Methods for magnetically labeling stem and other cells for detection by in vivo magnetic resonance imaging

Advances and clinical challenges of mesenchymal stem cell therapy

In recent years, cell therapy has provided desirable properties for promising new drugs. Mesenchymal stem cells are promising candidates for developing genetic engineering and drug delivery strategies due to their inherent properties, including immune regulation, homing ability and tumor tropism. The therapeutic potential of mesenchymal stem cells is being investigated for cancer therapy, inflammatory and fibrotic diseases, among others. Mesenchymal stem cells are attractive cellular carriers for synthetic nanoparticles for drug delivery due to their inherent homing ability. In this review, we comprehensively discuss the various genetic and non-genetic strategies of mesenchymal stem cells and their derivatives in drug delivery, tumor therapy, immune regulation, tissue regeneration and other fields. In addition, we discuss the current limitations of stem cell therapy and the challenges in clinical translation, aiming to identify important development areas and potential future directions.

Intranasal delivery of stem cells labeled by nanoparticles in neurodegenerative disorders: Challenges and opportunities

Neurodegenerative disorders occur through progressive loss of function or structure of neurons, with loss of sensation and cognition values. The lack of successful therapeutic approaches to solve neurologic disorders causes physical disability and paralysis and has a significant socioeconomic impact on patients. In recent years, nanocarriers and stem cells have attracted tremendous attention as a reliable approach to treating neurodegenerative disorders. In this regard, nanoparticle-based labeling combined with imaging technologies has enabled researchers to survey transplanted stem cells and fully understand their fate by monitoring their survival, migration, and differentiation. For the practical implementation of stem cell therapies in the clinical setting, it is necessary to accurately label and follow stem cells after administration. Several approaches to labeling and tracking stem cells using nanotechnology have been proposed as potential treatment strategies for neurological diseases. Considering the limitations of intravenous or direct stem cell administration, intranasal delivery of nanoparticle-labeled stem cells in neurological disorders is a new method of delivering stem cells to the central nervous system (CNS). This review describes the challenges and limitations of stem cell-based nanotechnology methods for labeling/tracking, intranasal delivery of cells, and cell fate regulation as theragnostic labeling. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease.

Click-Ready Perfluorocarbon Nanoemulsion for 19F MRI and Multimodal Cellular Detection

We describe an in vivo imaging probe platform that is readily modifiable to accommodate binding of different molecular targeting moieties and payloads for multimodal image generation. In this work, we demonstrate the utility of perfluorocarbon (PFC) nanoemulsions incorporating dibenzocyclooctyne (DBCO) by enabling postemulsification functionalization via a click reaction with azide-containing ligands. The addition of DBCO-lipid to the surfactant in PFC nanoemulsions did not affect nanoemulsion size or nanoemulsion stability. As proof-of-concept, fluorescent dye-azides were conjugated to PFC nanoemulsions, demonstrating the feasibility of functionalization the by click reaction. Uptake of the fluorescent PFC by macrophages was demonstrated both in vitro in cultured macrophages and in situ in an acute inflammation mouse model, where fluorescence imaging and ¹H/¹⁹F magnetic resonance imaging (MRI) were used for in vivo detection. Overall, these data demonstrate the potential of PFC nanoemulsions incorporating DBCO as a versatile platform for generating functionalized probes.

Experimental and mathematical modelling of magnetically labelled mesenchymal stromal cell delivery

A key challenge for stem cell therapies is the delivery of therapeutic cells to the repair site. Magnetic targeting has been proposed as a platform for defining clinical sites of delivery more effectively. In this paper, we use a combined in vitro experimental and mathematical modelling approach to explore the magnetic targeting of mesenchymal stromal cells (MSCs) labelled with magnetic nanoparticles using an external magnet. This study aims to (i) demonstrate the potential of magnetic tagging for MSC delivery, (ii) examine the effect of red blood cells (RBCs) on MSC capture efficacy and (iii) highlight how mathematical models can provide both insight into mechanics of therapy and predictions about cell targeting in vivo. In vitro MSCs are cultured with magnetic nanoparticles and circulated with RBCs over an external magnet. Cell capture efficacy is measured for varying magnetic field strengths and RBC percentages. We use a 2D continuum mathematical model to represent the flow of magnetically tagged MSCs with RBCs. Numerical simulations demonstrate qualitative agreement with experimental results showing better capture with stronger magnetic fields and lower levels of RBCs. We additionally exploit the mathematical model to make hypotheses about the role of extravasation and identify future in vitro experiments to quantify this effect.

Scaffold-free biofabrication of adipocyte structures with magnetic levitation

Tissue engineering research aims to repair the form and/or function of impaired tissues. Tissue engineering studies mostly rely on scaffold-based techniques. However, these techniques have certain challenges, such as the selection of proper scaffold material, including mechanical properties, sterilization, and fabrication processes. As an alternative, we propose a novel scaffold-free adipose tissue biofabrication technique based on magnetic levitation. In this study, a label-free magnetic levitation technique was used to form three-dimensional (3D) scaffold-free adipocyte structures with various fabrication strategies in a microcapillary-based setup. Adipogenic-differentiated 7F2 cells and growth D1 ORL UVA stem cells were used as model cells. The morphological properties of the 3D structures of single and cocultured cells were analyzed. The developed procedure leads to the formation of different patterns of single and cocultured adipocytes without a scaffold. Our results indicated that adipocytes formed loose structures while growth cells were tightly packed during 3D culture in the magnetic levitation platform. This system has potential for ex vivo modeling of adipose tissue for drug testing and transplantation applications for cell therapy in soft tissue damage. Also, it will be possible to extend this technique to other cell and tissue types.

Cellular Magnetic Resonance Imaging for Tracking Metastatic Cancer Cells in the Brain

Cellular magnetic resonance imaging (MRI) enables visualization of cells in vivo. This is accomplished by labeling cells with superparamagnetic iron oxide nanoparticles. Here, we describe the steps for labeling human cancer cells with iron for tracking them after injection into nude mice. We also provide details for validation of cell labeling, ultrasound guided intra-cardiac injection, and MRI.

Mesenchymal stem cell-based drug delivery strategy: from cells to biomimetic

Owing to the diversity and ease of preparation of nanomaterials, the rational nanocarriers with a rational design have become increasingly popular in medical researches. Although nanoparticle-based drug delivery exhibits great potential, there are some challenges facing like rapid plasma clearance, triggering or aggravation of immune response, etc. Herein, cell-based targeted drug delivery systems have drawn more and more attention owing to low immunogenicity and intrinsic mutation rate, and innate ability to allow targeted delivery. Mesenchymal stem cells (MSCs) have been used in gene and drug delivery. The use of MSCs is a promising approach for the development of gene transfer systems and drug loading strategies because of their intrinsic properties, including homing ability and tumor tropism. By combining the inherent cell properties and merits of synthetic nanoparticles (NPs), cell membrane coated NPs emerge as the time requires. Overall, we provide a comprehensive overview of the utility of MSCs in drug and gene delivery as well as MSC membrane coated nanoparticles for therapy and drug delivery, aiming to figure out the significant room for development and highlight the potential future directions.

