Abstract
Breast cancer is commonly thought of as a "women's disease". However, men are increasingly diagnosed with the disease, and their mortality rates are disparately higher than those of female patients. The abundance and composition of the immune microenvironment are determinants of breast cancer progression and survival. It is well documented that there are sex-specific differences in the immune response to several diseases, including various cancers. However, the effects of these differences in the context of breast cancer remain to be explored. This study demonstrates sex differences in the hormonal and immune landscape of the MMTV-PyMT transgenic murine model of female and male ER+ breast cancer using single-cell RNA sequencing (scRNA-Seq), whole-slide immunohistochemistry, and flow cytometry. Mammary tumors of transgenic male mice had increased estrogen receptor alpha expression and enriched nuclear binding signatures compared to female tumors. In the tumor immune compartment, male mice had lower intratumoral leukocyte infiltration. Yet, scRNA-Seq analysis reveals a more immunostimulatory microenvironment and increased antitumor immune populations in the primary and metastatic lungs as compared to transgenic females. Despite a more favorable innate immune profile, the metastatic burden was increased in male mice. Our data support a sex-dependent immune response in mammary carcinoma associated with the tumor, and likely host, hormonal environment. With emerging therapeutics targeting the tumor immune microenvironment, characterizing immune profiles is critical for optimizing their use in all breast cancer patients.