INTRODUCTION: Lymphatic vasculature provides a route for metastasis to secondary sites in the body. The role of the lymphatic endothelium in mediating the entry of breast cancer cells into the vasculature remains unclear.

METHODS: In this study, we formed aggregates of MDA-MB-231 human breast carcinoma cells next to human microvascular lymphatic endothelial cell (LEC)-lined cavities in type I collagen gels to model breast microtumors and lymphatic vessels, respectively. We tracked invasion and escape of breast microtumors into engineered lymphatics or empty cavities under matched flow rates for up to sixteen days.

RESULTS: After coming into contact with a lymphatic vessel, tumor cells escape by moving between the endothelium and the collagen wall, between endothelial cells, and/or into the endothelial lumen. Over time, tumor cells replace the LECs within the vessel wall and create regions devoid of endothelium. The presence of lymphatic endothelium slows breast tumor invasion and escape, and addition of LEC-conditioned medium to tumors is sufficient to reproduce nearly all of these inhibitory effects.

CONCLUSIONS: This work sheds light on the interactions between breast cancer cells and lymphatic endothelium during vascular escape and reveals an inhibitory role for the lymphatic endothelium in breast tumor invasion and escape.

SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12195-022-00745-9.