Production of particles with the ultrastructural appearance of C-type virions persisted for at least 6 h in actinomycin D-treated cells infected with murine leukemia virus. This phenomenon occurred despite severe inhibition of viral RNA synthesis. Virus particles present in a 6-h harvest sedimented in sucrose gradients with the buoyant density characteristic of RNA tumor viruses (1.16 g/cm') and exhibited high levels of reverse transcriptase activity in response to the exogenous template polyriboadenylic acid-oligo deoxythymidylic acid in the range of untreated controls. However, RNase-sensitive endogenous activity was only Y/5 the level found in controls. This observation correlated with a marked
Treatment of AKR cells that had spontaneously become producers of a murine leukemia virus with a partially purified mouse interferon (> 5 X 107 international mouse reference units per mg of protein)inhibited endogenous virus production. This inhibitory effect decreased over a 72-hr period in a manner similar to interferon-induced antiviral activity diiected against vesicular stomatitis virus in AKR cells. Despite the inhibitory effect of interferon on infectious murine leukemia virus and viral reverse transcriptase (RNA-dependent DNA yqlymerase) titers in the culture fluids, intracellular levels of viral group-specific antigens were significantly increased. These results suggest that interferon treatment in AKR cells inhibited the assembly or release of the virus.Interferon is thought to inhibit virus replication by a complex mechanism. The optimal development of an intracellular antiviral state after interferon treatment takes several hours and is dependent on cell-directed RNA and protein synthesis. The existence' of this antiviral state gives rise to very few measurable changes in the metabolic qr physiologic state of the cells unless they are infected with an interferon-sensitive virus. In this case, the virus multiplies poorly compared to its replication in cells that had not been treated with interferon (1). Most reports suggest that the basis for this interferoninduced inhibition of-virus replication is an inhibition of the transcription or translation of virus genetic information (2,3). RNA tumor viruses are known to be sensitive to interferon (4). During a series of studies on the effect of interferon on the replication of mouse leukemia viruses, however, we made the unexpected observation that interferon treatment markedly inhibited the replication of a murine leukemia virus (MLV) in AKR cells that spontaneously had become chronically infected by an endogenous .MLV several cell generations before the interferon treatment. In addition, this inhibitory effect did not appear to be the direct result of an inhibition of the translation of virus genetic information, since the intracellular level of viral group-specific (gs) antigens was increased in cells in which the release of infectious virus and of viral reverse transcriptase were inhibited. MATERIALS AND METHODSThe AKR cells (here termed AKR,C-)' were supplied by Dr. W. Rowe. These were originally negative for virus production and negative for viral p30 (gs) antigen, and their culture fluids were negative for viral reverse transcriptase activity. These AKR,C-cells do, however, contain the AKR-murine sarcoma virus genome (5). The cells (now termed AKR,C+) spontaneously became virus producers, and more than 85%owere then positive for the virus group-specific antigens (gs-1 and gs-3) in an immunofluorescence assay. Type-C budding virus was easily detected when these cultures were examined by electron microscopy, and high titers of virus reverse transcriptase activity were found in AKR,C+ culture fluids (Table 1).The interferon used was prepared by ...
SUMMARYLy cells were treated with I oo reference units[ml of mouse interferon and then infected with a wild-type vesicular stomatitis virus (VSV) at a multiplicity of IO to 6o p.f.u./cell. Prolonged infection of cultures ensued, lasting from 14 to at least 6o days. Less than 1% of the cells produced infectious virus, but more than IO% produced detectable levels of VSV antigens. No small virion RNA forms (< 42S) characteristic of defective interfering (DI) virus particles were detected. The virus produced appeared to be temperature sensitive. There was decreased plaquing efficiency at 37 or 39 °C compared to 32 °C and termination of the chronic infection due to c.p.e, within a few days after shift to 32 °C. The cultures resisted superinfection with wild-type VSV or with a heterologous virus, encephalomyocarditis (EMC) virus. Treatment of cultures with rabbit anti-mouse interferon globulin resulted in a marked increase in virus titres and termination of the chronic infection. Prolonged VSV infection in this system may be related both to the emergence of temperature-sensitive mutants and to endogenous interferon production rather than to cyclical generation of DI particles.
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