HKU1 internal protein ameliorate infection in mice

Coronaviruses express a repertoire of accessory proteins for evading host immune responses. Among these accessory proteins, a small internal (I) protein is expressed by members of the genus Betacoronavirus. Previous studies reported that the I proteins of SARS-CoV, MERS-CoV and SARS-CoV-2 inhibit type I interferon (IFN-I) expression through distinct mechanisms and play different roles in pathogenesis. In addition, human coronaviruses HKU1 and OC43 are betacroaonvairuses that predominantly cause common cold and encode the I protein as one of their accessory proteins; the I proteins of hCoV-HKU1 and hCoV-OC43 have not been previously characterized. However, the lack of robust reverse genetic systems, tissue culture and animal models limit the study of hCoV-HKU1 and hCoV-OC43 pathogenesis. Here, we examined the role of hCoV-HKU1 and hCoV-OC43 I proteins in pathogenesis using a prototypic coronavirus. We introduced the I proteins of hCoV-HKU1 and hCoV-OC43 independently to a neurotropic strain of mouse hepatitis virus (MHV-J2.2). MHV-J2.2 infection is well-characterized with clearly defined immune responses which allows the study of I proteins in the context of authentic coronavirus infection. We showed that the I protein of hCoV-HKU1 but not that of hCoV-OC43 ameliorated MHV-J2.2 infection while the I protein of MERS-CoV causes exacerbated disease. Further analysis revealed that infection with MHV-J2.2 expressing the I protein of MERS-CoV leads to increased neutrophil infiltration to the site of infection and virus titers in mice; diminished virus titers was observed in the presence of hCoV-HKU1 I protein. Overall, our findings suggest that the I protein of different betacoronaviruses play unique roles in pathogenesis. Mouse brains were homogenized in TRIZOL 3 days after infection with J2.2 virus expressing HKU1 protein or its control.