglobulins (to competitively inhibit the FcγIIa receptor)131. VITT-related venous thrombosis is not confined to classical sites such as the lungs and deep veins of the leg but is often

disseminated to the splanchnic, cerebral and ophthalmic veins132. The reasons for thrombosis at these uncommon locations in VITT are currently unclear. Although a role for complement in VITT

has not been explored, there are several intriguing features of VITT that are suggestive of potential complement involvement. For example, as the main driver of VITT is a platelet-bound

anti-PF4 IgG antibody, recognition of the IgG–platelet immune complexes by C1q can initiate activation of the classical pathway. This could trigger subsequent activation of the alternative

pathway and the coagulation system (Fig. 1) and create the thrombotic environment seen in VITT. Furthermore, the unusual locations of thrombotic precipitations observed in VITT are similar

to those seen in paroxysmal nocturnal haemoglobinuria, a condition caused by overactive complement due to deficiency in the complement regulator CD59 (refs133,134). Congruent with this, two

patients with VITT improved after treatment with the anti-C5 antibody eculizumab135. However, although plausible, the potential contributions of complement to VITT are currently hypothetical

and require further clinical and experimental exploration. INTRACELLULAR COMPLEMENT Complement circulating in plasma is produced and secreted by the liver91. However, complement components

are also produced by both immune92,93 and non-immune94,95,96 cells. In T cells, complement is produced both on activation through the T cell receptor and via ICAM1–LFA1 interactions during

diapedesis into tissues92,93. The latter scenario is relevant to COVID-19 as it represents a source of local complement in tissues, where plasma complement is absent, and is a requirement

for the bioenergetic reprogramming necessary for optimal effector function of immune cells92,97,98,99. Airway epithelial cells (AECs) also produce C3 and have the capacity to take up some C3

from exogenous sources96. In the context of cellular stress, for example from serum starvation, intracellular C3 protects AECs from apoptosis96, although the mechanism is currently unknown.

In the context of SARS-CoV-2 infection of AECs, expression of complement genes is among the biological pathways most highly induced by the virus100. Here, virus sensing triggers type I

interferon signalling through the JAK–STAT machinery and induces direct regulation of complement genes, notably including _C3_ and _CFB_ (encoding complement factor B)100. Factor B drives

the assembly of an inducible, intracellular alternative complement activation convertase, which processes C3 to C3a. Accordingly, SARS-CoV-2-infected cells produce C3a, and this can be

normalized by culture with ruxolitinib, an inhibitor of JAK1 and JAK2, or a cell-permeable inhibitor of factor B100. Importantly, immune cells close to SARS-CoV-2-infected AECs respond to

the milieu of heightened local C3 activation and upregulate genes induced by C3 fragment receptors (C3aR and CD46), generating proinflammatory mediators, a signature not detected in

circulating immune cells100. Whether increased C3 production and intracellular C3a generation by AECs triggered by SARS-CoV-2 infection has a cell-intrinsic effect has not yet been explored.

Collectively, AEC-derived complement is likely to represent a substantial local source of complement in the context of SARS-CoV-2 and is unlikely to be normalized by drugs targeting

complement components in the circulation. Given the contributions of canonical (plasma-circulating) and non-canonical (intracellular) complement to human immunological health101,102, it may

be worth exploring whether changes in complement activity with age may be a risk factor for severe COVID-19 (Box 2). BOX 2 RISK FACTORS FOR SEVERE COVID-19 One of the major unanswered

questions of the COVID-19 pandemic is why some people succumb to the disease while others experience only mild symptoms following infection. Surveillance data indicate that older adults

(over 70 years of age) and those with underlying health conditions have an increased risk of progression to severe COVID-19 (ref.136). Specifically, co-morbidities such as diabetes, heart

disease and hypertension (which are progressively more prevalent with age), obesity and smoking are associated with severe disease or fatal outcomes137. The underlying molecular mechanisms

that render these subgroups of patients more vulnerable are a focus of ongoing research — and complement perturbations should be considered as potential candidates. Complement plays an

active role in the development of hypertension138 and cardiovascular disease139, while smoking and obesity are both associated with chronically increased complement activation, which

perpetuates the associated diseases140,141. Thus, complement contributes to the pathological conditions priming individuals to development of severe COVID-19. More importantly, complement

may also contribute to the changes associated with the ageing immune system, referred to as ‘immunosenescence’142, and that augment vulnerability towards severe COVID-19 in otherwise healthy

individuals143. Specifically, the continued presence of low-grade inflammation (inflammageing) over a lifetime can fuel the onset or progression of age-related pathological inflammation and

autoimmunity. Indeed, the protein levels of classical and alternative pathway complement components in the circulation increase progressively with age144, and increases in the amounts of

circulating C3 and C1q proteins are considered biomarker candidates of ageing and age-related diseases145. Furthermore, increases in complement production in the brain contribute to

age-related synaptic loss and microglia dysfunction146, whereas an age-related increase in C1q protein levels in circulation impairs muscle regeneration over time147. Thus, it is feasible

that age-related changes in the amounts of specific complement proteins contribute to the detrimental imbalance in proinflammatory versus repair responses disproportionately observed in

older patients (over 70 years of age) with COVID-19. THERAPEUTIC PERSPECTIVES Early in the first wave of the COVID-19 pandemic, a number of case reports and small-scale clinical

interventions that were not randomized nor properly controlled advocated the use of complement-inhibiting therapies103,104,105,106,107,108,109, very similar to reports advocating the

wide-scale use of cytokine antagonists110. What has become clear over the past 18 months is that initial enthusiasm needs to be followed by rigorously executed clinical trials of appropriate

sample size across many countries before conclusions can be reached and clinical practice is altered. For COVID-19 therapy, it is emerging that timing is everything. The disease runs a

bimodal course with an early viral phase comprising mainly upper airway symptoms, followed in approximately 20% of patients by dyspnea and hypoxia, sometimes progressing to severe acute

respiratory distress syndrome. Despite the considerable numbers of patients who are eligible and willing to participate in clinical COVID-19 trials, only 20% of patients progress to severe

disease. Thus, the biggest hurdle to obtaining definitive answers with various intervention trials in severe COVID-19 has been to select the right timing and the right patient who might

benefit from the intervention, on the basis of either clinical or biochemically easy to measure (point of care) biomarkers. It is very likely that the hyperinflammation that characterizes

severe disease is driven by several molecular and cellular pathways that tend to self-amplify111, so in advanced disease, targeting only one pathway and hoping for improvement is likely to

be futile. Early intervention without causing immunosuppression is probably key. Currently, however, biomarkers predicting those who will go on to develop life-threatening COVID-19

