Pre-existing antibodies targeting a dominant linear antibody epitope on SARS-CoV-2 S2 cross-reacted with commensal gut bacteria and shaped immune responses elicited by a candidate vaccine

Abstract Pre-existing SARS-CoV-2 cross-reactive antibodies have been detected in both unexposed human and animals. However, the origins of these cross-reactive antibodies and their potential impacts on vaccine efficacy have not been completely clarified. In this study, we demonstrated that the S2 subunit was the predominant target of the pre-existing SARS-CoV-2 spike protein cross-reactive antibodies in both healthy human and naive SPF mice. Through linear epitope mapping, we identified a dominant antibody epitope on the connector domain of S2 (aa1145-aa1162), which could be recognized by antibodies pre-existed in unexposed human and mice. Six monoclonal antibodies against this linear epitope were isolated from naive SPF mice and were proved to cross-react with commensal gut bacteria collected from both human and mouse. Via immunizing mice with a candidate DNA vaccine encoding the full length of SARS-CoV-2 spike protein, we further demonstrated that high levels of pre-existing S2 cross-reactive antibodies did not impair the immunogenicity of the DNA vaccine. On the contrary, mice with high levels of pre-existing antibodies mounted stronger S2 specific binding antibody responses compared to mice with low levels of pre-existing antibodies. In addition, S1 specific T cell and binding antibody responses also tended to be enhanced in mice with high levels of pre-existing antibodies.