Cytolytic activity of interferon-gamma and its synergism with 5-fluorouracil

Highly purified natural or recombinant human immune interferon (IFN-gamma) was found to be directly cytolytic to certain tumor cell lines in vitro. Out of 5 human tumor cell lines and one normal fibroblast line tested, the colon adenocarcinoma line HT-29 and the rhabdomyosarcoma line A673 were highly sensitive to cytolysis by interferon, as determined by 125I-iododeoxyuridine release in a 72 h microcytotoxicity assay. Cytolysis was marked at IFN-gamma concentrations of less than I U/ml, and it reached a near-maximal level at 6.4 U/ml. A synergistic cytolysis on HT-29 cells of IFN-gamma and 5-fluorouracil (5-FU) was observed at 5-FU concentrations ranging from 64 to 640 micrograms/ml. In contrast, no synergism was observed between IFN-gamma and mitomycin C. The direct cytolytic activity and synergistic cytolysis with 5-FU of the IFN-gamma preparations used in the present study were abolished completely by treatment with a neutralizing monoclonal antibody specific for human IFN-gamma.