SPIO labeling of endothelial cells using ultrasound and targeted microbubbles at diagnostic pressures

In vivo cell tracking of therapeutic, tumor, and endothelial cells is an emerging field and a promising technique for imaging cardiovascular disease and cancer development. Site-specific labeling of endothelial cells with the MRI contrast agent superparamagnetic iron oxide (SPIO) in the absence of toxic agents is challenging. Therefore, the aim of this in vitro study was to find optimal parameters for efficient and safe SPIO-labeling of endothelial cells using ultrasound-activated CD31-targeted microbubbles for future MRI tracking. Ultrasound at a frequency of 1 MHz (10,000 cycles, repetition rate of 20 Hz) was used for varying applied peak negative pressures (10–160 kPa, i.e. low mechanical index (MI) of 0.01–0.16), treatment durations (0–30 s), time of SPIO addition (-5 min– 15 min with respect to the start of the ultrasound), and incubation time after SPIO addition (5 min– 3 h). Iron specific Prussian Blue staining in combination with calcein-AM based cell viability assays were applied to define the most efficient and safe conditions for SPIO-labeling. Optimal SPIO labeling was observed when the ultrasound parameters were 40 kPa peak negative pressure (MI 0.04), applied for 30 s just before SPIO addition (0 min). Compared to the control, this resulted in an approximate 12 times increase of SPIO uptake in endothelial cells in vitro with 85% cell viability. Therefore, ultrasound-activated targeted ultrasound contrast agents show great potential for effective and safe labeling of endothelial cells with SPIO.

MRI tracking of polyethylene glycol-coated superparamagnetic iron oxide-labelled placenta-derived mesenchymal stem cells toward glioblastoma stem-like cells in a mouse model

Mesenchymal stem cells (MSCs) that display homing and infiltration properties towards tumor cells are a promising cellular targeting vector for brain tumor therapy but are limited to local-regional delivery in current preclinical models. Here, we investigated whether placenta-derived MSCs (P-MSCs) are a superior cellular vector for systemic targeting of glioblastoma stem-like cells (GSCs), with an imaging modality to real-time monitor the trafficking P-MSCs to glioblastoma sites. Results demonstrated that P-MSCs had greater migratory activity towards GSCs and across blood-brain barrier compared with bone marrow-derived MSCs, and this activity was enhanced by hypoxia precondition. Chemokine ligand 5 was identified as a chemoattractant responsible for the glioblastoma tropism of P-MSCs. Polyethylene glycol-coated superparamagnetic iron oxide (PEG-SPIO) was synthesized for cellular labelling and imaging P-MSCs, displaying high cellular uptake and no cytotoxic effect on P-MSCs cell proliferation or stemness property. The homing effects of intravenously administered PEG-SPIO-labelled P-MSCs towards intracerebral GSCs were able to be detected in mice models through T2-weighted magnetic resonance imaging (MRI). This study suggests the possibility of innovative systemic P-MSC-based cell therapy for aggressive GSCs, developing a state-of-the-art theranostic technique for real-time tracking of therapeutic P-MSCs tumor infiltration through cellular MRI.

Poly-l-lysine-coated superparamagnetic nanoparticles: a novel method for the transfection of pro-BDNF into neural stem cells

Poly-l-lysine-coated superparamagnetic iron oxide nanoparticles (SPIONs-PLL) were prepared and used as a novel-carrier for the transfer of brain-derived neurotrophic factor (BDNF) into neural stem cells (NSCs) under the beneficial influence of an external magnetic field. Pro-BDNF, a gene from human brain cDNA libraries, was obtained by polymerase chain reaction and constructed in a mammalian expression vector (PSecTag2/HygroB). The nanoparticles (NPs) were examined using Fourier transform infrared spectroscopy, zeta potential, and Transmission electron microscopy. From the results, the levels of BDNF among the transfected and untransfected cells were 30.326 ± 5.9 and 5.85 ± 3.11 pg/mL, respectively, as detected by an ELISA method. Moreover, the enhanced green fluorescent protein vector was used to evaluate the gene expression efficiency for SPIONs-PLL as a non-viral carrier in NSCs. This was performed under the influence of a magnetic field and the transfection reagents (such as Lipofectamine 2000), which served as a positive control. The histological analysis revealed that the concentration of intracellular NPs was significantly higher than intercellular NPs. These results suggest that SPIONs-PLL can serve as a novel alternative for the transfection of BDNF-NSCs and could be used in gene therapy.

CEST-MRI studies of cells loaded with lanthanide shift reagents

PURPOSE

Magnetic resonance imaging has been used extensively to track in vivo implanted cells that have been previously labeled with relaxation enhancers. However, this approach is not suitable to track multiple cell populations, as it may lead to confounding results in case the contrast agent is released from the labeled cells. This paper demonstrates how the use of CEST agents can overcome these issues. After encapsulating paramagnetic lanthanide shift reagents, we may shift the absorption frequency of the intracellular water resonance (δ^In ), thus generating frequency-encoding CEST responsive cells that can be visualized in the MR image by applying the proper RF irradiation.

METHODS

Eu-HPDO3A, Dy-HPDO3A, and Tm-HPDO3A were used as shift reagents for labeling murine breast cancer cells and murine macrophages by hypotonic swelling and pinocytosis. The CEST-MR images were acquired at 7 T, and the saturation transfer effect was measured. Samples at different dilution of cells were analyzed to quantify the detection threshold. In vitro experiments of cell proliferation were carried out. Finally, murine breast cancer cells were injected subcutaneously in mice, and MR images were acquired to assess the proliferation index in vivo.

RESULTS

It was found that entrapment of the paramagnetic complexes into endosomes (i.e., using the pinocytosis route) leads to an enhanced shift of the intracellular water resonance. δ^In appears to be proportional to the effective magnetic moment (μ^eff ) and to the concentration of the loaded lanthanide complex. Moreover, a higher shift is present when the complexes are entrapped in the endosomes. The cell proliferation index was assessed both in vitro and in vivo by evaluating the reduction of δ^In value in the days after the cell labeling.

CONCLUSION

Cells can be visualized by CEST MRI after loading with paramagnetic shift reagent, by exploiting the large ensemble of the properly shifted intracellular water molecules. A better performance is obtained when the complexes are entrapped inside the endosomes. The observed (δ^In ) value is strongly correlated to the chemical nature of the probe, and to its concentration and cellular localization. Two applications of this method are reported in this paper: (1) for in vivo cell visualization and (2) for the monitoring of the cellular proliferation process, as this method is accompanied by a change in δ^In that may be exploited as a longitudinal reporter of the proliferation rate.

