The beneficial and pathogenic roles of complement in COVID-19

Introduction

The Coronavirus Disease 2019 (COVID-19) pandemic has created a health care crisis unmatched in our lifetimes. Caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), over 28 million cases have been reported worldwide, including more than 7.1 million infections in the United States (https://coronavirus.jhu.edu/map.html). This disease has resulted in at least one million deaths globally, over 204,000 of which are in the US. COVID-19 has a widely varied presentation (see Table 1 and ^1–4) – from asymptomatic carriers (20 to 40%), a common cold/influenza syndrome (30 to 70%) or a severe life-threatening disease (<10%). The last being characterized by respiratory failure, a hyperinflammatory internal milieu and multiorgan thrombotic complications.

Table 1.

Clinical Classes of COVID-19 Infection

(1) Asymptomatic	20 – 40%	Immune system shuts off the virus – no clinical signs or symptoms of infection – immunity induced – “natural” vaccination Complement activation good for host
(2) Upper Respiratory Tract (cold-like syndrome)	30 – 70%	Virus replicates in upper airway tissues leading to sore throat, cough, ± fever, sneezing, alteration in taste and smell, – “typical cold” syndrome – usually lasts 2 to 5 days – recovery in 2 to 3 weeks Complement activation good for host
(3) Moderately Severe Disease	5 – 10%	Viral load is greater – extension to deeper parts of the lung (pneumonia) and possibly to other tissues with prominent inflammatory picture Complement activation poor for host
(4) Severe Disease	1 – 5%   4A.   4B.	Heavy viral load – extension to extrapulmonary tissues, especially endothelial cell populations – thromboembolic phenomena Virus is never adequately controlled Virus eventually controlled and eliminated, but often with residual effects Complement activation poor for host

The complement system has recently gained interest as a contributing player to certain pathogenic features of severe COVID-19 infection particularly relative to a thrombotic microangiopathic (TMA) picture. Autopsy reports have also identified complement-mediated pathologic features consistent with TMA manifestations such as endothelial cell abnormalities and complement fragment deposition in the lungs, skin and kidney^1,2. In addition, recent transcriptional and genetic loci studies have provided further evidence for the involvement of the complement system in COVID-19. Indeed, therapies targeting complement components demonstrate preliminary success in treating severe COVID-19^3–5. But what has been somewhat ignored however is the antiviral contribution of the complement system in a response to COVID-19, particularly early in the infection.

Herein, we briefly summarize the complement system and its functions in immunity and disease, presenting data supporting both the requirement of complement to resolve COVID-19, and how its overactivation later in severe disease could drive multiorgan damage characteristic of fatal COVID-19. For the sake of brevity and space limitations, we have primarily referenced a limited number of reviews and only several citations in which there is a focus on the complement system (reviews^2,6–10 and original reports^{1,3–5,11–13}).

The complement system in immunity and disease

The complement system is a proteolytic cascade initiated by three pathways (classical, lectin and alternative), each uniquely triggered to generate a potent, highly regulated, innate immune response and to set the stage for a prompt and decisive adaptive immune response: 1) membrane perturbation featuring C4b- and C3b-mediated opsonization/phagocytosis as well as a lytic process mediated by the membrane attack complex (MAC, C5b-9), and 2) generation of a proinflammatory state largely mediated via the anaphylatoxins, C3a and C5a. Using these same two general mechanisms, the complement system also facilitates the clearance of apoptotic material and cellular debris. Further, intracellular complement activation enables cells to modulate metabolic pathways and thereby regulate immune responsiveness ⁶.

In systemic lupus erythematosus (SLE) and related autoimmune diseases, immune complexes generated by autoantibodies drive Type II and III hypersensitivity reactions which leverage primarily the classical pathway activation to initiate destructive inflammatory responses ^2,7. Interestingly, the failure to clear apoptotic debris, such as in complete C1q, C4 or C2 deficiency, is strongly predisposing to SLE ⁸. In age-related macular degeneration (AMD) and atypical hemolytic uremic syndrome (aHUS), loss-of-function (commonly haploinsufficiency) of complement regulatory proteins or gain-of-function in complement activating components promote excessive alternative pathway engagement, leading to retinal damage in AMD and microthrombi featuring endothelial injury in aHUS ^2,7.

Early control of SARS-CoV-2 likely requires complement activation

In the early phase of the innate immune response in COVID-19, a robust antiviral response to SARS-CoV-2 occurs in which the complement system plays an important role. Natural and cross-reacting Abs and lectins trigger complement activation to destroy complement coated virus and block viral entry. Mice infected with the related SARS-CoV virus generate numerous C3 activation products (C3a, C3b, iC3b, C3c, C3d) in the lung within a day ⁹. Infection of cells by SARS-CoV-2 may promote C3 activation, as suggested in a preprint demonstrating that the N protein of SARS-CoV-2 activated the mannan-binding lectin-associated serine protease (MASP-2)¹⁴. In another report utilizing an in vitro system, spike proteins 1 and 2 led to activation primarily of the alternative pathway on human cells¹³. These types of innate immune responses coupled with a rapid adaptive response lead to either no clinical signs of an infection or a common cold/influenza-like syndrome.