(prognostic biomarkers) among the less ill and that could predict those who will respond to therapy (theragnostic biomarkers) are still lacking. One notable exception is the serum

concentration of soluble urokinase plasminogen activator receptor, which strongly predicts clinical responsiveness to blockade with the IL-1 receptor antagonist anakinra112. In the specific

case of complement inhibition studies, it is still complicated in most hospitals to get point-of-care assessments of the complement cascade and in a timely manner. Clinical practice is also

rapidly changing during this pandemic, and use of the broad anti-inflammatory drug dexamethasone has become the standard of care, potentially making it harder to replicate earlier findings

regarding new compounds in small-scale studies in larger randomized trials that now include dexamethasone as the standard of care. As an example, despite many negative clinical outcome

trials, the effectiveness of interfering with IL-6 in COVID-19 became clear only from a meta-analysis of more than 30 randomized intervention trials, in which it was shown that IL-6 reduces

28-day mortality mainly in the most severely ill patients113. Counterintuitively, the clinical benefit was, however, seen mainly in those receiving dexamethasone or other steroids,

potentially because these patients had a worse prognosis114,115. Specifically, with regard to complement, other considerations include whether components of the complement system themselves

should be targeted (extracellularly or within cells) or whether mechanisms inducing complement production and/or activation should be targeted instead (for example, using JAK inhibitors or

factor B inhibitors; see earlier). These considerations are important because AEC-derived complement is likely to represent a substantial local source of complement in the context of

SARS-CoV-2 and is unlikely to be normalized by drugs targeting complement components in the circulation. Cell-permeating drugs targeting complement intracellularly (such as factor B

inhibitors) or upstream components inducing complement gene transcription (such as JAK inhibitors) could potentially be more efficacious in severe COVID-19. Indeed, although many trials are

ongoing, some data already support beneficial efficacy of JAK inhibitors in the context of severe COVID-19116,117,118,119. Finally, although the proliferation of preprint publications

somewhat mitigates publication bias, it still remains likely that negative data may be under-reported. Strategies to target complement in COVID-19 have mainly used available drugs that were

already undergoing clinical testing or were already approved for clinical use in complement-mediated diseases, such as thrombotic microangiopathy associated with transplantation, atypical

haemolytic uraemic syndrome, paroxysmal nocturnal haemoglobinuria, macular degeneration, complement-mediated nephropathies and myasthenia gravis. As a result, most of these have focused on

targeting the C5 system. Initially, a few case series of C5-targeting therapies (eculizumab, a monoclonal antibody targeting C5, by Alexion Pharmaceuticals; BDB-001, a monoclonal antibody

against C5a, by Staidson Biopharmaceuticals) were trialled in a total of 16 patients with severe to critical COVID-19, 14 of whom recovered26,106,109. A subsequent proof-of-concept

non-randomized trial in 40 patients with severe COVID-19 and controls showed a survival benefit in the group treated in an intensive care unit setting with eculizumab120. On the basis of

these studies, several randomized controlled trials (RCTs) have been initiated, and some have completed enrolment. Targeting C5 in COVID-19 has been trialled using the monoclonal antibodies

eculizumab (NCT04288713 and NCT04346797), ravulizumab (Ultomiris; NCT04570397 and NCT04369469) and tesidolumab (LFG316, Novartis)108, or using small protein or peptide antagonists of C5 such

as nomacopan (Akari Therapeutics; trial about to start in the UK and Brazil) or zilucoplan (ZILU-COV trial (NCT04382755, UCB Pharma). Interim analysis of a large phase III trial with

ravulizumab (NCT04369469) in patients with COVID-19 already receiving mechanical ventilation at randomization could not demonstrate clinical benefit, leading to a pause in the study. In the

ZILU-COV trial, in contrast, 81 patients with COVID-19 and signs of hypoxia not yet requiring mechanical ventilation were randomized to receive 32 mg of zilucoplan subcutaneous daily for 14

days in addition to prophylactic broad-spectrum antibiotics (to cover the risk of meningococcal disease) or standard-of-care treatment and the same antibiotic regimen121. Patients receiving

zilucoplan had improved oxygenation parameters at day 15, accompanied by a lowering of circulating cytokine levels. At day 28 there was also lower mortality in zilucoplan-treated patients,

although the study was underpowered to detect effects on mortality. Importantly, there was no risk of opportunistic infections in those receiving zilucoplan. This indicates that early timing

of C5 blockade may be crucial. Trials are also studying the impact of blocking either C5a or C5aR. An initial RCT using the C5a-targeting antibody vilobelimab (NCT04333420, InflaRx)

reported minimal improvement in oxygenation in 30 patients and controls, although there was a trend for increased survival in the vilobelimab arm122. This study has now been extended to a

large multicentre placebo-controlled RCT involving 390 patients receiving mechanical ventilation (NCT04333420) and is currently recruiting patients. Similarly, a large phase II/III RCT

involving 368 participants is addressing the C5a-specific antibody BDB-001 (NCT 04449588, Staidson Biopharmaceuticals) and is in progress. Despite promising results of C5aR blockade in a

preclinical model20, development of the C5aR1-specific monoclonal antibody avdoralimab (Innate Pharma, NCT04371367) was discontinued after disappointing effects of a phase II RCT in patients

with various degrees of COVID-19 severity. One explanation is that blockade of C5aR1 alone is not sufficient to halt the detrimental effects of complement in COVID-19 because the MAC is

still formed or because C5 can still signal through alternative receptors, such as C5aR2. Targeting the C3 pathway is also an attractive scenario. Use of the C3 inhibitor AMY-101 (Amyndas

Pharma) was initially reported in two case reports for treatment of four patients with severe COVID-19, all of whom subsequently recovered105,106. A larger phase II study with 144 patients

(NCT04395456) is planned but is not yet recruiting. The C3 antagonist APL9 (Apellis, NCT04402060) was discontinued after no difference in mortality was found in a phase I/II RCT in 65

patients with severe COVID-19. Other strategies of which the outcome is not yet known include blocking complement early in the cascade, such as by using the recombinant human C1 inhibitor

