CXCL12/stromal-cell-derived factor-1 effectively replaces endothelial progenitor cells to induce vascularized ectopic bone.

Bone defect healing is highly dependent on the simultaneous stimulation of osteogenesis and vascularization. In bone regenerative strategies, combined seeding of multipotent stromal cells (MSCs) and endothelial progenitor cells (EPCs) proves their mutual stimulatory effects. Here, we investigated whether stromal-cell-derived factor-1α (SDF-1α) stimulates vascularization by EPCs and whether SDF-1α could replace seeded cells in ectopic bone formation. Late EPCs of goat origin were characterized for their endothelial phenotype and showed to be responsive to SDF-1α in in vitro migration assays. Subsequently, subcutaneous implantation of Matrigel plugs that contained both EPCs and SDF-1α showed more tubule formation than constructs containing either EPCs or SDF-1α. Addition of either EPCs or SDF-1α to MSC-based constructs showed even more elaborate vascular networks after 1 week in vivo, with SDF-1α/MSC-laden groups showing more prominent interconnected networks than EPC/MSC-laden groups. The presence of abundant mouse-specific CD31/PECAM expression in these constructs confirmed ingrowth of murine vessels and discriminated between angiogenesis and vessel networks formed by seeded goat cells. Importantly, implantation of EPC/MSC or SDF-1α/MSC constructs resulted in indistinguishable ectopic bone formation. In both groups, bone onset was apparent at week 3 of implantation. Taken together, we demonstrated that SDF-1α stimulated the migration of EPCs in vitro and vascularization in vivo. Further, SDF-1α addition was as effective as EPCs in inducing the formation of vascularized ectopic bone based on MSC-seeded constructs, suggesting a cell-replacement role for SDF-1α. These results hold promise for the design of larger centimeter-scale, cell-free vascular bone grafts.