Abstract
Knowledge of the antibody-mediated immune response to SARS-CoV-2 is crucial to understand virus immunogenicity, establish seroprevalence, and determine whether subjects or recovered patients are at risk for infection/reinfection and would therefore benefit from vaccination. Here, we describe a novel and simple cell-ELISA specifically designed to measure viral spike S1-specific IgG produced in vitro by B cells in peripheral blood mononuclear cell (PBMC) cultures from a cohort of 45 asymptomatic (n = 24) and symptomatic (n = 21) individuals, with age ranging from 8 to 99 years. All subjects underwent ELISA serological screening twice, at the same time as the cell-ELISA (T2) as well as 35–60 days earlier (T1). Cryopreserved PBMCs of healthy donors obtained years before the COVID-19 pandemic were also included in the analysis. The preliminary results presented here show that out of 45 tested subjects, 16 individuals (35.5%) were positive to the cell-ELISA, 11 (24.5%) were concomitantly positive in the serological screening (T1 and/or T2), and only one person was exclusively positive in ELISA (T1) and negative in cell-ELISA, though values were close to the cutoff. Of note, five individuals (11.2%) tested negative in ELISA but positive in cell-ELISA and thus, they appear to have circulating B cells that produce antibodies against SARS-CoV-2, likely at levels that are undetectable in the serum, which challenges the negative results of the serological screening. The relative level of in vitro secreted IgG was measurable in positive subjects, ranging from 7 to 50 ng/well. Accordingly, all anti-SARS-CoV-2 antibody-positive subjects previously reported moderate to severe symptoms attributable to COVID-19, even though the RT-PCR data were rarely available to confirm viral infection. Overall, the described cell-ELISA might be an effective method for detecting subjects who encountered the virus in the past, and thus helpful to improve serological ELISA tests in the case of undetectable/equivocal circulating IgG levels, and a suitable and improved tool to better evaluate SARS-CoV-2-specific humoral immunity in the COVID-19 pandemic.
Highlights
Memory B cells and/or circulating plasma cells can be identified by their capability to secrete antigen-specific antibodies long after first encountering an antigen [1]; they can provide important information related to past exposure
Accurate tests able to identify the SARS-CoV-2 virus and monitor the presence of antiviral antibodies are key in detecting those who have had an immune response to the virus, to help the management and surveillance of the virus, and to fight the COVID-19 pandemic
Diagnosis for SARS-CoV-2 is currently performed after running qRT-PCR of selected viral genes from nasal/respiratory tract swabs, monitoring the immune humoral response by serological assays, and/or measuring circulating IgM/IgG antibodies using various immunoenzymatic ELISA platforms available on the market
Summary
Memory B cells and/or circulating plasma cells can be identified by their capability to secrete antigen-specific antibodies long after first encountering an antigen [1]; they can provide important information related to past exposure. This feature has been employed to setup in vitro. Viruses 2020, 12, 1274 assays to detect and count antibody-secreting B cells through ELISPOT platforms [2]. With this method, antibody-producing cells secrete antibodies that bind to an antigen-coated plastic surface.
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