Viral determinants of murine coronavirus-induced encephalomyelitis

Central nervous system (CNS) infection by mouse hepatitis virus (MHV) is a model for studying virus-induced acute encephalomyelitis. MHV strains differ dramatically in their neurovirulence, or ability to cause morbidity or mortality secondary to CNS infection. One of the viral genes that is thought to influence neurovirulence is the spike (S) glycoprotein of MHV. In the absence of a molecular clone of the MHV genome, however, it has been difficult to address the role of individual viral genes in pathogenesis. We have demonstrated that the spike gene is a major determinant of MHV neurovirulence using targeted RNA recombination. The recombinant virus S 4R, containing the S gene of the highly neurovirulent MHV-4 strain, exhibited a dramatically more neurovirulent phenotype than the isogenic recombinant SA59R, containing the S gene of the mildly neurovirulent MHV-A59 strain. To more finely map the determinants of neurovirulence within the spike, we generated isogenic recombinant viruses containing various MHV-4/MHV-A59 chimeric spike genes, and studied their phenotypes. All of the chimeric spike gene recombinants exhibited efficient replication in cell culture, yet many were severely attenuated for replication and for virulence in vivo. Thus it appeared that the interactions of multiple regions of the MHV-4 spike were necessary for efficient infection of and virulence in the murine CNS. To further elucidate the role of spike in MHV neurovirulence, we studied the neuropathological differences between S4R and SA59R infection. We characterized the regional virus distribution, cell tropism, number of infected cells, and inflammatory infiltrates in the CNS. We found that the high neurovirulence conferred by the MHV-4 spike relative to the MHV-A59 spike correlated with an increased number of viral antigen positive neurons and glia and an increased number of inflammatory cells, including CD4+ and CD8+ T cells, in the brain. Furthermore, infection of RAG1−/− mice suggested that the lymphocytic response to infection also contributes to the high neurovirulence conferred by the MHV-4 spike. These results demonstrate that the spike is a major determinant of MHV neurovirulence, identify features of neurovirulence conferred exclusively by the spike glycoprotein, and further our understanding of the role of the spike in neurovirulence