Augmentation of antitumor immunity by tumor cells transduced with a retroviral vector carrying the interleukin-2 and interferon-γ cDNAs

Therapeutic models using gene transfer into tumor cells have pursued three objectives: (1) to induce rejection of the tumor transduced with therapeutic genes, (2) to induce immune-mediated regression of metastatic disease, and (3) to induce long-lasting immunity to protect against challenge with tumor cells or clinical regrowth of micrometastatic disease. Because in vivo therapy for patients with cancer using gene transfer would, as a first step, attempt to eliminate the existing tumor, we have investigated whether antitumor immunity induced by tumor cells secreting a single cytokine could be increased by cotransfer of a second cytokine gene. To test this approach, CMS-5, a murine fibrosarcoma, was transduced with retroviral vectors carrying interleukin-2 (IL-2), interferon-γ (IFN-γ), or granulocyte-macrophage- colony-stimulating factor (GM-CSF) cDNA alone or IL-2 cDNA in combination with IFN-γ or GM-CSF cDNA. Single cytokine-secreting clones were selected to match levels of cytokine production by double cytokine-secreting clones so that similar amounts of cytokine were secreted. IFN-γ- and IL-2/IFN-γ- secreting CMS-5 cells showed increased levels of major histocompatibility complex class I expression compared with IL-2- and GM-CSF-secreting or parental CMS-5 cells. IL-2/IFN-γ-secreting CMS-5 cells were always rejected by syngeneic mice, whereas the same number of CMS-5 cells secreting only one of these cytokines or mixtures of single cytokine-secreting CMS-5 cells were not rejected. In vivo depletion of CD4⁺, CD8⁺, or natural-killer effector cell subpopulations showed that CD8⁺ cytotoxic T cells were primarily responsible for rejection of IL-2/IFN-γ-transduced tumor cells. Our data show the successful use of a single retroviral vector to stably transduce two cytokine genes into the same tumor cell, leading to an increased effect on the in vivo induction of antitumor immunity.