Cellular MR Imaging

Fluorescence-based assays for in vitro analysis of cell adhesion and migration

Cell adhesion and cell migration are two primary cellular phenomena for which in vitro approaches may be exploited to effectively dissect the individual events and underlying molecular mechanisms. The use of assays dedicated to the analysis of cell adhesion and migration in vitro also afford an efficient way of conducting larger basic and applied research screenings on the factors affecting these processes and are potentially exploitable in the context of routine diagnostic, prognostic, and predictive tests in the biological and medical fields. Therefore, there is a longstanding continuum in the interest in devising more rationale such assays and major contributions in this direction have been provided by the advent of procedures based on fluorescence cell tagging, the design of instruments capable of detecting fluorescent signals with high sensitivity, and informatic tools allowing sophisticated elaboration of data generated through these instruments. In this report, we describe three representative fluorescence-based model assays for the qualitative and quantitative assessment of cell adhesion and cell locomotion in static and dynamic conditions. The assays are easily performed, accurate and reproducible, and can be automated for high-to-medium throughput screenings of cell behavior in vitro. Performance of the assays involves the use of certain dedicated disposable accessories, which are commercially available, and a few instruments that, due to their versatility, can be regarded as constituents of a more generic laboratory setup.

Liver cell labelling with MRI contrast agents

Cell transplantation is a promising approach to improve the life of patients with liver disease. At present, however, techniques to track and visualise transplanted cells in patients are fairly limited and further development of non-invasive imaging technology is needed to advance the monitoring of liver cell grafts. Magnetic resonance imaging (MRI) is a non-invasive imaging technology that already allows the visualisation of particular cell fractions in the liver by using MR contrast agents. The use of contrast agents to pre-label liver cells prior to transplantation will potentially provide a method to identify, track and study the integration of engrafted cells non-invasively by MRI. Before this technique can find its clinical application, in vitro and pre-clinical in vivo studies need to be conducted to determine the safety and specificity of this approach.

Molecular magnetic resonance imaging of the genitourinary tract: recent results and future directions

This article focuses on preclinical and early clinical applications of renal cell MR imaging, on new developments in MR control of intrarenal gene therapy, and on several potential applications of molecular imaging techniques, mainly targeting cell receptors and enzyme activity, which could find exciting applications within the genitourinary tract.

In vivo "hot spot" MR imaging of neural stem cells using fluorinated nanoparticles

To optimize (19)F MR tracking of stem cells, we compared cellular internalization of cationic and anionic perfluoro-15-crown-5-ether (PFCE) nanoparticles using cell culture plates with different surface coatings. The viability and proliferation of anionic and cationic PFCE-labeled neural stem cells (NSCs) did not differ from unlabeled cells. Cationic PFCE nanoparticles ((19)F T1/T2 = 580/536 ms at 9.4 Tesla) were superior to anionic particles for intracellular fluorination. Best results were obtained with modified polystyrene culture dishes coated with both carboxylic and amino groups rather than conventional carboxyl-coated dishes. After injecting PFCE-labeled NSCs into the striatum of mouse brain, cells were readily identified in vivo by (19)F MRI without changes in signal or viability over a 2-week period after grafting. These results demonstrate that neural stem cells can be efficiently fluorinated with cationic PFCE nanoparticles without using transfection agents and visualized in vivo over prolonged periods with an MR sensitivity of approximately 140 pmol of PFCE/cell.

Detection and quantification of magnetically labeled cells by cellular MRI

Labeling cells with superparamagnetic iron oxide (SPIO) nanoparticles, paramagnetic contrast agent (gadolinium) or perfluorocarbons allows for the possibility of tracking single or clusters of labeled cells within target tissues following either direct implantation or intravenous injection. This review summarizes the practical issues regarding detection and quantification of magnetically labeled cells with various MRI contrast agents with a focus on SPIO nanoparticles.

Magnetic resonance imaging detects differences in migration between primary and immortalized neural stem cells

RATIONALE AND OBJECTIVES

The study was performed to evaluate the effect of magnetic resonance imaging (MRI) contrast agent (super paramagnetic iron oxide [SPIO]) on differentiation and migration of primary murine neural stem cells (NSCs) in comparison to a neural stem cell line (C17.2). Because detection of labeled cells depends on the concentration of SPIO particles per imaging voxel, the study was performed at various concentrations of SPIO particles to determine the concentration that could be used for in vivo detection of small clusters of grafted cells.

