Simple SummaryMetastasis occurs when a tumor spreads from its original site to another part of the body. Several factors are involved in this complex process, including proteins that regulate adhesion between circulating tumor cells and target organs (called adhesion molecules). Additionally, it is shown in the literature that many tumors release small vesicles in the plasma that may contribute to metastatization. In this study, the authors focused on brain metastasis using an in vitro model of the physiological barrier that protects the brain, essentially consisting of a co-culture of endothelial cells and astrocytes. They examined the ability of breast and lung tumor cells to cross this barrier, as well as the expression of adhesion molecules. Furthermore, they collected and analyzed the vesicles released by tumor cells to determine their ability to create ruptures in the barrier. The data indicated that cells with intermediate levels of adhesion molecule expression were most effective in migrating through the barrier. Additionally, the vesicles produced by tumor cells were able to induce the death of endothelial cells, the primary cellular component of the barrier. Data reported support the role of adhesion molecules and tumor-produced vesicles in brain metastasis, in addition to other factors.AbstractMetastatic brain disease (MBD) has seen major advances in clinical management, focal radiation therapy approaches and knowledge of biological factors leading to improved prognosis. Extracellular vesicles (EVs) have been found to play a role in tumor cross-talk with the target organ, contributing to the formation of a premetastatic niche. Human lung and breast cancer cell lines were characterized for adhesion molecule expression and used to evaluate their migration ability in an in vitro model. Conditioned culture media and isolated EVs, characterized by super resolution and electron microscopy, were tested to evaluate their pro-apoptotic properties on human umbilical vein endothelial cells (HUVECs) and human cerebral microvascular endothelial cells (HCMEC/D3) by annexin V binding assay. Our data showed a direct correlation between expression of ICAM1, ICAM2, β3-integrin and α2-integrin and the ability to firmly adhere to the blood–brain barrier (BBB) model, whereas the same molecules were down-regulated at a later step. Extracellular vesicles released by tumor cell lines were shown to be able to induce apoptosis in HUVEC while brain endothelial cells showed to be more resistant.