Effectiveness of micron‐sized superparamagnetic iron oxide particles as markers for detection of migration of bone marrow‐derived mesenchymal stromal cells in a stroke model

Purpose:

To evaluate the feasibility of using micron‐sized superparamagnetic iron oxide particles (MPIOs) as an effective labeling agent for monitoring bone marrow‐derived mesenchymal stromal cell (BMSC) migration in the brain using magnetic resonance imaging (MRI) in a rat model of stroke and whether the accumulation of MPIO‐labeled BMSCs can be differentiated from the accumulation of free MPIO particles or hemoglobin breakdown at a site of neuronal damage.

Materials and Methods:

In this study BMSCs were labeled with iron oxide and their pattern of migration following intravenous injection in a rat stroke model was monitored using a clinical MRI system followed by standard histopathology. The migration pattern was compared between intravenous injection of BMSCs alone, BMSCs labeled with MPIOs, and MPIO particles alone.

Results:

The results demonstrated that while MRI was highly sensitive in the detection of iron oxide particle‐containing cells in areas of neuronal ischemia, the true origin of cells containing iron oxide particles remains ambiguous. Therefore, detection of iron particles may not be a suitable strategy for the detection of BMSCs in the brain in a stroke model.

Conclusion:

This study suggests that the use of MPIOs as labeling agents are insufficient to conclusively determine the localization of iron within cells in regions of neuronal ischemia and hemorrhage. J. Magn. Reson. Imaging 2013;37:1409–1418. © 2012 Wiley Periodicals, Inc.     

MRI detection of macrophages labeled using micrometer-sized iron oxide particles

PURPOSE: To evaluate cellular labeling of immune cells using micron-sized iron oxide particles (MPIOs) and evaluate the MR relaxivity and MRI detection of the labeled cells. MATERIALS AND METHODS: Immune cells isolated from mice and rats were labeled with three different sizes of MPIO particles (0.35, 0.90, or 1.63 microm). These labeled cells were characterized using transmission electron microscopy (TEM), fluorescence microscopy, flow cytometry, MR relaxometry, and MRI. RESULTS: Macrophage uptake of MPIOs was found to be highest for the 1.63-microm size particles. MR relaxivity measurements indicated greater spin-spin relaxation for MPIO-labeled cells relative to cells labeled with nanometer-sized ultra-small superparamagnetic iron oxide (USPIO) particles with similar iron content. TEM and fluorescence microscopy indicated cellular uptake of multiple MPIO particles per cell. Macrophages labeled with 1.63-microm MPIOs had an average cellular iron uptake of 39.1 pg/cell, corresponding to approximately 35 particles per cell. CONCLUSION: Cells labeled with one or more MPIO particles could be readily detected ex vivo at 11.7 Tesla and after infusion of the MPIO-labeled macrophages into the kidney of a rat, hypointense regions of the outer cortex are observed, in vivo, by MRI at 4.7 Tesla.     

Gene expression profiling reveals early cellular responses to intracellular magnetic labeling with superparamagnetic iron oxide nanoparticles

With MRI (stem) cell tracking having entered the clinic, studies on the cellular genomic response toward labeling are warranted. Gene expression profiling was applied to C17.2 neural stem cells following superparamagnetic iron oxide/PLL (poly‐L ‐lysine) labeling over the course of 1 week. Relative to unlabeled cells, less than 1% of genes (49 total) exhibited greater than 2‐fold difference in expression in response to superparamagnetic iron oxide/PLL labeling. In particular, transferrin receptor 1 (Tfrc) and heme oxygenase 1 (Hmox1) expression was downregulated early, whereas genes involved in lysosomal function (Sulf1) and detoxification (Clu, Cp, Gstm2, Mgst1) were upregulated at later time points. Relative to cells treated with PLL only, cells labeled with superparamagnetic iron oxide/PLL complexes exhibited differential expression of 1399 genes. Though these differentially expressed genes exhibited altered expression over time, the overall extent was limited. Gene ontology analysis of differentially expressed genes showed that genes encoding zinc‐binding proteins are enriched after superparamagnetic iron oxide/PLL labeling relative to PLL only treatment, whereas members of the apoptosis/programmed cell death pathway did not display increased expression. Overexpression of the differentially expressed genes Rnf138 and Abcc4 were confirmed by quantitative real‐time polymerase chain reaction. These results demonstrate that, although early reactions responsible for iron homeostasis are induced, overall neural stem cell gene expression remains largely unaltered following superparamagnetic iron oxide/PLL labeling. Magn Reson Med 63:1031–1043, 2010. © 2010 Wiley‐Liss, Inc.     

