Monocytes form a vascular barrier and participate in vessel repair after brain injury.

Subpopulations of bone marrow-derived cells can be induced to assume a number of endothelial properties in vitro. However, their ability to form a functional vascular barrier has not been demonstrated. We report that human CD14+ peripheral blood monocytes cultured under angiogenic conditions develop a number of phenotypic and functional properties similar to brain microvascular endothelial cells. These cells express the tight junction proteins zonula occludens 1 (ZO-1) and occludin and form a barrier with a transcellular electrical resistance (TCER) greater than 100 ohm cm2 and low permeability to 4 kDa and 20 kDa dextrans. The TCER of the cellular barrier is decreased by bradykinin and histamine. We also demonstrate that these cells associate with repairing vasculature in areas of brain and skin injury. Our data suggest that CD14+ peripheral blood monocytes participate in the repair of the vascular barrier after brain injury.