Abstract 5031: Regulatory T cell ablation accelerates early stage breast cancer progression

Breast cancer is the most common malignancy in women worldwide. Early detection of pre-invasive breast tumor has dramatically improved due to the routine screening for women. Ductal carcinoma in situ is a non-obligate precursor of breast cancer, and it only progresses to invasive breast cancer in about 40% of patients. We have previously shown that transient ablation of regulatory T (Treg) cells in established breast tumors results in significant reduction of primary and lung metastatic tumor growth. In this study, we aim to investigate the specific role of Treg cells at the pre-malignant stages. Using a spontaneous, stage-wise MMTV-driven polyoma middle T (PyMT) model of murine mammary carcinogenesis, crossed to Foxp3DTR knock-in mice where injection of diphtheria toxin leads to a complete ablation of Treg cells for around 7 days, we found that ablation of Treg cells at the pre-invasive tumor stage resulted in a significant increase in the number of mammary glands developing tumors, as well as tumor growth compared to control mice. Histological examination of tissues revealed that the area of the incipient tumors was larger in Treg cell-ablated mice, and presented a more advanced tumor stage. In addition, Treg cell ablation resulted in a pronounced mammary inflammatory infiltrate, including an increase in tumor-associated macrophages and a soluble cytokine network conducive to tumor progression. Interestingly, Treg cell ablation increased the percentage of cancer stem/progenitor cells in the mammary compartment. Overall, our study demonstrates that unlike their role in established cancers, Treg cells ablation promotes breast cancer progression at early stages. Citation Format: Wei Du, Leandro Martinez, Valentina Robila, Nicholas Clark, Michael Idowu, Paula Bos, Paula Bos. Regulatory T cell ablation accelerates early stage breast cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5031.