Magnetic Resonance Imaging of Ferumoxide-Labeled Mesenchymal Stem Cells in Cartilage Defects: In Vitro and in Vivo Investigations

Osteoarthritis is a disabling joint disease affecting more than 69 million Americans1 and causing annual health care costs of $330,648,000,000.2 Cartilage defects in arthritic joints, which are the main cause of pain and disability in affected patients, cannot be reversed or repaired by conservative treatment.3 Autologous chondrocyte-based transplants are currently investigated for restoration of trauma-induced cartilage defects.4,5 However, clinical outcomes are limited owing to the cells’ inability to adapt to the unique cartilage microenvironment. Mesenchymal stem cells (MSCs) have emerged as a promising alternative for cartilage repair because they are autologous cells that can be harvested from bone marrow without further cartilage damage (chondrocyte transplants require harvesting additional cartilage from the target joint) and because of their fairly straightforward isolation and their ability to be expanded efficiently in culture.6 In addition, MSCs are capable of proliferating, adapting, and secreting chondrogenic matrix, which leads to improved engraftment outcomes.6 MSCs have been successfully implanted in human patellar defects with improvement in clinical symptoms.7 However, the behavior of MSCs embedded in various biomaterials in the long term and in the context of arthritic joints remains to be studied to ascertain predictable clinical outcomes.7–11 An imaging method that could monitor successful MSC engraftments or diagnose a treatment failure by direct depiction of the transplanted cells would be highly desirable. Among various available imaging techniques for cell tracking,10–13 magnetic resonance imaging (MRI) has the following advantages (1) it is the only imaging technique that provides direct cartilage depiction, (2) it is noninvasive and is not associated with radiation exposure, and (3) stem cell labeling and tracking techniques with clinically applicable magnetic resonance contrast agents are established.14,15 Previous studies optimized stem cell labeling techniques with iron oxide nanoparticles toward a compromise between a cellular iron oxide load that is high enough to provide sensitive cell depiction on MRIs but also low enough to ensure an unimpaired stem cell differentiation into chondrocytes.16–18 In addition, our group showed previously that iron oxide–labeled stem cells can be depicted in cartilage defects with MRI19–21 and that iron oxide–labeled viable and nonviable stem cells demonstrate different magnetic resonance signal characteristics in ex vivo settings.19,21 The purpose of our study was to translate knowledge from previous in vitro and ex vivo studies to in vivo applications by (1) comparing three different ferumoxide labeling techniques of MSCs, (2) evaluating if ferumoxide labeling allows in vivo tracking of matrix-associated stem cell implants (MASIs) in an animal model, and (3) comparing the MRI characteristics of ferumoxide-labeled viable and apoptotic human mesenchymal stem cells (hMSCs). We hypothesized that clinically applicable protamine transfection techniques improve labeling efficiencies compared to simple incubation protocols, that ferumoxide-labeled MSCs can be tracked in cartilage defects in vivo with MRI, and that iron oxide–labeled viable and apoptotic cell transplants show different magnetic resonance signal characteristics in vivo.