miR-146a Controls Immune Response in the Melanoma Microenvironment

MicroRNAs (miR) are small noncoding RNAs that regulate gene expression, posttranscription, and manipulate immune responses in different types of cancers. In this study, we identify miR-146a as a negative regulator of immune activation, comparable to immune-checkpoint molecules. miR146a levels were increased in melanoma microenvironmental tissue, and miR-146a(-/-) mice survived longer and developed less metastases in comparison with wild-type melanomabearing mice. T cells isolated from miR-146a(-/-) mice revealed higher expression levels of the miR-146a target gene Stat1 and the Stat1-regulated cytokine IFN gamma. Neutralization of IFN gamma in miR-146a(-/-) mice decreased survival and increased melanoma metastasis patterns to those of wild-type mice. In vitro, IFN gamma reduced melanoma cell migration, cell-cycle activity, and basal metabolic rate. Conversely, IFN gamma 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 gamma 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 antitumor immune response elicited by checkpoint therapy. Significance: These findings identify a microRNA-based mechanism by which melanoma cells escape the immune system, providing a new therapeutic strategy to improve the current management of patients with melanoma.