LAUSR

Tumor-associated macrophages (TAMs) promote triple-negative breast cancer (TNBC) progression. Here, we report BRCA1-IRIS overexpressing (IRISOE) TNBC cells secrete high levels of GM-CSF in a HIF-1a- and a NF-kB-dependent manner to recruit macrophages to IRISOE cells and polarize them to pro-tumor M2 TAMs. GM-CSF triggered TGF-b1 expression by M2 TAMs by activating STAT5, NF-kB and/or ERK signaling. Despite expressing high levels of TGF-b1 receptors on their surface, IRISOE TNBC cells channeled TGF-b1/TbRI/II signaling towards 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 TGF-b1. Co-injecting macrophages with IRISOE TNBC cells induced earlier metastasis in athymic mice accompanied by high levels of circulating GM-CSF and TGF-b1. IRISOE TNBC cells expressed low levels of calreticulin (the "eat me" signal for macrophages) and high levels CD47 (the "don't eat me" signal for macrophages) and PD-L1 (a T-cell inactivator) on their surface. Accordingly, IRISOE TNBC tumors had significantly few CD8+/PD-1+ cytotoxic T-cells and more CD25+/FOXP3+-regulatory T cells. These data show that the bi-directional 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.