Quantitative, Epitope-specific, Serological Screening of COVID-19 Patients Using a Novel Multiplexed Array-based Immunoassay Platform

Following the COVID-19 pandemic outbreak in late 2019, a large number of antibody tests were developed for use in seroprevalence studies aimed at determining the extent of current or previous SARS-CoV-2 virus infections in a given population. The vast majority of these tests are qualitative and use a single target for antibody detection, incorporating either full-length or truncated versions of the nucleocapsid (N) or spike (S) proteins from SARS-CoV-2. Importantly, mono-epitope tests - whether qualitative or quantitative - are unable to localise antibody binding or characterise the distribution and titres of epitope recognition by anti-SARS-CoV-2 antibodies within an individual or across a population. However, it seems plausible that if such information were available, it may correlate with the presence of potent, high-titre, neutralising antibodies that afford protection again imminent re-infection, as well as with the likelihood of developing a memory B-cell response that would provide more durable protection. We have developed a novel, quantitative, multi-antigen, multiplexed, array-based immunoassay platform, ImmuSAFE COVID+ (ImmuSAFE) comprising 6 functionally validated domains or regions of the N protein of SARS-CoV-2 expressed using Sengenics KREX technology. This array platform enables determination of both the position and breadth of anti-SARS-CoV-2 antibody responses following natural infection or vaccination. To validate our platform, 100 serum samples (confirmed sero-positive COVID-19 cases, n=50; pre-pandemic HIV positive controls, n=50) were tested for IgG seropositivity to the N antigen, yielding 100% specificity and 100% sensitivity. All 50 cases showed positive antibody reactivity towards at least one N protein epitope, whilst all 50 controls showed antibody reactivity below threshold values. Broad variation was also observed in the magnitude and breadth of antibodies present, represented as an Epitope Coverage score (EPC). A positive correlation was observed between increasing age and EPC values, with individuals under 40 years old having a mean EPC score of 3.1, whilst individuals above the age of 60 had a mean EPC of 5.1. This finding may have broad implications for the natural history of COVID-19 disease in different individuals.