Mutation of host cell determinants which discriminate between lytic and persistent mouse hepatitis virus infection results in a fusion-resistant phenotype.

The expression of mouse hepatitis virus (MHV) E2-specific mRNA, the E2 polypeptide and its associated cell fusing activity was monitored in various cell types inoculated with a recombinant vaccinia virus, designated vMS containing the E2 gene. The results suggest that host cell permissiveness to MHV infection correlates with cellular susceptibility to membrane fusion mediated by the MHV E2 glycoprotein. In addition, we utilized a genetic approach to the analysis of host cell functions involved in determining permissiveness to MHV. By using the chemical mutagen ethyl methanesulphonate, mouse fibroblast cell mutants were generated and selected for their resistance to cell killing by MHV. When challenged with MHV, all five mutants examined gave rise to persistent infections, in contrast to wild-type L-2 cells which were rapidly killed by the virus. The results provide genetic evidence in support of a previous correlation proposed between MHV permissiveness and two host determinants, namely susceptibility to MHV infection and to MHV-mediated cell fusion. Fusion resistance was specific to fusion mediated by the MHV E2 glycoprotein as shown in contact fusion assays between uninfected cells and cells infected either with MHV or with an E2-expressing recombinant vaccinia virus. In contrast, mutant cells were not resistant to fusion after treatment with polyethylene glycol. The observed high rate of generation of these mutants suggests that the conversion of a fully MHV-susceptible cell to a semi-resistant one is a fairly common event, possibly involving a single mutation. In this case, resistance to MHV infection and to E2-mediated membrane fusion may depend on a common host function. This result provides prospects for the precise genetic and biochemical characterization of the steps involved in host cell permissiveness to MHV infection.