Loss in Virulence of Yellow Fever Virus Serially Passed in HeLa Cells

Summary

Yellow fever virus that was passed one and three times in HeLa cells was characterized with respect to the following: (a) adaptation to growth in cell culture as indicated by the time required to obtain maximal titer, (b) level of titer, (c) production of a cytopathic effect (cpe), (d) stability of virus when aerosolized at either 50% or 80% relative humidity (RH), and (e) the loss of virulence of viral preparations for rhesus monkeys infected by the respiratory route. Four viral populations which possessed different combinations of these properties were obtained. In 11 of 12 experiments, one passage in cell culture yielded a virus population (Ty-1) that titered to 10⁷ MICLD₅₀/ml on day 5, that failed to cause cell lysis (cpe), that was equally stable at 50% and 80% RH, and that was lethal for monkeys. In contrast, in 4 of 6 experiments, 3 passages in cell culture yielded a virus pouplation (Ty-3) that titered 10⁸ MICLD₅₀ or greater on day 3, that induced a cpe, that had a greater sensitivity to 50% than to 80% RH, and that was non-lethal for monkeys. In the remaining experiments, one "atypical" viral population (ATy-1) was obtained from culture after one passage. In 2 other experiments, "atypical" viral populations (ATy-3) that were similar to each other but different from ATy-1 were obtained after 3 passages. The characteristics demonstrated by ATy-1 and ATy-3 suggested that these were intermediate to Ty-1 and Ty-3. The manner in which population characteristics became altered during the conversion of Ty-1 to Ty-3 in vitro suggested that some of the characteristics were closely associated with one another.