Identification of immunodominant epitopes on nucleocapsid and spike proteins of the SARS-CoV-2 in Iranian COVID-19 patients

Abstract

Given the emergence of SARS-CoV-2 virus as a life-threatening pandemic, identification of immunodominant epitopes of the viral structural proteins, particularly the nucleocapsid (NP) protein and receptor binding domain (RBD) of spike protein, is important to determine targets for immunotherapy and diagnosis. In this study, epitope screening was performed using a panel of overlapping peptides spanning the entire sequences of the RBD and NP proteins of SARS-CoV-2 in the sera from 66 COVID-19 patients and 23 healthy subjects by enzyme-linked immunosorbent assay (ELISA). Also, three non-overlapping peptides belonging to the S2 domain of spike protein were assessed. Our results showed that while reactivity of patients’ sera with reduced recombinant RBD protein was significantly lower than the native form of RBD (p<0.001), no significant differences were observed for reactivity of patients’ sera with reduced and non-reduced NP protein. Pepscan analysis revealed weak to moderate reactivity towards different RBD peptide pools, which was more focused on peptides encompassing aa 181-223 of RBD. NP peptides, however, displayed strong reactivity with a single peptide covering aa 151-170. These findings were confirmed by peptide depletion experiments using both ELISA and Western blotting. Altogether, our data suggest the involvement of mostly conformational disulfide bond-dependent immunodominant epitopes in RBD-specific antibody response, while the IgG response to NP is dominated by linear epitopes. Identification of antigenic epitope in NP and RBD of SARS-CoV-2 could provide important advances for the development of passive and active immunotherapy as well as diagnostic tools for the control of COVID-19 infection.