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miR-146a controls immune response in the melanoma microenvironment

    Micro-RNAs (miR) are small non-coding RNAs that regulate gene expression, post-transcription, and manipulate immune responses in different types of cancer. In this study, we identify miR-146a as a negative regulator of immune activation, comparable to immune checkpoint molecules. miR-146a levels were increased in melanoma microenvironmental tissue, and miR-146a-/- mice survived longer and developed less metastases in comparison to wild type melanoma-bearing mice. T cells isolated from miR-146a-/- mice revealed higher expression levels of the miR-146a target gene Stat1 and the Stat1-regulated cytokine Interferon-γ (IFN-γ). Neutralization of IFN-γ in miR-146a-/- mice decreased survival and increased melanoma metastasis patterns to those of wild-type mice. In vitro, IFN-γ reduced melanoma cell migration, cell cycle activity, and basal metabolic rate. Conversely, IFN-γ also increased PD-L1 levels on the melanoma cells, which may counterbalance some of the beneficial effects increasing immune escape in vivo. Combined treatment with a miR-146a antagomiR and anti-PD-1 resulted in improved survival over isotype-control or anti-PD-1 treatment alone. In summary, these data show that miR-146a plays a central role within the STAT1/IFN-γ axis in the melanoma microenvironment, affecting melanoma migration, proliferation, and mitochondrial fitness as well as PD-L1 levels. Additionally, combined inhibition of PD-1 and miR-146a could be a novel strategy to enhance anti-tumor immune response elicited by checkpoint therapy.

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