Characterization of humoral immune responses against SARS-CoV-2 accessory proteins in infected patients and mouse model

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, encodes several accessory proteins that have been shown to play crucial roles in regulating the innate immune response. However, their expressions in infected cells and immunogenicity in infected humans and mice are still not fully understood. In this study, we detected accessory protein-specific antibodies in COVID-19 patients' sera using various techniques, including Luciferase Immunoprecipitation System (LIPS), Immunofluorescence assay (IFA), and Western blot (WB). Proteins 3a, 3b, 7b, 8 and 9c specific antibodies can be detected by LIPS, but only protein 3a antibody was detected by IFA or WB. And antibodies against protein 3a and 7b only detected in ICU patients, which may serve as a marker for predicting the disease progression. Further, we investigated the expression of accessory proteins in SARS-CoV-2-infected cells and identified the expressions of proteins 3a, 6, 7a, 8, and 9b. We also analyzed their ability to induce antibodies in immunized mice and found that only proteins 3a, 6, 7a, 8, 9b and 9c were able to induce measurable antibody productions, but these antibodies lacked neutralizing activities and did not protect mice from SARS-CoV-2 infection. Our findings validate the expression of SARS-CoV-2 accessory proteins and elucidate their humoral immune response, providing a basis for the protein detection assays and their role in pathogenesis.