The cell-biological program termed the epithelial-mesenchymal transition (EMT) confers on cancer cells mesenchymal traits and an ability to enter the cancer stem cell (CSC) state. However, the interactions between CSCs and their surrounding microenvironment are poorly understood. Here we show that tumor-associated monocytes and macrophages (TAMs) create a CSC-niche via juxtacrine signaling with CSCs. We performed quantitative proteomic profiling and found that the EMT program upregulates the expression of CD90/Thy1 and EphA4, which mediate the physical interactions of CSCs with TAMs by directly binding with their respective counter-receptors on these cells. In response, the EphA4 receptor on the carcinoma cells activates Src and NF-κB, the latter results in the secretion of a variety of cytokines by the CSCs; these cytokines serve to sustain the stem-cell state. Indeed, admixed macrophages enhance the CSC activities of carcinoma cells. These findings underscore the significance of TAMs as important components of the CSC niche.

Distant metastasis, which results in >90% of cancer related deaths, is enabled by hematogenous dissemination of tumor cells via the circulation. This requires the completion of a sequence of complex steps including transit, initial arrest, extravasation, survival and proliferation. Increased understanding of the cellular and molecular players enabling each of these steps is key in uncovering new opportunities for therapeutic intervention during early metastatic dissemination. Here, we describe an in vitro model of the human microcirculation with the potential to recapitulate discrete steps of early metastatic seeding, including arrest, transendothelial migration and early micrometastases formation. The microdevice features self-organized human microvascular networks formed over 4–5 days, after which tumor can be perfused and extravasation events easily tracked over 72 hours, via standard confocal microscopy. Contrary to most in vivo and in vitro extravasation assays, robust and rapid scoring of extravascular cells combined with high-resolution imaging can be easily achieved due to the confinement of the vascular network to one plane close to the surface of the device. This renders extravascular cells clearly distinct and allows tumor cells of interest to be identified quickly compared to those in thick tissues. The ability to generate large numbers of devices (~36) per experiment coupled with fast quantitation further allows for highly parametric studies, which is required when testing multiple genetic or pharmacological perturbations. This is coupled with the capability for live tracking of single cell extravasation events allowing both tumor and endothelial morphological dynamics to be observed in high detail with a moderate number of data points. This Protocol Extension describes an adaptation of an existing Protocol describing a microfluidic platform that offers additional applications.