Microglial reprogramming enhances antitumor immunity and immunotherapy response in melanoma brain metastases

Melanoma is one of the solid tumor types with the highest risk of brain metastasis. However, the biology of melanoma brain metastasis and the contribution of the brain immune microenvironment to the responses to therapies remain insufficiently characterized. By using preclinical models and single-cell transcriptomics, we identify microglia, as critical regulators of melanoma biology in the brain. We show that activation of the Rela/NF-kB pathway in microglia promotes melanoma brain metastasis and that targeting this pathway elicits microglia reprogramming towards a pro-inflammatory phenotype that enhances anti-tumor immunity and reduces brain metastatic burden. Additionally, canonical and pro-inflammatory microglial markers in melanoma brain metastasis correlate with better responses to immune checkpoint inhibitors in patients and we show that Rela/NF-kB targeting improves responses to these therapies in the brain. Thus, we propose targeting Rela/NF-kB in activated microglia as a strategy to promote anti-tumor immunity and to improve the response to immune checkpoint inhibitors in melanoma brain metastasis. Overall design: To investigate the role of microglia in the progression of melanoma brain metastases (MBM) we developed murine B16F10 MBM in Cx3cr1-yfp reporter mice We injected animals with saline (Sham-control animals) and animals with melanoma cell via the carotid artery to provoke brain colonisation and therefore obtain MBM. We obtained 4 different samples, composed each of a pool of three different animals with mixed genders. The samples are composed of FACS isolated subpopulations of Cx3cr1+ and double Tmem119+ or CD49D+ subpopulations within the MBM. The experimental groups are the following: CMG (Control microglia); animals injected with saline and operated via the intracarotid artery as well, isolated using CX3cr1+/Tmem119+. TMG (tumor microglia) and T-MAC (Tumor associated macrophages); animals injected with B16F10 cells via the intracarotid artery. Cells isolated using Cx3cr1+ and Tmem119+ or Cx3cr1+ and CD49D respectively. The comparative anaysis have been perform to compare CMG vs either TMG or TA-MAC.