FMN-coated fluorescent USPIO for cell labeling and non-invasive MR imaging in tissue engineering

Non-invasive magnetic resonance imaging (MRI) is gaining significant attention in the field of tissue engineering, since it can provide valuable information on in vitro production parameters and in vivo performance. It can e.g. be used to monitor the morphology, location and function of the regenerated tissue, the integrity, remodeling and resorption of the scaffold, and the fate of the implanted cells. Since cells are not visible using conventional MR techniques, ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles are routinely employed to label and monitor the cells embedded in tissue-engineered implants. We here set out to optimize cell labeling procedures with regard to labeling efficiency, biocompatibility and in vitro validation during bioreactor cultivation, using flavin mononucleotide (FMN)-coated fluorescent USPIO (FLUSPIO). Efficient FLUSPIO uptake is demonstrated in three different cell lines, applying relatively short incubation times and low labeling concentrations. FLUSPIO-labeled cells were successfully employed to visualize collagen scaffolds and tissue-engineered vascular grafts. Besides promoting safe and efficient cell uptake, an exquisite property of the non-polymeric FMN-coating is that it renders the USPIO fluorescent, providing a means for in vitro, in vivo and ex vivo validation via fluorescence microscopy and fluorescence reflectance imaging (FRI). FLUSPIO cell labeling is consequently considered to be a suitable tool for theranostic tissue engineering purposes.