Abstract Dissemination of tumor cells is an essential step in metastasis. Direct contact between a macrophage, mammalian-enabled (MENA)–overexpressing tumor cell, and endothelial cell [Tumor MicroEnvironment of Metastasis (TMEM)] correlates with metastasis in breast cancer patients. Here we show, using intravital high-resolution two-photon microscopy, that transient vascular permeability and tumor cell intravasation occur simultaneously and exclusively at TMEM. The hyperpermeable nature of tumor vasculature is described as spatially and temporally heterogeneous. Using real-time imaging, we observed that vascular permeability is transient, restricted to the TMEM, and required for tumor cell dissemination. VEGFA signaling from TIE2hi TMEM macrophages causes local loss of vascular junctions, transient vascular permeability, and tumor cell intravasation, demonstrating a role for the TMEM within the primary mammary tumor. These data provide insight into the mechanism of tumor cell intravasation and vascular permeability in breast cancer, explaining the value of TMEM density as a predictor of distant metastatic recurrence in patients. Significance: Tumor vasculature is abnormal with increased permeability. Here, we show that VEGFA signaling from TIE2hi TMEM macrophages results in local, transient vascular permeability and tumor cell intravasation. These data provide evidence for the mechanism underlying the association of TMEM with distant metastatic recurrence, offering a rationale for therapies targeting TMEM. Cancer Discov; 5(9); 932–43. ©2015 AACR. See related commentary by Kadioglu and De Palma, p. 906. This article is highlighted in the In This Issue feature, p. 893

Real-Time Imaging Reveals Local, Transient Vascular Permeability, and Tumor Cell Intravasation Stimulated by TIE2hi Macrophage–Derived VEGFA