Non-invasive analysis of myoblast transplants in rodent cardiac muscle

Superparamagnetic Iron Oxide Nanoparticles in Musculoskeletal Biology

The use of platelet-rich plasma and mesenchymal stem cells has garnered much attention in orthopedic medicine, focusing on the biological aspects of cell function. However, shortly after systemic delivery, or even a local injection, few of the transplanted stem cells or platelets remain at the target site. Improvement in delivery, and the ability to track and monitor injected cells, would greatly improve clinical translation. Nanoparticles can effectively and quickly label most cells in vitro, and evidence to date suggests such labeling does not compromise the proliferation or differentiation of cells. A specific type of nanoparticle, the superparamagnetic iron oxide nanoparticle (SPION), is already employed as a magnetic resonance imaging (MRI) contrast agent. SPIONs can be coupled with cells or bioactive molecules (antibodies, proteins, drugs, etc.) to form an injectable complex for in vivo use. The biocompatibility, magnetic properties, small size, and custom-made surface coatings also enable SPIONs to be used for delivering and monitoring of small molecules, drugs, and cells, specifically to muscle, bone, or cartilage. Because SPIONs consist of cores made of iron oxides, targeting of SPIONs to a specific muscle, bone, or joint in the body can be enhanced with the help of applied gradient magnetic fields. Moreover, MRI has a high sensitivity to SPIONs and can be used for noninvasive determination of successful delivery and monitoring distribution in vivo. Gaps remain in understanding how the physical and chemical properties of nanomaterials affect biological systems. Nonetheless, SPIONs hold great promise for regenerative medicine, and progress is being made rapidly toward clinical applications in orthopedic medicine.

Skeletal muscle satellite cell migration to injured tissue measured with 111In-oxine and high-resolution SPECT imaging

The delivery of adult skeletal muscle stem cells, called satellite cells, to several injured muscles via the circulation would be useful, however, an improved understanding of cell fate and biodistribution following their delivery is important for this goal to be achieved. The objective of this study was to evaluate the ability of systemically delivered satellite cells to home to injured skeletal muscle using single-photon emission computed tomography (SPECT) imaging of (111)In-labeled satellite cells. Satellite cells labeled with (111)In-oxine and green fluorescent protein (GFP) were injected intravenously after bupivicaine-induced injury to the tibialis anterior muscle. Animals were imaged with a high-resolution SPECT system called FastSPECT II for up to 7 days after transplantation. In vivo FastSPECT II imaging demonstrated a three to five-fold greater number of transplanted satellite cells in bupivicaine-injured muscle as compared to un-injured muscle after transplantation; a finding that was verified through autoradiograph analysis and quantification of GFP expression. Satellite cells also accumulated in other organs including the lung, liver, and spleen, as determined by biodistribution measurements. These data support the ability of satellite cells to home to injured muscle and support the use of SPECT and autoradiograph imaging techniques to track systemically transplanted (111)In labeled satellite cells in vivo, and suggest their homing may be improved by reducing their entrapment in filter organs.

Noninvasive in vivo tracking of mesenchymal stem cells and evaluation of cell therapeutic effects in a murine model using a clinical 3.0 T MRI

Cardiac cell therapy with mesenchymal stem cells (MSCs) represents a promising treatment approach for end-stage heart failure. However, little is known about the underlying mechanisms and the fate of the transplanted cells. The objective of the presented work is to determine the feasibility of magnetic resonance imaging (MRI) and in vivo monitoring after transplantation into infarcted mouse hearts using a clinical 3.0 T MRI device. The labeling procedure of bone marrow-derived MSCs with micron-sized paramagnetic iron oxide particles (MPIOs) did not affect the viability of the cells and their cell type-defining properties when compared to unlabeled cells. Using a clinical 3.0 T MRI scanner equipped with a dedicated small animal solenoid coil, 10(5) labeled MSCs could be detected and localized in the mouse hearts for up to 4 weeks after intramyocardial transplantation. Weekly ECG-gated scans using T1-weighted sequences were performed, and left ventricular function was assessed. Histological analysis of hearts confirmed the survival of labeled MSCs in the target area up to 4 weeks after transplantation. In conclusion, in vivo tracking of labeled MSCs using a clinical 3.0 T MRI scanner is feasible. In combination with assessment of heart function, this technology allows the monitoring of the therapeutic efficacy of regenerative therapies in a small animal model.