Ruconest (Pharming, NCT04705831, NCT0530136 and NCT04414631) or the MASP2-specific antibody narsoplimab (Omeros, NCT04488081) to block the lectin pathway of complement activation. Similarly,

given the potential for SARS-CoV-2 spike protein to activate complement via the alternative pathway12,13,14, a phase II clinical trial of remdesivir, with or without a factor D inhibitor,

is currently under way (Alexion, NCT04988035). An important cautionary note that also needs to be considered in the context of strategies that inhibit a non-redundant component of normal

immunity is the risk of life-threatening, invasive infections that are distinct from COVID-19 itself. As examples, a fatal case of _Klebsiella pneumoniae_ infection following compassionate

use of a C5-blocking monoclonal antibody in one of five patients108 and a more than doubling of infectious complications, including bacteraemia and ventilator-associated pneumonia, in a

clinical trial of eculizumab have been reported120. These infectious complications occurred despite adequate prior antibiotic prophylaxis and vaccination. Thus, the potential gains of

complement inhibition need also to be balanced against the potential risks and harms that may also ensue. CONCLUSIONS Both clinical and basic science studies suggest that uncontrolled

activity of the complement system may be a central player in the pathogenesis of COVID-19. Clinical trial evidence is now beginning to substantiate this suspicion. This clearly is an

exciting development; however, it should still be viewed with caution because, similarly to IL-6-targeting therapies, it is likely that clinical trials will indicate that the benefits of

complement-targeting therapies will depend on several confounders, namely severity of disease, timing of initiation and co-administered drugs. Thus, at present there is no compelling

evidence to strongly advocate complement-targeting treatments for all patients presenting with severe COVID-19. This will likely change in the future as the niche for complement targeting

becomes clearer and as we gain a better understanding of the distinct spatio-temporal contributions of complement following SARS-CoV-2 infection, its crosstalk with the coagulation system

and the role of genetic polymorphisms in both systems. REFERENCES * Dong, E., Du, H. & Gardner, L. An interactive web-based dashboard to track COVID-19 in real time. _Lancet Infect.

Dis._ 20, 533–534 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Zhou, F. et al. Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a

retrospective cohort study. _Lancet_ 395, 1054–1062 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Whitaker, M. et al. Persistent symptoms following SARS-CoV-2 infection in a random

community sample of 508,707 people. Preprint at _medRxiv_ https://doi.org/10.1101/2021.06.28.21259452 (2021). Article  Google Scholar  * Vasarmidi, E., Tsitoura, E., Spandidos, D. A.,

Tzanakis, N. & Antoniou, K. M. Pulmonary fibrosis in the aftermath of the COVID-19 era (review). _Exp. Ther. Med._ 20, 2557–2560 (2020). CAS  PubMed  PubMed Central  Google Scholar  *

Noris, M. & Remuzzi, G. Overview of complement activation and regulation. _Semin. Nephrol._ 33, 479–492 (2013). CAS  PubMed  PubMed Central  Google Scholar  * Noris, M., Benigni, A.

& Remuzzi, G. The case of complement activation in COVID-19 multiorgan impact. _Kidney Int._ 98, 314–322 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Lo, M. W., Kemper, C.

& Woodruff, T. M. COVID-19: complement, coagulation, and collateral damage. _J. Immunol._ 205, ji2000644 (2020). Google Scholar  * Skendros, P. et al. Complement and tissue

factor-enriched neutrophil extracellular traps are key drivers in COVID-19 immunothrombosis. _J. Clin. Invest._ 130, 6151–6157 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Perico,

L. et al. Immunity, endothelial injury and complement-induced coagulopathy in COVID-19. _Nat. Rev. Nephrol._ 17, 46–64 (2021). PubMed  Google Scholar  * Ali, Y. M. et al. Lectin pathway

mediates complement activation by SARS-CoV-2 proteins. _Front. Immunol._ 12, 714511 (2021). THIS STUDY PROVIDES THE FIRST DEMONSTRATION THAT SARS-COV-2 DIRECTLY ACTIVATES COMPLEMENT VIA

MASP1 AND MASP2. CAS  PubMed  PubMed Central  Google Scholar  * Holter, J. C. et al. Systemic complement activation is associated with respiratory failure in COVID-19 hospitalized patients.

_Proc. Natl Acad. Sci. USA_ 117, 25018–25025 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Yu, J. et al. Complement dysregulation is associated with severe COVID-19 illness.

_Haematologica_ https://doi.org/10.3324/haematol.2021.279155 (2021). Article  PubMed  PubMed Central  Google Scholar  * Yu, J. et al. Direct activation of the alternative complement pathway

by SARS-CoV-2 spike proteins is blocked by factor D inhibition. _Blood_ 136, 2080–2089 (2020). PubMed  Google Scholar  * Satyam, A. et al. Activation of classical and alternative complement

pathways in the pathogenesis of lung injury in COVID-19. _Clin. Immunol._ 226, 108716 (2021). CAS  PubMed  PubMed Central  Google Scholar  * Robbins, R. A., Russ, W. D., Rasmussen, J. K.

& Clayton, M. M. Activation of the complement system in the adult respiratory distress syndrome1–4. _Am. Rev. Respir. Dis._ 135, 651–658 (1987). CAS  PubMed  Google Scholar  * Zilow, G.,

Sturm, J. A., Rother, U. & Kirschfink, M. Complement activation and the prognostic value of C3a in patients at risk of adult respiratory distress syndrome. _Clin. Exp. Immunol._ 79,

151–157 (1990). CAS  PubMed  PubMed Central  Google Scholar  * Zilow, G., Joka, T., Obertacke, U., Rother, U. & Kirschfink, M. Generation of anaphylatoxin C3a in plasma and

bronchoalveolar lavage fluid in trauma patients at risk for the adult respiratory distress syndrome. _Crit. Care Med._ 20, 468–473 (1992). CAS  PubMed  Google Scholar  * Duchateau, J. et al.

Complement activation in patients at risk of developing the adult respiratory distress syndrome1–3. _Am. Rev. Respir. Dis._ 130, 1058–1064 (1984). CAS  PubMed  Google Scholar  * Ohta, R. et

al. Serum concentrations of complement anaphylatoxins and proinflammatory mediators in patients with 2009 H1N1 influenza. _Microbiol. Immunol._ 55, 191–198 (2011). CAS  PubMed  Google

Scholar  * Carvelli, J. et al. Association of COVID-19 inflammation with activation of the C5a-C5aR1 axis. _Nature_ 588, 146–150 (2020). THIS IS A KEY PUBLICATION DEMONSTRATING THAT

INCREASED C5A LEVELS IN THE CIRCULATION AND HYPEREXPRESSION AND HYPERACTIVATION OF C5AR1 ON BLOOD AND PULMONARY IMMUNE CELLS CONTRIBUTE TO COVID-19 SEVERITY. CAS  PubMed  PubMed Central 

Google Scholar  * Cugno, M. et al. Complement activation in patients with COVID-19: a novel therapeutic target. _J. Allergy Clin. Immun._ 146, 215–217 (2020). CAS  PubMed  Google Scholar  *

Shen, B. et al. Proteomic and metabolomic characterization of COVID-19 patient sera. _Cell_ 182, 59–72.e15 (2020). THIS IS A FUNDAMENTAL DATA SOURCE ALLOWING NOVEL INSIGHTS INTO DISEASE

MECHANISMS OF COVID-19 BEYOND THE BATTERY OF SINGLE-CELL RNA-SEQUENCING DATA AVAILABLE. CAS  PubMed  PubMed Central  Google Scholar  * Sinkovits, G. et al. Complement overactivation and

consumption predicts in-hospital mortality in SARS-CoV-2 infection. _Front. Immunol._ 12, 663187 (2021). CAS  PubMed  PubMed Central  Google Scholar  * Ramlall, V. et al. Immune complement

and coagulation dysfunction in adverse outcomes of SARS-CoV-2 infection. _Nat. Med._ 26, 1609–1615 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Magro, C. et al. Complement

associated microvascular injury and thrombosis in the pathogenesis of severe COVID-19 infection: a report of five cases. _Transl. Res._ 220, 1–13 (2020). CAS  PubMed  PubMed Central  Google

Scholar  * Gao, T. et al. Highly pathogenic coronavirus N protein aggravates lung injury by MASP-2-mediated complement over-activation. Preprint at _medRxiv_

https://doi.org/10.1101/2020.03.29.20041962 (2020). Article  PubMed  PubMed Central  Google Scholar  * Riphagen, S., Gomez, X., Gonzalez-Martinez, C., Wilkinson, N. & Theocharis, P.