Monitoring bone marrow‐originated mesenchymal stem cell traffic to myocardial infarction sites using magnetic resonance imaging

How stem cells promote myocardial repair in myocardial infarction (MI) is not well understood. The purpose of this study was to noninvasively monitor and quantify mesenchymal stem cells (MSC) from bone marrow to MI sites using magnetic resonance imaging (MRI). MSC were dual‐labeled with an enhanced green fluorescent protein and micrometer‐sized iron oxide particles prior to intra‐bone marrow transplantation into the tibial medullary space of C57Bl/6 mice. Micrometer‐sized iron oxide particles labeling caused signal attenuation in T₂*‐weighted MRI and thus allowed noninvasive cell tracking. Longitudinal MRI demonstrated MSC infiltration into MI sites over time. Fluorescence from both micrometer‐sized iron oxide particles and enhanced green fluorescent protein in histology validated the presence of dual‐labeled cells at MI sites. This study demonstrated that MSC traffic to MI sites can be noninvasively monitored in MRI by labeling cells with micrometer‐sized iron oxide particles. The dual‐labeled MSC at MI sites maintained their capability of proliferation and differentiation. The dual‐labeling, intra‐bone marrow transplantation, and MRI cell tracking provided a unique approach for investigating stem cells' roles in the post‐MI healing process. This technique can potentially be applied to monitor possible effects on stem cell mobilization caused by given treatment strategies. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Temporal and noninvasive monitoring of inflammatory‐cell infiltration to myocardial infarction sites using micrometer‐sized iron oxide particles

Micrometer‐sized iron oxide particles (MPIO) are a more sensitive MRI contrast agent for tracking cell migration compared to ultrasmall iron oxide particles. This study investigated the temporal relationship between inflammation and tissue remodeling due to myocardial infarction (MI) using MPIO‐enhanced MRI. C57Bl/6 mice received an intravenous MPIO injection for cell labeling, followed by a surgically induced MI seven days later (n = 7). For controls, two groups underwent either sham‐operated surgery without inducing an MI post‐MPIO injection (n = 7) or MI surgery without MPIO injection (n = 6). The MRIs performed post‐MI showed significant signal attenuation around the MI site for the mice that received an intravenous MPIO injection for cell labeling, followed by a surgically induced MI seven days later, compared to the two control groups (P < 0.01). The findings suggested that the prelabeled inflammatory cells mobilized and infiltrated into the MI site. Furthermore, the linear regression of contrast‐to‐noise ratio at the MI site and left ventricular ejection function suggested a positive correlation between the labeled inflammatory cell infiltration and cardiac function attenuation during post‐MI remodeling (r² = 0.98). In conclusion, this study demonstrated an MRI technique for noninvasively and temporally monitoring inflammatory cell migration into the myocardium while potentially providing additional insight concerning the pathologic progression of a myocardial infarction. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Sizing it up: cellular MRI using micron-sized iron oxide particles.