DNA surface modified gadolinium phosphate nanoparticles as MRI contrast agents

Oligonucleotide modified gadolinium phosphate nanoparticles have been prepared and their magnetic resonance relaxivity properties measured. Nanoparticles of GdPO4·H2O were synthesized in a water/oil microemulsion using IGEPAL CO-520 as surfactant, resulting in 50 to 100 nm particles that are highly dispersible and stable in water. Using surface modification chemistry previously established for zirconium phosphonate surfaces, the particles are directly modified with 5'-phosphate terminated oligonucleotides, and the specific interaction of the divalent phosphate with Gd(3+) sites at the surface is demonstrated. The ability of the modified nanoparticles to act as MRI contrast agents was determined by performing MR relaxivity measurements at 14.1 T. Solutions of nanopure water, Feridex, and Omniscan (FDA approved contrast agents) in 0.25% agarose were used for comparison and control purposes. MRI data confirm that GdPO4·H2O nanoparticles have relaxivities (r1, r2) comparable to those of commercially available contrast agents. In addition, the data suggest that biofunctionalization of the surface of the nanoparticles does not prevent their function as MRI contrast agents.

Value of MR contrast media in image-guided body interventions

In the past few years, there have been multiple advances in magnetic resonance (MR) instrumentation, in vivo devices, real-time imaging sequences and interventional procedures with new therapies. More recently, interventionists have started to use minimally invasive image-guided procedures and local therapies, which reduce the pain from conventional surgery and increase drug effectiveness, respectively. Local therapy also reduces the systemic dose and eliminates the toxic side effects of some drugs to other organs. The success of MR-guided procedures depends on visualization of the targets in 3D and precise deployment of ablation catheters, local therapies and devices. MR contrast media provide a wealth of tissue contrast and allows 3D and 4D image acquisitions. After the development of fast imaging sequences, the clinical applications of MR contrast media have been substantially expanded to include pre- during- and post-interventions. Prior to intervention, MR contrast media have the potential to localize and delineate pathologic tissues of vital organs, such as the brain, heart, breast, kidney, prostate, liver and uterus. They also offer other options such as labeling therapeutic agents or cells. During intervention, these agents have the capability to map blood vessels and enhance the contrast between the endovascular guidewire/catheters/devices, blood and tissues as well as direct therapies to the target. Furthermore, labeling therapeutic agents or cells aids in visualizing their delivery sites and tracking their tissue distribution. After intervention, MR contrast media have been used for assessing the efficacy of ablation and therapies. It should be noted that most image-guided procedures are under preclinical research and development. It can be concluded that MR contrast media have great value in preclinical and some clinical interventional procedures. Future applications of MR contrast media in image-guided procedures depend on their safety, tolerability, tissue specificity and effectiveness in demonstrating success of the interventions and therapies.

MR fluoroscopy in vascular and cardiac interventions (review)

Vascular and cardiac disease remains a leading cause of morbidity and mortality in developed and emerging countries. Vascular and cardiac interventions require extensive fluoroscopic guidance to navigate endovascular catheters. X-ray fluoroscopy is considered the current modality for real time imaging. It provides excellent spatial and temporal resolution, but is limited by exposure of patients and staff to ionizing radiation, poor soft tissue characterization and lack of quantitative physiologic information. MR fluoroscopy has been introduced with substantial progress during the last decade. Clinical and experimental studies performed under MR fluoroscopy have indicated the suitability of this modality for: delivery of ASD closure, aortic valves, and endovascular stents (aortic, carotid, iliac, renal arteries, inferior vena cava). It aids in performing ablation, creation of hepatic shunts and local delivery of therapies. Development of more MR compatible equipment and devices will widen the applications of MR-guided procedures. At post-intervention, MR imaging aids in assessing the efficacy of therapies, success of interventions. It also provides information on vascular flow and cardiac morphology, function, perfusion and viability. MR fluoroscopy has the potential to form the basis for minimally invasive image-guided surgeries that offer improved patient management and cost effectiveness.

