Abstract 449: Progesterone-induced immunosuppression thwarts immunosurveillance to tumors and promotes lung metastasis in a breast cancer model.

Exposure to sexual hormones is one of the strongest risk factors for breast cancer. Increased risk is entwined with a greater number of reproductive cycles and the use of progestin-containing hormone replacement therapy. In addition, the importance of progesterone in dampening immune response was illustrated in several models of infection and pregnancy. Based on the tolerogenic properties of progesterone (Pg) and its promoting role in breast cancer, in this work we investigated whether progesterone may thwart immunosurveillance and promote tumor metastasis by fostering an immunosuppressive microenvironment in breast cancer. Exogenously added Pg: (10−8M) or its synthetic analogue medroxiprogesterone acetate (MPA: 10−8M) induced the expression of the immunosuppressive protein galectin-1 (Gal1) in the hormone-dependent human breast cancer cell lines T47D and MCF-7 and in the mouse mammary adenocarcinoma C4HD. This effect was abrogated by pre-treatment with RU486 (10−7M) indicating that the progesterone receptor (PR) was involved. Interestingly, in vitro MPA treatment of human PBMC or mouse splenocytes controlled the expansion of regulatory T cells (Tregs) (CD4+CD25+Foxp3+) and skewed the balance toward a Th2-type cytokine profile. Furthermore, differentiation from naive T-cells to Tregs cells was enhanced in the presence of MPA, IL-2 and suboptimal concentrations of TGF-β. In addition, in vivo treatment with MPA (15mg 30-day release depot) of C4HD tumor-bearing mice increased both the expression of Gal1 and the frequency of Treg cells in tumor-draining lymph nodes and the tumor microenvironment. While the increase in Treg cell frequency was associated with an effect on Treg cell SMAD4 expression that potentiates TGF-β signalling, homing of Treg cells to tumor sites was supported through the induction of tumor-derived CCL22. Finally, we used a PR negative breast tumor model (4T1) to evaluate the effect of MPA on the tumor microenvironment and the consequences of this effect on tumor progression. In vivo treatment with MPA promoted lung metastasis by inducing an invasive phenotype in breast cancer cells. Co-culture of Treg cells differentiated in vivo in the presence of MPA with 4T1 cells promoted its invasive activity and modulate several genes associated with EMT, including SNAIL. In summary, these results demonstrate that progesterone hierarchically fosters an immunosuppressive tumor microenvironment that promotes metastasis by co-ordinately regulating Gal1 expression, stimulating the TGF-β pathway and augmenting the frequency of Treg cells. Our findings provide new insights into the role of progesterone in breast cancer as a crucial promoter of tumor-immune escape. Citation Format: Mariana Salatino, Tomas Dalotto, Diego O. Croci, Santiago P. Mendez Huergo, Sebastian L. Dergan Dylon, Juan P. Cerliani, Marta A. Toscano, Gabriel A. Rabinovich. Progesterone-induced immunosuppression thwarts immunosurveillance to tumors and promotes lung metastasis in a breast cancer model. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 449. doi:10.1158/1538-7445.AM2013-449