Mesenchymal stem cells transmigrate across brain microvascular endothelial cell monolayers through transiently formed inter-endothelial gaps.

Mesenchymal stem cells (MSCs) hold much promise for cell therapy for neurological diseases such as cerebral ischemia and Parkinson's disease. Intravenously administered MSCs accumulate in lesions within the brain parenchyma, but little is known of the details of MSC transmigration across the blood-brain barrier (BBB). To study MSC transmigration across the BBB, we developed an in vitro culture system consisting of rat brain microvascular endothelial cells (BMECs) and bone marrow-derived MSCs using Transwell or Millicell culture inserts. Using this system, we first investigated the influence of the number of MSCs added to the upper chamber on BMEC barrier integrity. The addition of MSCs at a density of 1.5 × 10⁵ cells/cm² led to disruption of the BMEC monolayer structure and decreased barrier function as measured by the transendothelial electrical resistance (TEER). When applied at a density of 1.5 × 10⁴ cells/cm², neither remarkable disruption of the BMEC monolayers nor a significant decrease in TEER was observed until at least 12 h. After cultivation for 24 h under this condition, MSCs were found in the subendothelial space or beneath the insert membrane, suggesting that MSCs transmigrate across BMEC monolayers. Time-lapse imaging revealed that MSCs transmigrated across the BMEC monolayers through transiently formed intercellular gaps between the BMECs. These results show that our in vitro culture system consisting of BMECs and MSCs is useful for investigating the molecular and cellular mechanisms underlying MSC transmigration across the BBB.