Interferon Regulatory Factor-8 (IRF8) Expression in Macrophages Governs an Anti-Metastatic Program in Breast Cancer

High macrophage infiltration in cancer is associated with reduced survival in animal models and in patients. This reflects a shift in the macrophage response from a tumor-suppressive to tumor-supportive program governed by transcriptional events regulated by the inflammatory milieu. Although several transcription factors are known to drive a pro-metastatic program, those that govern an anti-metastatic program are less understood. Interferon regulatory factor-8 (IRF8) is integral for macrophage responses against infections. Using a genetic loss-of-function approach, we tested the hypothesis that IRF8 expression in macrophages governs their capacity to inhibit metastasis. We found that: 1) metastasis was significantly increased in mice with IRF8-deficient macrophages; 2) IRF8-deficient macrophages displayed a neutrophil-like program enriched for metastasis-associated genes; and 3) lower IRF8 expression correlated with reduced survival in human breast cancer and melanoma with high or low macrophage infiltration. These data demonstrate a previously unrecognized role for IRF8 in macrophage biology to control metastasis. Overall design: Bone marrow-derived macrophages were treated with IFN-? (100u/mL) or IL-4 (20ng/mL) for 24h, RNA isolated and RNA-Seq performed on the samples.