Effects of MRI contrast agents on the stem cell phenotype

Magnetic resonance imaging and fluorescence labeling of clinical-grade mesenchymal stem cells without impacting their phenotype: study in a rat model of stroke

Human mesenchymal stem cells (hMSCs) have strong potential for cell therapy after stroke. Tracking stem cells in vivo following a graft can provide insight into many issues regarding optimal route and/or dosing. hMSCs were labeled for magnetic resonance imaging (MRI) and histology with micrometer-sized superparamagnetic iron oxides (M-SPIOs) that contained a fluorophore. We assessed whether M-SPIO labeling obtained without the use of a transfection agent induced any cell damage in clinical-grade hMSCs and whether it may be useful for in vivo MRI studies after stroke. M-SPIOs provided efficient intracellular hMSC labeling and did not modify cell viability, phenotype, or in vitro differentiation capacity. Following grafting in a rat model of stroke, labeled hMSCs could be detected using both in vivo MRI and fluorescent microscopy until 4 weeks following transplantation. However, whereas good label stability and unaffected hMSC viability were observed in vitro, grafted hMSCs may die and release iron particles in vivo.

Cell motility of neural stem cells is reduced after SPIO-labeling, which is mitigated after exocytosis

MRI is used for tracking of superparamagnetic iron oxide (SPIO)-labeled neural stem cells. Studies have shown that long-term MR tracking of rapidly dividing cells underestimates their migration distance. Time-lapse microscopy of random cellular motility and cell division was performed to evaluate the effects of SPIO-labeling on neural stem cell migration. Labeled cells divided symmetrically and exhibited no changes in cell viability, proliferation, or apoptosis. However, SPIO-labeling resulted in decreased motility of neural stem cells as compared with unlabeled controls. When SPIO-labeled neural stem cells and human induced pluripotent stem cells were transplanted into mouse brain, rapid exocytosis of SPIO by live cells was observed as early as 48 h postengraftment, with SPIO-depleted cells showing the farthest migration distance. As label dilution is negligible at this early time point, we conclude that MRI underestimation of cell migration can also occur as a result of reduced cell motility, which appears to be mitigated following SPIO exocytosis.

Labeling human embryonic stem-cell-derived cardiomyocytes for tracking with MR imaging

BACKGROUND

Human embryonic stem cells (hESC) can generate cardiomyocytes (CM), which offer promising treatments for cardiomyopathies in children. However, challenges for clinical translation result from loss of transplanted cell from target sites and high cell death. An imaging technique that noninvasively and repetitively monitors transplanted hESC-CM could guide improvements in transplantation techniques and advance therapies.

OBJECTIVE

To develop a clinically applicable labeling technique for hESC-CM with FDA-approved superparamagnetic iron oxide nanoparticles (SPIO) by examining labeling before and after CM differentiation.

MATERIALS AND METHODS

Triplicates of hESC were labeled by simple incubation with 50 μg/ml of ferumoxides before or after differentiation into CM, then imaged on a 7T MR scanner using a T2-weighted multi-echo spin-echo sequence. Viability, iron uptake and T2-relaxation times were compared between groups using t-tests.

RESULTS

hESC-CM labeled before differentiation demonstrated significant MR effects, iron uptake and preserved function. hESC-CM labeled after differentiation showed no significant iron uptake or change in MR signal (P < 0.05). Morphology, differentiation and viability were consistent between experimental groups.

CONCLUSION

hESC-CM should be labeled prior to CM differentiation to achieve a significant MR signal. This technique permits monitoring delivery and engraftment of hESC-CM for potential advancements of stem cell-based therapies in the reconstitution of damaged myocardium.

Angiographic Demonstration of Neoangiogenesis after Intra-arterial Infusion of Autologous Bone Marrow Mononuclear Cells in Diabetic Patients with Critical Limb Ischemia

Critical limb ischemia in diabetic patients is associated with high rates of morbidity and mortality. Suboptimal responses to the available medical and surgical treatments are common in these patients, who also demonstrate limited vascular homeostasis. Neovasculogenesis induced by stem cell therapy could be a useful approach for these patients. Neovasculogenesis and clinical improvement were compared at baseline and at 3 and 12 months after autologous bone marrow-derived mononuclear cell (BMMNC) transplantation in diabetic patients with peripheral artery disease. We conducted a prospective study to evaluate the safety and efficacy of intra-arterial administration of autologous BMMNCs (100–400 × 106 cells) in 20 diabetic patients with severe below-the-knee arterial ischemia. Although the time course of clinical effects differed among patients, after 12 months of follow-up all patients presented a notable improvement in the Rutherford-Becker classification, the University of Texas diabetic wound scales, and the Ankle-Brachial Index in the target limb. The clinical outcome was consistent with neovasculogenesis, which was assessed at 3 months by digital subtraction angiography and quantified by MetaMorph software. Unfortunately, local cell therapy in the target limb had no beneficial effect on the high mortality rate in these patients. In diabetic patients with critical limb ischemia, intra-arterial perfusion of BMMNCs is a safe procedure that generates a significant increase in the vascular network in ischemic areas and promotes remarkable clinical improvement.

