Complement lectin pathway activation is associated with COVID-19 disease severity, independent of MBL2 genotype subgroups

Control of complement-induced inflammatory responses to SARS-CoV-2 infection by anti-SARS-CoV-2 antibodies

Dysregulated immune responses contribute to the excessive and uncontrolled inflammation observed in severe COVID-19. However, how immunity to SARS-CoV-2 is induced and regulated remains unclear. Here, we uncover the role of the complement system in the induction of innate and adaptive immunity to SARS-CoV-2. Complement rapidly opsonizes SARS-CoV-2 particles via the lectin pathway. Complement-opsonized SARS-CoV-2 efficiently induces type-I interferon and pro-inflammatory cytokine responses via activation of dendritic cells, which are inhibited by antibodies against the complement receptors (CR) 3 and 4. Serum from COVID-19 patients, or monoclonal antibodies against SARS-CoV-2, attenuate innate and adaptive immunity induced by complement-opsonized SARS-CoV-2. Blocking of CD32, the FcγRII antibody receptor of dendritic cells, restores complement-induced immunity. These results suggest that opsonization of SARS-CoV-2 by complement is involved in the induction of innate and adaptive immunity to SARS-CoV-2 in the acute phase of infection. Subsequent antibody responses limit inflammation and restore immune homeostasis. These findings suggest that dysregulation of the complement system and FcγRII signaling may contribute to severe COVID-19.

mRNA-LNP COVID-19 Vaccine Lipids Induce Complement Activation and Production of Proinflammatory Cytokines: Mechanisms, Effects of Complement Inhibitors, and Relevance to Adverse Reactions

A small fraction of people vaccinated with mRNA-lipid nanoparticle (mRNA-LNP)-based COVID-19 vaccines display acute or subacute inflammatory symptoms whose mechanism has not been clarified to date. To better understand the molecular mechanism of these adverse events (AEs), here, we analyzed in vitro the vaccine-induced induction and interrelations of the following two major inflammatory processes: complement (C) activation and release of proinflammatory cytokines. Incubation of Pfizer-BioNTech's Comirnaty and Moderna's Spikevax with 75% human serum led to significant increases in C5a, sC5b-9, and Bb but not C4d, indicating C activation mainly via the alternative pathway. Control PEGylated liposomes (Doxebo) also induced C activation, but, on a weight basis, it was ~5 times less effective than that of Comirnaty. Viral or synthetic naked mRNAs had no C-activating effects. In peripheral blood mononuclear cell (PBMC) cultures supplemented with 20% autologous serum, besides C activation, Comirnaty induced the secretion of proinflammatory cytokines in the following order: IL-1α < IFN-γ < IL-1β < TNF-α < IL-6 < IL-8. Heat-inactivation of C in serum prevented a rise in IL-1α, IL-1β, and TNF-α, suggesting C-dependence of these cytokines' induction, although the C5 blocker Soliris and C1 inhibitor Berinert, which effectively inhibited C activation in both systems, did not suppress the release of any cytokines. These findings suggest that the inflammatory AEs of mRNA-LNP vaccines are due, at least in part, to stimulation of both arms of the innate immune system, whereupon C activation may be causally involved in the induction of some, but not all, inflammatory cytokines. Thus, the pharmacological attenuation of inflammatory AEs may not be achieved via monotherapy with the tested C inhibitors; efficacy may require combination therapy with different C inhibitors and/or other anti-inflammatory agents.

Emerging role of complement in COVID-19 and other respiratory virus diseases

The complement system, a key component of innate immunity, provides the first line of defense against bacterial infection; however, the COVID-19 pandemic has revealed that it may also engender severe complications in the context of viral respiratory disease. Here, we review the mechanisms of complement activation and regulation and explore their roles in both protecting against infection and exacerbating disease. We discuss emerging evidence related to complement-targeted therapeutics in COVID-19 and compare the role of the complement in other respiratory viral diseases like influenza and respiratory syncytial virus. We review recent mechanistic studies and animal models that can be used for further investigation. Novel knockout studies are proposed to better understand the nuances of the activation of the complement system in respiratory viral diseases.

The Lectin Pathway of the Complement System-Activation, Regulation, Disease Connections and Interplay with Other (Proteolytic) Systems

The complement system is the other major proteolytic cascade in the blood of vertebrates besides the coagulation-fibrinolytic system. Among the three main activation routes of complement, the lectin pathway (LP) has been discovered the latest, and it is still the subject of intense research. Mannose-binding lectin (MBL), other collectins, and ficolins are collectively termed as the pattern recognition molecules (PRMs) of the LP, and they are responsible for targeting LP activation to molecular patterns, e.g., on bacteria. MBL-associated serine proteases (MASPs) are the effectors, while MBL-associated proteins (MAps) have regulatory functions. Two serine protease components, MASP-1 and MASP-2, trigger the LP activation, while the third component, MASP-3, is involved in the function of the alternative pathway (AP) of complement. Besides their functions within the complement system, certain LP components have secondary ("moonlighting") functions, e.g., in embryonic development. They also contribute to blood coagulation, and some might have tumor suppressing roles. Uncontrolled complement activation can contribute to the progression of many diseases (e.g., stroke, kidney diseases, thrombotic complications, and COVID-19). In most cases, the lectin pathway has also been implicated. In this review, we summarize the history of the lectin pathway, introduce their components, describe its activation and regulation, its roles within the complement cascade, its connections to blood coagulation, and its direct cellular effects. Special emphasis is placed on disease connections and the non-canonical functions of LP components.

