Immune checkpoint inhibitors (ICB) have significantly prolonged patient survival across multiple tumor types, particularly in melanoma. Interestingly, gender specific differences in response to ICB have been observed with males getting more benefit than females, although the mechanism(s) underlying this difference are unknown. Mining published transcriptomic datasets, we determined that response to ICBs is influenced by the functionality of intratumoral macrophages. This puts into context our observation that estrogens (E2) working through the estrogen receptor (ERα) stimulate melanoma growth in murine models by skewing macrophage polarization towards an immune-suppressive state that promotes CD8 + T cell dysfunction/exhaustion and ICB resistance. This activity was not evident in mice harboring a macrophage specific depletion of ERα confirming a direct role for estrogen signaling within myeloid cells in establishing an immunosuppressed state. Inhibition of ERα using fulvestrant, a selective estrogen receptor downregulator (SERD) decreases tumor growth, stimulates adaptive immunity and increases the antitumor efficacy of ICBs. Further, a gene signature that reads on ER activity in macrophages predicted survival in ICB treated melanoma patients. These results highlight the importance of E2/ER as a regulator of intratumoral macrophage polarization; an activity that can be therapeutically targeted to reverse immune suppression and increase ICB efficacy.

The immune system functions in a sexually dimorphic manner with females exhibiting more robust immune responses than males. However, how female sex hormones affect immune function in normal homeostasis and in autoimmunity is poorly understood. In this review we discuss how estrogens affect innate and adaptive immune cell activity and how dysregulation of estrogen signaling underlies the pathobiology of some autoimmune diseases and cancers. The potential roles of the major circulating estrogens, and each of the three estrogen receptors (ERα, ERβ and GPER) in the regulation of the activity of different immune cells are considered. This provides the framework for a discussion of the impact of ER modulators (aromatase inhibitors, Selective Estrogen Receptor Modulators (SERMs) and Selective Estrogen Receptor Downregulators (SERDs)) on immunity. Synthesis of this information is timely given the considerable interest of late in defining the mechanistic basis of gender biased responses/outcomes in patients with different cancers treated with immune checkpoint blockade. It will also be instructive with respect to the further development of ER modulators that modulate immunity in a therapeutically useful manner.

Despite advances in surgery and targeted therapies, the prognosis for women with high-grade serous ovarian cancer remains poor. Moreover, unlike other cancers, immunotherapy has minimally impacted outcomes in patients with ovarian cancer. Progress in this regard has been hindered by the lack of relevant syngeneic ovarian cancer models to study tumor immunity and evaluate immunotherapies. To address this problem, we developed a luciferase labeled murine model of high-grade serous ovarian cancer, STOSE.M1 luc. We defined its growth characteristics, immune cell repertoire, and response to anti PD-L1 immunotherapy. As with human ovarian cancer, we demonstrated that this model is poorly sensitive to immune checkpoint modulators. By developing the STOSE.M1 luc model, it will be possible to probe the mechanisms underlying resistance to immunotherapies and evaluate new therapeutic approaches to treat ovarian cancer.

Significant differences exist in the incidence and severity of various inflammatory diseases between males and females. Further, sex hormones such as estrogens, through their ability to regulate both innate and adaptive immune responses, have been shown to impact the pathobiology of many diseases. Notwithstanding this knowledge, it is not clear how and to what extent ER signaling within immune cells impacts tumor immunity and if and how this can be exploited therapeutically. To address this question, we performed tumor growth studies using various murine estrogen (E2)-nonresponsive breast cancer cell lines. Female mice were ovariectomized to remove the endogenous source of estrogens, and in turn, supplemented with either placebo or slow-releasing E2 pellets. One week later, tumor cells were injected orthotopically and tumor volume measured every two to three days for 14 days. Despite being non-responsive in vitro, E2 administration accelerates the growth of these tumors in vivo, and this activity was ablated using ER-antagonists. To evaluate the contribution of immune cells in estrogen-biology in these tumors, we performed similar studies in NOD-scid IL2Rgammanull (NSG) mice, which lack functional innate and adaptive immune responses. E2 failed to increase tumor growth in these mice, implicating immune cells as mediators of E2 action on tumors. Immunoprofiling of syngeneic tumors revealed a robust and consistent reduction in the number of major histocompatibility complex (MHC) Class II-expressing cells in the tumor microenvironment of E2-treated mice. Single-cell RNA-sequencing of the tumors revealed that the most prevalent MHC-II-expressing cell impacted were dendritic cells. Flow cytometry analysis confirmed the downregulation of MHCII expression on dendritic cells and other myeloid cell populations isolated from E2-treated tumors. MHCII expression on the surface of dendritic cells is important for the presentation of tumor antigens to T cells the decrease of which may compromise anti-tumor immunity. Paradoxically, using bone-marrow derived dendritic cells it was observed that E2 treatment increased the expression of MHC Class II complex, but this positive response was ablated when cells were cultured in tumor-conditioned media. This result suggests that although estrogens have a positive impact on dendritic cell function under normal circumstances these are attenuated by tumor-produced factors. Our ongoing work is focused on a dissection of estrogen signaling in dendritic cells and an evaluation of the impact of mechanistically distinct modulators on these processes and how they influence tumor immunity. Citation Format: Felicia Lim, Corinne Haines, ChingYi Chang, Donald Mcdonnell. Assessing estrogen receptor signaling and function in dendritic cells within the tumor microenvironment [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr B05.