Infection of LLC-MK 2 cells with the International Health Division strain of vaccinia virus caused rounding of all cells within 2 to 3 hr after infection. Puromycin, at a concentration of 330 µg./ml., completely prevented the early virus-induced cytopathological changes as well as virus multiplication; however, at 33 µg./ml. the compound did not prevent the cytopathic effects of vaccinia virus, in spite of the fact that virus multiplication was largely inhibited. Treatment of cells for 4 hr with puromycin at 330 µg./ml. did not alter the rate of cell division when the compound was removed. When the antibiotic (330 µg./ml.) was removed from infected cultures after a treatment period of 4 hr, virus-induced cell damage began almost immediately after removal and by 1 hr all cells were rounded.

Actinomycin D, 5 µg./ml., also protected the cells from the early vaccinia-virus-induced cytopathic effects. At 0.5 µg./ml., although growth of virus was inhibited, no protection against viral cytopathic effects was observed.

In LLC-MK 2 cells virus-induced morphological damage was not prevented by -fluorophenylalanine, 5-fluoro-2′-deoxyuridine, and isatin-β-thiosemicarbazone—compounds known to inhibit the reproduction of vaccinia virus. Ultraviolet-irradiated virus, which had lost its infectivity, was still capable of causing early cell damage.

These findings suggest that the early virus-induced cytopathological effects, previously considered the ‘toxic’ effects of the virus, are brought about by the synthesis of virus-induced protein(s).


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