Direct and Indirect Effects of Interferon on in Vivo Murine Tumor Cell Growth

We cloned two sublines (S1 and R1) of murine Meth A fibrosarcoma cells with respect to their sensitivity to a murine α/β-interferon (IFN) preparation. The growth of S1 cells was suppressed and that of R1 cells was hardly affected by IFN in vitro . This was also the case with cells enclosed in cell-impermeable diffusion chambers in peritoneal cavities. Nevertheless, IFN suppressed the growth of not only S1 cells but also R1 cells in mice inoculated i.p. with these cells, and the survival rates of both S1 cell recipients and R1 cell recipients were markedly improved. S1 cells were observed microscopically to be injured by the direct effect of IFN in vitro and in vivo , but R1 cells in in vitro culture with IFN and those surviving in vivo in the presence of IFN appeared to proliferate well. In the peritoneal cavity of R1 recipients treated daily with IFN, the recruitment of macrophages was enhanced in comparison with untreated R1 recipients. Adherent peritoneal exudate cells obtained from IFN-treated, R1-bearing mice were highly suppressive for the in vitro growth of not only R1 cells but also allogeneic and human cells. The role of macrophages in the indirect effect of IFN on tumor cell growth is discussed.