Utility of dual-modality bioluminescence and MRI in monitoring stem cell survival and impact on post myocardial infarct remodeling

RATIONALE AND OBJECTIVES

Firefly luciferase (Fluc) reporter gene is an authentic marker for surviving stem cells. However, it is unable to visualize the intramyocardial delivery of stem cells or their impact on cardiac function. The investigators demonstrate that bioluminescence imaging (BLI) combined with magnetic resonance imaging (MRI) allows better assessment of cell delivery and the impact on post-myocardial infarction remodeling.

MATERIALS AND METHODS

Murine embryonic stem cells (0.3 million) were double-labeled with Fluc and superparamagnetic iron oxide particles and injected into the infarct border zone of athymic rat hearts. BLI and MRI were performed serially up to 2 months after injection, followed by immunohistochemistry.

RESULTS

Dual-modality imaging was able to verify the initial intramyocardial delivery of the cells and their survival status. Over time, BLI signal increased in seven of nine hearts and disappeared in the other two hearts. The divergence of BLI signal over time was supported by MRI findings. Left ventricular ejection fraction and fractional shortening estimated by MRI suggested that cell engraftment mediated a positive impact on post-myocardial infarction remodeling. Two months after intramyocardial injection, superparamagnetic iron oxide-associated signals facilitated the localization of the injection site.

CONCLUSIONS

Dual-modality imaging has the unique ability to monitor cell delivery, survival status, graft morphology, and impact on post-myocardial infarction remodeling.

Superparamagnetic iron oxide labeling of stem cells for MRI tracking and delivery in cardiovascular disease

In the mid-1980s, iron oxide nanoparticles were developed as contrast agents for diagnostic imaging. In the last two decades, established methods to label cells with superparamagnetic iron oxides (SPIOs) have been developed to aid in targeted delivery and tracking of stem cell therapies. The surge in cellular therapy clinical trials for cardiovascular applications has seen a similar rise in the number of preclinical animal studies of SPIO-labeled stem cells in an effort to understand the mechanisms of cardiovascular regenerative therapy and stem cell biodistribution. The adoption of a limited number of methods of direct labeling of stem cells with SPIOs is due in large part to the desire to rapidly translate these techniques to clinical trials. In this review, we will outline the most commonly adopted methods for iron oxide labeling of stem cells for cardiovascular applications and describe strategies for magnetic resonance imaging (MRI) of magnetically labeled cells in the heart.

In vivo imaging of embryonic stem cell therapy

Embryonic stem cells (ESCs) have the most pluripotent potential of any stem cell. These cells, isolated from the inner cell mass of the blastocyst, are "pluripotent," meaning that they can give rise to all cell types within the developing embryo. As a result, ESCs have been regarded as a leading candidate source for novel regenerative medicine therapies and have been used to derive diverse cell populations, including myocardial and endothelial cells. However, before they can be safely applied clinically, it is important to understand the in vivo behavior of ESCs and their derivatives. In vivo analysis of ESC-derived cells remains critically important to define how these cells may function in novel regenerative medicine therapies. In this review, we describe several available imaging modalities for assessing cell engraftment and discuss their strengths and limitations. We also analyze the applications of these modalities in assessing the utility of ESCs in regenerative medicine therapies.

