Title: Single-cell analysis of the brain microenvironment in breast cancer brain metastasis
Abstract: Breast cancer brain metastasis affects 10-20% of breast cancer patients with a median survival time of less than 2 years and very few treatment options. An emerging possibility for these patients is drugs that alter the microenvironment of brain metastases to facilitate an inhibitory immune response. Interestingly, previous studies have shown drugs that reprogram elements of the microenvironment (such a tumor associated macrophages) result in more effective treatment outcomes than drugs that eliminate cell types entirely. While promising, this treatment approach requires careful characterization of cell phenotypes and the subpopulations that make up the tumor niche as well as an understanding of how these cells can influence one another. To address this descriptive need, we performed a cardiac injection of human breast cancer cells (MDA-MB-231BR2) in Foxn1nu/nu mice to mimic breast cancer metastasis and collected myeloid, astrocyte, and cancer cells using fluorescence activated cell sorting (FACS) from 3 unaffected and 3 metastatic mouse brains. We then performed single-cell RNA sequencing on >50,000 cells and identified multiple cell states enhanced in metastatic brains. Notably, we find subpopulations of microglia (the resident macrophages of the brain) with increased proliferation, translation, immune recruitment, and interferon response. Computational analysis predicts that immune recruitment is an early microglial response to metastasis and most subpopulations appear late, likely after peripheral infiltration. Future work will be focused on investigating the relationship between microglial activation and peripheral immune recruitment as well as identifying cancer-secreted factors that can directly influence microglia states.