Hyperinflammatory shock in children during COVID-19 pandemic. _Lancet_ 395, 1607–1608 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Verdoni, L. et al. An outbreak of severe

Kawasaki-like disease at the Italian epicentre of the SARS-CoV-2 epidemic: an observational cohort study. _Lancet_ 395, 1771–1778 (2020). THIS IS AMONG THE FIRST STUDIES SHOWING THAT

SARS-COV-2 CAN INDUCE A SPECTRUM OF MULTISYSTEM INFLAMMATORY SYNDROMES. CAS  PubMed  PubMed Central  Google Scholar  * Syrimi, E. et al. The immune landscape of SARS-CoV-2-associated

multisystem inflammatory syndrome in children (MIS-C) from acute disease to recovery. _iScience_ 24, 103215 (2021). CAS  PubMed  PubMed Central  Google Scholar  * Varga, Z. et al.

Endothelial cell infection and endotheliitis in COVID-19. _Lancet_ 395, 1417–1418 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Su, H. et al. Renal histopathological analysis of 26

postmortem findings of patients with COVID-19 in China. _Kidney Int._ 98, 219–227 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Bryce, C. et al. Pathophysiology of SARS-CoV-2:

targeting of endothelial cells renders a complex disease with thrombotic microangiopathy and aberrant immune response. The Mount Sinai COVID-19 autopsy experience. Preprint at _medRxiv_

https://doi.org/10.1101/2020.05.18.20099960 (2020). Article  Google Scholar  * Timmermans, S. A. M. E. G. et al. C5b9 formation on endothelial cells reflects complement defects among

patients with renal thrombotic microangiopathy and severe hypertension. _J. Am. Soc. Nephrol._ 29, 2234–2243 (2018). CAS  PubMed  PubMed Central  Google Scholar  * Jiang, Y. et al. Blockade

of the C5a–C5aR axis alleviates lung damage in hDPP4-transgenic mice infected with MERS-CoV. _Emerg. Microbes Infec_ 7, 77 (2018). Google Scholar  * Rockx, B. et al. Early upregulation of

acute respiratory distress syndrome-associated cytokines promotes lethal disease in an aged-mouse model of severe acute respiratory syndrome coronavirus infection. _J. Virol._ 83, 7062–7074

(2009). CAS  PubMed  PubMed Central  Google Scholar  * Gralinski, L. E. et al. Complement activation contributes to severe acute respiratory syndrome coronavirus pathogenesis. _mBio_ 9,

e01753–18 (2018). THIS STUDY PROVIDES THE FIRST EVIDENCE IN A SMALL ANIMAL IN VIVO MODEL THAT COMPLEMENT ACTIVATION CONTRIBUTES TO SARS-COV-2-INDUCED AIRWAY PATHOLOGY BY DEMONSTRATING THAT

C3-DEFICIENT, VIRUS-INFECTED MICE HAVE LESS PATHOLOGY COMPARED WITH WILD-TYPE MICE. PubMed  PubMed Central  Google Scholar  * Cameron, M. J. et al. Lack of innate interferon responses during

SARS coronavirus infection in a vaccination and reinfection ferret model. _PLoS ONE_ 7, e45842 (2012). CAS  PubMed  PubMed Central  Google Scholar  * Ip, W. K. E. et al. Mannose-binding

lectin in severe acute respiratory syndrome coronavirus infection. _J. Infect. Dis._ 191, 1697–1704 (2005). CAS  PubMed  Google Scholar  * Ling, M. T. et al. Mannose-binding lectin

contributes to deleterious inflammatory response in pandemic H1N1 and avian H9N2 infection. _J. Infect. Dis._ 205, 44–53 (2012). CAS  PubMed  Google Scholar  * Middeldorp, S. et al.

Incidence of venous thromboembolism in hospitalized patients with COVID-19. _J. Vasc. Surg. Venous Lymphatic Disord._ 9, 536 (2021). Google Scholar  * Iffah, R. & Gavins, F. N. E.

Thromboinflammation in coronavirus disease 2019: the clot thickens. _Brit J. Pharmacol._ https://doi.org/10.1111/bph.15594 (2021). Article  Google Scholar  * Wadman, M., Couzin-Frankel, J.,

Kaiser, J. & Matacic, C. A rampage through the body. _Science_ 368, 356–360 (2020). CAS  PubMed  Google Scholar  * Pellegrini, D. et al. Microthrombi as a major cause of cardiac injury

in COVID-19: a pathologic study. _Circulation_ 143, 1031–1042 (2021). CAS  PubMed  Google Scholar  * Al-Samkari, H. et al. COVID-19 and coagulation: bleeding and thrombotic manifestations of

SARS-CoV-2 infection. _Blood_ 136, 489–500 (2020). CAS  PubMed  Google Scholar  * Pujhari, S., Paul, S., Ahluwalia, J. & Rasgon, J. L. Clotting disorder in severe acute respiratory

syndrome coronavirus 2. _Rev. Med. Virol._ 31, e2177 (2021). CAS  PubMed  Google Scholar  * Lax, S. F. et al. Pulmonary arterial thrombosis in COVID-19 with fatal outcome: results from a

prospective, single-center, clinicopathologic case series. _Ann. Intern. Med._ 173, 350–361 (2020). PubMed  Google Scholar  * Shi, J. et al. Coagulation dysfunction in ICU patients with

coronavirus disease 2019 in Wuhan, China: a retrospective observational study of 75 fatal cases. _Aging_ 13, 1591–1607 (2020). PubMed  PubMed Central  Google Scholar  * Violi, F. et al.