Characterization of the inflammatory response in a photothrombotic stroke model by MRI: implications for stem cell transplantation

PURPOSE

The aim of this study was to evaluate the specificity of magnetic resonance imaging (MRI) contrast in a photothrombotic (PT) stroke model with and without engraftment of superparamagnetic iron oxide (SPIO)-labeled stem cells.

PROCEDURES

We monitored animals with PT stroke versus animals with PT stroke and stem cell engraftment by T(2)/T(2)*w MRI 4-8 h and 2, 4, 6/7 and 14 days after PT induction. Results were correlated with immunohistochemistry.

RESULTS

T(2)*w MRI images showed hypointense contrast due to the accumulation of inflammatory cells and corresponding iron accumulation and glial scar formation in the border zone of the lesion, similar as what was observed for SPIO-labeled cells. Histological analysis was thus indispensable to distinguish between labeled transplanted cells and immune cells.

CONCLUSION

These results raise caution regarding the non-invasive monitoring of SPIO-labeled transplanted stem cells by MRI in models that result in a strong inflammatory response.

In vivo imaging of adult neurogenesis

As our understanding of adult neurogenesis increases, hopes rise that neurological disorders and neuronal losses might be addressed one day by neural stem cell-based regenerative therapies. However, evaluating the efficacy and safety of any neurogenesis-based intervention requires a means to monitor neurogenesis in vivo and, so far, no such imaging techniques are available for human studies. Nevertheless, using imaging techniques presently available to clinicians, i.e. magnetic resonance imaging, positron emission tomography and optical imaging, significant progress has been made in this direction over the last decade. This review describes the current state-of-the-art for each imaging technique, and shows that detection of neurogenesis could theoretically be achieved using current imaging devices. Indeed, in vivo imaging of neurogenesis has already been achieved in mice using transgenic model systems. However, the imaging of human neurogenesis still requires the development of methods to reliably target the neural stem cells and the neuronal precursors in vivo.

In vivo monitoring of adult neurogenesis in health and disease

Adult neurogenesis, i.e., the generation of new neurons in the adult brain, presents an enormous potential for regenerative therapies of the central nervous system. While 5-bromo-2'-deoxyuridine labeling and subsequent histology or immunohistochemistry for cell-type-specific markers is still the gold standard in studies of neurogenesis, novel techniques, and tools for in vivo imaging of neurogenesis have been recently developed and successfully applied. Here, we review the latest progress on these developments, in particular in the area of magnetic resonance imaging (MRI) and optical imaging. In vivo in situ labeling of neural progenitor cells (NPCs) with micron-sized iron oxide particles enables longitudinal visualization of endogenous progenitor cell migration by MRI. The possibility of genetic labeling for cellular MRI was demonstrated by using the iron storage protein ferritin as the MR reporter-gene. However, reliable and consistent results using ferritin imaging for monitoring endogenous progenitor cell migration have not yet been reported. In contrast, genetic labeling of NPCs with a fluorescent or bioluminescent reporter has led to the development of some powerful tools for in vivo imaging of neurogenesis. Here, two strategies, i.e., viral labeling of stem/progenitor cells and transgenic approaches, have been used. In addition, the use of specific promoters for neuronal progenitor cells such as doublecortin increases the neurogenesis-specificity of the labeling. Naturally, the ultimate challenge will be to develop neurogenesis imaging methods applicable in humans. Therefore, we certainly need to consider other modalities such as positron emission tomography and proton magnetic resonance spectroscopy ((1)H-MRS), which have already been implemented for both animals and humans. Further improvements of sensitivity and neurogenesis-specificity are nevertheless required for all imaging techniques currently available.

In vitro evaluation of magnetic resonance imaging contrast agents for labeling human liver cells: implications for clinical translation

PURPOSE

Magnetic resonance imaging (MRI) is a promising approach for non-invasive monitoring after liver cell transplantation. We compared in vitro labeling of human liver cells with nano-sized (SPIO) and micron-sized iron oxide particles (MPIO).

PROCEDURES

The cellular iron load was quantified and phantom studies were performed using 3.0-T MRI. Transferrin receptor and ferritin gene expression, reactive oxygen species (ROS) formation, transaminase leakage, and urea synthesis were investigated over 6 days.

RESULTS

Incubation with MPIO produced stronger signal extinctions in MRI at similar iron loads within shorter labeling time. MPIO had no negative effects on the cellular iron homeostasis or cell performance, whereas SPIO caused temporary ROS formation and non-physiologic activation of the iron metabolic pathway.

CONCLUSIONS

Our findings suggest that MPIO are suited for clinical translation of strategies for cellular imaging with MRI. Attention should be paid to iron release and oxidative stress caused by biodegradable contrast agents.