Pathogenesis of chimeric MHV4/MHV-A59 recombinant viruses: the murine coronavirus spike protein is a major determinant of neurovirulence

The mouse hepatitis virus (MHV) spike glycoprotein, S, has been implicated as a major determinant of viral pathogenesis. In the absence of a full-leng th molecular clone, however, it has been difficult to address the role of i ndividual viral genes in pathogenesis, By using targeted RNA recombination to introduce the S gene of MHV4, a highly neurovirulent strain, into the ge nome of MHV-A59, a mildly neurovirulent strain, we have been able to direct ly address the role of the S gene in neurovirulence. In cell culture, the r ecombinants containing the MHV4 S gene, S4R22 and S4R21, exhibited a small- plaque phenotype and replicated to low levels, similar to wild-type MHV4. I ntracranial inoculation of C57BL/6 mice with S4R22 and S4R21 revealed a mar ked alteration in pathogenesis. Relative to wild-type control recombinant v iruses (wtR13 and wtR9), containing the MHV-A59 S gene, the MHV4 S gene rec ombinants exhibited a dramatic increase in virulence and an increase in bot h viral antigen staining and inflammation in the central nervous system, Th ere was not, however, an increase in the level of viral replication in the brain. These studies demonstrate that the MHV4 S gene alone is sufficient t o confer a highly neurovirulent phenotype to a recombinant virus deriving t he remainder of its genome from a mildly neurovirulent virus, MHV-A59. This definitively confirms previous findings, suggesting that the spike is a ma jor determinant of pathogenesis.