Viruses, as might be expected, have developed evasion tactics to limit complement activation ¹⁰. For example, poxviruses express a protein that is structurally and functionally similar to human complement regulatory proteins and flaviviruses produce proteins to prevent the engagement of complement proteins with regulators. Other pathogens “hijack” our own regulators to protect themselves. Flaviviruses produce proteins that down-regulate the complement system’s activating capabilities. It remains unknown whether coronaviruses also possess antiviral activity.

Inadequate complement activation characterizes late COVID-19

Inadequate control of SARS-CoV-2 replication early in infection will lead to a heavier viral load and, an exuberant immune response will naturally follow, including complement activation. A robust proinflammatory cytokine (IL-6, IL-8) response has been observed in severe infections (often associated with a so-called “cytokine release or storm syndrome”). This result could be secondary in part to the viral damage to infected tissues (even though viral replication is halted) but an exuberant immune response to the accumulating debris occurs ^3,4,9.

Further, data implicating a prominent role for the complement system in severe COVID-19 came from early clinicopathologic studies. C5b-9, C4d and MASP-2 deposition was noted in affected tissues, including the skin in those with reticuliform and purpuric lesions and the lung microvasculature in those who died due to respiratory failure.¹ While COVID-19 lung pathology is distinct from typical adult respiratory distress syndrome (ARDS), complement activation within the lung has commonly been observed in ARDS suggesting a possible common etiology (reviewed in ¹²). Complement activation fragments have also been observed to be elevated in serum of patients with severe COVID-19 (Reviewed in ²). Additionally, histopathologic features with similarities to other complement-mediated diseases indicate that complement deposition is a pathologic actor in severe COVID-19. Endothelial cell abnormalities, such as cellular swelling with foamy degeneration in the setting of a TMA, have been observed in numerous organs, consistent with C5b-9 mediated injury and a hypercoagulable state. These data are consistent with the murine model of SARS-CoV, in which C3-deficient mice experienced less lung inflammation and injury⁹.

One mechanism by which complement may be overactivated in severe COVID-19 lies in the interplay between neutrophils and complement. Activated neutrophils generate extracellular chromatin-rich structures called neutrophil traps (NETs), which bind pathogenic material. NETs promote alternative pathway activation as they contain C3, factor B and properdin. Exuberant neutrophil activation and NET formation have been noted in severe COVID-19 patients (reviewed in ²).

Conclusion

It is clear that complement activation occurs in COVID-19 and likely serves different roles depending on the time course of the infection. COVID-19 clinical management early on needs to support innate immunity including complement activation against the virus but curtail subsequent potential damage to the tissues or organs in the severe syndrome (assuming the viral replication has been largely stopped). Early on, complement is required to assist antiviral responses, such as clearing of complement coated virions. Later, in the subset of patients with severe (too much virus and/or tissue damage) COVID-19, complement drives several of the pathogenic features observed in these patients. This has substantial implications in the use of complement therapeutics. Thus, the timing of administration may lead to poor or improved outcomes. On one hand, complement inhibition in early disease may lead to worse clinical outcomes or poor protective responses, while in late disease, complement inhibition may improve outcomes. Indeed, preliminary reports of complement inhibition in severe COVID-19 have been encouraging. For example, a novel inhibitor to C3, AMY-101, which prevents C3 cleavage to C3a and C3b, has anecdotal success in severe COVID-19. This had led to an active phase II trial in COVID-19 patients with respiratory distress^3,5. Furthermore, monoclonal antibodies targeting C5 activation (eculizumab and ravulizumab) have also demonstrated efficacy in COVID-19 with several active phase III clinical trials currently recruiting^4,5. Additional answers will emerge as these and other formal clinical trials start reporting their results, providing an improved understanding of how to best manipulate the complement system in COVID-19.

Comments:

1. Group 4A is where at least, early on in the infection, immune enhancement might be indicated.

2. Group 4B is where it could be advantageous, especially if the viral replication has been controlled, to dampen down the inflammatory response.

3. This same good and bad timing needs to be considered in two other settings. One is the transfusion of plasma from a recovered patient that will form immune complexes with the intact virus (good) but also with viral remnants and damaged tissues to potentially exacerbate the situation. A similar consideration is the use of steroids (or other immune inflammatory suppressions) that also could inhibit the immune response to COVID-19. Here again, the timing of the administration could be critical as is knowledge relative to the viral load and replicating capability.