There is rapidly increasing interest in the use of MRI to track cell migration in intact animals. Currently, cell labeling is usually accomplished by endocytosis of nanometer-sized, dextran-coated iron oxide particles. The limitations of using nanometer-sized particles, however, are that millions of particles are required to achieve sufficient contrast, the label can be diluted beyond observability by cell division, and the label is biodegradable. These problems make it difficult to label cells other than macrophages in vivo, and to conduct long-term engraftment studies. It was recently demonstrated that micron-sized iron oxide particles (MPIOs) can be taken up by a number of cell types. In this study we examined the MRI properties of single MPIOs with sizes of 0.96, 1.63, 2.79, 4.50, and 5.80 microm. Furthermore, the capacity of cells to endocytose these MPIOs was investigated, and the MRI properties of the labeled cells at 7.0 and 11.7 Tesla were measured as a function of image resolution and echo time (TE). Cells labeled with MPIOs generally contained iron levels of approximately 100 pg, which is approximately threefold higher than those obtained with the best strategies to label cells using nanometer-sized particles. On occasion, some cells had levels as high as approximately 400 pg. We demonstrate that these large particles and the cells labeled with them can be detected by spin echo (SE)-based imaging methods. These measurements indicate that MPIOs should be useful for improving cell tracking by MRI.     

Release activation of iron oxide nanoparticles: (REACTION) A novel environmentally sensitive MRI paradigm

Smart contrast agents for MRI‐based cell tracking would enable the use of MRI methodologies to not only detect the location of cells but also gene expression. Here, we report on a new enzyme/contrast agent paradigm which involves the enzymatic degradation of the polymer coating of magnetic nanoparticles to release encapsulated magnetic cores. Cells were labeled with particles coated with a polymer, which is cleavable by a specific enzyme. This coat restricts the approach of water to the particle, preventing the magnetic core from efficiently relaxing protons. The reactive enzyme was delivered to cells and changes in cellular T₂ and T₂* relaxation times of ～ 35% and ～ 50% were achieved in vitro. Large enhancements of dark contrast volume (240%) and contrast‐to‐noise ratio (48%) within the contrast regions were measured, in vivo, for cells co‐labeled with enzyme and particles. These results warrant exploration of genetic avenues toward achieving release activation of iron oxide nanoparticles. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

An experimental study on MR imaging of atherosclerotic plaque with SPIO marked endothelial cells in a rabbit model

Purpose:

To investigate how to label macrophages in atherosclerotic plaques with superparamagnetic iron oxide (SPIO) nanoparticles and trace SPIO with MR imaging.

Materials and Methods:

Atherosclerotic lesions of a rabbit model were induced by a combination of high‐fat and high‐cholesterol diet and subsequent endothelial abrasion of the abdominal aorta. SPIO particles were pretreated with poly‐L‐lysine. SPIO nanoparticles and SPIO‐labeled human endothelial cells (ECV‐304) were IV injected into model animals, respectively. The MRI scans and histopathological examination were performed 12 h and 24 h after the injection. The imaging and histopathological data were analyzed.

Results:

Prussian blue staining of the vessel specimens indicated that SPIO particles were not found in the atheroma but in the Kupffer's cells of the liver after SPIO injection. However, the accumulation of SPIO particles in the atheroma was confirmed in animals received SPIO‐labeled endothelial cell transplantation. The best quality MR scan sequences of rabbit abdominal aorta were T₂WI fat suppression, T₁WI, and DIR series, on which of MR image had a higher quality. Signal loss of the original incrassate plaque in the vessel wall on T₂WI was found in 6 of 10 animals received SPIO‐labeled endothelial cell transplantation.

