Utilization of Receptor-Binding Domain of SARS-CoV-2 Spike Protein Expressed in Escherichia coli for the Development of Neutralizing Antibody Assay.

The ongoing COVID-19 pandemic has resulted from widespread infection by the SARS-CoV-2 virus. As new variants of concern continue to emerge, understanding the correlation between the level of neutralizing antibodies (NAb) and clinical protection from SAR-CoV-2 infection could be critical in planning the next steps in COVID-19 vaccine programs. This study explored the potential usefulness of E. coli as an alternative expression system that can be used to produce a SARS-CoV-2 receptor-binding domain (RBD) for the development of an affordable and flexible NAb detection assay. We expressed the RBD of Beta, Delta, and Omicron variants in the E. coli BL21(DE3) strain and purified them from whole bacterial cells using His-tag-mediated affinity chromatography and urea-assisted refolding. Next, we conducted a head-to-head comparison of the binding activity of our E. coli-produced RBD (E-RBD) with commercial HEK293-produced RBD (H-RBD). The results of a direct binding assay revealed E-RBD and H-RBD binding with ACE2-hFc in similar signal strengths. Furthermore, in the NAb detection assay, % inhibition obtained from both E-RBD and H-RBD demonstrated comparable results in all the investigated assays, suggesting that non-glycosylated RBD produced from E. coli may offer a cost-effective alternative to the use of more expensive glycosylated RBD produced from human cells in the development of such an assay.