Non-surgical stem cell delivery strategies and in vivo cell tracking to injured myocardium

Heart failure is a major economic and public health problem. Despite the recent advances in drug therapy and coronary revascularization, the lost cardiomyocytes due to necrosis and apoptosis are not replaced by new myocardial tissue. Cell therapy is an interesting therapeutic option as it potentially improves contractility and restores regional ventricular function. Early clinical data demonstrated that cell transplantation, mainly delivered through non-surgical methods, is safe and feasible. However, several important issues need to be elucidated. This includes, next to determining the best cell type, the optimal delivery strategy, the biodistribution and the survival of implanted stem cells after transplantation. In this view, pre-clinical animal experiments are indispensable. Reporter genes, magnetic or radioactive labeling of stem cells have been developed to observe the fate and the distribution of transplanted cells using non-invasive imaging techniques. Several studies have demonstrated that these direct and non-direct labeling techniques may become an important tool in cell therapy. Integration of cell delivery and cell tracking will probably be a key for the success of cell therapy in patients. This review will provide a comprehensive overview on the various cell tracking and non-surgical cell delivery techniques, which are highly important in view of experimental and clinical studies.

A biological global positioning system: considerations for tracking stem cell behaviors in the whole body

Many recent research studies have proposed stem cell therapy as a treatment for cancer, spinal cord injuries, brain damage, cardiovascular disease, and other conditions. Some of these experimental therapies have been tested in small animals and, in rare cases, in humans. Medical researchers anticipate extensive clinical applications of stem cell therapy in the future. The lack of basic knowledge concerning basic stem cell biology-survival, migration, differentiation, integration in a real time manner when transplanted into damaged CNS remains an absolute bottleneck for attempt to design stem cell therapies for CNS diseases. A major challenge to the development of clinical applied stem cell therapy in medical practice remains the lack of efficient stem cell tracking methods. As a result, the fate of the vast majority of stem cells transplanted in the human central nervous system (CNS), particularly in the detrimental effects, remains unknown. The paucity of knowledge concerning basic stem cell biology--survival, migration, differentiation, integration in real-time when transplanted into damaged CNS remains a bottleneck in the attempt to design stem cell therapies for CNS diseases. Even though excellent histological techniques remain as the gold standard, no good in vivo techniques are currently available to assess the transplanted graft for migration, differentiation, or survival. To address these issues, herein we propose strategies to investigate the lineage fate determination of derived human embryonic stem cells (hESC) transplanted in vivo into the CNS. Here, we describe a comprehensive biological Global Positioning System (bGPS) to track transplanted stem cells. But, first, we review, four currently used standard methods for tracking stem cells in vivo: magnetic resonance imaging (MRI), bioluminescence imaging (BLI), positron emission tomography (PET) imaging and fluorescence imaging (FLI) with quantum dots. We summarize these modalities and propose criteria that can be employed to rank the practical usefulness for specific applications. Based on the results of this review, we argue that additional qualities are still needed to advance these modalities toward clinical applications. We then discuss an ideal procedure for labeling and tracking stem cells in vivo, finally, we present a novel imaging system based on our experiments.

In vivo imaging of stem cells and Beta cells using direct cell labeling and reporter gene methods

Cellular transplantation therapy offers a means to stimulate cardiovascular repair either by direct (graft-induced) or indirect (host-induced) tissue regeneration or angiogenesis. Typically, autologous or donor cells of specific subpopulations are expanded exogenously before administration to enrich the cells most likely to participate in tissue repair. In animal models of cardiovascular disease, the fate of these exogenous cells can be determined using histopathology. Recently, methods to label cells with contrast agents or transduce cells with reporter genes to produce imaging beacons has enabled the serial and dynamic assessment of the survival, fate, and engraftment of these cells with noninvasive imaging. Although cell tracking methods for cardiovascular applications have been most studied in stem or progenitor cells, research in tracking of whole islet transplants and particularly insulin producing beta cells has implications to the cardiovascular community attributable to the vascular changes associated with diabetes mellitus. In this review article, we will explore some of the state-of-the art methods for stem, progenitor, and beta cell tracking.

