INVESTIGATION OF THE HUMORAL IMMUNE RESPONSES IN SARS-COV-2 INFECTED AND VACCINATED INDIVIDUALS

Abstract

Background: Protective and lasting immunity to viral infections or vaccines are usually achieved through the combined actions of both cellular and humoral immune responses. Although there exists clear evidence supporting the importance of neutralizing antibodies (NAbs) in conferring protection in SARS-CoV-2 infected and vaccinated individuals, these NAbs responses wane rapidly over time and appear to be less effective against the new SARS-CoV-2 variants. In that case and given the myriad roles of antibodies, it can be presumed that antibodies could also mediate protection against SARS-CoV-2 via effector mechanisms such as antibody-dependent cellular cytotoxicity (ADCC), providing new correlates of protective immunity. Objectives: This project aims to investigate and characterize the humoral immune responses in previously infected SARS-CoV-2 patients and COVID-19 vaccinated individuals to identify the correlates of protective immunity. Methods: First, the performance of nine IgM and IgG ELISA kits, five automated immunoassays, and two surrogate virus neutralization tests (sVNT) was assessed using a wide range of sera samples collected from previously infected and vaccinated individuals. Then, humoral responses, including binding and NAbs, and ADCC activity against different SARS-CoV-2 antigens were characterized for a selected group of sera samples (n=90 from infected; n=77 from vaccinated individuals). Results: We observed a heterogeneous assay performance for IgM serological assays, whereas IgG assays targeting the SARS-CoV-2 spike (S) protein showed a good to excellent overall performance. Both sVNTs targeting the SARS-CoV-2 receptor binding domain (RBD) demonstrated excellent performance and correlation with the pseudovirus neutralization reference test. These results suggest that that these assays could serve as robust platforms for reliable and high-throughput screening of RBDtargeting NAbs. As for ADCC responses, it was shown that ADCC was elicited within the first 10 days post-infection and 30 days post-vaccination after the second dose and remained detectable after two months in both groups. Patients with severe disease had significantly higher ADCC levels against both the S and nucleocapsid (N) proteins than asymptomatic and vaccinated individuals. Notably, no significant difference in the ADCC levels was observed between individuals vaccinated with BNT162b2 and mRNA-1273 vaccines, despite binding antibody and NAbs responses being significantly higher in mRNA-1273-vaccinated individuals. Additionally, there was no significant difference in ADCC levels across the different age groups and between genders. Conclusion: Several commercial immunoassays were identified as reliable assays for the detection of anti-SARS-CoV-2 humoral responses. The extent to which these conventional serology assays correlate with neutralization was shown to depend on the targeted antigen and on the time of sample collection post disease onset. Further, significant Fc?R-binding ADCC response were detected in infected and vaccinated individuals but was more prominent in symptomatic COVID-19 patients, suggesting that ADCC assay could be used to estimate the extra-neutralization activity of antibodies targeting SARS-CoV-2.