Tumor-derived TGF-β Mediates Conversion of CD4⁺Foxp3⁺ Regulatory T Cells in a Murine Model of Pancreas Cancer

Abstract

CD4⁺25⁺Foxp3⁺ regulatory T cells (T_reg) play a critical role in the induction of tolerance to tumor-associated antigens and suppression of antitumor immunity. How T_reg are induced in cancer is poorly understood. We reported previously that T_reg are significantly elevated in the peripheral blood of patients with pancreas cancer and that in a murine pancreas cancer model induction of T_reg seems to be transforming growth factor (TGF)-β dependent. Here we provide additional evidence that T_reg are increased locally within the tumor microenvironment by a mechanism that seems dependent on TGF-β receptor expression and the presence of tumor derived TGF-β. The murine pancreas cancer cell line Pan02 produces high levels of TGF-β both in vitro and in vivo. In contrast, the esophageal murine cancer cell line, Eso2, does not. Immunohistochemical staining of Foxp3 in explanted tumors shows an identifiable population of T_reg in the Pan02 (TGF-β positive) tumors but not Eso2 (TGF-β negative). Naive CD4⁺25⁻Foxp3⁻ T cells, when adoptively transferred into Rag^−/− mice, are converted into Foxp3⁺ T_reg in the presence of Pan02 but not Eso2 tumors. Induction of T_reg in Pan02 mice is blocked by systemic injection of an anti–TGF-β antibody. If Rag^−/− mice are instead reconstituted with naive CD4⁺25⁻ T cells expressing a mutated TGF-β receptor, induction of Foxp3⁺ T_reg in Pan02 bearing mice is blocked. Collectively, these observations further support the role of TGF-β in the induction of T_reg in pancreas adenocarcinoma.