Effects of leukocyte and fibroblast interferon on events in the fibroblast cell cycle

Serum-depleted human foetal skin fibroblasts were stimulated by addition of 10% foetal calf serum to proliferate synchronously for at least one cell cycle. This proliferation was suppressed by leukocyte or fibroblast interferon (IF), which prolonged the G1 phase and diminished the rate of DNA synthesis during the S phase in a dose-dependent manner. When used in identical concentration, as judged in terms of units of antiviral activity, fibroblast IF had more pronounced effects on cell cycle events than leukocyte IF. Interferon exerted its effect in early G1, before the cells were irreversibly committed to DNA synthesis.