Tracking Stem Cell Implants in Cartilage Defects of Minipigs by Using Ferumoxytol-enhanced MRI.

Background Cartilage repair outcomes of matrix-associated stem cell implants (MASIs) in patients have been highly variable. Conventional MRI cannot help distinguish between grafts that will and grafts that will not repair the underlying cartilage defect until many months after the repair. Purpose To determine if ferumoxytol nanoparticle labeling could be used to depict successful or failed MASIs compared with conventional MRI in a large-animal model. Materials and Methods Between January 2016 and December 2017, 10 Göttingen minipigs (n = 5 male; n = 5 female; mean age, 6 months ± 5.1; age range, 4-20 months) received implants of unlabeled (n = 12) or ferumoxytol-labeled (n = 20) viable and apoptotic MASIs in cartilage defects of the distal femur. All MASIs were serially imaged with MRI on a 3.0-T imaging unit at week 1 and weeks 2, 4, 8, 12, and 24, with calculation of T2 relaxation times. Cartilage regeneration outcomes were assessed by using the MR observation of cartilage repair tissue (MOCART) score (scale, 0-100), the Pineda score, and histopathologic quantification of collagen 2 production in the cartilage defect. Findings were compared by using the unpaired Wilcoxon rank sum test, a linear regression model, the Fisher exact test, and Pearson correlation. Results Ferumoxytol-labeled MASIs showed significant T2 shortening (22.2 msec ± 3.2 vs 27.9 msec ± 1.8; P < .001) and no difference in cartilage repair outcomes compared with unlabeled control MASIs (P > .05). At week 2 after implantation, ferumoxytol-labeled apoptotic MASIs showed a loss of iron signal and higher T2 relaxation times compared with ferumoxytol-labeled viable MASIs (26.6 msec ± 4.9 vs 20.8 msec ± 5.3; P = .001). Standard MRI showed incomplete cartilage defect repair of apoptotic MASIs at 24 weeks. Iron signal loss at 2 weeks correlated with incomplete cartilage repair, diagnosed at histopathologic examination at 12-24 weeks. Conclusion Ferumoxytol nanoparticle labeling can accelerate the diagnosis of successful and failed matrix-associated stem cell implants at MRI in a large-animal model. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Sneag and Potter in this issue.

Ferumoxytol-based Dual-modality Imaging Probe for Detection of Stem Cell Transplant Rejection.

Purpose: Stem cell transplants are an effective approach to repair large bone defects. However, comprehensive techniques to monitor the fate of transplanted stem cells in vivo are lacking. Such strategies would enable corrective interventions at an early stage and greatly benefit the development of more successful tissue regeneration approaches. In this study, we designed and synthesized a dual-modality imaging probe (Feru-AFC) that can simultaneously localize transplanted stem cells and diagnose immune rejection-induced apoptosis at an early stage in vivo. Methods: We used a customized caspase-3 cleavable peptide-dye conjugate to modify the surface of clinically approved ferumoxytol nanoparticles (NPs) to generate the dual-modality imaging probe with fluorescence "light-up" feature. We labeled both mouse mesenchymal stem cells (mMSCs, matched) and pig mesenchymal stem cells (pMSCs, mismatched) with the probe and transplanted the labeled cells with biocompatible scaffold at the calvarial defects in mice. We then employed intravital microscopy (IVM) and magnetic resonance imaging (MRI) to investigate the localization, engraftment, and viability of matched and mismatched stem cells, followed by histological analyses to evaluate the results obtained from in vivo studies. Results: The Feru-AFC NPs showed good cellular uptake efficiency in the presence of lipofectin without cytotoxicity to mMSCs and pMSCs. The fluorescence of Feru-AFC NPs was turned on inside apoptotic cells due to the cleavage of peptide by activated caspase-3 and subsequent release of fluorescence dye molecules. Upon transplantation at the calvarial defects in mice, the intense fluorescence from the cleaved Feru-AFC NPs in apoptotic pMSCs was observed with a concomitant decrease in the overall cell number from days 1 to 6. In contrast, the Feru-AFC NP-treated mMSCs exhibited minimum fluorescence and the cell number also remained similar. Furthermore, in vivo MRI of the Feru-AFC NP-treated mMSC and pMSCs transplants could clearly indicate the localization of matched and mismatched cells, respectively. Conclusions: We successfully developed a dual-modality imaging probe for evaluation of the localization and viability of transplanted stem cells in mouse calvarial defects. Using ferumoxytol NPs as the platform, our Feru-AFC NPs are superparamagnetic and display a fluorescence "light-up" signature upon exposure to activated caspase-3. The results show that the probe is a promising tool for long-term stem cell tracking through MRI and early diagnosis of immune rejection-induced apoptosis through longitudinal fluorescence imaging.