Arterial and venous thrombosis in coronavirus 2019 disease (Covid-19): relationship with mortality. _Intern. Emerg. Med._ 16, 1231–1237 (2021). PubMed  PubMed Central  Google Scholar  *

Pérez-García, F. et al. Age-adjusted endothelial activation and stress index for coronavirus disease 2019 at admission is a reliable predictor for 28-day mortality in hospitalized patients

with coronavirus disease 2019. _Front. Med._ 8, 736028 (2021). Google Scholar  * Contou, D. et al. Causes and timing of death in critically ill COVID-19 patients. _Crit. Care_ 25, 79 (2021).

PubMed  PubMed Central  Google Scholar  * Romanova, E. S. et al. Cause of death based on systematic post-mortem studies in patients with positive SARS-CoV-2 tissue PCR during the COVID-19

pandemic. _J. Intern. Med._ 290, 655–665 (2021). CAS  PubMed  Google Scholar  * Spiezia, L. et al. COVID-19-related severe hypercoagulability in patients admitted to intensive care unit for

acute respiratory failure. _Thromb. Haemost._ 120, 998–1000 (2020). PubMed  PubMed Central  Google Scholar  * Ackermann, M. et al. Pulmonary vascular endothelialitis, thrombosis, and

angiogenesis in Covid-19. _N. Engl. J. Med._ 383, 120–128 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Aimes, R. et al. Endothelial cell serine proteases expressed during vascular

morphogenesis and angiogenesis. _Thromb. Haemost._ 89, 561–572 (2003). CAS  PubMed  Google Scholar  * Nicin, L. et al. Cell type-specific expression of the putative SARS-CoV-2 receptor ACE2

in human hearts. _Eur. Heart J._ 41, 1804–1806 (2020). CAS  PubMed  Google Scholar  * Delorey, T. M. et al. COVID-19 tissue atlases reveal SARS-CoV-2 pathology and cellular targets. _Nature_

595, 107–113 (2021). CAS  PubMed  PubMed Central  Google Scholar  * Vinci, R. et al. From angiotensin-converting enzyme 2 disruption to thromboinflammatory microvascular disease: a paradigm

drawn from COVID-19. _Int. J. Cardiol._ 326, 243–247 (2021). CAS  PubMed  Google Scholar  * Monteil, V. et al. Inhibition of SARS-CoV-2 infections in engineered human tissues using

clinical-grade soluble human ACE2. _Cell_ 181, 905–913.e7 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Wang, P. et al. A cross-talk between epithelium and endothelium mediates

human alveolar–capillary injury during SARS-CoV-2 infection. _Cell Death Dis._ 11, 1042 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Bryce, C. et al. Pathophysiology of SARS-CoV-2:

the Mount Sinai COVID-19 autopsy experience. _Mod. Pathol._ 34, 1456–1467 (2021). CAS  PubMed  PubMed Central  Google Scholar  * Sia, S. F. et al. Pathogenesis and transmission of

SARS-CoV-2 in golden hamsters. _Nature_ 583, 834–838 (2020). WITH THIS PUBLICATION, THE AUTHORS PROVIDE A HAMSTER IN VIVO MODEL OF MILD SARS-COV-2 INFECTION WHICH MIMICS HUMAN DISEASE BETTER

THAN COMPARABLE MOUSE MODELS. CAS  PubMed  PubMed Central  Google Scholar  * Schimmel, L. et al. Endothelial cells are not productively infected by SARS-CoV-2. _Clin. Transl. Immunol._ 10,

e1350 (2021). CAS  Google Scholar  * Goshua, G. et al. Endotheliopathy in COVID-19-associated coagulopathy: evidence from a single-centre, cross-sectional study. _Lancet Haematol._ 7,

e575–e582 (2020). PubMed  PubMed Central  Google Scholar  * O’Sullivan, J. M., Gonagle, D. M., Ward, S. E., Preston, R. J. S. & O’Donnell, J. S. Endothelial cells orchestrate COVID-19

coagulopathy. _Lancet Haematol._ 7, e553–e555 (2020). PubMed  PubMed Central  Google Scholar  * Ma, L. et al. Increased complement activation is a distinctive feature of severe SARS-CoV-2

infection. _Sci. Immunol._ 6, eabh2259 (2021). PubMed Central  Google Scholar  * Cotter, A. H., Yang, S.-J. T., Shafi, H., Cotter, T. M. & Palmer-Toy, D. E. Elevated von Willebrand

factor antigen is an early predictor of mortality and prolonged length of stay for coronavirus disease 2019 (COVID-19) inpatients. _Arch. Pathol. Lab. Med._

https://doi.org/10.5858/arpa.2021-0255-sa (2021). Article  Google Scholar  * Marchetti, M. et al. Endothelium activation markers in severe hospitalized COVID-19 patients: role in mortality

risk prediction. _TH Open_ 5, e253–e263 (2021). PubMed  PubMed Central  Google Scholar  * Fox, S. E. et al. Pulmonary and cardiac pathology in African American patients with COVID-19: an

autopsy series from New Orleans. _Lancet Respir. Med._ 8, 681–686 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Pfister, F. et al. Complement activation in kidneys of patients with

COVID-19. _Front. Immunol._ 11, 594849 (2021). PubMed  PubMed Central  Google Scholar  * Diorio, C. et al. Evidence of thrombotic microangiopathy in children with SARS-CoV-2 across the

spectrum of clinical presentations. _Blood Adv._ 4, 6051–6063 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Noris, M., Mescia, F. & Remuzzi, G. STEC-HUS, atypical HUS and TTP

are all diseases of complement activation. _Nat. Rev. Nephrol._ 8, 622–633 (2012). CAS  PubMed  Google Scholar  * Noris, M. et al. Dynamics of complement activation in aHUS and how to

monitor eculizumab therapy. _Blood_ 124, 1715–1726 (2014). CAS  PubMed  PubMed Central  Google Scholar  * Mat, O. et al. Kidney thrombotic microangiopathy after COVID-19 associated with C3

gene mutation. _Kidney Int. Rep._ 6, 1732–1737 (2021). PubMed  PubMed Central  Google Scholar  * Ville, S. et al. Atypical HUS relapse triggered by Covid-19. _Kidney Int._ 99, 267–268

(2020). PubMed  PubMed Central  Google Scholar  * Riedl, M. et al. Complement activation induces neutrophil adhesion and neutrophil-platelet aggregate formation on vascular endothelial

cells. _Kidney Int. Rep._ 2, 66–75 (2017). PubMed  Google Scholar  * Skeie, J. M., Fingert, J. H., Russell, S. R., Stone, E. M. & Mullins, R. F. Complement component C5a activates ICAM-1

expression on human choroidal endothelial cells. _Invest. Ophth Vis. Sci._ 51, 5336–5342 (2010). Google Scholar  * Kilgore, K. S., Ward, P. A. & Warren, J. S. Neutrophil adhesion to

human endothelial cells is induced by the membrane attack complex: the roles of P-selectin and platelet activating factor. _Inflammation_ 22, 583–598 (1998). THIS IS PIONEERING WORK

CONNECTING MAC FORMATION WITH ENDOTHELIAL CELL AND PLATELET ACTIVATION. CAS  PubMed  Google Scholar  * Foreman, K. E. et al. C5a-induced expression of P-selectin in endothelial cells. _J.

