Background: Cytokine genes encode proteins that modulate immune system responses. Modification of tumor cells by the introduction of cytokine genes has been used as a strategy to augment host immunity. Interleukin 7 (IL-7) gene transfer enhances the immune response to tumor cells and can result in tumor regression. Transforming growth factor-β1 (TGF-β1) is a potent immunosuppressive cytokine produced by many tumors. We have previously reported that recombinant IL-7 decreases the expression of TGF-β1 by murine macrophages. Purpose: This study investigates the inhibition of tumor-derived TGF-β1 production as a possible mechanism for the enhanced antitumor immunity that accompanies IL-7 gene transfer. Methods: A fibrosarcoma cell line (FSA-JmIL-7) genetically modified to produce IL-7 was used to evaluate the effects of IL-7 on tumor production of TGF-β1. The control cell line (FSA-Jneo) originated from the same parental fibrosarcoma cell line (FSA) and was produced by transduction with the same retroviral vector without the IL-7 gene. FSA-Jneo and FSA-JmIL-7 tumor cells were evaluated for the expression of TGF-β1 messenger RNA (mRNA). To determine if the observed change in TGF-β1 mRNA was associated with an alteration in protein secretion, we compared supernatants from tumor cell cultures for TGF-β1 production. Specific anti-TGF-βl monoclonal antibody (MAb) was used to confirm the role of TGF-B1 in these assays. Results: Compared with FSA parental and FSA-Jneo cells, FSA-JmIL-7 cells expressed TGF-β1 mRNA at a lower level. Compared with supernatants from FSA-Jneo cells, FSA-JmIL-7 supernatants contained consistently lower levels of TGF-β1 activity (P<.05). In addition, FSA-Jneo supernatants suppressed lymphocyte proliferation to a significantly greater degree than supernatants from FSA-JmIL-7 cells (P<.05). Studies with anti-TGF-β1 MAb added to the supernatants confirmed the role of TGF-β1 in inhibition of lymphocyte proliferation. Conclusion: These findings suggest that IL-7 gene transfer inhibits the production of TGF-β1 by tumor cells and thus may enhance the efficacy of the host's antitumor immune response. Implication: The regulation of endogenous tumor-derived cytokines in response to cytokine gene transfer may contribute to altered immune responses in the tumor microenvironment and thus may be an important additional parameter to assess in gene therapy. [J Natl Cancer Inst 87: 593–597, 1995]