Novel b-cell epitopes of non-neutralizing antibodies in the receptor-binding domain of the S-protein of SARS-CoV-2 with differing effects on the severity of the course of COVID-19

Abstract

Antibodies against the receptor-binding domain of the SARS-CoV-2 spike protein (RBD of S-protein) contribute significantly to the humoral immune response during coronavirus infection (COVID-19) and after vaccination. The main focus of the study of the epitope composition of RBD is concentrated on the epitopes recognized by viral neutralizing antibodies. The role of antibodies that bind to RBD but are unable to neutralize the virus in the formation of the immune response remains unclear. In this investigation, the immunochemical properties of two mouse monoclonal antibodies RS17 and S11 against RBD were examined. Both antibodies were shown to have high affinity, but they did not neutralize the virus. The epitopes of these antibodies were localized using phage display: the epitope recognized by the RS17 antibody is located at the N-terminal site of RBD (348-SVYAVNRKRIS-358); the epitope recognized by the S11 antibody is inside the receptor-binding motif of RBD (452-YRLFRKSN-459). Three groups of sera were tested for antibodies competing with non-neutralizing antibodies S11 and RS17: 1) from unvaccinated volunteers, who did not suffer from COVID-19 previously; 2) from people who had had a mild form of COVID-19; 3) from people who had had a severe form of COVID-19. Antibodies competing with the S11 antibody were shown to occur with equal frequency in each of the serum groups studied. At the same time, the presence of antibodies competing with antibody RS17 in the sera was associated with the severity of COVID-19 and was significantly more frequent in the group of sera obtained from patients with severe COVID-19. In conclusion, despite the clear significance of anti-RBD antibodies for the formation of an effective immune response against SARS-CoV-2, it is important to analyze their viral neutralizing activity and to confirm the absence of negative features of obtained anti-RBD antibodies after vaccination.