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MHV-A59 fusion mutants are attenuated and display altered hepatotropism
S T Hingley1, J L Gombold, E Lavi
1Department of Microbiology, University of Pennsylvania School of Medicine, Philadelphia 19104-6076.
Abstract:
Mouse hepatitis virus strain A59 causes a persistent productive, but nonlytic, infection of cultured glial cells. We have mutants isolated from persistently infected glial cell cultures which have been shown to be fusion-defective due to a histidine to aspartic acid mutation (H716D) near the cleavage site of the peplomer protein, S. Here, we examine the pathogenicity of these mutants and show differences in hepatotropism and virulence compared to wild-type virus (WT). Two mutants chosen for detailed study, B11 and C12, were impaired in their abilities to cause hepatitis and/or replicate in the liver of susceptible mice. Furthermore, B11 and C12 display two separate hepatotropic phenotypes. The ability of B11 to replicate in the liver was dependent on infectious dose and route of inoculation, while C12 consistently displayed decreased hepatotropism regardless of dose and route of inoculation. However, B11 and C12 were shown to replicate in the CNS of infected animals similarly to WT. Like WT, the mutants produced meningoencephalitis during acute infection, with viral antigen exhibiting a similar distribution in the brain, and demyelination during chronic infection. Sequence analysis of wild-type, mutant, and revertant S proteins indicates that (1) a mutation in the N terminal subunit of S (S1), resulting in a glutamine to leucine amino acid substitution (Q159L), may affect hepatotropism and (2) a cleavage site mutation which determines fusogenicity is not responsible for altered hepatotropism. Furthermore, since B11, C12, and a nonattenuated fusion mutant (B12) have identical S protein sequences, there must be additional mutations outside of S which influence both virulence and hepatotropism.
Insights
Mutant mouse hepatitis virus strains show altered liver infection (hepatotropism) and virulence. Mutations outside the S protein influence these changes, while CNS infection remains similar to wild-type.
Area of Science:
- Virology
- Immunology
- Neuroscience
Background:
- Mouse hepatitis virus (MHV) strain A59 establishes persistent, nonlytic infections in glial cells.
- Specific MHV mutants with a histidine to aspartic acid substitution (H716D) near the S protein cleavage site are fusion-defective.
- These mutants exhibit altered pathogenicity, including differences in hepatotropism and virulence compared to wild-type (WT) virus.
Purpose of the Study:
- To investigate the pathogenicity of MHV mutants with defects in S protein fusogenicity.
- To compare the hepatotropism and virulence of these mutants to WT MHV in mice.
- To identify genetic determinants of MHV hepatotropism and virulence.
Main Methods:
- Infection of susceptible mice with WT MHV and two selected mutants (B11, C12).
- Assessment of viral replication in the liver and CNS.
- Analysis of clinical signs, including hepatitis, meningoencephalitis, and demyelination.
- Sequence analysis of the viral S protein in WT, mutants, and revertants.
Main Results:
- Mutants B11 and C12 showed impaired hepatitis and reduced liver replication compared to WT.
- B11 and C12 exhibited distinct hepatotropic phenotypes, with B11's liver replication being dose- and route-dependent, while C12 consistently showed decreased hepatotropism.
- Both mutants replicated similarly to WT in the CNS, causing acute meningoencephalitis and chronic demyelination.
- Sequence analysis suggested a mutation in the S1 subunit (Q159L) may influence hepatotropism, but the cleavage site mutation was not responsible.
- Identical S protein sequences in mutants B11, C12, and a non-attenuated mutant B12 indicated that mutations outside the S protein affect virulence and hepatotropism.
Conclusions:
- Mutations outside the S protein are critical for determining MHV virulence and hepatotropism.
- While S protein mutations can influence hepatotropism, the cleavage site mutation is not the sole determinant.
- MHV mutants with altered S protein fusogenicity retain the capacity for CNS infection, leading to meningoencephalitis and demyelination.