Endogenous interferon-gamma acts directly on tumor cells in vivo to suppress growth

Background: Recent evidence implicates endogenous interferon-gamma (IFN-gamma) in the host response to an immunogenic tumor (MCA105); antibody blockade of host IFN-gamma increased tumor growth rate (Doherty ct al., Ann. Surg. Oncol. 3: 198-203, 1996). Those experiments did not attempt to determine the site of IFN-gamma activity (the host, the tumor, or both). Materials and Methods: MCA101 murine tumor cells were transfected with a plasmid expression vector containing an antisense construct, to the IFN-gamma receptor (IFN-gamma R) or a control construct. Clones were isolated and tested for IFN-gamma stimulation of MHC I expression, sensitivity to IFN-gamma growth effects in vitro and specific [I-125]IFN-gamma binding. Results: The antisense strategy was successful in decreasing the number of cell-surface IFN-gamma binding sites and in vitro response to IFN-gamma. Finally, in vivo experiments demonstrated significantly increased untransfected tumor growth rate in animals after blockade of endogenous IFN-gamma by a single dose of anti-IFN-gamma antibody and more rapid growth of the IFN-gamma R-deficient cells compared to controls. Conclusion: endogenous IFN-gamma has a direct effect on this less immunogenic tumor in vivo which serves to slow growth and which is, at least partially, mediated through interferon-gamma receptors on the tumor cell. (C) 1996 Academic Press, Inc.