In situ analysis of the metastatic immune milieu in breast cancer brain metastasis at the single-cell level.

Abstract

Abstract Despite advances in early detection and treatment, breast cancer remains the second-leading cause of death among women. Among these cases, incidence of breast cancer brain metastasis (BCBM) carries a poor prognosis with a mean survival of 6 months following diagnosis, demonstrating a drastic unmet need for effective treatments. As the most abundant resident immune cell in the brain, microglia are poised to be effective first-responders to combat tumor infiltration, yet their response to metastases has not been fully characterized. Prevailing literature suggests that microglia promote metastatic outgrowth, however these works fail to adequately discriminate microglia from other brain resident and infiltrating macrophages. Employing a mouse model harboring a deletion in a super-enhancer of macrophage colony-stimulating factor 1 receptor which results in a near-complete loss of microglia, we find more tumor outgrowth compared to animals with microglia. We subsequently compared the metastatic immune milieu in the microglia replete and deficient mice using single-cell RNA sequencing and Akoya’s CODEX (CO-Detection by indEXing) high-parameter immunofluorescence multiplexing platform. We demonstrate that microglia undergo a pro-inflammatory shift in response to BCBM and infiltrating lymphocytes fail to express the inducible costimulatory molecule, ICOS, in the absence of microglia. To understand how microglia promote productive T cell responses to BCBM, we used CODEX to interrogate 20+ different biomarkers in BCBM with or without microglia. Through spatial phenotyping of immune cells from mice with and without microglia we uncover the importance of the microglia/lymphocyte crosstalk in controlling BCBM outgrowth. Supported by NIH Grant (R01 CA237376-02)