Embryonic stem cell grafting in normal and infarcted myocardium: serial assessment with MR imaging and PET dual detection

PURPOSE

To use magnetic resonance (MR) imaging and positron emission tomography (PET) dual detection of cardiac-grafted embryonic stem cells (ESCs) to examine (a) survival and proliferation of ESCs in normal and infarcted myocardium, (b) host macrophage versus grafted ESC contribution to serial MR imaging signal over time, and (c) cardiac function associated with the formation of grafts and whether improvement in cardiac function is related to cardiac differentiation of ESCs.

MATERIALS AND METHODS

All animal procedures were approved by the institutional animal care and use committee. Murine ESCs were stably transfected with a mutant version of herpes simplex virus type 1 thymidine kinase, HSV1-sr39tk, and also were labeled with superparamagnetic iron oxide (SPIO) particles. Cells were injected directly in the border zone of the infarcted heart or in corresponding regions of normal hearts in athymic rats. PET and MR imaging were performed longitudinally for 4 weeks in the same animals.

RESULTS

ESCs survived and underwent proliferation in the infarcted and normal hearts, as demonstrated by serial increases in 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl) guanine PET signals. In parallel, the hypointense areas on MR images at the injection sites decreased over time. Double staining for host macrophages and SPIO particles revealed that the majority of SPIO-containing cells were macrophages at week 4 after injection. Left ventricular ejection fraction increased in the ESC-treated rats but decreased in culture media-treated rats, and border-zone function was preserved in ESC-treated animals; however, cardiac differentiation of ESCs was less than 0.5%.

CONCLUSION

Dual-modality imaging permits complementary information in regard to cell survival and proliferation, graft formation, and effects on cardiac function.

SUPPLEMENTAL MATERIAL

http://radiology.rsnajnls.org/cgi/content/full/250/3/821/DC1.

An in vivo multimodal imaging study using MRI and PET of stem cell transplantation after myocardial infarction in rats

PURPOSE

The purpose of the study is to track iron-oxide nanoparticle-labelled adult rat bone marrow-derived stem cells (IO-rBMSCs) by magnetic resonance imaging (MRI) and determine their effect in host cardiac tissue using 2-deoxy-2-[F-18]fluoro-D: -glucose-positron emission tomography (FDG-PET).

PROCEDURES

Infarcted rats were randomised to receive (1) live IO-rBMSCs by direct local injection, or (2) dead IO-rBMSCs as controls; (3) sham-operated rats received live IO-rBMSCs. The rats were then imaged from 2 days to 6 weeks post-cell implantation using both MRI at 9.4T and FDG-PET.

RESULTS

Implanted IO-rBMSCs were visible in the heart by MRI for the duration of the study. Histological analysis confirmed that the implanted IO-rBMSCs were present for up to 6 weeks post-implantation. At 1 week post-IO-rBMSC transplantation, PET studies demonstrated an increase in FDG uptake in infarcted regions implanted with live IO-rBMSC compared to controls.

CONCLUSIONS

Noninvasive multimodality imaging allowed us to visualise IO-rBMSCs and establish their affect on cardiac function in a rat model of myocardial infarction (MI).

Cellular MRI and its role in stem cell therapy

Hematopoietic, stromal and organ-specific stem cells are under evaluation for therapeutic efficacy in cell-based therapies of cardiac, neurological and other disorders. It is critically important to track the location of directly transplanted or infused cells that can serve as gene carrier/delivery vehicles for the treatment of disease processes and be able to noninvasively monitor the temporal and spatial homing of these cells to target tissues. Moreover, it is also necessary to determine their engraftment efficiency and functional capability following transplantation. There are various in vivo imaging modalities used to track the movement and incorporation of administered cells. Tagging stem cells with different contrast agents can make these cells probes for different imaging modalities. Recent reports have shown that stem cells labeled with iron oxides can be used as cellular MRI probes demonstrating the cell trafficking to target tissues. In this review, we will discuss the status and future prospect of stem cell tracking by cellular MRI for cell-based therapy.