Iron oxide labeling does not affect differentiation potential of human bone marrow mesenchymal stem cells exhibited by their differentiation into cardiac and neuronal cells

Mesenchymal stem cells (MSCs) have shown promising outcomes in cardiac and neuronal diseases. Efficient and noninvasive tracking of MSCs is essential to harness their therapeutic potential. Iron oxide nanoparticles (IONPs) have emerged as effective means to label stem cells and visualize them using magnetic resonance imaging (MRI). It is known that IONPs do not affect viability and cell proliferation of stem cells. However, very few studies have demonstrated differentiation potential of iron oxide-labeled MSCs and their differentiation into specific lineages that can contribute to cellular therapies. The differentiation of IONP-labeled human bone marrow mesenchymal stem cells (hBM-MSCs) into cardiac and neuronal lineages has never been studied. In this study, we have shown that IONP-labeled hBM-MSCs retain their differentiation potential to cardiac and neuronal cell lineages. We also confirmed that labeling hBM-MSCs with IONP does not affect their characteristic properties such as viability, cellular proliferation rate, surface marker profiling, and trilineage differentiation capacity. This study shows that IONP can be efficiently tracked, and its labeling does not alter stemness and differentiation potential of hBM-MSCs. Thus, the labeled hBM-MSCs can be used in clinical therapies and regenerative medicine.

Quantifications of in vivo labeled stem cells based on measurements of magnetic moments

Cells labeled by super paramagnetic iron-oxide (SPIO) nanoparticles are more easily seen in gradient echo MR images, but it has not been shown that the amount of nanoparticles or the number of cells can be directly quantified from MR images. This work utilizes a previously developed and improved Complex Image Summation around a Spherical or Cylindrical Object (CISSCO) method to quantify the magnetic moments of several clusters of SPIO nanoparticle labeled cells from archived rat brain images. With the knowledge of mass magnetization of the cell labeling agent and cell iron uptake, the number of cells in each nanoparticle cluster can be determined. Using a high pass filter with a reasonable size has little effect on each measured magnetic moment from the CISSCO method. These procedures and quantitative results may help improve the efficacy of cell-based treatments in vivo.

Autonomous magnetic labelling of functional mesenchymal stem cells for improved traceability and spatial control in cell therapy applications

Mesenchymal stem cells (MSCs) represent a valuable resource for regenerative medicine treatments for orthopaedic repair and beyond. Following developments in isolation, expansion and differentiation protocols, efforts to promote clinical translation of emerging cellular strategies now seek to improve cell delivery and targeting. This study shows efficient live MSC labelling using silica‐coated magnetic particles (MPs), which enables 3D tracking and guidance of stem cells. A procedure developed for the efficient and unassisted particle uptake was shown to support MSC viability and integrity, while surface marker expression and MSC differentiation capability were also maintained. In vitro, MSCs showed a progressive decrease in labelling over increasing culture time, which appeared to be linked to the dilution effect of cell division, rather than to particle release, and did not lead to detectable secondary particle uptake. Labelled MSC populations demonstrated magnetic responsiveness in vitro through directed migration in culture and, when seeded onto a scaffold, supporting MP‐based approaches to cell targeting. The potential of these silica‐coated MPs for MRI cell tracking of MSC populations was validated in 2D and in a cartilage repair model following cell delivery. These results highlight silica‐coated magnetic particles as a simple, safe and effective resource to enhance MSC targeting for therapeutic applications and improve patient outcomes. © 2016 The Authors Journal of Tissue Engineering and Regenerative Medicine Published by John Wiley & Sons Ltd.

Labeling of mesenchymal stem cells for MRI with single-cell sensitivity

Sensitive cell detection by magnetic resonance imaging (MRI) is an important tool for the development of cell therapies. However, clinically approved contrast agents that allow single-cell detection are currently not available. Therefore, we compared very small iron oxide nanoparticles (VSOP) and new multicore carboxymethyl dextran-coated iron oxide nanoparticles (multicore particles, MCP) designed by our department for magnetic particle imaging (MPI) with discontinued Resovist^® regarding their suitability for detection of single mesenchymal stem cells (MSC) by MRI. We achieved an average intracellular nanoparticle (NP) load of >10 pg Fe per cell without the use of transfection agents. NP loading did not lead to significantly different results in proliferation, colony formation, and multilineage in vitro differentiation assays in comparison to controls. MRI allowed single-cell detection using VSOP, MCP, and Resovist^® in conjunction with high-resolution T2*-weighted imaging at 7 T with postprocessing of phase images in agarose cell phantoms and in vivo after delivery of 2,000 NP-labeled MSC into mouse brains via the left carotid artery. With optimized labeling conditions, a detection rate of ~45% was achieved; however, the experiments were limited by nonhomogeneous NP loading of the MSC population. Attempts should be made to achieve better cell separation for homogeneous NP loading and to thus improve NP-uptake-dependent biocompatibility studies and cell detection by MRI and future MPI. Additionally, using a 7 T MR imager equipped with a cryocoil resulted in approximately two times higher detection. In conclusion, we established labeling conditions for new high-relaxivity MCP, VSOP, and Resovist^® for improved MRI of MSC with single-cell sensitivity.

Magnetic resonance imaging and cell-based neurorestorative therapy after brain injury

Restorative cell-based therapies for experimental brain injury, such as stroke and traumatic brain injury, substantially improve functional outcome. We discuss and review state of the art magnetic resonance imaging methodologies and their applications related to cell-based treatment after brain injury. We focus on the potential of magnetic resonance imaging technique and its associated challenges to obtain useful new information related to cell migration, distribution, and quantitation, as well as vascular and neuronal remodeling in response to cell-based therapy after brain injury. The noninvasive nature of imaging might more readily help with translation of cell-based therapy from the laboratory to the clinic.

Magnetic Nanoparticles Labeled Mesenchymal Stem Cells: A Pragmatic Solution toward Targeted Cancer Theranostics

Mesenchymal stem cells (MSCs) have gained much interest to be used as targeting vehicle in cancer therapy due to the intrinsic tumor-homing behavior associated with them. In this scenario, superparamagnetic nanoparticles are emerging as an ideal probe for noninvasive cell tracking for different stem cell applications. In the study, it is demonstrated that the formulated aqueous dispersible glyceryl monooleate coated magnetic nanoparticles (MNPs) can act as a better labeling and efficient tracking agent without affecting the inherent properties of MSCs. The MNPs-MSCs facilitate the stem cell tracking by magnetic resonance imaging at a very low cell number having high T₂ relaxivity and potentiates the use of MNPs-MSCs as a prospective diagnostic tool. Most importantly, the homing of MNPs-MSCs toward inflammation site, subcutaneous prostate tumor (small as well as large tumor), and in orthotopic prostate tumor suggests the clinical relevance of the system. In addition, intraperitoneal delivery of MNPs-MSCs shows enhanced tumor accumulation and less sequestration in liver as revealed by in vivo imaging and histological studies. The results here demonstrate that MNPs-MSCs may prove as a better targeted delivery agent for early diagnosis of tumors even of smaller size.