MATERIALS AND METHODS

Murine primary NSCs or C17.2 cells were labeled with different concentrations of SPIO particles (0, 25, 100, and 250 microg Fe/mL) and in vitro assays were performed to assess cell differentiation. In vivo MRI was performed 7 weeks after neonatal transplantation of labeled cells to evaluate the difference in migration capability of the two cell populations.

RESULTS

Both the primary NSCs and the C17.2 cells differentiated to similar number of neurons (Map2ab-positive cells). Similar patterns of engraftment of C17.2 cells were seen in transplanted mice regardless of the SPIO concentration used. In vivo MRI detection of grafted primary and C17.2 cells was only possible when cells were incubated with 100 microg/mL or higher concentration of SPIO. Extensive migration of C17.2 cells throughout the brain was observed, whereas the migration of the primary NSCs was more restricted.

CONCLUSIONS

Engraftment of primary NSCs can be detected noninvasively by in vivo MRI, and the presence of SPIO particles do not affect the viability, differentiation, or engraftment pattern of the donor cells.

Initial evaluation of the use of USPIO cell labeling and noninvasive MR monitoring of human tissue-engineered vascular grafts in vivo

This pilot study examines noninvasive MR monitoring of tissue-engineered vascular grafts (TEVGs) in vivo using cells labeled with iron oxide nanoparticles. Human aortic smooth muscle cells (hASMCs) were labeled with ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles. The labeled hASMCs, along with human aortic endothelial cells, were incorporated into eight TEVGs and were then surgically implanted as aortic interposition grafts in a C.B-17 SCID/bg mouse host. USPIO-labeled hASMCs persisted in the grafts throughout a 3 wk observation period and allowed noninvasive MR imaging of the human TEVGs for real-time, serial monitoring of hASMC retention. This study demonstrates the feasibility of applying noninvasive imaging techniques for evaluation of in vivo TEVG performance.

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.

Nanoimmunoliposome delivery of superparamagnetic iron oxide markedly enhances targeting and uptake in human cancer cells in vitro and in vivo

To circumvent the problem of reduction of the supermagnetic properties of superparamagnetic iron oxide (SPIO) nanoparticles after chemical modification to conjugate targeting molecules, we have adapted a tumor-targeting nanoimmunoliposome platform technology (scL) to encapsulate and deliver SPIO (scL-SPIO) in vitro and in vivo without chemical modification. Scanning probe microscopy, confocal microscopy, and Prussian blue staining were used to analyze the scL-SPIO and assess intracellular uptake and distribution of SPIO in vitro. In vivo targeting and tumor-specific uptake of scL-SPIO was examined using fluorescent-labeled SPIO. We demonstrated that SPIO encapsulation in the scL complex results in an approximately 11-fold increase in SPIO uptake in human cancer cells in vitro, with distribution to cytoplasm and nucleus. Moreover, the scL nanocomplex specifically and efficiently delivered SPIO into tumor cells after systemic administration, demonstrating the potential of this approach to enhance local tumor concentration and the utility of SPIO for clinical applications.

In vivo MRI tracking of exogenous monocytes/macrophages targeting brain tumors in a rat model of glioma

This study has shown that murine monocytes/macrophages (Mo/Ma) can be labeled simply and efficiently with large, green-fluorescent, micrometer-sized particles of iron-oxide (MPIO). Neither size nor proliferation rate of the Mo/Ma is significantly affected by this labeling. The labeled Mo/Ma have been administered intravenously to rats that had developed a glioma following stereotactic injection of C6 cells. The labeled Mo/Ma were shown to target the brain tumors, a process that could be monitored non-invasively using T2*-weighted MRI. MRI observations were confirmed by Prussian blue staining, lectin staining and fluorescence histology. Overall, the results of this study suggest that the use of Mo/Ma may be envisaged in the clinic for vectorizing therapeutic agents toward gliomas.

A chronic 1 year assessment of MRI contrast agent-labelled neural stem cell transplants in stroke

Non-invasive identification of transplanted neural stem cells in vivo by pre-labelling with contrast agents may play an important role in the translation of cell therapy to the clinic. Understanding the impact of these labels on the cells' ability to repair is therefore vital. In rats with middle cerebral artery occlusion (MCAo), a model of stroke, the transhemispheric migration of MHP36 cells labelled with the bimodal contrast agent GRID was detected on magnetic resonance images (MRI) up to 4 weeks following transplantation. However, compared to MHP36 cells labelled with the red fluorescent dye PKH26, GRID-labelled transplants did not significantly improve behaviour, and performance was akin to non-treated animals. Likewise, the evolution of anatomical damage as assessed by serial, T(2)-weighted MRI over 1 year indicated that GRID-labelled transplants resulted in a slight increase in lesion size compared to MCAo-only animals, whereas the same, PKH26-labelled cells significantly decreased lesion size by 35%. Although GRID labelling allows the in vivo identification of transplanted cells up to 1 month after transplantation, it is likely that some is gradually degraded inside cells. The translation of cellular imaging therefore does not only require the in vitro assessment of contrast agents on cellular functions, but also requires the chronic, in vivo assessment of the label on the stem cells' ability to repair in preclinical models of neurological disease.