In vitro model of bromodeoxyuridine or iron oxide nanoparticle uptake by activated macrophages from labeled stem cells: implications for cellular therapy

There is increasing interest in using exogenous labels such as bromodeoxyuridine (BrdU) or superparamagnetic iron oxide nanoparticles (SPION) to label cells to identify transplanted cells and monitor their migration by fluorescent microscopy or in vivo magnetic resonance imaging (MRI), respectively. Direct implantation of cells into target tissue can result in >80% cell death due to trauma or apoptosis. Bystander uptake of labeled cells by activated macrophages (AM) can confound the interpretation of results. This study investigated the frequency of BrdU or SPION uptake by AM using the Boyden chamber model of inflammation. SPION/BrdU-labeled bone marrow stromal cells or HeLa cells, AM, and mouse fibroblasts (MF) or human fibroblasts (HF) were mixed in various ratios in Matrigel in the upper chamber and incubated for up to 96 hours. The AM were chemotactically induced to migrate to the lower chamber. Fluorescence-activated cell sorting analysis of AM from lower and upper chambers, in the presence of either MF or HF using anti-CD68, anti-BrdU, anti-dextran antibodies, revealed 10%-20% dextran-positive or 10% BrdU-positive AM after 96 hours of incubation. Transfer of iron to AM accounted for <10% of the total iron in labeled cells. The uptake of BrdU and SPION was dependent on the ratio of labeled cells to inflammatory cells and microenvironmental conditions. Direct implantation of BrdU/SPION-labeled cells into target tissue can result in uptake of label by AM; therefore, care should be taken to validate by histology transplanted cells for bystander cell markers and correlation with MRI results.

In vivo imaging of transplanted hepatocytes with a 1.5-T clinical MRI system--initial experience in mice

The feasibility of in vitro mature mouse hepatocyte labeling with a novel iron oxide particle was assessed and the ability of 1.5-T magnetic resonance imaging (MRI) to track labeled mouse hepatocytes in syngenic recipient livers following intraportal cell transplantation was tested. Mouse hepatocytes were incubated with anionic iron oxide nanoparticles at various iron concentrations. Cell viability was assessed and iron oxide particle uptake quantified. Labeled hepatocytes were intraportally injected into 20 mice, while unlabeled hepatocytes were injected into two mice. Liver T2 values, spleen-to-muscle relative signal intensity (RI( spleen/muscle )), and liver-to-muscle relative signal intensity (RI( liver/muscle )) on gradient-echo T2-weighted imaging after injection of either labeled or unlabeled hepatocytes were compared with an ANOVA test followed by Fisher's a posteriori PLSD test. Livers, spleens and lungs were collected for histological analysis. Iron oxide particle uptake was saturable with a maximum iron content of 20 pg per cell and without viability alteration after 3 days of culture. Following labeled-cell transplantation, recipient livers showed well-defined nodular foci of low signal intensity on MRI--consistent with clusters of labeled hepatocytes on pathological analysis--combined with a significant decrease in both liver T2 values and liver-to-muscle RI( liver/muscle ) (P = 0.01) with minimal T2 values demonstrated 8 days after transplantation. Conventional MRI can demonstrate the presence of transplanted iron-labeled mature hepatocytes in mouse liver.

Noninvasive tracking of cardiac embryonic stem cells in vivo using magnetic resonance imaging techniques