Conclusion:

SPIO‐labeled endothelial cells were superior to SPIO for MR imaging of atherosclerotic plaques. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Use of magnetic nanoparticles to monitor alginate‐encapsulated βTC‐tet cells

Noninvasive monitoring of tissue‐engineered constructs is an important component in optimizing construct design and assessing therapeutic efficacy. In recent years, cellular and molecular imaging initiatives have spurred the use of iron oxide‐based contrast agents in the field of NMR imaging. Although their use in medical research has been widespread, their application in tissue engineering has been limited. In this study, the utility of monocrystalline iron oxide nanoparticles (MIONs) as an NMR contrast agent was evaluated for βTC‐tet cells encapsulated within alginate/poly‐L‐lysine/alginate (APA) microbeads. The constructs were labeled with MIONs in two different ways: 1) MION‐labeled βTC‐tet cells were encapsulated in APA beads (i.e., intracellular compartment), and 2) MION particles were suspended in the alginate solution prior to encapsulation so that the alginate matrix was labeled with MIONs instead of the cells (i.e., extracellular compartment). The data show that although the location of cells can be identified within APA beads, cell growth or rearrangement within these constructs cannot be effectively monitored, regardless of the location of MION compartmentalization. The advantages and disadvantages of these techniques and their potential use in tissue engineering are discussed. Magn Reson Med 61:282–290, 2009. © 2009 Wiley‐Liss, Inc.     

In vivo and ex vivo MR imaging of slowly cycling melanoma cells

Slowly cycling cells are believed to play a critical role in tumor progression and metastatic dissemination. The goal of this study was to develop a method for in vivo detection of slowly cycling cells. To distinguish these cells from more rapidly proliferating cells that constitute the vast majority of cells in tumors, we used the well‐known effect of label dilution due to division of cells with normal cycle and retention of contrast agent in slowly dividing cells. To detect slowly cycling cells, melanoma cells were labeled with iron oxide particles. After labeling, we observed dilution of contrast agent in parallel with cell proliferation in the vast majority of normally cycling cells. A small and distinct subpopulation of iron‐retaining cells was detected by flow cytometry after 20 days of in vitro proliferation. These iron‐retaining cells exhibited high expression of a biological marker of slowly cycling cells, JARID1B. After implantation of labeled cells as xenografts into immunocompromised mice, iron‐retaining cells were detected in vivo and ex vivo by magnetic resonance imaging that was confirmed by Prussian Blue staining. Magnetic resonance imaging detects not only iron retaining melanoma cells but also iron positive macrophages. Proposed method opens up opportunities to image subpopulation of melanoma cells, which is critical for continuous tumor growth. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Assessment of cell infiltration in myocardial infarction: A dose‐dependent study using micrometer‐sized iron oxide particles

Myocardial infarction (MI) is a leading cause of death and disabilities. Inflammatory cells play a vital role in the process of postinfarction remodeling and repair. Inflammatory cell infiltration into the infarct site can be monitored using T‐weighted MRI following an intravenous administration of iron oxide particles. In this study, various doses of micrometer‐sized iron oxide particles (1.1–14.5 μg Fe/g body weight) were injected into the mouse blood stream before a surgical induction of MI. Cardiac MRIs were performed at 3, 7, 14, and 21 days postinfarction to monitor the signal attenuation at the infarct site. A dose‐dependent phenomenon of signal attenuation was observed at the infarct site, with a higher dose leading to a darker signal. The study suggests an optimal temporal window for monitoring iron oxide particles‐labeled inflammatory cell infiltration to the infarct site using MRI. The dose‐dependent signal attenuation also indicates an optimal iron oxide dose of approximately 9.1–14.5 μg Fe/g body weight. A lower dose cannot differentiate the signal attenuation, whereas a higher dose would cause significant artifacts. This iron oxide‐enhanced MRI technique can potentially be used to monitor cell migration and infiltration at the pathological site or to confirm any cellular response following some specific treatment strategies. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

In vivo single scan detection of both iron-labeled cells and breast cancer metastases in the mouse brain using balanced steady-state free precession imaging at 1.5 T

Purpose:

To simultaneously detect iron‐labeled cancer cells and brain tumors in vivo in one scan, the balanced steady‐state free precession (b‐SSFP) imaging sequence was optimized at 1.5 T on mice developing brain metastases subsequent to the injection of micron‐sized iron oxide particle‐labeled human breast cancer cells.

