Evaluation of a SARS-CoV-2 Capture IgM Antibody Assay in Convalescent Sera

Anti-SARS-CoV-2 IgM Antibody Levels Measured by an In-House ELISA in a Convalescent Latin Population Persist over Time and Exhibit Neutralizing Capacity to Several Variants of Concern

BACKGROUND

The coronavirus, SARS-CoV-2, is the causative agent for COVID-19, first registered in Wuhan, China and responsible for more than 6 million deaths worldwide. Currently, RT-PCR is the gold-standard method for diagnosing COVID-19. However, serological tests are needed for screening acute disease diagnosis and screening large populations during the COVID-19 outbreak.

OBJECTIVES

Herein, we described the development and validation of an in-house enzyme-linked immunosorbent assay (ELISA) for detecting the levels of anti-spike-1-RBD IgM antibody (CovIgM-ELISA) in well-defined serum/plasma panel for screening and identifying subjects infected with SARS-CoV-2 in a Latin population.

METHOD

In-house CovIgM-ELISA has the format of an indirect ELISA. It was optimized by checkerboard titration using recombinant SARS-CoV-2 spike-S1-RBD protein as an antigen.

RESULTS

We found that, compared to the RT-PCR as the standard method, the in-house CovIgM-ELISA displayed sensitivities of 96.15% and 93.22% for samples collected up to 30 or 60 days after infection, respectively, as well as 95.59% specificity with 97.3% accuracy. The agreement kappa value (κ) of our CovIgM-ELISA was substantial when compared to RT-PCR (κ = 0.873) and the anti-SARS-CoV-2 IgM ELISA (InBios Int) (κ = 0.684). The IgM levels detected in the population positively correlated with the neutralizing activity against the wild-type, Alpha and Delta variants of concern, but failed to neutralize Omicron.

CONCLUSIONS

These data indicate that our in-house CovIgM-ELISA is a compatible performing assay for the detection of SARS-CoV-2 infection.

A novel highly specific biotinylated MAC-ELISA for detection of anti-SARS-CoV-2 nucleocapsid antigen IgM antibodies during the acute phase of COVID-19

The gold standard for diagnosing COVID-19 in the acute phase is RT-qPCR. However, this molecular technique can yield false-negative results when nasopharyngeal swab collection is not conducted during viremia. To mitigate this challenge, the enzyme-linked immunosorbent assay (ELISA) identifies anti-SARS-CoV-2 IgM antibodies in the initial weeks after symptom onset, facilitating early COVID-19 diagnosis. This study introduces a novel and highly specific IgM antibody capture ELISA (MAC-ELISA), which utilizes biotinylated recombinant SARS-CoV-2 nucleocapsid (N) antigen produced in plants. Our biotinylated approach streamlines the procedure by eliminating the requirement for an anti-N-conjugated antibody, circumventing the need for peroxidase-labeled antigens, and preventing cross-reactivity with IgM autoantibodies such as rheumatoid factor. Performance evaluation of the assay involved assessing sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy using 682 RT-qPCR-positive samples, categorized by weeks relative to symptoms onset. Negative controls included 205 pre-pandemic serum samples and 46 serum samples from patients diagnosed with other diseases. Based on a cut-off of 0.087 and ROC curve analysis, the highest sensitivity of 81.2% was observed in the 8-14 days post-symptom (dps) group (2nd week), followed by sensitivities of 73.8% and 68.37% for the 1-7 dps (1st week) and 15-21 dps groups (3rd week), respectively. Specificity was consistently 100% across all groups. This newly developed biotinylated N-MAC-ELISA offers a more streamlined and cost-effective alternative to molecular diagnostics. It enables simultaneous testing of multiple samples and effectively identifies individuals with false-negative results.

On the caveats of a multiplex test for SARS-CoV-2 to detect seroconversion after infection or vaccination

The Covid-19 pandemic, caused by SARS-CoV-2, has resulted in over 6 million reported deaths worldwide being one of the biggest challenges the world faces today. Here we present optimizations of all steps of an enzyme-linked immunosorbent assay (ELISA)-based test to detect IgG, IgA and IgM against the trimeric spike (S) protein, receptor binding domain (RBD), and N terminal domain of the nucleocapsid (N-NTD) protein of SARS-CoV-2. We discuss how to determine specific thresholds for antibody positivity and its limitations according to the antigen used. We applied the assay to a cohort of 126 individuals from Rio de Janeiro, Brazil, consisting of 23 PCR-positive individuals and 103 individuals without a confirmed diagnosis for SARS-CoV-2 infection. To illustrate the differences in serological responses to vaccinal immunization, we applied the test in 18 individuals from our cohort before and after receiving ChAdOx-1 nCoV-19 or CoronaVac vaccines. Taken together, our results show that the test can be customized at different stages depending on its application, enabling the user to analyze different cohorts, saving time, reagents, or samples. It is also a valuable tool for elucidating the immunological consequences of new viral strains and monitoring vaccination coverage and duration of response to different immunization regimens.