Positive contrast of SPIO-labeled cells by off-resonant reconstruction of 3D radial half-echo bSSFP

This article describes a new acquisition and reconstruction concept for positive contrast imaging of cells labeled with superparamagnetic iron oxides (SPIOs). Overcoming the limitations of a negative contrast representation as gained with gradient echo and fully balanced steady state (bSSFP), the proposed method delivers a spatially localized contrast with high cellular sensitivity not accomplished by other positive contrast methods. Employing a 3D radial bSSFP pulse sequence with half-echo sampling, positive cellular contrast is gained by adding artificial global frequency offsets to each half-echo before image reconstruction. The new contrast regime is highlighted with numerical intravoxel simulations including the point-spread function for 3D half-echo acquisitions. Furthermore, the new method is validated on the basis of in vitro cell phantom measurements on a clinical MRI platform, where the measured contrast-to-noise ratio (CNR) of the new approach exceeds even the negative contrast of bSSFP. Finally, an in vivo proof of principle study based on a mouse model with a clear depiction of labeled cells within a subcutaneous cell islet containing a cell density as low as 7 cells/mm(3) is presented. The resultant isotropic images show robustness to motion and a high CNR, in addition to an enhanced specificity due to the positive contrast of SPIO-labeled cells.

Tracking of mesenchymal stem cells labeled with gadolinium diethylenetriamine pentaacetic acid by 7T magnetic resonance imaging in a model of cerebral ischemia

Progress in the development of stem cell and gene therapy requires repeatable and non-invasive techniques to monitor the survival and integration of stem cells in vivo with a high temporal and spatial resolution. The purpose of the present study was to examine the feasibility of using the standard contrast agent gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) to label rat mesenchymal stem cells (MSCs) for stem cell tracking. MSCs, obtained from the bilateral femora of rats, were cultured and propagated. The non-liposomal lipid transfection reagent effectene was then used to induce the intracellular uptake of Gd-DTPA. Electron microscopy was used to detect the distribution of Gd-DTPA particles in the MSCs. The labeling efficiency of the Gd-DTPA particles in the MSCs was determined using spectrophotometry, and MTT and trypan blue exclusion assays were used to evaluate the viability and proliferation of the labeled MSCs. T1-weighted magnetic resonance imaging (MRI) was used to observe the labeled cells in vitro and in the rat brain. Gd-DTPA particles were detected inside the MSCs using transmission electron microscopy and a high labeling efficiency was observed. No difference was observed in cell viability or proliferation between the labeled and unlabeled MSCs (P>0.05). In the in vitro T1-weighted MRI and in the rat brain, a high signal intensity was observed in the labeled MSCs. The T1-weighted imaging of the labeled cells revealed a significantly higher signal intensity compared with that of the unlabeled cells (P<0.05) and the T1 values were significantly lower. The function of the labeled MSCs demonstrated no change following Gd-DTPA labeling, with no evident adverse effect on cell viability or proliferation. Therefore, a change in MR signal intensity was detected in vitro and in vivo, suggesting Gd-DTPA can be used to label MSCs for MRI tracking.

MRI tracking stem cells transplantation for coronary heart disease

Cardiovascular disease is the leading cause of mortality worldwide. Stem cell transplantation has become a new treatment option for cardiovascular disease because the stem cells are able to migrate to damaged cardiac tissue, repair the myocardial infarction area and ultimately reduce the role of the infarct-related mortality. Cardiac magnetic resonance imaging (MRI) is a new robust non-invasive imaging technique that can detect anatomical information and myocardial dysfunction, study the mechanism of stem cells therapy with superb spatial/temporal resolution, relatively safe contrast material and lack of radiation. This review describes the advantages and disadvantages of cardiac MRI applied in stem cells transplantation and discusses how to translate this technique into clinical therapy.

Sources of Data/Study Selection: Data from cross-sectional and prospective studies published between the years 2001-2013 on the topic were included. Data searches included both human and animal studies.

Data Extraction: The data was extracted from online resources of statistic reports, Pub med, THE MEDLINE, Google Scholar, Medical and Radiological journals.

Conclusion: MRI is an appealing technique for cell trafficking depicting engraftment, differentiation and survival.

The effect of superparamagnetic iron oxide with iRGD peptide on the labeling of pancreatic cancer cells in vitro: a preliminary study

The iRGD peptide loaded with iron oxide nanoparticles for tumor targeting and tissue penetration was developed for targeted tumor therapy and ultrasensitive MR imaging. Binding of iRGD, a tumor homing peptide, is mediated by integrins, which are widely expressed on the surface of cells. Several types of small molecular drugs and nanoparticles can be transfected into cells with the help of iRGD peptide. Thus, we postulate that SPIO nanoparticles, which have good biocompatibility, can also be transfected into cells using iRGD. Despite the many kinds of cell labeling studies that have been performed with SPIO nanoparticles and RGD peptide or its analogues, only a few have applied SPIO nanoparticles with iRGD peptide in pancreatic cancer cells. This paper reports our preliminary findings regarding the effect of iRGD peptide (CRGDK/RGPD/EC) combined with SPIO on the labeling of pancreatic cancer cells. The results suggest that SPIO with iRGD peptide can enhance the positive labeling rate of cells and the uptake of SPIO. Optimal functionalization was achieved with the appropriate concentration or concentration range of SPIO and iRGD peptide. This study describes a simple and economical protocol to label panc-1 cells using SPIO in combination with iRGD peptide and may provide a useful method to improve the sensitivity of pancreatic cancer imaging.

Cellular magnetic resonance with iron oxide nanoparticles: long-term persistence of SPIO signal in the CNS after transplanted cell death

AIM

To study the specificity of cellular MRI based on superparamagnetic iron oxide particles (SPIOs), especially within the CNS.

MATERIALS & METHODS

A microglial cell line was engineered for the expression of a suicide gene, the receptor of diphtheria toxin (DT), and two reporter genes, green fluorescent protein and luciferase, in order to induce, in a controlled manner, cell death and test it through bioluminescence. SPIO-labeled DT-sensitive and control DT-insensitive cells were transplanted into the brains of mice, which underwent serial MRI and bioluminescence studies before and up to 90 days after DT-induced cell death.

RESULTS

No variations in SPIO signal voids were detected along longitudinal monitoring in brain hemispheres transplanted with DT-sensitive cells. Ex vivo analyses showed persistence of iron nanoparticle deposits at transplantation sites.

CONCLUSION

Due to the long-term persistence of signal after transplanted cell death, caution is advised when SPIOs are employed for cell tracking.

Effect of transplantation route on stem cell migration to fibrotic liver of rats via cellular magnetic resonance imaging

BACKGROUND AIMS

Assessing mesenchymal stromal cells (MSCs) after grafting is essential for understanding their migration and differentiation processes. The present study sought to evaluate via cellular magnetic resonance imaging (MRI) if transplantation route may have an effect on MSCs engrafting to fibrotic liver of rats.

METHODS

Rat MSCs were prepared, labeled with superparamagnetic iron oxide and scanned with MRI. Labeled MSCs were transplanted via the portal vein or vena caudalis to rats with hepatic fibrosis. MRI was performed in vitro before and after transplantation. Histologic examination was performed. MRI scan and imaging parameter optimization in vitro and migration under in vivo conditions were demonstrated.