Materials and Methods:

b‐SSFP sequence parameters (repetition time, flip angle, and receiver bandwidth) were varied and the signal‐to‐noise ratio, contrast between the brain and tumors, and the number of detected iron‐labeled cells were evaluated.

Results:

Optimal b‐SSFP images were acquired with a 26 msec repetition time, 35° flip angle, and bandwidth of ±21 kHz. b‐SSFP images were compared with T₂‐weighted 2D fast spin echo (FSE) and 3D spoiled gradient recalled echo (SPGR) images. The mean tumor‐brain contrast‐to‐noise ratio and the ability to detect iron‐labeled cells were the highest in the b‐SSFP images.

Conclusion:

A single b‐SSFP scan can be used to visualize both iron‐labeled cells and brain metastases. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

19F MRI detection of acute allograft rejection with in vivo perfluorocarbon labeling of immune cells

Current diagnosis of organ rejection following transplantation relies on tissue biopsy, which is not ideal due to sampling limitations and risks associated with the invasive procedure.We have previously shown that cellular magnetic resonance imaging (MRI) of iron‐oxide labeled immune‐cell infiltration can provide a noninvasive measure of rejection status by detecting areas of hypointensity on T‐weighted images. In this study, we tested the feasibility of using a fluorine‐based cellular tracer agent to detect macrophage accumulation in rodent models of acute allograft rejection by fluorine‐19 (¹⁹F) MRI and magnetic resonance spectroscopy. This study used two rat models of acute rejection, including abdominal heterotopic cardiac transplant and orthotopic kidney transplant models. Following in vivo labeling of monocytes and macrophages with a commercially available agent containing perfluoro‐15‐crown‐5‐ether, we observed ¹⁹F‐signal intensity in the organs experiencing rejection by ¹⁹F MRI, and conventional ¹H MRI was used for anatomical context. Immunofluorescense and histology confirmed macrophage labeling. These results are consistent with our previous studies and show the complementary nature of the two cellular imaging techniques. With no background signal, ¹⁹F MRI/magnetic resonance spectroscopy can provide unambiguous detection of fluorine labeled cells, and may be a useful technique for detecting and quantifying rejection grade in patients. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Imaging iron‐loaded mouse glioma tumors with bSSFP at 3 T

The poor prognosis for patients with high‐grade glioma is partly due to the invasion of tumor cells into surrounding brain tissue. The goal of the present work was to develop a mouse model of glioma that included the potential to track cell invasion using MRI by labeling GL261 cells with iron oxide contrast agents prior to intracranial injection. Two types of agents were compared with several labeling schemes to balance between labeling with sufficient iron to curb the dilution effect of cell division while avoiding overwhelming signal loss that could prevent adequate visualization of tumor boundaries. The balanced steady‐state free precession (bSSFP) pulse sequence was evaluated for its suitability for imaging glioma tumors and compared to T₂‐weighted two‐dimensional fast spin echo (FSE) and T₁‐weighted spoiled gradient recalled echo (SPGR) at 3 T in terms of signal‐to‐noise ratio and contrast‐to‐noise ratio efficiencies. Ultimately, a three‐dimensional bSSFP protocol consisting of a set of two images with complementary contrasts was developed, allowing excellent tumor visualization with minimal iron contrast when using pulse repetition time = 6 ms and α = 40°, and extremely high sensitivity to iron when using pulse repetition time = 22 ms and α = 20°. Quantitative histologic analysis validated that the strong signal loss seen in balanced steady state free precession pulse sequence images of iron‐loaded tumors correlated well with the presence of iron. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Long‐term MR cell tracking of neural stem cells grafted in immunocompetent versus immunodeficient mice reveals distinct differences in contrast between live and dead cells

