A tissue-engineered 3D microvessel model reveals the dynamics of mosaic vessel formation in breast cancer.

In solid tumors, vascular structure and function varies from the core to the periphery. This structural heterogeneity has been proposed to influence the mechanisms by which tumor cells enter the circulation. Blood vessels exhibit regional defects in endothelial coverage, which can result in cancer cells directly exposed to flow and potentially promoting intravasation. Consistent with prior reports, we observed in human breast tumors and in a mouse model of breast cancer that approximately 6% of vessels consisted of both endothelial cells and tumor cells, so-called mosaic vessels. Due in part to the challenges associated with observing tumor-vessel interactions deep within tumors in real-time, the mechanisms by which mosaic vessels form remain incompletely understood. We developed a tissue-engineered model containing a physiologically realistic microvessel in co-culture with mammary tumor organoids. This approach allows real-time and quantitative assessment of tumor-vessel interactions under conditions that recapitulate many in vivo features. Imaging revealed that tumor organoids integrate into the endothelial cell lining, resulting in mosaic vessels with gaps in the basement membrane. While mosaic vessel formation was the most frequently observed interaction, tumor organoids also actively constricted and displaced vessels. Furthermore, intravasation of cancer cell clusters was observed following the formation of a mosaic vessel. Taken together, our data reveal that cancer cells can rapidly reshape, destroy, or integrate into existing blood vessels, thereby affecting oxygenation, perfusion, and systemic dissemination. Our novel assay also enables future studies to identify targetable mechanisms of vascular recruitment and intravasation.