MHV-A59 FUSION MUTANTS ARE ATTENUATED AND DISPLAY ALTERED HEPATOTROPISM

Mouse hepatitis virus strain A59 causes a persistent productive, but n onlytic, infection of cultured glial cells. We have mutants isolated f rom persistently infected glial cell cultures which have been shown to be fusion-defective due to a histidine to aspartic acid mutation (H71 6D) near the cleavage site of the peplomer protein, S. Here, we examin e the pathogenicity of these mutants and show differences in hepatotro pism and virulence compared to wild-type virus (WT). Two mutants chose n for detailed study, B11 and C12, were impaired in their abilities to cause hepatitis and/or replicate in the liver of susceptible mice. Fu rthermore, B11 and C12 display two separate hepatotropic phenotypes. T he ability of B11 to replicate in the liver was dependent on infectiou s dose and route of inoculation, while C12 consistently displayed decr eased hepatotropism regardless of dose and route of inoculation. Howev er, B11 and C12 were shown to replicate in the CNS of infected animals similarly to WT. Like WT, the mutants produced meningoencephalitis du ring acute infection, with viral antigen exhibiting a similar distribu tion in the brain, and demyelination during chronic infection. Sequenc e analysis of wild-type, mutant, and revertant S proteins indicates th at (1) a mutation in the N terminal subunit of S (S1), resulting in a glutamine to leucine amino acid substitution (Q159L), may affect hepat otropism and (2) a cleavage site mutation which determines fusogenicit y is not responsible for altered hepatotropism. Furthermore, since B11 , C12, and a nonattenuated fusion mutant (B12) have identical S protei n sequences, there must be additional mutations outside of S which inf luence both virulence and hepatotropism. (C) 1994 Academic Press, Inc.