The effect of human fibroblast interferon (IFN), a heterologous IFN, on the uncoating of murine leukaemia virus (MLV) was investigated in exogenously infected NIH/3T3 mouse cells. Virus uncoating was determined by following the disappearance of the penetrating virus particles from the cytoplasm of the infected cells. Intracellular virus particles were estimated by sedimenting them at high speed from the cytoplasmic fraction of the infected cells and assaying their reverse transcriptase activity. In untreated control cells, uncoating started immediately after penetration, but in cells treated with human IFN, uncoating was delayed for 2 or 3 h. This delay led to prolongation of the infectious cycle, with delayed release of progeny virus. The delay in release did not result from inhibition by IFN of the process of release, since in NIH/3T3 cells chronically infected with MLV, treatment with IFN had no effect on virus release.
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