Antitumor effects of interferon in mice injected with interferon‐sensitive and interferon‐resistant friend leukemia cells. I.

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Abstract
Interferon‐sensitive (745) and interferon‐resistant (3Cl‐8) Friend leukemia cells (FLC) are highly tumorigenic for DBA/2 mice. The phenotype of interferon sensitivity or resistance does not change with in vivo passage. Daily administration of mouse interferon markedly enhanced the survival time of mice injected with either 745 or 3Cl‐8 cells. Use of quantitative methods for determining the number of FLC (colony formation in agarose and immunofluorescence) permitted us to show that potent, partially purified or highly purified mouse interferon (s.a. 0.5 to 1 × 109 u/mg protein) induced a 100‐ to 1,000‐fold decrease in the number of tumor cells in the peritoneal cavity in the days following inoculation of 745 or 3Cl‐8 cells. Interferon decreased the number of FLC even when treatment was initiated at a time when tumor cells were multiplying exponentially in the peritoneal cavity. There was no evidence that interferon acted as an inducer of FLC differentiation in vivo. The finding that interferon was equally effective in mice inoculated with interferonresistant cells as in mice inoculated with interferon‐sensitive cells suggests that in this experimental system interferon does not act directly on the tumor cells, but that the interferon‐induced antitumor activity is mediated by the host.