Effect of interferon on human cells releasing oncornaviruses: An assay for human interferon

Human RD-114 cells, chronically infected with a feline RNA tumor virus, were treated with different concentrations of human interferon in order to determine the kinetics of development of the antiviral state in human cells. The amount of virus released was monitored by reverse transcriptase (RT) activity in the culture medium. Our results demonstrated that 4-6 hr after the addition of 60 units/ml of interferon, a 70-80% inhibition in the amount of virus released is observed. The antiviral state induced in RD-114 cells appears to be rather stable since, even 24 hr after interferon removal, an inhibition of over 70% in virus release is observed in cells treated with 60 units/ml. Human interferon and mouse interferon showed species specificity when tested for their effect on virus release from RD-114 and mouse NIH/3T3 (murine leukemia virus) cells. Human interferon was titrated on RD-114 cells by either monitoring the RT activity in the culture medium or determining the reduction in the number of plaques formed on cell monolayers, after infection with vesicular stomatitis virus. Both assays were identical in their sensitivity. The use of the RT assay for detection of human fibroblast interferon during its purification by ion-exchange chromatography is illustrated.