The effect of blocking angiogenesis on anterior cruciate ligament healing following stem cell transplantation.

Ruptured human anterior cruciate ligaments (ACL) contain vascular stem cells capable of enhancing the healing of tendon grafts. In the current study we explored the role that neo-angiogenesis plays in ACL healing. ACL-derived CD34+ cells were isolated via Fluorescence Activated Cell Sorting (FACS) from the rupture sites of human ACLs. The cells were then virally transduced to express either vascular endothelial growth factor (VEGF) or soluble FLT-1 (sFLT-1), which is an antagonist of VEGF. We established five groups: CD34+VEGF(100%), where 100% of the cells were transduced with VEGF, CD34+VEGF(25%), where only 25% of the cells were transduced with VEGF, CD34+, CD34+sFLT-1, and a No cells group. The CD34+sFLT1 group had a significant reduction in biomechanical strength compared to the CD34+ group at 4 and 8 weeks; whereas the biomechanical strength of the CD34+VEGF(25%) group was significantly greater than the CD34+ group at week 4; however, no difference was observed by week 8. Immunohistochemical staining demonstrated a significantly lower number of isolectin B4 and hCD31 positive cells, markers associated with angiogenesis, in the CD34+sFLT1 group, and a higher number of isolectin B4 and hCD31 positive cells in the CD34+VEGF(100%) and CD34+VEGF(25%) groups compared to the CD34+ group. Graft maturation was significantly delayed in the CD34+sFLT1 group and accelerated in the CD34+VEGF(25%) group compared to the CD34+ group. In conclusion, blocking VEGF reduced angiogenesis, graft maturation and biomechanical strength following ACL reconstruction. Native expression of VEGF by the CD34+ cells improved tendon graft maturation and biomechanical strength; however, over-expression of VEGF impeded improvements in biomechanical strength.