Abstract
Variola virus growth and hyperplastic focus formation in HeLa cells cultured at temperatures higher than 37C were studied. Virus growth was inhibited by 98% at 37.5C, whereas the efficiency of focus formation was not changed. At 38.0C the growth of virus was almost completely inhibited, while the number of foci formed was reduced only to 30 to 50% of those formed at 37C. At temperatures higher than 38.5C both virus growth and focus formation were completely inhibited.
The interaction between variola virus and HeLa cells leading to focus formation was then analysed by exposing the infected cultures to 40C for a limited period of time at various intervals after infection. Virus adsorption and penetration into HeLa cells were rather accelarated at 40C. Shifting the incubation temperature of the infected cultures from 37C to 40C at any time after infection resulted in interruption of the virus growth. It was also indicated that 30 to 36 hr at 37C were necessary for visible foci to develop even after the shift up, suggesting that micro-foci completed during this period and that subsequent cell aggregation took place at 40C. Shift down experiments from 40C to 37C indicated that initial incubation at 40C for longer than 36 hr impaired greatly the efficiency of focus formation after shift down, probably due to the thermal degradation of penetrated virus.
