Bone Marrow Monocyte Lineage Cells Adhere on Injured Endothelium in a Monocyte Chemoattractant Protein-1-Dependent Manner and Accelerate Reendothelialization as Endothelial Progenitor Cells

Peripheral blood (PB)-derived CD14⁺ monocytes were shown to transdifferentiate into endothelial cell (EC) lineage cells and contribute to neovascularization. We investigated whether bone marrow (BM)- or PB-derived CD34^-/CD14⁺ cells are involved in reendothelialization after carotid balloon injury. Although neither hematopoietic nor mesenchymal stem cells were included in human BM-derived CD34^-/CD14⁺ monocyte lineage cells (BM-MLCs), they expressed EC-specific markers (Tie2, CD31, VE-cadherin, and endoglin) to an extent identical to mature ECs. When BM-MLCs were cultured with vascular endothelial growth factors, hematopoietic markers were drastically decreased and new EC-specific markers (Flk and CD34) were induced. BM-MLCs were intra-arterially transplanted into balloon-injured arteries of athymic nude rats. When BM-MLCs were activated by monocyte chemoattractant protein-1 (MCP-1) in vivo or in vitro, they adhered onto injured endothelium, differentiated into EC-like cells by losing hematopoietic markers, and inhibited neointimal hyperplasia. Ability to prevent neointimal hyperplasia was more efficient than that of BM-derived CD34⁺ cells. MCP-dependent adhesion was not observed in PB-derived CD34^-/ CD14⁺ monocytes. Regenerated endothelium exhibited a cobblestone appearance, blocked extravasation of dye, and induced NO-dependent vasorelaxation. Basal adhesive activities on HUVECs under laminar flow and β₁-integrin expression (basal and active forms) were significantly increased in BM-MLCs compared with PB-derived monocytes. MCP-1 markedly enhanced adhesive activity of BM-MLCs (2.8-fold) on HUVECs by activating β₁-integrin conformation. Thus, BM-MLCs can function as EC progenitors that are more potent than CD34⁺ cells and acquire the ability to adhere on injured endothelium in a MCP-1-dependent manner, leading to reendothelialization associated with inhibition of intimal hyperplasia. This will open a novel window to MCP-1-mediated biological actions and vascular regeneration strategies by cell therapy.