Estradiol Reprograms Microglia to Create an Immune-Suppressed Niche Permissive to Breast Cancer Brain Metastasis

Young age is an independent risk factor for the development of breast cancer brain metastases (BM). Prior work showed that 17β-estradiol (E2), the predominant premenopausal hormone, promotes BM of tumors intrinsically unresponsive to E2, in part through modulating estrogen receptor-alpha expressing (ERα⁺) glial cells. However, how E2 reshapes the brain tumor microenvironment (TME), particularly microglia-mediated immunity, and its impact to BM progression remains unclear.

scRNA sequencing and multiparametric flow cytometry were used to define the impact of E2 and E2-suppression on brain immune-cell populations across different stages of BM progression using spontaneous and experimental models of BM. Depletion of microglia and T cell co-cultures were used to study microglia's role in E2-induced BM. The effects of E2-suppression alone or in combination with whole brain radiotherapy were tested in preclinical models mimicking late-stage BM.

E2 repressed immune surveillance and immune activation programs in microglia from early to late stages of brain metastatic progression, suppressing recruitment of effector immune cells to BM. Estrogen suppression, in turn reactivated anti-tumoral signaling in microglia and increased recruitment of effector immune cells to the brain. Microglia from E2-stimulated BM-bearing mice showed decreased ability to induce interferon cytotoxic function and expansion of activated T cells. Conversely, E2-suppression reactivated an effective anti-tumoral response and synergized with RT to significantly decrease BM progression.

These findings reveal a previously unrecognized mechanism by which E2 accelerates BC-BM progression through microglial immunosuppression and support evaluation of endocrine therapies as adjunct treatments for ER⁻ brain metastases.

Standard of care for BM includes stereotactic radiosurgery (SRS) alone or in combination with surgery, systemic chemotherapy or targeted therapies. Our studies show that ovariectomy (which eliminates ovarian E2) and aromatase inhibitors (AIs, which eliminate peripheral E2 synthesis) reduce progression of BM when used in combination with WBRT and in immuno-competent models. We demonstrate E2 promotes an immunosuppressive brain microenvironment from early stages of metastatic progression, in part through modulation of myeloid cells and repression of recruitment of effector T cells to the brain. Thus, these studies suggest that FDA-approved E2-depletion therapies (aromatase inhibitors and selective-estrogen modulators) could be used in combination with brain irradiation to decrease BM progression.