Background: TGF-β is a pleotropic cytokine, which has emerged as a potential target in cancer treatment due to its dual role in tumorigenesis and homeostasis. There are three isoforms of TGF-β (TGF-β1, TGF-β2 and TGF-β3), which are secreted by immune and nonimmune cells as a latent complex. Depending on the local context, TGF-β adopts opposing roles in carcinogensis and in modulating the immune system. These dueling roles of TGF-β are dependent on its secretion and activation. Local radiation therapy (RT) can activate TGF-β via reactive oxygen species. Such TGF-β expression is linked to radioresistance and dose-limiting toxicities, reducing the effectiveness of RT. In these studies, we aim to characterize the effect of RT on the temporal and cell-specific expression patterns of TGF-β isoforms in mouse tumor models. This will inform treatment regimens combining isoform specific anti-TGF-β therapy with RT.

Methods: Fluorescence-activated cell sorting (FACS): C57BL/6 mice were implanted on the hind limb with B16-F10 melanoma cells. On day 10, tumors were irradiated locally with 15 Gy. Expression of TGF-β isoforms was measured at 1, 3 and 5 days post-RT by FACS. In vivo: C57BL/6 mice were implanted with tumors and irradiated as described. Mice were treated (10/group) with anti-TGF-β1, anti-TGF-β3 or a pan-TGF-β antibody beginning 1 day after RT given intraperitoneally (200 ug/mouse) every other day for 8 doses. Tumor growth and overall survival were monitored. A similar experiment was conducted in the 4T1 breast cancer model, in which mice were treated 1 day prior to radiation.

Results: FACS data indicated that TGF-β1 and TGF-β3 expression increases on most immune cells in the tumor 1 day after RT, decreases 3 days after RT and reaches a peak 5 days after RT. Preliminary in vivo studies demonstrate that both αTGF-β1 and αTGF-β3 as monotherapies have activity against B16 melanoma. In combination with RT, αTGF-β3 shows greater antitumor activity compared to αTGF-β1 in melanoma. Similar observations were obtained in a 4T1 breast model; however, αTGF-β3 alone and in combination with RT as well as αTGF-β1 + RT showed a significant delay against tumor growth. No significant differences in survival were seen in either tumor model.

Conclusions: TGF-β1 and TGF-β3 are expressed on numerous lymphoid and myeloid cells in B16 tumors and spleens. TGF-β isoform expression peaks 5 days post-RT. Anti-TGF-β therapy is effective in delaying tumor growth and may synergize with RT in certain cancers. This demonstrates rationale for the use of anti-TGF-β therapy to enhance the effectiveness of RT in cancer.

Citation Format: Aditi Gupta, Sadna Budhu, Rachel Giese, Jacques van Snick, Catherine Uyttenhove, Gerd Ritter, Jedd Wolchok, Taha Merghoub. Targeting specific TGF-β isoforms in combination with radiation therapy leads to differential antitumor effects in mouse models of cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4716.