Characterization of two linear epitopes SARS CoV-2 spike protein formulated in tandem repeat

The vital roles of diagnostic tools and vaccines are prominent in controlling COVID-19. Spike protein of the SARS CoV-2, specifically the epitopes in that protein, are the critical components of the vaccines and immunological diagnostic tools. Two epitopes in the spike protein, the S14P5 and S21P2, identified previously are of great interest because they are linear and elicit neutralizing antibodies. The present study formulated each epitope in the tandem-repeat structure to increase their immunogenicity and facilitate their production. The tandem repeats (TR) were expressed efficiently in E. coli, yielding 58 mg and 46 mg per liter culture for TR-S14P5 and TR-S212, respectively. ELISA using either one of the repeating epitopes can be used as a serological test to identify individuals infected by the SARS-CoV-2 virus. The area under curves (AUC), based on testing 157 serum samples from COVID-19 patients and 26 from COVID-19-free individuals, were 0.806 and 0.889 for TR-S14P5 and TR-S21P2-based ELISAs, respectively. For 100% diagnostic specificity, the sensitivity was only 70%. The low sensitivity supposedly resulted from some samples being from early infection prior to antibody conversion. Both recombinant epitopes were highly immunogenic in rabbits, and the immune sera recognized inactivated SARS CoV-2 virus in dot-blot assays. These antibodies should be useful as a reagent for detecting SARS-CoV-2 antigens. Furthermore, the TR-S14P5 and TR-S21P2, being conserved and denaturation-resistant, are envisaged to be ideal for intra-nasal vaccines, which are required to complement current COVID-19 to overcome rapidly mutated SARS CoV-2.