RESULTS

Strong MRI susceptibility effects could be found on gradient echo-weighted, or T2∗-weighted, imaging sequences from 24 h after labeling to passage 4 of labeled MSCs in vitro. In vivo, MRI findings of the portal vein group indicated lower signal in liver on single shot fast spin echo-weighted, or T2-weighted, imaging and T2∗-weighted imaging sequences. The low liver MRI signal increased gradually from 0-3 h and decreased gradually from 3 h to 14 days post-transplantation. The distribution pattern of labeled MSCs in liver histologic sections was identical to that of MRI signal. It was difficult to find MSCs in tissues near the portal area on day 14 after transplantation; labeled MSCs appeared in fibrous tuberculum at the edge of the liver. No MRI signal change and a positive histologic examination were observed in the vena caudalis group.

CONCLUSIONS

The portal vein route seemed to be more beneficial than the vena caudalis on MSC migration to fibrotic liver of rats via MRI.

Magnetic resonance imaging tracking of ferumoxytol-labeled human neural stem cells: studies leading to clinical use

Numerous stem cell-based therapies are currently under clinical investigation, including the use of neural stem cells (NSCs) as delivery vehicles to target therapeutic agents to invasive brain tumors. The ability to monitor the time course, migration, and distribution of stem cells following transplantation into patients would provide critical information for optimizing treatment regimens. No effective cell-tracking methodology has yet garnered clinical acceptance. A highly promising noninvasive method for monitoring NSCs and potentially other cell types in vivo involves preloading them with ultrasmall superparamagnetic iron oxide nanoparticles (USPIOs) to enable cell tracking using magnetic resonance imaging (MRI). We report here the preclinical studies that led to U.S. Food and Drug Administration approval for first-in-human investigational use of ferumoxytol to label NSCs prior to transplantation into brain tumor patients, followed by surveillance serial MRI. A combination of heparin, protamine sulfate, and ferumoxytol (HPF) was used to label the NSCs. HPF labeling did not affect cell viability, growth kinetics, or tumor tropism in vitro, and it enabled MRI visualization of NSC distribution within orthotopic glioma xenografts. MRI revealed dynamic in vivo NSC distribution at multiple time points following intracerebral or intravenous injection into glioma-bearing mice that correlated with histological analysis. Preclinical safety/toxicity studies of intracerebrally administered HPF-labeled NSCs in mice were also performed, and they showed no significant clinical or behavioral changes, no neuronal or systemic toxicities, and no abnormal accumulation of iron in the liver or spleen. These studies support the clinical use of ferumoxytol labeling of cells for post-transplant MRI visualization and tracking.

Wrapped omentum with periosteum concurrent with adipose derived adult stem cells for bone tissue engineering in dog model

Adipose derived adult stem cells (ASCs) are multipotent cells that are able to differentiate into osteoblasts in presence of certain factors. The histological characteristics of periosteum makes it a specific tissue with a unique capacity to be engineered. Higher flexibility of the greater omentum is useful for reconstructive surgery. These criteria make it suitable for tissue engineering. The present study was designed to evaluate bone tissue engineering with periosteal free graft concurrent with ASCs and pedicle omentum in dog model. Twelve young female indigenous dogs were used in this experiment. In omental group (n = 4), end of omentum was wrapped by periosteum of the radial bone in abdomen of each dog. In omental-autogenously ASCs group (n = 4), 1 ml of ASCs was injected into the wrapped omentum with periosteum while in omental-allogenously ASCs group (n = 4), 1 ml of allogenous ASCs was injected. Lateral view radiographs were taken from the abdominal cavity postoperatively at the 2nd, 4th, 6th and 8th weeks post-surgery. Eight weeks after operation the dogs were re-anesthetized and the wrapped omenum by periosteum in all groups was found and removed for histopathological evaluation. Our results showed that omentum-periosteum, omental-periosteum-autogenous ASCs and omental-periosteum-allogenous ASCs groups demonstrated bone tissue formation in the abdominal cavity in dog model. The radiological, macroscopical and histological findings of the present study by the end of 8 weeks post-surgery indicate bone tissue engineering in all three groups in an equal level. The present study has shown that the wrapped omentum with periosteum concurrent with ASCs (autogenous or allogenous ASCs) lead to a favorable bone tissue formation. We suggested that it may be useful when pedicle graft omentum used concurrent with periosteum in the bone defect reconstruction, and this phenomenon should be studied in future.

A comprehensive analysis of transfection-assisted delivery of iron oxide nanoparticles to dendritic cells

Polylysine (PL) has been used to facilitate dendritic cell (DC) uptake of super paramagnetic iron oxide (SPIO) nanoparticles for use in magnetic resonance imaging (MRI). In this work, we examined the effect of PL on cell toxicity and induction of cell maturation as manifested by the up-regulation of surface molecules. We found that PL became toxic to bone marrow-derived DCs (BMDCs) at the 10 μg/ml threshold. Incubation of BMDCs with 20 μg/ml of PL for 1h resulted in approximately 90% cell death. However, addition of SPIO nanoparticles rescued DCs from PL-induced death as the combination of SPIO with PL did not cause cytotoxicity until the PL concentration was 1000 μg/ml. Prolonged exposure to PL induced BMDC maturation as noted by the expression of surface molecules such as MHC class II, CD40, CCR7 and CD86. However, the combination of SPIO and PL did not induce BMDC maturation at 1h. However prolonged exposure to SPIO nanoparticles induced CD40 expression and protein expression of TNFα and KC. The data suggest that the use of PL to enhance the labeling of DCs with SPIO nanoparticles is a dedicated work. Appropriate calibration of the incubation time and concentrations of PL and SPIO nanoparticles is crucial to the development of MRI technology for noninvasive imaging of DCs in vivo.

FROM THE CLINICAL EDITOR

The authors of this study present detailed data on toxicity and efficiency of polylysine-facilitated uptake of USPIO-s by dendritic cells for cell-specific MR imaging.

Tracking and evaluation of dendritic cell migration by cellular magnetic resonance imaging

Cellular magnetic resonance imaging (MRI) is a means by which cells labeled ex vivo with a contrast agent can be detected and tracked over time in vivo. This technology provides a noninvasive method with which to assess cell-based therapies in vivo. Dendritic cell (DC)-based vaccines are a promising cancer immunotherapy, but its success is highly dependent on the injected DC migrating to a secondary lymphoid organ such as a nearby lymph node. There the DC can interact with T cells to elicit a tumor-specific immune response. It is important to verify DC migration in vivo using a noninvasive imaging modality, such as cellular MRI, so that important information regarding the anatomical location and persistence of the injected DC in a targeted lymph node can be provided. An understanding of DC biology is critical in ascertaining how to label DC with sufficient contrast agent to render them detectable by MRI. While iron oxide nanoparticles provide the best sensitivity for detection of DC in vivo, a clinical grade iron oxide agent is not currently available. A clinical grade (19) Fluorine-based perfluorcarbon nanoemulsion is available but is less sensitive, and its utility to detect DC migration in humans remains to be demonstrated using clinical scanners presently available. The ability to quantitatively track DC migration in vivo can provide important information as to whether different DC maturation and activation protocols result in improved DC migration efficiency which will determine the vaccine's immunogenicity and ultimately the tumor immunotherapy's outcome in humans.