Clin. Invest._ 94, 1147–1155 (1994). CAS  PubMed  PubMed Central  Google Scholar  * Ikeda, K. et al. C5a induces tissue factor activity on endothelial cells. _Thromb. Haemost._ 77, 394–398

(1997). CAS  PubMed  Google Scholar  * Bettoni, S. et al. Interaction between multimeric von Willebrand factor and complement: a fresh look to the pathophysiology of microvascular

thrombosis. _J. Immunol._ 199, 1021–1040 (2017). CAS  PubMed  Google Scholar  * Leppkes, M. et al. Vascular occlusion by neutrophil extracellular traps in COVID-19. _Ebiomedicine_ 58, 102925

(2020). PubMed  PubMed Central  Google Scholar  * Bont, C. M., de, Boelens, W. C. & Pruijn, G. J. M. NETosis, complement, and coagulation: a triangular relationship. _Cell Mol.

Immunol._ 16, 19–27 (2019). PubMed  Google Scholar  * Xu, J. et al. Extracellular histones are major mediators of death in sepsis. _Nat. Med._ 15, 1318–1321 (2009). CAS  PubMed  PubMed

Central  Google Scholar  * Semeraro, F. et al. Extracellular histones promote thrombin generation through platelet-dependent mechanisms: involvement of platelet TLR2 and TLR4. _Blood_ 118,

1952–1961 (2011). CAS  PubMed  PubMed Central  Google Scholar  * Bosmann, M. et al. Extracellular histones are essential effectors of C5aR- and C5L2-mediated tissue damage and inflammation

in acute lung injury. _FASEB J._ 27, 5010–5021 (2013). THIS IS THE FIRST STUDY TO CONNECT LOCAL HISTONE H3 AND H4 RELEASE AS DAMPS THAT DRIVE C5 ACTIVATION AND SUBSEQUENT INDUCTION OF TISSUE

PATHOLOGIES IN RAT AND MOUSE MODELS OF ACUTE LUNG INJURY. CAS  PubMed  PubMed Central  Google Scholar  * Caudrillier, A. et al. Platelets induce neutrophil extracellular traps in

transfusion-related acute lung injury. _J. Clin. Invest._ 122, 2661–2671 (2012). CAS  PubMed  PubMed Central  Google Scholar  * Middleton, E. A. et al. Neutrophil extracellular traps

contribute to immunothrombosis in COVID-19 acute respiratory distress syndrome. _Blood_ 136, 1169–1179 (2020). THIS STUDY IS AMONG THE FIRST TO DEMONSTRATE THAT NEUTROPHILS AND SPECIFICALLY

NEUTROPHIL NETS CONTRIBUTE TO COVID-19 PATHOLOGY. CAS  PubMed  Google Scholar  * Barnes, B. J. et al. Targeting potential drivers of COVID-19: neutrophil extracellular traps. _J. Exp. Med._

217, e20200652 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Perico, L. et al. SARS-CoV-2 spike protein 1 activates microvascular endothelial cells and complement system leading to

thrombus formation. _SSRN Electron. J._ https://doi.org/10.2139/ssrn.3864027 (2021). Article  Google Scholar  * Macor, P. et al. Multiple-organ complement deposition on vascular endothelium

in COVID-19 patients. _Biomed_ 9, 1003 (2021). CAS  Google Scholar  * Walport, M. J. Complement. First two parts. _N. Engl. J. Med._ 344, 1058–1066 (2001). CAS  PubMed  Google Scholar  *

Kolev, M. et al. Diapedesis-induced integrin signaling via LFA-1 facilitates tissue immunity by inducing intrinsic complement C3 expression in immune cells. _Immunity_ 52, 513–527.e8 (2020).

CAS  PubMed  PubMed Central  Google Scholar  * Liszewski, M. K. et al. Intracellular complement activation sustains T cell homeostasis and mediates effector differentiation. _Immunity_ 39,

1143–1157 (2013). CAS  PubMed  PubMed Central  Google Scholar  * Friščić, J. et al. The complement system drives local inflammatory tissue priming by metabolic reprogramming of synovial

fibroblasts. _Immunity_ 54, 1002–1021.e10 (2021). PubMed  Google Scholar  * Pratt, J. R., Abe, K., Miyazaki, M., Zhou, W. & Sacks, S. H. In situ localization of C3 Synthesis in

experimental acute renal allograft rejection. _Am. J. Pathol._ 157, 825–831 (2000). CAS  PubMed  PubMed Central  Google Scholar  * Kulkarni, H. S. et al. Intracellular C3 protects human

airway epithelial cells from stress-associated cell death. _Am. J. Resp. Cell Mol._ 60, 144–157 (2019). THIS STUDY SUPPORTS THE EMERGING CONCEPT OF INTRACELLULAR COMPLEMENT AS KEY DRIVER OF

HEALTHY CELL HOMEOSTASIS BY SHOWING THAT INTRACELLULAR C3 INDUCES CELL LIFE-SAVING AUTOPHAGY IN EPITHELIAL CELLS EXPOSED TO OXIDATIVE STRESS. CAS  Google Scholar  * Arbore, G. et al.

Complement receptor CD46 co-stimulates optimal human CD8+ T cell effector function via fatty acid metabolism. _Nat. Commun._ 9, 4186 (2018). PubMed  PubMed Central  Google Scholar  * Kolev,

M. et al. Complement regulates nutrient influx and metabolic reprogramming during Th1 cell responses. _Immunity_ 42, 1033–1047 (2015). CAS  PubMed  PubMed Central  Google Scholar  * Arbore,

G. et al. T helper 1 immunity requires complement-driven NLRP3 inflammasome activity in CD4+ T cells. _Science_ 352, aad1210 (2016). PubMed  PubMed Central  Google Scholar  * Yan, B. et al.

SARS-CoV-2 drives JAK1/2-dependent local complement hyperactivation. _Sci. Immunol._ 6, eabg0833 (2021). THIS IS THE FIRST STUDY SHOWING THAT SARS-COV-2 INDUCES PATHOLOGICAL C3

HYPERACTIVATION IN INFECTED TYPE II PNEUMOCYTES AND THIS RESULTS IN A SHIFT IN THE BALANCE FROM C3 HOMEOSTATIC ACTIVITY TO PATHOLOGICAL ACTIVITY THAT CONTRIBUTES TO COVID-19 PATHOLOGY.

