Comparative analysis of IFN-γ B7.1 and antisense TGF-β gene transfer on the tumorigenicity of a poorly immunogenic metastatic mammary carcinoma

Cancer progression is attributed in part to immune evasion strategies that include lack of co-stimulation, down-regulation of cell surface MHC molecules, and secretion of immunosuppressive factors, such as transforming growth factor-β (TGF-β). Gene therapy has been employed to counter these mechanisms of immune evasion by transference of B7.1, IFN-γ or antisense TGF-β genes into tumor cells, resulting in cell surface expression of B7.1, upregulation of MHC class I and class II molecules, or elimination of tumor-derived TGF-β, respectively. Although each of these transgenes has been shown to alter tumorigenicity in murine models, a direct comparison of their efficacy has not been performed. In this study, we have employed a very aggressive, poorly immunogenic and highly metastatic mammary model, 4T1, to compare the efficacy of B7.1, IFN-γ and antisense TGF-β gene transfer in stimulating an anti-tumor response. We demonstrate that both IFN-γ and antisense TGF-β gene expression significantly reduced the tumorigenicity of these cells compared to mock transduced cells, with IFN-γ having a greater effect. In contrast, B7.1 gene transfer did not affect the tumorigenicity of 4T1 cells. The anti-tumor response directed against antisense TGF-β-expressing 4T1 tumors was mediated by CD4⁺ and CD8⁺ T cells. However, CD8⁺ T cells, and not CD4⁺ T cells, appeared to mediate the anti-tumor response against IFN-γ-expressing tumors. Treatment of tumor-bearing animals with IFN-γ or antisense TGF-β gene-modified tumor cell vaccines reduced the number of clonogenic metastases to the lungs and liver compared to treatment with mock-transduced cells. Finally, in a residual disease model in which the primary tumor was excised and mice were vaccinated with irradiated tumor cells, treatment of mice with vaccinations consisting of 4T1 cells expressing both antisense TGF-β and IFN-γ genes was the most effective in prolonging survival.