Abstract 4915: Evaluation of the immune response following treatment with anti-CTLA-4 antibody, radiation therapy or the combination in a murine model of breast cancer
Authors: Franklin Maryland R. , Thayer Matt , Draper David , Saims Dan , Wise Scott 
Published: July 2016

Abstract
While breast cancers are considered poorly immunogenic, several approaches utilizing immunotherapies are being undertaken in the clinic to evaluate their potential for improving outcomes. Radiation therapy (RT) is a highly utilized clinical treatment modality in breast cancer, is known to modify the tumor microenvironment and has been shown to potentially synergize with immunotherapies. To evaluate this potential synergy preclinically, we utilized the 4T1 murine mammary carcinoma model to investigate the immune response following treatment with either the 9H10 clone of an anti-CTLA-4 antibody (9H10), RT (8Gy, QDx3) or the combination of the two. Tumors were established in the mammary fat pad and treatments initiated when tumors reached a mean volume of 63-72mm3. We found that both monotherapies resulted in a period of stable disease followed by tumor regrowth. The combination did not demonstrate added benefit over either single agent treatment. As the 4T1 model is known to be highly metastatic, with tumors seen in lung, liver and lymph nodes, we closely examined each mouse at the time of euthanasia for evidence of metastatic disease. None of the treatments effectively reduced metastasis. To attempt to gain a better mechanistic understanding around the lack of added benefit of the combination, we profiled different immune cell subsets in the tumor by flow cytometry. We found that within the tumor infiltrating lymphocytes (TILs), 9H10 antibody and RT treatment resulted in an increased percentage of CD4+ T cells. Conversely a decreased percentage of CD8+ T cells was observed. However, the greatest effect exerted by 9H10 and RT was on the myeloid derived suppressive cell (MDSC) population. Although RT slightly decreased the percentage of MDSC within tumors and 9H10 had no effect, the two treatments synergized to reduce the number of tumor-infiltrating MDSC. Interestingly, a similar synergistic effect was observed when we analyzed the expression of the proliferation marker Ki-67 within CD8+ T cells. We also found that RT, and to a lesser extent 9H10, increased the percentage of CD45+ cells expressing the T cell inhibitory receptors PD1, PD-L1, and CTLA-4. No additive effect on the expression of these receptors was observed with combined therapy. Taken together, our results suggest that anti-CTLA-4 treatment combined with RT triggers both pro- and anti-tumor signaling pathways thus providing a possible explanation for the marginal anti-tumor responses we observed with these therapies in this disease model. Furthermore, this model could be very beneficial to evaluate other agents either in combination with 9H10 or RT or as a triple combination with both 9H10 and RT.
Citation Format: Maryland R. Franklin, Matt Thayer, David Draper, Dan Saims, Scott Wise. Evaluation of the immune response following treatment with anti-CTLA-4 antibody, radiation therapy or the combination in a murine model of breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4915.