PubMed  PubMed Central  Google Scholar  * Hajishengallis, G., Reis, E. S., Mastellos, D. C., Ricklin, D. & Lambris, J. D. Novel mechanisms and functions of complement. _Nat. Immunol._

18, 1288–1298 (2017). CAS  PubMed  PubMed Central  Google Scholar  * Hess, C. & Kemper, C. Complement-mediated regulation of metabolism and basic cellular processes. _Immunity_ 45,

240–254 (2016). CAS  PubMed  PubMed Central  Google Scholar  * Kulasekararaj, A. G. et al. Terminal complement inhibition dampens the inflammation during COVID-19. _Brit J. Haematol._ 190,

e141–e143 (2020). CAS  Google Scholar  * Laurence, J. et al. Anti-complement C5 therapy with eculizumab in three cases of critical COVID-19. _Clin. Immunol._ 219, 108555 (2020). CAS  PubMed

  PubMed Central  Google Scholar  * Mastaglio, S. et al. The first case of COVID-19 treated with the complement C3 inhibitor AMY-101. _Clin. Immunol._ 215, 108450 (2020). CAS  PubMed  PubMed

Central  Google Scholar  * Mastellos, D. C. et al. Complement C3 vs C5 inhibition in severe COVID-19: early clinical findings reveal differential biological efficacy. _Clin. Immunol._ 220,

108598 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Urwyler, P. et al. Treatment of COVID-19 with conestat alfa, a regulator of the complement, contact activation and

kallikrein-kinin system. _Front. Immunol._ 11, 2072 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Zelek, W. M. et al. Complement inhibition with the C5 blocker LFG316 in severe

COVID-19. _Am. J. Resp. Crit. Care_ 202, 1304–1308 (2020). CAS  Google Scholar  * Diurno, F. et al. Eculizumab treatment in patients with COVID-19: preliminary results from real life ASL

Napoli 2 Nord experience. _Eur. Rev. Med. Pharm._ 24, 4040–4047 (2020). CAS  Google Scholar  * Cavalli, G. et al. Interleukin-1 blockade with high-dose anakinra in patients with COVID-19,

acute respiratory distress syndrome, and hyperinflammation: a retrospective cohort study. _Lancet Rheumatol._ 2, e325–e331 (2020). PubMed  PubMed Central  Google Scholar  * Merad, M.,

Subramanian, A. & Wang, T. T. An aberrant inflammatory response in severe COVID-19. _Cell Host Microbe_ 29, 1043–1047 (2021). CAS  PubMed  PubMed Central  Google Scholar  *

Kyriazopoulou, E. et al. Early treatment of COVID-19 with anakinra guided by soluble urokinase plasminogen receptor plasma levels: a double-blind, randomized controlled phase 3 trial. _Nat.

Med._ 27, 1752–1760 (2021). CAS  PubMed  PubMed Central  Google Scholar  * WHO Rapid Evidence Appraisal for COVID-19 Therapies (REACT) Working Group. Association between administration of

IL-6 antagonists and mortality among patients hospitalized for COVID-19. _JAMA_ 326, 499–518 (2021). Google Scholar  * RECOVERY Collaborative Group. Dexamethasone in hospitalized patients

with Covid-19. _N. Engl. J. Med._ 384, 693–704 (2020). THIS SEMINAL TRIAL SHOWS THAT ANTI-INFLAMMATORY DRUG APPLICATION CAN AMELIORATE COVID-19. Google Scholar  * REMAP-CAP Investigators.

Interleukin-6 receptor antagonists in critically ill patients with Covid-19. _N. Engl. J. Med._ 384, 1491–1502 (2021). Google Scholar  * Kalil, A. C. et al. Baricitinib plus remdesivir for

hospitalized adults with Covid-19. _N. Engl. J. Med._ 384, 795–807 (2021). CAS  PubMed  Google Scholar  * Cao, Y. et al. Ruxolitinib in treatment of severe coronavirus disease 2019

(COVID-19): a multicenter, single-blind, randomized controlled trial. _J. Allergy Clin. Immun._ 146, 137–146.e3 (2020). CAS  PubMed  Google Scholar  * Cantini, F. et al. Baricitinib therapy

in COVID-19: a pilot study on safety and clinical impact. _J. Infect._ 81, 318–356 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Bronte, V. et al. Baricitinib restrains the immune

dysregulation in severe COVID-19 patients. _J. Clin. Invest._ 130, 6409–6416 (2020). CAS  PubMed  PubMed Central  Google Scholar  * Annane, D. et al. Eculizumab as an emergency treatment for

adult patients with severe COVID-19 in the intensive care unit: a proof-of-concept study. _Eclinicalmedicine_ 28, 100590 (2020). PubMed  PubMed Central  Google Scholar  * Declercq, J. et

al. Zilucoplan in patients with acute hypoxic respiratory failure due to COVID-19 (ZILU-COV): A structured summary of a study protocol for a randomised controlled trial. _Trials_ 21, 934

(2020). CAS  PubMed  PubMed Central  Google Scholar  * Vlaar, A. P. J. et al. Anti-C5a antibody IFX-1 (vilobelimab) treatment versus best supportive care for patients with severe COVID-19

(PANAMO): an exploratory, open-label, phase 2 randomised controlled trial. _Lancet Rheumatol._ 2, e764–e773 (2020). PubMed  PubMed Central  Google Scholar  * FDA. COVID-19 vaccines.

https://www.fda.gov/emergency-preparedness-and-response/coronavirus-disease-2019-covid-19/covid-19-vaccines (2021). * EMA. COVID-19 vaccines.

https://www.ema.europa.eu/en/human-regulatory/overview/public-health-threats/coronavirus-disease-covid-19/treatments-vaccines/vaccines-covid-19/covid-19-vaccines-authorised (2021). *

Bloomberg. Bloomberg vaccine tracker. https://www.bloomberg.com/graphics/covid-vaccine-tracker-global-distribution/#global (2021). * Greinacher, A. et al. Thrombotic thrombocytopenia after

ChAdOx1 nCov-19 vaccination. _N. Engl. J. Med._ 384, 2092–2101 (2021). CAS  PubMed  Google Scholar  * Choi, P. Y.-I. Thrombotic thrombocytopenia after ChAdOx1 nCoV-19 vaccination. _N. Engl.

J. Med._ 385, e11 (2021). PubMed  Google Scholar  * Muir, K.-L., Kallam, A., Koepsell, S. A. & Gundabolu, K. Thrombotic thrombocytopenia after Ad26.COV2.S vaccination. _N. Engl. J. Med._

384, 1964–1965 (2021). CAS  PubMed  Google Scholar  * Sadoff, J., Davis, K. & Douoguih, M. Thrombotic thrombocytopenia after Ad26.COV2.S vaccination — response from the manufacturer.