Rapid spectrophotometric technique for quantifying iron in cells labeled with superparamagnetic iron oxide nanoparticles: potential translation to the clinic

Labeling cells with superparamagnetic iron oxide (SPIO) nanoparticles provides the ability to track cells by magnetic resonance imaging. Quantifying intracellular iron concentration in SPIO labeled cells would allow for the comparison of agents and techniques used to magnetically label cells. Here we describe a rapid spectrophotometric technique (ST) to quantify iron content of SPIO-labeled cells, circumventing the previous requirement of an overnight acid digestion. Following lysis with 10% sodium dodecyl sulfate (SDS) of magnetically labeled cells, quantification of SPIO doped or labeled cells was performed using commonly available spectrophotometric instrument(s) by comparing absorptions at 370 and 750 nm with correction for turbidity of cellular products to determine the iron content of each sample. Standard curves demonstrated high linear correlation (R(2) = 0.998) between absorbance spectra of iron oxide nanoparticles and concentration in known SPIO-doped cells. Comparisons of the ST with inductively coupled plasma-mass spectroscopy (ICP-MS) or nuclear magnetic resonance relaxometric (R(2)) determinations of intracellular iron contents in SPIO containing samples resulted in significant linear correlation between the techniques (R(2) vs ST, R(2) > 0.992, p < 0.0001; ST vs ICP-MS, R(2) > 0.995, p < 0.0001) with the limit of detection of ST for iron = 0.66 µg ml(-1) for 10(6) cells ml(-1). We have developed a rapid straightforward protocol that does not require overnight acid digestion for quantifying iron oxide content in magnetically labeled cells using readily available analytic instrumentation that should greatly expedite advances in comparing SPIO agents and protocols for labeling cells.

MR and optical imaging of early micrometastases in lymph nodes: triple labeling with nano-sized agents yielding distinct signals

Few imaging methods are available for depicting in vivo cancer cell migration within the lymphatic system. Detection of such early micrometastases requires extremely high target to background. In this study, we dual-labeled human breast cancer cells (MDA-MB468) with a small particle of iron oxide (SPIO) and a quantum dot (QD), and tracked these cells in the lymphatic system in mice using in vivo MRI and optical imaging. A generation-6 gadolinium-dendrimer-based MRI contrast agent (Gd-G6) was employed for visualizing regional lymphatic channels and nodes. Since Gd-G6 shortened T(1) leading to high signal, whereas SPIO-labeled cancer cells greatly lowered signal, a small number of cells were simultaneously visualized within the draining lymphatic basins. One million dual-labeled cancer cells were subcutaneously injected into the paws of mice 24 h prior to imaging. Then whole body images were acquired pre- and post-intracutaneous injection of Gd-G6 with 3D-T(1) w-FFE and balanced-FFE sequences for cancer cell tracking and MR lymphangiography. In vivo MRI clearly visualized labeled cancer cells migrating from the paw to the axillary lymph nodes using draining lymphatics. In vivo optical imaging using a fluorescence surgical microscope demonstrated tiny cancer cell clusters in the axillary lymph node with high spatial resolution. Thus, using a combination of MRI and optical imaging, it is possible to depict macro- and early micrometastases within the lymphatic system. This platform offers a versatile research tool for investigating and treating lymphatic metastases in animal models.

Effects of adipose tissue stem cell concurrent with greater omentum on experimental long-bone healing in dog

Repair of large bone defects resulting from trauma, tumors, and osteitis is a current challenge to surgeons. Adipose-derived adult stem cells (ASCs) are multipotent cells that are able to differentiate into osteoblasts in the presence of certain factors. In this study, the role of greater omentum as a scaffold incorporation of ASCs was evaluated in long-bone defect healing in dog model. Sixteen 3-4-year-old, male adult mongrel dogs, weighing 25.2 ± 3.5 kg, were used in this study. In the control group (n = 4), the defect was left empty. In the omental group (n = 4), the defect was filled with harvested omentum. In the omental-ASCs group (n = 4), the defect was filled with omentum and 1 mL of ASCs was injected into the grafted omentum. In the omental-culture medium group (n = 4), 1 mL of culture medium was injected into the grafted omentum. Finally, the injured radial bones were fixed with plate and screw. Radiographs of each forelimb was taken postoperatively on the first day and at the second, fourth, sixth, and eighth weeks postinjury to evaluate bone formation, union, and remodeling of the defect. The operated radii were removed on the 56th postoperative day and were histopathologically evaluated. In this study, both omental-culture medium and omental-ASCs groups demonstrated superior osteogenic potential in healing the radial bone defect. Compared to those of the omental and control groups, more advanced bone healing criteria were present in the omental-culture medium and omental-ASCs groups at radiological and histopathological levels at 8 weeks postsurgery.

pH-titratable superparamagnetic iron oxide for improved nanoparticle accumulation in acidic tumor microenvironments

A wide variety of nanoparticle platforms are being developed for the diagnosis and treatment of malignancy. While many of these are passively targeted or rely on receptor-ligand interactions, metabolically directed nanoparticles provide a complementary approach. It is known that both primary and secondary events in tumorigenesis alter the metabolic profile of developing and metastatic cancers. One highly conserved metabolic phenotype is a state of up-regulated glycolysis and reduced use of oxidative phosphorylation, even when oxygen tension is not limiting. This metabolic shift, termed the Warburg effect, creates a "hostile" tumor microenvironment with increased levels of lactic acid and low extracellular pH. In order to exploit this phenomenon and improve the delivery of nanoparticle platforms to a wide variety of tumors, a pH-responsive iron oxide nanoparticle was designed. Specifically, glycol chitosan (GC), a water-soluble polymer with pH-titratable charge, was conjugated to the surface of superparamagnetic iron oxide nanoparticles (SPIO) to generate a T(2)*-weighted MR contrast agent that responds to alterations in its surrounding pH. Compared to control nanoparticles that lack pH sensitivity, these GC-SPIO nanoparticles demonstrated potent pH-dependent cellular association and MR contrast in vitro. In murine tumor models, GC-SPIO also generated robust T(2)*-weighted contrast, which correlated with increased delivery of the agent to the tumor site, measured quantitatively by inductively coupled plasma mass spectrometry. Importantly, the increased delivery of GC-SPIO nanoparticles cannot be solely attributed to the commonly observed enhanced permeability and retention effect since these nanoparticles have similar physical properties and blood circulation times as control agents.

