Metastatic dissemination was proposed to occur only in invasive cancer. However, detection of cancer cells in the bone marrow of patients with pre-invasive breast ductal carcinoma in situ (DCIS) showed that dissemination could, through unknown mechanisms, take place earlier. Late evolved invasive breast cancer cells disseminated by recruiting circulating monocyte-derived tumor associated macrophages (TAMs). We therefore investigated whether macrophages might actively participate in early dissemination. We show that pre-malignant cancer cells in MMTV-HER2 mice produced CCL2 in an NFkB dependent manner and thereby recruited resident mammary tissue macrophages into the duct, months before monocyte-derived TAMs arrived. Intra-epithelial mammary tissue macrophages induced an epithelial-to-mesenchymal transition in early HER2+ cells in a Wnt-dependent manner and thereby drove early dissemination. Consequently, depletion of macrophages from pre-malignant MMTV-HER2 mice significantly reduced early dissemination of pre-malignant cancer cells as measured by reduction in levels of circulating cancer cells and disseminated cancer cells in lungs. Interestingly, macrophage depletion during pre-malignant stages alone was sufficient to reduce the onset of late metastasis in MMTV-HER2 mice, indicating that early disseminating cancer cells contributed to late metastasis formation. In humans DCIS samples we frequently found intra-epithelial macrophages inside of ducts and their presence correlated with reduced E-Cadherin levels. Finally, a first pilot study indicated that those patients whose DCIS lesions contained macrophage +/E-Cadherin low microenvironments frequently had DCCs in the bone marrow. Taken together, we reveal that resident mammary tissue macrophages can promote early dissemination, and unravel a mechanism how early cancer spread might proceed in breast cancer patients and in other cancers where metastasis are diagnosed and the primary tumor is never detected (cancer of unknown primary). We also propose that early DCCs play a long-term causal role in metastasis development when patients are diagnosed in late stages, implying that the heterogeneity of metastases is higher than we might anticipate.

Citation Format: Nina Linde, Arthur Mortha, Nicole Saenger, Maria Soledad Sosa, Kathryn Harper, Ethan Tardio, Tanja Fehm, Thomas Karn, Miriam Merad, Julio A. Aguirre-Ghiso. Macrophages orchestrate early dissemination of HER2+ cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3233.