Magnetic resonance imaging of human endothelial progenitors reveals opposite effects on vascular and muscle regeneration into ischaemic tissues.

AIMS The assessment of progenitor cell survival and efficacy after transplantation is one of the major challenges in cardiovascular cell therapy. Translation of currently used imaging techniques to patients is not immediate. Possible options include iron oxide particle loading into cells to be tracked using magnetic resonance (MR) and by MR-based water diffusion anisotropy analysis. The aim of the present study was to assess, using these techniques, the localization and survival of human 'early' endothelial progenitor cells (EPCs) and their effects on vascular and skeletal muscle regeneration in a mouse model of hind limb ischaemia. METHODS AND RESULTS A paramagnetic iron oxide particle loading protocol of human peripheral blood-derived early EPCs was devised. The iron+ EPCs maintained their phenotype and in vitro functional activity. In addition, the presence of iron+ cells was observed by MR until 7 days after injection into a pharmacologically immunosuppressed mouse model of hind limb ischaemia. Immunohistochemistry with human major histocompatibility complex antibodies revealed the absence of human cells at 7 days post-ischaemia. EPC death was confirmed by staining of iron+ cells with an anti-mouse CD68 antibody and by qPCR performed on DNA extracted from injected ischaemic limbs, at different times following injection. Surprisingly, early EPC injection enhanced arteriogenesis but caused a significant increase in ischaemic tissue inflammation and a retarded muscle regeneration, as evidenced by water diffusion anisotropy analysis and histology. CONCLUSION In line with recent reports, our results show that the use of iron-based contrast agents does not allow detection of long-term EPC engraftment into ischaemic tissues. They further show that early EPCs exert a potent arteriogenic effect on ischaemic tissues that is not dependent on their prolonged survival. Unexpectedly, injection of these cells elicited a long-term inflammatory response that reflected a delayed muscle healing process.