Cationic Gd-DTPA liposomes for highly efficient labeling of mesenchymal stem cells and cell tracking with MRI

In the current study cell labeling was performed with water-soluble gadolinium (Gd)-DTPA containing liposomes, to allow for cell tracking by MRI. Liposomes were used to assure a highly concentrated intracellular build up of Gd, aiming to overcome the relatively low MRI sensitivity of Gd (compared to T2 contrast agents). Liposomes were positively charged (cationic) to facilitate uptake by binding to anionic charges in the cell membrane of bone marrow-derived mesenchymal stem cells (MSCs). We determined the cellular Gd load by variations in labeling time (1, 4, and 24 h) and liposome concentration (125, 250, 500, 1000 μM lipid), closely monitoring effects on cell viability, proliferation rate, and differentiation ability. Labeling was both time and dose dependent. Labeling for 4 h was most efficient regarding the combination of processing time and final cellular Gd uptake. Labeling for 4 h with low-dose concentration (125 μM lipid, corresponding to 52 ± 3 μM Gd) yielded an intracellular load of 30 ± 2.5 pg Gd cell(-1), without any effects on cell viability, proliferation, and cell differentiation. Gd liposomes, colabeled with fluorescent dyes, exhibited a prolonged cellular retention, with an endosomal distribution pattern. In vitro assay over 20 days demonstrated a drop in the average Gd load per cell, as a result of mitosis. However, there was no significant change in the sum of the Gd load in all daughter cells at endpoint (20 days), indicating an excellent cellular retention of Gd. MSCs labeled with Gd liposomes were imaged with MRI at both 1.5T and 3.0T, resulting in excellent visualization both in vitro and in vivo. Prolonged in vivo imaging of 500,000 Gd-labeled cells was possible for at least 2 weeks (3.0T). In conclusion, Gd-loaded cationic liposomes (125 μM lipid) are an excellent candidate to label cells, without detrimental effects on cell viability, proliferation, and differentiation, and can be visualized by MRI.

Dual contrast magnetic resonance imaging tracking of iron-labeled cells in vivo

Negative-contrast magnetic resonance imaging (MRI) methods utilizing magnetic susceptibility contrast agents have become one of the most widely used approaches in cellular imaging research. However, visualizing and tracking super-paramagnetic iron oxide nanoparticle (SPIO)-labeled cells on the basis of negative-contrast can limit specificity and sensitivity. Therefore, there has been a strong motivation to explore MRI methods for cellular imaging with either positive or dual contrast (both positive and negative) for identifying labeled cells; these methods offer the potential to improve significantly the sensitivity and specificity of MRI-based cell-tracking approaches. In this review, current state-of-the-art positive- and dual-contrast MRI techniques and contrast agents are described specifically for applications involving in vivo cellular tracking and imaging.

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.

The biology of brain metastases-translation to new therapies

Brain metastases are a serious obstacle in the treatment of patients with solid tumors and contribute to the morbidity and mortality of these cancers. It is speculated that the frequency of brain metastasis is increasing for several reasons, including improved systemic therapy and survival, and detection of metastases in asymptomatic patients. The lack of preclinical models that recapitulate the clinical setting and the exclusion of patients with brain metastases from most clinical trials have slowed progress. Molecular factors contributing to brain metastases are being elucidated, such as genes involved in cell adhesion, extravasation, metabolism, and cellular signaling. Furthermore, the role of the unique brain microenvironment is beginning to be explored. Although the presence and function of the blood-brain barrier in metastatic tumors is still poorly understood, it is likely that some tumor cells are protected from therapeutics by the blood-tumor barrier, creating a sanctuary site. This Review discusses what is known about the biology of brain metastases, what preclinical models are available to study the disease, and which novel therapeutic strategies are being studied in patients.

Quantitative T2* imaging of metastatic human breast cancer to brain in the nude rat at 3 T

This study uses quantitative T(2)* imaging to track ferumoxides--protamine sulfate (FEPro)-labeled MDA-MB-231BR-Luc (231BRL) human breast cancer cells that metastasize to the nude rat brain. Four cohorts of nude rats were injected intracardially with FEPro-labeled, unlabeled or tumor necrosis factor-related apoptosis-inducing ligand(TRAIL)-treated (to induce apoptosis) 231BRL cells, or saline, in order to develop metastatic breast cancer in the brain. The heads of the rats were imaged serially over 3-4 weeks using gradient multi-echo and turbo spin-echo pulse sequences at 3 T with a solenoid receive-only 4-cm-diameter coil. Quantitative T(2)* maps of the whole brain were obtained by the application of single-exponential fitting to the signal intensity of T(2)* images, and the distribution of T(2)* values in brain voxels was calculated. MRI findings were correlated with Prussian blue staining and immunohistochemical staining for iron in breast cancer and macrophages. Quantitative analysis of T(2)* from brain voxels demonstrated a significant shift to lower values following the intracardiac injection of FEPro-labeled 231BRL cells, relative to animals receiving unlabeled cells, apoptotic cells or saline. Quartile analysis based on the T(2)* distribution obtained from brain voxels demonstrated significant differences (p < 0.0083) in the number of voxels with T(2)* values in the ranges 10-35  ms (Q1), 36-60  ms (Q2) and 61-86  ms (Q3) from 1 day to 3 weeks post-infusion of labeled 231BRL cells, compared with baseline scans. There were no significant differences in the distribution of T(2)* obtained from serial MRI in rats receiving unlabeled or TRAIL-treated cells or saline. Histologic analysis demonstrated isolated Prussian blue-positive breast cancer cells scattered in the brains of rats receiving labeled cells, relative to animals receiving unlabeled or apoptotic cells. Quantitative T(2)* analysis of FEPro-labeled metastasized cancer cells was possible even after the hypointense voxels were no longer visible on T(2)*-weighted images.

Variations in labeling protocol influence incorporation, distribution and retention of iron oxide nanoparticles into human umbilical vein endothelial cells

Various studies have shown that various cell types can be labeled with iron oxide particles and visualized by magnetic resonance imaging (MRI). However, reported protocols for cell labeling show a large variation in terms of labeling dose and incubation time. It is therefore not clear how different labeling protocols may influence labeling efficiency. Systematic assessment of the effects of various labeling protocols on labeling efficiency of human umbilical vein endothelial cells (HUVEC) using two different types of iron oxide nanoparticles, i.e. super paramagnetic iron oxide particles (SPIOs) and microparticles of iron oxide (MPIOs), demonstrated that probe concentration, incubation time and particle characteristics all influence the efficiency of label incorporation, label distribution, label retention and cell behavior. For SPIO the optimal labeling protocol consisted of a dose of 12.5 µg iron/2 ml/9.5 cm(2) and an incubation time of 24 h, resulting in an average iron load of 12.0 pg iron/per cell (uptake efficiency of 9.6%). At 4 h many SPIOs are seen sticking to the outside of the cell instead of being taken up by the cell. For MPIO optimal labeling was obtained with a dose of 50 µg iron/2 ml/9.5 cm(2). Incubation time was of less importance since most of the particles were already incorporated within 4 h with a 100% labeling efficiency, resulting in an intracellular iron load of 626 pg/cell. MPIO were taken up more efficiently than SPIO and were also better tolerated. HUVEC could be exposed to and contain higher amounts of iron without causing significant cell death, even though MPIO had a much more pronounced effect on cell appearance. Using optimal labeling conditions as found for HUVEC on other cell lines, we observed that different cell types react differently to identical labeling conditions. Consequently, for each cell type separately an optimal protocol has to be established.

