Characterization of the Indirect Antitumor Effect of γ-Interferon Using Ascites-associated Macrophages in a Human Tumor Clonogenic Assay

The indirect antitumor effects of recombinant γ-interferon (IFN-γ) were investigated using an in vitro tumor clonogenic assay modified to include ascites-associated macrophages (AAM). Untreated AAM stimulated tumor colony growth; conversely, AAM treated with IFN-γ at clinically achievable doses demonstrate a significant growth-inhibiting effect. The indirect antiproliferative activity was dependent on the density of AAM. Supernatants from IFN-γ-pretreated AAM cultures derived from 11 different ovarian cancer patients significantly inhibited the colony growth of ovarian cancer cell line BG-1, as well as five of six other cell lines. Physicochemical characteristics of the supernatant indicated that a significant part of the antiproliferative activity is heat sensitive, destroyed by proteolytic enzymes, and is dependent on RNA and protein synthesis for production. Neutralizing antiserum against tumor necrosis factor significantly reduced the antiproliferative activity of the supernatants. Production of this factor by AAM was induced by exposure to 1000 units/ml of IFN-γ for 15 min, although activity in the supernatants was not detected until 8 h after exposure to IFN-γ. Potency of the supernatants reached a peak 12 h after priming and ceased by 22 h. Production of antiproliferative activity was maintained over 5 days by intermittent treatment of AAM with IFN-γ. Combinations of IFN-γ and supernatant from IFN-γ-treated AAM showed potentiated antiproliferative activity against BG-1 in an additive to synergistic manner. Antitumor effects of IFN-γ may be dependent on tumor-associated macrophages and treatment scheduling.