Abstract A44: Pharmacologic targeting of estrogen receptor in melanoma to enhance antitumor immunity

Metastatic melanoma is one of the most aggressive, morbid cancers with a median survival of 6-9 months. The development of MAPK-pathway inhibitors and immune checkpoint blockade (ICB) agents (α-PD1 and α-CTLA4) has significantly improved outcome in patients with melanoma. However, de novo and acquired resistance to these therapies remains a major impediment to achieving durable clinical responses in most patients. Epidemiologic studies have implicated sex steroids in melanoma etiology; however, the mechanisms by which these hormones impact the development and progression of melanoma are unclear. The aim of this study is to evaluate the extent to which estrogens, such as 17β-estradiol (E2), working through its cognate nuclear receptor(s), impact the pathobiology of melanoma. Using estrogen receptor alpha negative (ERα-) syngeneic (GEMM6, B16F10 and YuMM5.2 cells) and a transgenic B-RafCA/WT; Ptenf/f; mTyr-Cre/ERT2 (iBP) mouse model of melanoma, we have determined that manipulating circulating E2 levels in these mice models has a dramatic effect on tumor growth. This E2-mediated increase in tumor growth was not observed in NOD-scid/IL2null mice, suggesting that this hormone exerts its actions by modulating intratumoral immune cell repertoire and function. Indeed, multiparametric flow cytometry analysis of tumor-infiltrating immune cells indicates that E2 increases M2 polarization of macrophages in GEMM6 and iBP mouse models and decreases proinflammatory M1 macrophages in the B16F10 and YuMM 5.2 tumor models. Characterization of (a) tumor associated-macrophages (TAMs) from E2-treated mice and (b) bone marrow-derived macrophages (BMDM) from non-tumor bearing WT C57BL/6J mice differentiated in the presence of tumor conditioned media and E2, revealed increased expression of immune-suppressive cytokines (Tnf, Retnla, Il10 and Ptgs2). The E2 treated cells also demonstrated an enhanced ability to suppress CD8+ T-cell proliferation, an activity that could be reversed by addition of the ER antagonist ICI182780. Either pharmacologic depletion of TAMs by clodronate liposomes in GEMM6 tumor model or myeloid cell selective ablation of ERα expression negated the effects of E2 on tumor growth (B16F10, YuMM5.2). Further, CD8+T cell depletion (YuMM 5.2) increased tumor growth in ovariectomized animals to a level observed in E2-treated animals, suggesting that TAMs in E2-treated mice suppress CD8+ T-cell function and enhance tumor growth. Finally, treatment with ICI182780 inhibited the effects of E2 on tumor growth and immune cell repertoire (GEMM6, B16F10, and YuMM 5.2), confirming the role of ER in melanoma biology. Taken together, these results suggest that E2, working through ERα, promotes an immune-suppressive microenvironment during melanoma progression. These findings have informed the design of a clinical study that will assess the benefit of using antiestrogens, together with ICBs or standard of care, to treat melanoma. Citation Format: Binita Chakraborty, Jovita Bymerwa, Robert Baldi, Wen Liu, Ching-Yi Chang, Donald McDonnell. Pharmacologic targeting of estrogen receptor in melanoma to enhance antitumor immunity [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr A44.