_N. Engl. J. Med._ 384, 1965–1966 (2021). CAS  PubMed  Google Scholar  * See, I. et al. US case reports of cerebral venous sinus thrombosis with thrombocytopenia after Ad26.COV2.S

vaccination, March 2 to April 21, 2021. _JAMA_ 325, 2448–2456 (2021). CAS  PubMed  Google Scholar  * Graf, T. et al. Immediate high-dose intravenous immunoglobulins followed by direct

thrombin-inhibitor treatment is crucial for survival in Sars-Covid-19-adenoviral vector vaccine-induced immune thrombotic thrombocytopenia VITT with cerebral sinus venous and portal vein

thrombosis. _J. Neurol._ 268, 4483–4485 (2021). CAS  PubMed  PubMed Central  Google Scholar  * Pomara, C. et al. Post-mortem findings in vaccine-induced thrombotic thombocytopenia.

_Haematologica_ 106, 2291–2293 (2021). CAS  PubMed  PubMed Central  Google Scholar  * Hill, A., DeZern, A. E., Kinoshita, T. & Brodsky, R. A. Paroxysmal nocturnal haemoglobinuria. _Nat.

Rev. Dis. Prim._ 3, 17028 (2017). PubMed  Google Scholar  * Scheuerle, A. F., Serbecic, N. & Beutelspacher, S. C. Paroxysmal nocturnal hemoglobinuria may cause retinal vascular

occlusions. _Int. Ophthalmol._ 29, 187–190 (2009). PubMed  Google Scholar  * Tiede, A. et al. Prothrombotic immune thrombocytopenia after COVID-19 vaccine. _Blood_ 138, 350–353 (2021). CAS 

PubMed  PubMed Central  Google Scholar  * CDC COVID-19 Response Team. Preliminary estimates of the prevalence of selected underlying health conditions among patients with coronavirus disease

2019 — United States, February 12–March 28, 2020. _MMWR Morb. Mortal. Wkly Rep._ 69, 382–386 (2020). PubMed Central  Google Scholar  * Jordan, R. E., Adab, P. & Cheng, K. K. Covid-19:

risk factors for severe disease and death. _BMJ_ 368, m1198 (2020). PubMed  Google Scholar  * Wenzel, U. O., Kemper, C. & Bode, M. The role of complement in arterial hypertension and

hypertensive end organ damage. _Br. J. Pharmacol._ 178, 2849–2862 (2021). CAS  PubMed  Google Scholar  * Lappegård, K. T. et al. A vital role for complement in heart disease. _Mol. Immunol._

61, 126–134 (2014). PubMed  Google Scholar  * Arason, G. J. et al. Smoking and a complement gene polymorphism interact in promoting cardiovascular disease morbidity and mortality. _Clin.

Exp. Immunol._ 149, 132–138 (2007). CAS  PubMed  PubMed Central  Google Scholar  * Copenhaver, M., Yu, C.-Y. & Hoffman, R. P. Complement components, C3 and C4, and the metabolic

syndrome. _Curr. Diabetes Rev._ 15, 44–48 (2018). Google Scholar  * Goronzy, J. J. & Weyand, C. M. Understanding immunosenescence to improve responses to vaccines. _Nat. Immunol._ 14,

428–436 (2013). CAS  PubMed  PubMed Central  Google Scholar  * Pietrobon, A. J., Teixeira, F. M. E. & Sato, M. N. Immunosenescence and inflammaging: risk factors of severe COVID-19 in

older people. _Front. Immunol._ 11, 579220 (2020). CAS  PubMed  PubMed Central  Google Scholar  * da Costa, M. G. et al. Age and sex-associated changes of complement activity and complement

levels in a healthy Caucasian population. _Front. Immunol._ 9, 2664 (2018). Google Scholar  * Cardoso, A. L. et al. Towards frailty biomarkers: candidates from genes and pathways regulated

in aging and age-related diseases. _Ageing Res. Rev._ 47, 214–277 (2018). CAS  PubMed  Google Scholar  * Shi, Q. et al. Complement C3 deficiency protects against neurodegeneration in aged

plaque-rich APP/PS1 mice. _Sci. Transl. Med._ 9, eaaf6295 (2017). PubMed  PubMed Central  Google Scholar  * Naito, A. T. et al. Complement C1q activates canonical Wnt signaling and promotes

aging-related phenotypes. _Cell_ 149, 1298–1313 (2012). CAS  PubMed  PubMed Central  Google Scholar  Download references ACKNOWLEDGEMENTS Research described in this article was supported (in

part) by the Intramural Research Program of the US National Institute of Diabetes and Digestive and Kidney Diseases (project number ZIA/DK075149 (B.A).) and the US National Heart, Lung, and

Blood Institute (project number ZIA/Hl006223 (C.K.)), by an FWO COVID-19 grant to B.N.L., and by a COVID-19 grant from the Fondazione Italcementi Cav. Lav. CARLO PESENTI to M.N. AUTHOR

INFORMATION AUTHORS AND AFFILIATIONS * Immunoregulation Section, Kidney Diseases Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health,

Bethesda, MD, USA Behdad Afzali * Istituto di Ricerche Farmacologiche “Mario Negri”, Clinical Research Center for Rare Diseases “Aldo e Cele Daccò”, Ranica, Italy Marina Noris * “Centro Anna

Maria Astori”, Bergamo, Italy Marina Noris * Laboratory of Immunoregulation and Mucosal Immunology, VIB-UGent Center for Inflammation Research, Ghent, Belgium Bart N. Lambrecht * Department

of Internal Medicine and Pediatrics, Ghent University, Ghent, Belgium Bart N. Lambrecht * Department of Pulmonary Medicine, Erasmus University Medical Center, Rotterdam, Netherlands Bart N.

Lambrecht * Complement and Inflammation Research Section (CIRS), National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, USA Claudia Kemper * Institute for

Systemic Inflammation Research, University of Lübeck, Lübeck, Germany Claudia Kemper Authors * Behdad Afzali View author publications You can also search for this author inPubMed Google

Scholar * Marina Noris View author publications You can also search for this author inPubMed Google Scholar * Bart N. Lambrecht View author publications You can also search for this author

inPubMed Google Scholar * Claudia Kemper View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS The authors contributed equally to all aspects of

the article. CORRESPONDING AUTHORS Correspondence to Behdad Afzali, Marina Noris, Bart N. Lambrecht or Claudia Kemper. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no

competing interests. ADDITIONAL INFORMATION PEER REVIEW INFORMATION _Nature Reviews Immunology_ thanks M. Bosmann and the other, anonymous, reviewer(s) for their contribution to the peer

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and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Afzali, B., Noris, M., Lambrecht, B.N. _et al._ The state of complement in COVID-19. _Nat Rev Immunol_ 22, 77–84 (2022).

https://doi.org/10.1038/s41577-021-00665-1 Download citation * Accepted: 25 November 2021 * Published: 15 December 2021 * Issue Date: February 2022 * DOI:

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