A magnetic bead immunoassay to detect human IgG reactive to SARS-CoV-2 Spike S1 RBD produced in Escherichia coli

Immunological assays to detect SARS-CoV-2 Spike receptor binding domain antigen seroconversion in humans are important tools to monitor the levels of protecting antibodies in the population in response to infection and/or immunization. Here we describe a simple, low cost and high throughput Ni²⁺ magnetic bead immunoassay to detect human IgG reactive to Spike S1 RBD Receptor Binding Domain produced in Escherichia coli. A 6xHis tagged Spike S1 RBD was expressed in E. coli and purified by affinity chromatography. The protein was mobilized on the surface of Ni²⁺ magnetic beads and used to investigate the presence of reactive IgG in the serum obtained from pre-pandemic and COVID-19 confirmed cases. The method was validated with a cohort of 290 samples and an area under the receiver operating characteristics curve of 0.94 was obtained. The method operated with>82% sensitivity at 98% specificity and was also able to track human IgG raised in response to vaccination with Comirnaty with 85% sensitivity. The IgG signal obtained with the described method was well correlated with the signal obtained when pre fusion Spike produced in HEK cell lines were used as antigen. This novel low-cost and high throughput immunoassay may act as an important tool to investigate protecting IgG antibodies against SARS-CoV-2 in the human population.