Stem cell labeling for magnetic resonance imaging

In vivo applications of cells for the monitoring of their cell dynamics increasingly use non-invasive magnetic resonance imaging. This imaging modality allows in particular to follow the migrational activity of stem cells intended for cell therapy strategies. All these approaches require the prior labeling of the cells under investigation for excellent contrast against the host tissue background in the imaging modality. The present review discusses the various routes of cell labeling and describes the potential to observe both cell localization and their cell-specific function in vivo. Possibilities for labeling strategies, pros and cons of various contrast agents are pointed out while potential ambiguities or problems of labeling strategies are emphasized.

Efficient stem cell labeling for MRI studies

Iron oxide particles are especially suited for cell tracking experiments due to their extraordinarily molar relaxivity as compared with other paramagnetic nuclei. We have compared different iron oxide particles (Sinerem, Endorem and magnetic microspheres) for their suitability to label embryonic stem cells (D3 cell line). In addition to detectability thresholds, particular attention has been paid to the evaluation of long-term stability of the labelling procedure (up to 4 weeks) as well as to toxic and other adverse effects on cell viability. Comparative studies were performed using neural progenitor cells (C17.2) and dendritic cells. The present study indicates strong dependence of the label efficiency and stability on the iron oxide particles and cell lines in use.

Molecular MRI of hematopoietic stem-progenitor cells: in vivo monitoring of gene therapy and atherosclerosis

A characteristic feature of atherosclerotic cardiovascular disease is the diffuse involvement of arteries across the entire human body and the presence of multiple, simultaneous lesions. The diffuse nature of this disease creates a unique challenge for early diagnosis and effective treatment. We believe that recent progress in the field of molecular MRI has opened new avenues towards solving the problem. A new technology has been developed that uses molecular MRI to monitor the migration and homing of hematopoietic stem-progenitor cells to injured arteries and atherosclerosis. In this Review, we introduce several novel technical developments in the field of molecular MRI of atherosclerosis, including advanced techniques for magnetic labeling of stem-progenitor cells and molecular MRI of hematopoietic bone marrow cells migrating to injured arteries and homing to atherosclerotic plaques. In addition, we examine molecular MRI of vascular gene therapy mediated by stem-progenitor cells. These new techniques provide the basis for the further development of in vivo MRI techniques to monitor stem-cell-mediated vascular gene therapy for multiple and diffuse atherosclerotic cardiovascular lesions.

Cellular magnetic resonance imaging: in vivo imaging of melanoma cells in lymph nodes of mice

Metastasis is responsible for most deaths due to malignant melanoma. The clinical significance of micrometastases in the lymph is a hotly debated topic, but an improved understanding of the lymphatic spread of cancer remains important for improving cancer survival. Cellular magnetic resonance imaging (MRI) is a newly emerging field of imaging research that is expected to have a large impact on cancer research. In this study, we demonstrate the cellular MRI technology required to reliably image the lymphatic system in mice and to detect iron-labeled metastatic melanoma cells within the mouse lymph nodes. Melanoma cells were implanted directly into the inguinal lymph nodes in mice, and micro-MRI was performed using a customized 1.5-T clinical MRI system. We show cell detection of as few as 100 iron-labeled cells within the lymph node, with injections of larger cell numbers producing increasingly obvious regions of signal void. In addition, we show that cellular MRI allows monitoring of the fate of these cells over time as they develop into intranodal tumors. This technology will allow noninvasive investigations of cellular events in cancer metastasis within an entire animal and will facilitate progress in understanding the mechanisms of metastasis within the lymphatic system.

Non-invasive genetic imaging for molecular and cell therapies of cancer

Gene therapy is a very attractive strategy in experimental cancer therapy. Ideally, the approach aims to deliver therapeutic genes selectively to cancer cells. However, progress in the improvement of gene therapy formulations has been hampered by difficulties in measuring transgene delivery and in quantifying transgene expression in vivo. In clinical trials, endpoints rely almost exclusively on the analysis of biopsies, which provide limited information. Non-invasive monitoring of gene delivery and expression is a very attractive approach as it can be repeated over time in the same patient to provide spatiotemporal information on gene expression on a whole body scale. Thus, imaging methods can uniquely provide researchers and clinicians the ability to directly and serially assess morphological, functional and metabolic changes consequent to molecular and cellular based therapies. This review highlights the various methods currently being developed in preclinical models.