High production MBL2 polymorphisms protect against COVID-19 complications in critically ill patients: A retrospective cohort study

Mannose-binding lectin (MBL) binds to SARS-CoV-2, inhibits infection of susceptible cells, and activates the complement system via the lectin pathway. In this study, we investigated the association of MBL2 polymorphisms with the risk of hospitalization and clinical worsening in patients with COVID-19. A total of 550 patients with COVID-19 were included (94 non-hospitalized and 456 hospitalized). Polymorphisms in MBL2 exon 1 (codons 52, 54 and 57) and promoter region (-550, -221, and +4) were determined by real-time PCR. MBL and complement proteins were measured by Luminex. A higher frequency of the H/H genotype and the HYPA haplotype was observed in non-hospitalized patients when compared to hospitalized. In addition, critically ill patients carrying haplotypes associated with high MBL levels (HYPA/HYPA + HYPA/LYPA + HYPA/LYQA + LYPA/LYQA + LYPA/LYPA + LYQA/LYQA + LXPA/HYPA + LXPA/LYQA + LXPA/LYPA) were protected against lower oxygen saturation levels (P = 0.02), use of invasive ventilation use (P = 0.02, OR 0.38), and shock (P = 0.01, OR 0.40), independent of other potential confounders adjusted by multivariate analysis. Our results suggest that variants in MBL2 associated with high MBL levels may play a protective role in the clinical course of COVID-19.

Differentiating between activation via the lectin or the classical complement pathway in patients with systemic lupus erythematosus

Complement activation is a hallmark of systemic lupus erythematosus (SLE) and can proceed through the classical (CP), lectin (LP), or alternative pathway (AP). When managing SLE patients, pathway-specific complement activation is rarely monitored as clinical assays are unavailable. In this study, we aim to differentiate between CP- or LP-mediated complement activation in SLE patients by quantifying pathway-specific protein complexes, namely C1s/C1-inhibitor (C1-INH) (CP-specific activation) and MASP-1/C1-INH (LP-specific activation). Levels for both complexes were assessed in 156 SLE patients and 50 controls using two newly developed ELISAs. We investigated whether pathway-specific complement activation was associated with disease activity and lupus nephritis (LN). Disease activity stratification was performed using SLEDAI scores assessed at inclusion. C1s/C1-INH concentrations were significantly increased in active SLE patients (SLEDAI ≥6) when compared with SLE patients with low disease activity (SLEDAI <6, P < 0.01) and correlated with SLEDAI score (r = .29, P < 0.01). In active LN, MASP-1/C1-INH plasma concentrations were significantly increased compared with nonactive LN (P = 0.02). No differences in MASP-1/C1-INH plasma concentrations were observed between active SLE patients and patients with low disease activity (P = 0.11) nor did we observe a significant correlation with disease activity (r = 0.12, P = 0.15). Our data suggest that the CP and the LP are activated in SLE. The CP is activated in active SLE disease, whereas activation of the LP might be more specific to disease manifestations like LN. Our results warrant further research into specific complement pathway activation in SLE patients to potentially improve specific-targeted and tailored-treatment approaches.

Optimizing the use of vilobelimab for the treatment of COVID-19

INTRODUCTION

On 4 April 2023i4 April 2023, the United States Food and Drug Administration issued an emergency use authorization for the use of vilobelimab (GohibicTM) for the treatment of COVID-19 in hospitalized adults when initiated within 48 hours of receiving invasive mechanical ventilation or extracorporeal membrane oxygenation.

AREAS COVERED

Vilobelimab is a human-mouse chimeric IgG4 kappa antibody that targets human complement component 5a, a part of the immune system that is thought to play an important role in the systemic inflammation due to SARS-CoV-2 infection that leads to COVID-19 disease progression.

EXPERT OPINION

A pragmatic, adaptive, randomized, multicenter phase II/III study evaluating vilobelimab for the treatment of severe COVID-19 found that patients receiving invasive mechanical ventilation and usual care who were treated with vilobelimab had a lower risk of death by day 28 and day 60 compared to those receiving placebo. This manuscript explores what is known about vilobelimab and explores how this treatment may be used in the future to treat severe COVID-19.