Endothelial Progenitor Cells Are Capable of Vascular Tube-Like Structure Formation Upon Direct Contact with Living but not Ischemic Cardiac Tissue

The umbilical cord blood derived endothelial progenitor cells (EPCs) contribute to vascular regeneration in experimental models of ischemia. However, their ability to participate in cardiovascular tissue restoration has not been elucidated yet. We employed a novel co-culture system to investigate whether human EPCs have the capacity to integrate into living and ischemic cardiac tissue, and participate to neovascularization. EPCs were cocultured with either living or ischemic murine embryonic ventricular slices, in the presenceor absence of a pro-angiogenic growth factor cocktail consisting of VEGF, IGF-1, EGF and bFGF. Tracking of EPCs within the co-cultures was performed by cell transfection with green fluorescent protein or by immunostaining performed with anti-human vWF, CD31, nuclei and mitochondria antibodies. EPCs generated vascular tube-like structures in direct contact with the living ventricular slices. Furthermore, the pro-angiogenic growth factor cocktail reduced significantly tubes formation. Co-culture of EPCs with the living ventricular slices in a transwell system did not lead to vascular tube-like structures formation, demonstrating that the direct contact is necessary and that the soluble factors secreted by the living slices were not sufficient for their induction. No vascular tubes were formed when EPCs were co-cultured with ischaemic ventricular slices, even in the presence of the pro-angiogenic cocktail. In conclusion, EPCs form vascular tube-like structures in contact with living cardiac tissue and the direct cell-to-cell interaction is a prerequisite for their induction. Understanding the cardiac niche and micro-environmental interactions that regulate EPCs integration and neovascularization is essential for applying these cells to cardiovascular regeneration.