Measuring SARS-CoV-2 Neutralizing Antibodies

The SARS-CoV-2-neutralizing activity of donated plasma and purified IgG was determined as previously described (30 (link), 40 (link), 59 (link)). In brief, VeroE6^TMPRSS2 cells were seeded in 96-well flat microtiter culture plates at the density of 1 × 10⁴ cells/well. On the following day, the virus (SARS-CoV-2^05-2N) was mixed to the various concentrations of the plasma or purified IgG fractions and incubated for 20 min at 37°C. The preincubated mixture was inoculated to the cells at a multiplicity of infection (MOI) of 0.01. The cells were cultured for 3 days, and the number of viable cells in each well was measured using Cell Counting Kit-8 (Dojindo, Kumamoto, Japan). The potency of SARS-CoV-2 inhibition by plasma or purified IgG was determined based on its inhibitory effect on virally induced cytopathicity in VeroE6^TMPRSS2 cells. The amounts of SARS-CoV-2-S1-binding antibodies in each plasma sample were determined by using anti-SARS-CoV-2 ELISA (IgG) (Euroimmun, Lübeck, Germany). The total human IgG concentration was determined by using Human IgG ELISA Kit (Abcam, Cambridge, UK).