Neural stem cell (NSC)‐based therapy is actively being pursued in preclinical and clinical disease models. Magnetic resonance imaging (MRI) cell tracking promises to optimize current cell transplantation paradigms, however, it is limited by dilution of contrast agent during cellular proliferation, transfer of label from dying cells to surrounding endogenous host cells, and/or biodegradation of the label. Here, we evaluated the applicability of magnetic resonance imaging for long‐term tracking of transplanted neural stem cells labeled with superparamagnetic iron oxide and transfected with the bioluminescence reporter gene luciferase. Mouse neural stem cells were transplanted into immunodeficient, graft‐accepting Rag2 mice or immunocompetent, graft‐rejecting Balb/c mice. Hypointense voxel signals and bioluminescence were monitored over a period of 93 days. Unexpectedly, in mice that rejected the cells, the hypointense MR signal persisted throughout the entire time‐course, whereas in the nonrejecting mice, the contrast cleared at a faster rate. In immunocompetent, graft‐rejecting Balb/c mice, infiltrating leukocytes, and microglia were found surrounding dead cells and internalizing superparamagnetic iron oxide clusters. The present results indicate that live cell proliferation and associated label dilution may dominate contrast clearance as compared with cell death and subsequent transfer and retention of superparamagnetic iron oxide within phagocytes and brain interstitium. Thus, interpretation of signal changes during long‐term MR cell tracking is complex and requires caution. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Relaxation effects of ferucarbotran‐labeled mesenchymal stem cells at 1.5T and 3T: Discrimination of viable from lysed cells

Human mesenchymal stem cells (hMSCs) were labeled with Ferucarbotran by simple incubation and cultured for up to 14 d. Iron content was determined by spectrometry and the intracellular localization of the contrast agent uptake was studied by electron and confocal microscopy. At various time points after labeling, ranging from 1 to 14 d, samples with viable or lysed labeled hMSCs, as well as nonlabeled controls, underwent MRI. Spin‐echo (SE) and gradient‐echo (GE) sequences with multiple TRs and TEs were used at 1.5T and 3T on a clinical scanner. Spectrometry showed an initial iron oxide uptake of 7.08 pg per cell. Microscopy studies revealed lysosomal compartmentalization. Contrast agent effects of hMSCs were persistent for up to 14 d after labeling. A marked difference in the T₂ effect of compartmentalized iron oxides compared to free iron oxides was found on T₂‐weighted sequences, but not on T‐weighted sequences. The observed differences may be explained by the loss of compartmentalization of iron oxide particles, the uniformity of distribution, and the subsequent increase in dephasing of protons on SE images. These results show that viable cells with compartmentalized iron oxides may—in principle—be distinguished from lysed cells or released iron oxides. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Capacity of human monocytes to phagocytose approved iron oxide MR contrast agents in vitro

To evaluate the capacity of human monocytes to phagocytose various approved iron oxide based magnetic resonance (MR) contrast agents and to optimize in vitro labeling of these cells. Human monocytes were incubated with two superparamagnetic iron oxide particles (SPIO) as well as two ultrasmall SPIO (USPIO) at varying iron oxide concentrations and incubation times. Iron uptake in monocytes was proven by histology, quantified by atomic emission absorption spectrometry and depicted with T2* weighted fast field echo (FFE) MR images at 1.5 T. Additionally, induction of apoptosis in iron oxide labeled monocytes was determined by YO-PRO-1 staining. Cellular iron uptake was significantly (P<0.01) higher after incubation with SPIO compared with USPIO. For SPIO, the iron oxide uptake was significantly (P<0.01) higher after incubation with the ionic Ferucarbotran as compared with the non-ionic Ferumoxides. Efficient cell labeling was achieved after incubation with Ferucarbotran at concentrations 500 g Fe/ml and incubation times 1 h, resulting in a maximal iron oxide uptake of up to 50 pg Fe/cell without impairment of cell viability. In vitro labeling of human monocytes for MR imaging is most effectively obtained with the approved SPIO Ferucarbotran. Potential subsequent in vivo cell tracking applications comprise, e.g. specific targeting of inflammatory processes.     

