Tumor-associated macrophages (TAM) promote triplenegative breast cancer (TNBC) progression. Here, we report BRCA1-IRIS-overexpressing (IRISOE) TNBC cells secrete high levels of GM-CSF in a hypoxia-inducible factor-1a (HIF1a)-and a NF-kB-dependent manner to recruit macrophages to IRISOE cells and polarize them to protumor M2 TAMs. GM-CSF triggered TGFb1 expression by M2 TAMs by activating STAT5, NF-kB, and/or ERK signaling. Despite expressing high levels of TGFb1 receptors on their surface, IRISOE TNBC cells channeled TGFb1/TbRI/II signaling toward AKT, not SMAD, which activated stemness/EMT phenotypes. In orthotopic and syngeneic mouse models, silencing or inactivating IRIS in TNBC cells lowered the levels of circulating GM-CSF, suppressed TAM recruitment, and decreased the levels of circulating TGFb1. Coinjecting macrophages with IRISOE TNBC cells induced earlier metastasis in athymic mice accompanied by high levels of circulating GM-CSF and TGFb1. IRISOE TNBC cells expressed low levels of calreticulin (the "eat me" signal for macrophages) and high levels of CD47 (the "do not eat me" signal for macrophages) and PD-L1 (a T-cell inactivator) on their surface. Accordingly, IRISOE TNBC tumors had significantly few CD8 þ /PD-1 þ cytotoxic T cells and more CD25 þ /FOXP3 þ regulatory T cells. These data show that the bidirectional interaction between IRISOE cells and macrophages triggers an immunosuppressive microenvironment within TNBC tumors that is favorable for the generation of immune-evading/ stem-like/IRISOE TNBC metastatic precursors. Inhibiting this interaction may inhibit disease progression and enhance patients' overall survival.Significance: The BRCA1-IRIS oncogene promotes breast cancer aggressiveness by recruiting macrophages and promoting their M2 polarization.

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