Potential Differences in Cleavage of the S Protein and Type 1 Interferon Together Control Human Coronavirus Infection, Propagation, and Neuropathology within the Central Nervous System

Human coronaviruses (HCoV) are respiratory pathogens which have been known since the 1960s. In December 2019, a new betacoronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was reported, and it is responsible for one of the biggest pandemics of the last 2 centuries. Available evidence suggests that similar to the case with the HCoV-OC43 strain, SARS-CoV-2 neuroinvasion is associated with potential neurological disorders. Coronavirus infection of the central nervous system (CNS) is largely controlled by a viral factor, the spike glycoprotein (S), and a host factor, innate immunity. However, the interaction between these two factors remains elusive. Proteolytic cleavage of the S protein can occur at the interface between receptor binding (S1) and fusion (S2) domains (S1/S2), as well as in a position adjacent to a fusion peptide within S2 (S2′). In this study, using HCoV-OC43 as a surrogate for SARS-CoV-2, we determined that both S protein sites are involved in neurovirulence and are required for optimal CNS infection. Whereas efficient cleavage at S1/S2 is associated with decreased virulence, the potentially cleavable putative S2′ site is essential for efficient viral infection. Furthermore, type 1 interferon (IFN-1)-related innate immunity also plays an important role in the control of viral spread toward the spinal cord, by preventing infection of ependymal cells. Our results underline the link between the differential S cleavage and IFN-1 in the prevention of viral spread, to control the severity of infection and pathology in both immunocompetent and immunodeficient mice. Taken together, these results point toward two potential therapeutic antiviral targets: cleavage of the S protein in conjunction with efficient IFN-1-related innate immunity to prevent or at least reduce neuroinvasion, neural spread, and potential associated neurovirulence of human coronaviruses. IMPORTANCE Human coronaviruses (HCoV) are recognized respiratory pathogens. The emergence of the novel pathogenic member of this family in December 2019 (SARS-CoV-2, which causes COVID-19) poses a global health emergency. As with other coronaviruses reported previously, invasion of the human central nervous system (CNS), associated with diverse neurological disorders, was suggested for SARS-CoV-2. Herein, using the related HCoV-OC43 strain, we show that the viral spike protein constitutes a major neurovirulence factor and that type 1 interferon (IFN-1), in conjunction with cleavage of S protein by host proteases, represents an important host factor that participates in the control of CNS infection. To our knowledge, this is the first demonstration of a direct link between cleavage of the S protein, innate immunity, and neurovirulence. Understanding mechanisms of viral infection and spread in neuronal cells is essential to better design therapeutic strategies, and to prevent infection by human coronaviruses such as SARS-CoV-2 in the human CNS, especially in vulnerable populations such as the elderly and immunocompromised individuals.