Complement augments antibody neutralization of SARS-CoV-2 variants

The complement system is a major effector system of the humoral immune response, but its capacity to cooperate with antibodies to inhibit the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a host remains poorly characterized. Here, we combined authentic virus neutralization, antibody engineering, and single viral particle analysis by flow virometry to show that complement enhances antibody neutralization of SARS-CoV-2. Addition of complement increased neutralization titers of sera from convalescent individuals (n = 14), vaccinated individuals (n = 15), and individuals with hybrid immunity (n = 15) against D614G, BA.1, XBB.1.5, and JN.1 variants by up to 42-fold. D614G and BA.1 displayed a high level of neutralization, which was only slightly improved by adding complement. Conversely, neutralization of XBB.1.5 and JN.1 was weak but strongly enhanced by complement. Among the 32 JN.1 nonneutralizers, 22 regained detectable neutralization in the presence of complement. The enhancement mainly relied on cross-reactive anti-S2 antibodies, but monoclonal antibodies targeting the receptor binding domain and N-terminal domain could also be augmented. Viral inhibition required antibody Fc hexamerization, C1q recruitment, and C3 deposition, but it was independent of the membrane attack complex formed by C9. Consistently, complement components were deposited at the surfaces of viral particles with no detectable virolysis. In sera from individuals with severe COVID-19, both complement-mediated neutralization and anti-S2 antibody levels were associated with improved survival. Overall, our data show that complement potentiates the ability of antispike antibodies to neutralize immune-evasive SARS-CoV-2 variants.