Intracellular bimodal nanoparticles based on quantum dots for high-field MRI at 21.1 T

Multimodal, biocompatible contrast agents for high magnetic field applications represent a new class of nanomaterials with significant potential for tracking of fluorescence and MR in vitro and vivo. Optimized for high-field MR applications-including biomedical imaging at 21.1 T, the highest magnetic field available for MRI-these nanoparticles capitalize on the improved performance of chelated Dy(3+) with increasing magnetic field coupled to a noncytotoxic Indium Phosphide/Zinc Sulfide (InP/ZnS) quantum dot that provides fluorescence detection, MR responsiveness, and payload delivery. By surface modifying the quantum dot with a cell-penetrating peptide sequence coupled to an MR contrast agent, the bimodal nanomaterial functions as a self-transfecting high-field MR/optical contrast agent for nonspecific intracellular labeling. Fluorescent images confirm sequestration in perinuclear vesicles of labeled cells, with no apparent cytotoxicity. These techniques can be extended to impart cell selectivity or act as a delivery vehicle for genetic or pharmaceutical interventions.

Non-invasive tracking of human haemopoietic CD34(+) stem cells in vivo in immunodeficient mice by using magnetic resonance imaging

OBJECTIVE

To assess migration of CD34(+) human stem cells to the bone marrow of athymic mice by using magnetic resonance (MR) imaging and Resovist, a contrast agent containing superparamagnetic iron oxide (SPIO) particles.

METHODS

All animal and human procedures were approved by our institution's ethics committee, and women had given consent to donate umbilical cord blood (UCB). Balb/c-AnN Foxn1(nu)/Crl mice received intravenous injection of 1 x 10(6) (n=3), 5 x 10(6) (n=3) or 1 x 10(7) (n=3) human Resovist-labelled CD34(+) cells; control mice received Resovist (n=3). MR imaging was performed before, 2 and 24 h after transplantation. Signal intensities of liver, muscle and bone marrow were measured and analysed by ANOVA and post hoc Student's t tests. MR imaging data were verified by histological and immunological detection of both human cell surface markers and carboxydextrancoating of the contrast agent.

RESULTS

CD34(+) cells were efficiently labelled by Resovist without impairment of functionality. Twenty-four hours after administration of labelled cells, MR imaging revealed a significant signal decline in the bone marrow, and histological and immunological analyses confirmed the presence of transplanted human CD34(+) cells.

CONCLUSION

Intravenously administered Resovist-labelled CD34(+) cells home to bone marrow of mice. Homing can be tracked in vivo by using clinical 1.5-T MR imaging technology.

Quantitative magnetic resonance fluorine imaging: today and tomorrow

Fluorine (19F) is a promising moiety for quantitative magnetic resonance imaging (MRI). It possesses comparable magnetic resonance (MR) sensitivity to proton (1H) but exhibits no tissue background signal, allowing specific and selective assessment of the administrated 19F-containing compounds in vivo. Additionally, the MR spectra of 19F-containing compounds exhibited a wide range of chemical shifts (>200 ppm). Therefore, both MR parameters (e.g., spin-lattice relaxation rate R1) and the absolute quantity of molecule can be determined with 19F MRI for unbiased assessment of tissue physiology and pathology. This article reviews quantitative 19F MRI applications for mapping tumor oxygenation, assessing molecular expression in vascular diseases, and tracking labeled stem cells.

Mesenchymal stem cell labeling and in vitro MR characterization at 1.5 T of new SPIO contrast agent: Molday ION Rhodamine-B™

In vivo detection of transplanted stem cells is requisite for improving stem cell-based treatments by developing a thorough understanding of their therapeutic mechanisms. MRI tracking of magnetically labeled cells is non-invasive and is suitable for longitudinal studies. Molday ION Rhodamine-B™ (MIRB) is a new superparamagnetic iron oxide (SPIO) contrast agent specifically formulated for cell labeling and is readily internalized by non-phagocytic cells. This investigation characterizes mesenchymal stem cell (MSC) labeling and MR imaging properties of this new SPIO agent. Effects of MIRB on MSC viability and differentiation as well as cellular loading properties were assessed for MSC labeled with MIRB at concentrations from 5 to 100 µg Fe/ml. Labeled MSC were evaluated, in vitro, on a clinical 1.5 T MRI. Optimal scanning sequences and imaging parameters were determined based on contrast-to-noise ratio and contrast modulation. Relaxation rates (1/T(2)*) for gradient-echo sequences were approximated and an idealized limit of detection was established. MIRB labeling did not affect MSC viability or the ability to differentiate into either bone or fat. Labeling efficiency was found to be approximately 95% for labeling concentrations at or above 20 µg Fe/ml. Average MIRB per MSC ranged from 0.7 pg Fe for labeling MIRB concentration of 5 µg Fe/ml and asymptotically approached a value of 20-25 pg Fe/MSC as labeling concentration increased to 100 µg Fe/ml. MRI analysis of MIRB MSC revealed long echo time, gradient echo sequences to provide the most sensitivity. Limit of detection for gradient echo sequences was determined to be less than 1000 MSC, with approximately 15 pg Fe/MSC (labeled at 20 µg Fe/ml). These investigations have laid the groundwork and established feasibility for the use of this contrast agent for in vivo MRI detection of MSC. Properties evaluated in this study will be used as a reference for tracking labeled MSC for in vivo studies.

Superparamagnetic iron oxide nanoparticles labeling of bone marrow stromal (mesenchymal) cells does not affect their "stemness"

Superparamagnetic iron oxide nanoparticles (SPION) are increasingly used to label human bone marrow stromal cells (BMSCs, also called “mesenchymal stem cells”) to monitor their fate by in vivo MRI, and by histology after Prussian blue (PB) staining. SPION-labeling appears to be safe as assessed by in vitro differentiation of BMSCs, however, we chose to resolve the question of the effect of labeling on maintaining the “stemness” of cells within the BMSC population in vivo. Assays performed include colony forming efficiency, CD146 expression, gene expression profiling, and the “gold standard” of evaluating bone and myelosupportive stroma formation in vivo in immuncompromised recipients. SPION-labeling did not alter these assays. Comparable abundant bone with adjoining host hematopoietic cells were seen in cohorts of mice that were implanted with SPION-labeled or unlabeled BMSCs. PB+ adipocytes were noted, demonstrating their donor origin, as well as PB+ pericytes, indicative of self-renewal of the stem cell in the BMSC population. This study confirms that SPION labeling does not alter the differentiation potential of the subset of stem cells within BMSCs.

lacZ as a genetic reporter for real-time MRI

Molecular imaging based on MRI is currently hampered by the lack of genetic reporters for in vivo imaging. We determined that the commercially available substrate S-Gal can be used to detect genetically engineered beta-galactosidase expressing cells by MRI. The effect and specificity of the reaction between beta-galactosidase and S-Gal on MRI contrast were determined both in vitro and in vivo. beta-galactosidase activity in the presence of S-Gal resulted in enhanced T(2) and T*(2) MR-contrast, which was amplified with increasing magnetic field strengths (4.7-17.6 T) in phantom studies. Using both lacZ(+) transgenic animals and lacZ(+) tissue transplants, we were able to detect labeled cells in live animals in real time. Similar to phantom studies, detection of the labeled cells/tissues in vivo was enhanced at high magnetic fields. These results demonstrate that the genetic reporter, lacZ, can be used as an in vivo marker gene using high-field-strength MRI.