Endothelial progenitor cells from peripheral blood support bone regeneration by provoking an angiogenic response

Neovascularization is crucial for fracture healing and plays an important role in long-time graft survival in tissue engineering applications. Endothelial progenitor cells (EPCs) can be isolated from peripheral blood avoiding donor site morbidity, which makes them attractive for autologous cell-based engineering of neovessels. However, contradictory results are published concerning the vasculogenic potential of this cell type. We could previously show that implanted human endothelial vein cells (HUVECs) gave rise to the formation of a complex functional human neovasculature in a heterotopic (subcutaneous) as well as in an orthotopic (calvarial defect) model of severe combined immunodeficiency (SCID) mice, where vessel formation could even be increased by coimplanting mesenchymal stem cells (MSCs) functioning as perivascular cells. In this study, we investigated whether coimplantation of MSCs which have been predifferentiated in vitro into SMCs (SMC-MSCs) may enable pbEPCs to form blood vessels upon implantation and, if this would be the case, whether the resulting enhanced vascularization may support bone regeneration. For this purpose, pbEPCs and SMC-MSCs were mono- or cocultured in collagen matrices and seeded into scaffolds consisting of decalcified processed bovine cancellous bone (PBCB, Tutobone). Neovascularization and osteogenesis were evaluated using a calvarial bone defect-model in SCID mice. Our experiments could show that the missing vasculogenic potential of pbEPCs is not rescued by coimplantation of SMCs derived from MSCs predifferentiated along the vascular smooth muscle lineage. However, implantation of both cell types alone, or in combination induced an angiogenic response, which correlated in a positive manner with bone formation within the implants.