title: Understanding the Role of the Complement System in Ebola Virus and SARS-CoV-2 Pathogenesis
Creators: name: Mellors, Jack
Publisher: name: University of Liverpool Repository
Year: 2022

description: The role of the complement system in viral infections is often complex, with significant implications for pathogenesis and disease. The complement system can form part of the early innate immune response through the binding of glycosylated viral proteins, or through spontaneous activation on viral surfaces. The complement system can also be activated by antibodies in complex with viral antigens. These mechanisms have the potential to inhibit virus interactions with host proteins, mediate opsonisation, promote inflammation and chemotaxis, cause the agglutination of virions, lyse virions, and lyse virus-infected cells. Despite the diverse and significant roles of the complement system in viral infection, it is a relatively under-researched aspect of antiviral immunity. The complement system has been associated with more severe symptoms and fatal outcomes of Ebola virus (EBOV) disease (EVD) and Coronavirus disease (COVID)-19. However, the underlying mechanisms of the complement system in response to EBOV and SARS-CoV-2 (the causative agent of COVID-19), and the wider implications for immunity, are poorly understood. We first investigated the antibody-independent mechanisms of the complement system in response to Ebolavirus and Coronavirus glycoproteins (GPs), to better understand the underlying mechanisms of complement activation in the early stages of infection. Using novel ELISAs and western blot assays, we identified MBL binding to a range of Ebolavirus and Coronavirus GPs, and demonstrated their potential to activate the complement system, eventuating in formation of the membrane attack complex (MAC). We also utilised PCR assays, next-generation sequencing, and LC-MS/MS, to identify potential differences in the structure and expression of complement proteins in EVD survivors. We found broad diversity in the SNPs of several complement proteins but were restricted by the sample size to determine significance. These findings showed potential mechanisms for antibody-independent complement activation that could influence the pathogenesis of EBOV and SARS-CoV-2 in the early stages of infection. Next, we evaluated the antibody-dependent mechanisms of the complement system. We developed novel flow cytometry assays to assess the ability of EVD convalescent, COVID-19 convalescent, and SARS-CoV-2 vaccinated plasma to mediate antibody-dependent complement deposition (ADCD) in response to the respective Ebolavirus and Coronavirus GPs. We found a differential response in ADCD between EVD plasma that was influenced by neutralisation titre, IgG titre, and/or the Ebolavirus GP present. For SARS-CoV-2, we found that ChAdOx1 nCoV-19 vaccine-induced antibodies could mediate ADCD, and that levels of ADCD correlated with disease severity in COVID-19 convalescent individuals. These findings are important for understanding the variability of responses in mediating the complement system, with particular relevance to recrudescence, re-infection, infection post-vaccination, and cross-reactivity. Lastly, we evaluated the significance of these antibody-independent and antibody-dependent complement mechanisms on wild-type EBOV and SARS-CoV-2 neutralisation. Independent of antibodies, the complement system did not influence virus neutralisation. However, in the presence of low-neutralising, virus-specific antibodies, we observed an enhancement in neutralisation of both EBOV and SARS-CoV-2 when the complement system was present. Neutralisation assays are a fundamental aspect of identifying therapeutic antibodies and determining correlates of protection, with further implications for vaccine licensure. Our observed effect of the complement system on neutralisation has implications for the initial assessments of therapeutic candidates, evaluating vaccine-induced immune responses, defining correlates of protection, and could be a consideration for the therapeutic use of complement inhibitors.