Endothelial Progenitor Cells Secrete SDF-1 That Recruit Endogenous Stem Cells

Objectives: Endothelial progenitor cells (EPCs) are endothelial cell precursors that initiate angiogenesis thereby participating in osteogenesis. Our previous studies showed a 5mm gain of extra-cortical bone following EPC transplantation. The aim of this study was to investigate the paracrine role of EPCs in osteogenesis by recruitment of endogenous cells.
Methods: Human EPCs (hEPCs) were isolated from the blood of healthy volunteers. hEPCS were seeded onto β-tri-calcium phosphate (β-TCP) granules before transplantation into subcutaneous pouches in nude mice (n = 30). Mice were allocated to control (β-TCP) and test (β-TCP with hEPC) groups and sacrificed after 10 days, 3 and 8 weeks. Recruitment of endogenous cells to the transplantation site was examined using immunohistochemical staining for mouse endothelial cells (CD31) and mesenchymal stromal cells (MSC, CD73). Intensity of stain was quantified using Image Pro. Interactions between EPCs and MSCs were examined in-vitro by transwell migration assay, and proteomic analysis of hEPCs conditioned medium was performed.
Results: Three weeks after transplantation, mouse endothelial (CD31) and mouse MSCs (CD73) levels were higher in the test transplants (p = 0.005, p = 0.007, respectively), suggesting a strong paracrine effect. In-vitro, hEPCs conditioned medium stimulated migration of MSCs and EPCs compared to standard medium (EGM-2) (p<0.01). Proteomic analysis of hEPCs conditioned medium from four donors revealed ~400 proteins, including SDF-1, VEGF, CTGF, CYR61. These proteins have high intensity ratio in the samples compared to the medium (control). In a transwell migration assay, SDF-1 enhanced EPCs and MSCs migration (p=0.001, p=0.0002 respectively) whereas AMD-3100 (SDF-1 antagonist) reduced cell migration (p=0.001).
Conclusions: .hEPCs recruit EPCs and MSCs by a paracrine effect modulated by SDF-1