Cost-effective serological test to determine exposure to SARS-CoV-2: ELISA based on the receptor-binding domain of the spike protein (Spike-RBDN318-V510) expressed in Escherichia coli

Massive worldwide serological testing for SARS-CoV-2 is needed to determine the extent of virus exposure in a particular region, the ratio of symptomatic to asymptomatic infected persons, and the duration and extent of immunity after infection. To achieve this aim, the development and production of reliable and cost-effective SARS-CoV-2 antigens is critical. Here, we report the bacterial production of the peptide S-RBD N318-V510, which contains the receptor binding domain of the SARS-CoV-2 spike protein. We purified this peptide using a straightforward approach involving bacterial lysis, his-tag mediated affinity chromatography, and imidazole-assisted refolding. The antigen performances of S-RBD N318-V510 and a commercial full-length spike protein were compared in two distinct ELISAs. In direct ELISAs, where the antigen was directly bound to the ELISA surface, both antigens discriminated sera from non-exposed and exposed individuals. However, the discriminating resolution was better in ELISAs that used the full-spike antigen than the S-RBD N318-V510. Attachment of the antigens to the ELISA surface using a layer of anti-histidine antibodies gave equivalent resolution for both S-RBD N318-V510 and the full length spike protein. Our results demonstrate that ELISA-functional SARS-CoV-2 antigens can be produced in bacterial cultures. S-RBD N318-V510 is amenable to massive production and may represent a cost-effective alternative to the use of structurally more complex antigens in serological COVID-19 testing.