Evaluation of spike protein antigens for SARS-CoV-2 serology

Abstract

BackgroundSpike protein domains are being used in various serology-based assays to detect prior exposure to SARS-CoV-2 virus. However, there has been limited comparison of human antibody titers against various spike protein antigens among COVID-19 infected patients.MethodsWe compared four spike proteins (RBD, S1, S2 and a stabilized spike trimer (ST)) representing commonly used antigens for their reactivity to human IgG antibodies using indirect ELISA in serum from COVID-19 patients and pre-2020 samples. ST ELISA was also compared against the EUROIMMUN IgG ELISA test. Further, we estimated time appropriate IgG and IgA seropositivity rates in COVID-19 patients using a panel of sera samples collected longitudinally from the day ofonset of symptoms (DOS).ResultsAmong the four spike antigens tested, the ST demonstrated the highest sensitivity (86.2%; 95% CI: 77.8-91.7%), while all four antigens showed high specificity to COVID-19 sera (94.7-96.8%). 13.8% (13/94) of the samples did not show seroconversion in any of the four antigen-based assays. In a double-blinded head-to-head comparison, ST based IgG ELISA displayed a better sensitivity (87.5%, 95%CI: 76.4-93.8%) than the EUROIMMUN IgG ELISA (67.9%, 95% CI: 54.8-78.6%). Further, in ST-based assays, we found 48% and 50% seroconversion in the first six days (from DOS) for IgG and IgA antibodies, respectively, which increased to 84% (IgG) and 85% (IgA) for samples collected ≥22 days DOS.ConclusionsComparison of spike antigens demonstrates that spike trimer protein is a superior option as an ELISA antigen for COVID-19 serology.HighlightsSpike trimer displays the highest antibody titer in SARS-CoV-2 infections among spike protein antigens.Spike trimer IgG ELISA displays a sensitivity of 50% within six days and 86.2% after 14 days from onset of symptoms.IgA and IgG responses to spike trimer antigen were comparable and concomitant in time after infection.16% (IgG) and 15% (IgA) of COVID-19 RT-PCR positive patients did not seroconvert even after 21 days from onset of symptoms.

Expression and characterization of SARS-CoV-2 spike proteins

“…The presence of the furin cleavage site is additionally critical for understanding the viral transmissibility in clinical variants such as D614G, where increased furin site accessibility and cleavage may be responsible for increase infectivity 59 . ELISAs utilizing the HexaPro stabilized trimer showed increased sensitivity for seroconversion compared with individual spike subunits 60 . Additionally, HexaPro does not transition into the post-fusion conformation, eliminating any newly exposed post-fusion antigenic sites 18 .…”