A persistent infection with Saint Louis encephalitis (SLE) virus in a poikilothermic cell line TH-1 (turtle heart cells) was studied. Infected TH-1 cells were subcultured weekly at 31 °C for 1 year and continued to produce low levels (10 to 10 p.f.u./ml) of virus without obvious cytopathic effects or marked cyclic events. Indirect fluorescent antibody and infectious centre assays indicated that < 1% of the cells were producing detectable virus proteins or infectious virus. Defective-interfering particles, temperature-sensitive mutants and DNA provirus were not detected. Interferon (IFN) mediation of the persistent infection was considered since the persistently infected cells (PIC) and normal TH-1 cells were resistant to heterologous virus challenge after treatment with virus-free culture fluid from PIC. A direct relationship was found between the m.o.i. and the amount of IFN produced, plateauing at an m.o.i. of approx. 10. The reptilian IFN was physically and chemically similar to mammalian and avian IFN. Certain biological markers of the SLE virus changed during the persistent infection. It was less virulent for mice, showed distinct differences in cell culture host range and had increased thermal lability.


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