Despite rapid advances in the stem cell field, the ability to identify and track transplanted or migrating stem cells in vivo is limited. To overcome this limitation, we used magnetic resonance imaging (MRI) to detect and follow transplanted stem cells over a period of 28 days in mice using an established myocardial infarction model. Pluripotent mouse embryonic stem (mES) cells were expanded and induced to differentiate into beating cardiomyocytes in vitro. The cardiac-differentiated mES cells were then loaded with superparamagnetic fluorescent microspheres (1.63 microm in diameter) and transplanted into ischemic myocardium immediately following ligation and subsequent reperfusion of the left anterior descending coronary artery. To identify the transplanted stem cells in vivo, MRI was performed using a Varian Inova 4.7 Tesla scanner. Our results show that (a) the cardiac-differentiated mES were effectively loaded with superparamagnetic microspheres in vitro, (b) the microsphere-loaded mES cells continued to beat in culture prior to transplantation, (c) the transplanted mES cells were readily detected in the heart in vivo using noninvasive MRI techniques, (d) the transplanted stem cells were detected in ischemic myocardium for the entire 28-day duration of the study as confirmed by MRI and post-mortem histological analyses, and (e) concurrent functional MRI indicated typical loss of cardiac function, although significant amelioration of remodeling was noted after 28 days in hearts that received transplanted stem cells. These results demonstrate that it is feasible to simultaneously track transplanted stem cells and monitor cardiac function in vivo over an extended period using noninvasive MRI techniques.

MR imaging of Her-2/neu protein using magnetic nanoparticles

The aim of this study was to assess whether Her-2/neu expressing tumour cells can be detected in vitro as well as in animal tumour models with magnetic resonance imaging at 1.5 T. Magnetic nanoparticles (with relaxivities R 1, R 2 of 3.7 ± 0.4 (mM s)(-1), 277 ± 32 (mM s)(-1) at 21 °C, respectively) coupled to anti-Her-2/neu antibodies or gamma globulin IgG (high or non-affinity probe, respectively) were used. After incubation of Her-2/neu expressing cells (SKBR3) with high or non-affinity probes (20 min), values of R 1 = 0.34 ± 0.02 (mM s)(-1) and R 2 = 63.02 ± 30 (mM s)(-1) were obtained. Electron microscopy and atomic absorption spectrometry examinations verified the presence of relatively high iron levels in cells incubated with the high affinity probe compared to controls. For in vivo MRI, high or non-affinity probes (≈1.7 mg Fe/animal) were injected into the tail vein of mice (n = 16) bearing SKBR3 tumours. A distinct decrease in the normalized MR signal ratio between tumour and reference area (approximately -17 ± 2%) after application of the high affinity probe was observed. In conclusion, in vivo detection of Her-2/neu expressing tumours is feasible in a clinical MR scanner by using immunoconjugated magnetic nanoparticles.

Iron particles for noninvasive monitoring of bone marrow stromal cell engraftment into, and isolation of viable engrafted donor cells from, the heart

Stem cells offer a promising approach to the treatment of myocardial infarction and prevention of heart failure. We have used iron labeling of bone marrow stromal cells (BMSCs) to noninvasively track cell location in the infarcted rat heart over 16 weeks using cine-magnetic resonance imaging (cine-MRI) and to isolate the BMSCs from the grafted hearts using the magnetic properties of the donor cells. BMSCs were isolated from rat bone marrow, characterized by flow cytometry, transduced with lentiviral vectors expressing green fluorescent protein (GFP), and labeled with iron particles. BMSCs were injected into the infarct periphery immediately following coronary artery ligation, and rat hearts were imaged at 1, 4, 10, and 16 weeks postinfarction. Signal voids caused by the iron particles in the BMSCs were detected in all rats at all time points. In mildly infarcted hearts, the volume of the signal void decreased over the 16 weeks, whereas the signal void volume did not decrease significantly in severely infarcted hearts. High-resolution three-dimensional magnetic resonance (MR) microscopy identified hypointense regions at the same position as in vivo. Donor cells containing iron particles and expressing GFP were identified in MR-targeted heart sections after magnetic cell separation from digested hearts. In conclusion, MRI can be used to track cells labeled with iron particles in damaged tissue for at least 16 weeks after injection and to guide tissue sectioning by accurately identifying regions of cell engraftment. The magnetic properties of the iron-labeled donor cells can be used for their isolation from host tissue to enable further characterization.