Iron oxide nanoparticle-labeled rat smooth muscle cells: cardiac MR imaging for cell graft monitoring and quantitation

PURPOSE: To perform a quantitative analysis of anionic maghemite nanoparticle-labeled cells in vitro and determine the effect of labeling on signal intensity at magnetic resonance (MR) imaging. MATERIALS AND METHODS: The study was approved by the institutional animal care and use committee at H?pital Bichat. In vitro cell proliferation, iron content per cell, and MR signal intensity of cells were measured in agarose phantoms for 0-14 days of culture after labeling of rat smooth muscle cells with anionic maghemite nanoparticles. Next, iron oxide-labeled smooth muscle cells were injected into healthy hearts and hearts with ischemic injury in seven live Fisher rats. Ex vivo MR imaging experiments in excised hearts 2 and 48 hours after injection were performed with a 1.5-T medical imaging system by using T2-weighted gradient-echo and spin-echo sequences. Histologic sections were obtained after MR imaging. Correlation analyses between division factor of iron load and cell amplification factor and between 1/T2 and number of labeled cells or number of days in culture were performed by using linear regression. RESULTS: Viability of smooth muscle cells was not affected by magnetic labeling. Transmission electron micrographs of cells revealed the presence of iron oxide nanoparticles in vesicles up to day 14 of culture. Intracellular iron concentration decreased in parallel with cell division (r2 = 0.99) and was correlated with MR signal intensity (r2 = 0.95). T2*-weighted MR images of excised rat hearts showed hypointense signal in myocardium at 2 and 48 hours after local injection of labeled cells. Subsequent histologic staining evidenced iron oxide nanoparticles within cells and confirmed the presence of the original cells at 2 and 48 hours after implantation. CONCLUSION: Magnetic labeling of smooth muscle cells with anionic maghemite nanoparticles allows detection of cells with MR imaging after local transplantation in the heart.     

Magnetic nanoparticles for imaging dendritic cells

We report the development of superparamagnetic iron oxide (SPIOs) nanoparticles and investigate the migration of SPIO‐labeled dendritic cells (DCs) in a syngeneic mouse model using magnetic resonance (MR) imaging. The size of the dextran‐coated SPIO is roughly 30 nm, and the DCs are capable of independent uptake of these particles, although not at levels comparable to particle uptake in the presence of a transfecting reagent. On average, with the assistance of polylysine, the particles were efficiently delivered inside DCs within one hour of incubation. The SPIO particles occupy approximately 0.35% of cell surface and are equivalent to 34.6 pg of iron per cell. In vivo imaging demonstrated that the labeled cells migrated from the injection site in the footpad to the corresponding popliteal lymph node. The homing of labeled cells in the lymph nodes resulted in a signal drop of up to 79%. Furthermore, labeling DCs with SPIO particles did not compromise cell function, we demonstrated that SPIO‐enhanced MR imaging can be used to track the migration of DCs effectively in vivo. Magn Reson Med 63:1383–1390, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo single cell detection of tumor‐infiltrating lymphocytes with a clinical 1.5 Tesla MRI system

We demonstrate the feasibility of detecting individual tumor‐infiltrating cells in vivo, by means of cellular magnetic labeling and a 1.5 Tesla clinical MRI device equipped with a high‐resolution surface coil. Using a recently developed high‐temperature superconducting (HTS) surface coil, single cells were detected in vitro in voxels of (60 μm)³ at magnetic loads as low as 0.2 pg of iron per cell. The same imaging protocol was used in vivo to monitor infiltration of ovalbumin‐expressing tumors by transferred OVA antigen‐specific cytotoxic lymphocytes with low iron load. Magn Reson Med 60:1292–1297, 2008. © 2008 Wiley‐Liss, Inc.