Wild-type temperature-sensitive and -resistant visna viruses: isolation and biological comparison

Area of Science:

• Virology
• Immunology
• Animal Pathology

Background:

• Visna virus and progressive pneumonia virus cause persistent infections in sheep.
• Wild-type virus populations exhibit heterogeneous plaque sizes in sheep choroid plexus (SCP) cell cultures.
• Plaque heterogeneity suggests the presence of distinct viral variants within populations.

Purpose of the Study:

• To investigate the biological and genetic characteristics of visna virus variants.
• To determine the role of different plaque-forming variants in persistent ovine lentiviral infections.
• To understand the implications of temperature-sensitive variants on host immune responses.

Main Methods:

• Plaque purification and characterization of visna virus strains (V-K485, V-K796).
• Comparative analysis of large plaque-forming (Lpf) and small plaque-forming (Spf) virus phenotypes.
• Temperature sensitivity assays and population analyses of wild-type virus strains.
• Assessment of viral replication kinetics and cytopathic effects in SCP cells.

Main Results:

• Visna virus populations contain both Lpf and Spf variants, which are antigenically similar.
• Spf virus is less cytopathic and temperature-sensitive (thermosensitive) compared to Lpf virus.
• Wild-type populations harbor Spf variants, and cultivation at 37°C selects against these thermosensitive variants.
• Spf variants exhibit similar latent periods and early synthesis rates to Lpf variants.

Conclusions:

• The Spf variant is a temperature-sensitive visna virus.
• Spf variants are maintained in wild-type visna virus populations and are not selected against in vivo.
• Temperature-sensitive Spf variants likely contribute to persistent visna virus infections in sheep.
• Spf variants may delay the development of cell-mediated immunity against viral proteins in infected sheep.