MRI detection of paramagnetic chemical exchange effects in mice kidneys in vivo

In this report, the On resonance PARamagnetic CHemical Exchange Effects (OPARACHEE) method was implemented in vivo using WALTZ-16* as a preparation pulse with a standard spin echo sequence to detect the accumulation and clearance of the TmDOTA-4AmC(-) in mouse kidney. The performance of the technique in vivo is described in terms of the magnitude of the contrast effect versus the bolus agent concentration and signal-to-noise ratio (SNR) levels. The lowest injected concentration of TmDOTA-4AmC(-), 200 microL of a 2-mM stock solution (corresponds to approximately 0.2 mM agent in plasma), reduced the total water signal in the kidney papilla by 45% 3 min after the a bolus injection. The results show that the OPARACHEE methodology employing low-amplitude RF trains can detect paramagnetic exchanging agents in vivo.

Very-low-frequency electron paramagnetic resonance (EPR) imaging of nitroxide-loaded cells

Recent advances in electron paramagnetic resonance (EPR) imaging have made it possible to image, in real time in vivo, cells that have been labeled with nitroxide spin probes. We previously reported that cells can be loaded to high (millimolar) intracellular concentrations with (2,2,5,5-tetramethylpyrrolidin-1-oxyl-3-ylmethyl)amine-N,N-diacetic acid by incubation with the corresponding acetoxymethyl (AM) ester. Furthermore, the intracellular lifetime (t(1/e)) of this nitroxide is 114 min-sufficiently long to permit in vivo imaging studies. In the present study, at a gradient of approximately 50 mT/m, we acquire and compare EPR images of a three-tube phantom, filled with either a 200-microM solution of the nitroxide, or a suspension of cells preincubated with the nitroxide AM ester. In both cases, 3-mm resolution images can be acquired with excellent signal-to-noise ratios (SNRs). These findings indicate that cells well-loaded with nitroxide are readily imageable by EPR imaging, and that in vivo tracking studies utilizing such cells should be feasible.

[Cell tracking. Principles and applications]

Cell based therapies such as stem cell therapies or adoptive immunotherapies are currently being explored as a potential treatment for a variety of diseases such as Parkinson's disease, diabetes or cancer. However, quantitative and qualitative evaluation of adoptively transferred cells is indispensable for monitoring the efficiency of the treatment. Current approaches mostly analyze transferred cells from peripheral blood, which cannot assess whether transferred cells actually home to and stay in the targeted tissue. Using cell-labeling methods such as direct labeling or transfection with a marker gene in conjunction with various imaging modalities (MRI, optical or nuclear imaging), labeled cells can be followed in vivo in real-time, and their accumulation as well as function in vivo can be monitored and quantified accurately. This method is usually referred to as "cell tracking" or "cell trafficking" and is also being applied in basic biological sciences, exemplified in the evaluation of genes contributing to metastasis. This review focuses on principles of this promising methodology and explains various approaches by highlighting recent examples.

MRI of intravenously injected bone marrow cells homing to the site of injured arteries

The aim of this study was to test the feasibility of using MRI to detect magnetically labeled, intravenously injected bone marrow (BM) cells homing to injured arteries. In the first phase, BM cells from LacZ-transgenic or green fluorescent protein (GFP)-transgenic mice were transplanted into eight recipient mice. The left femoral arteries of recipient mice were injured using a cuff-constriction or endothelium-damage approach, and the right femoral arteries were uninjured to serve as controls. The location and distribution of migrated LacZ-BM or GFP-BM cells were confirmed with histology. In the second phase, BM-derived cells from LacZ-transgenic mice were labeled with superparamagnetic iron oxide (Feridex) and then transplanted into eight recipient mice with cuff-induced injuries in the left femoral arteries. Migrated Feridex/LacZ-BM cells were monitored in vivo using a 4.7 T MR scanner. Subsequently, high-resolution ex vivo MRI was performed on 9.4 T and 11.7 T. LacZ-positive or GFP-positive cells in the thickened adventitia of the injured arteries were evident on histology. Both in vivo and ex vivo MRI showed larger regions of hypointensity with Feridex-labeled cells at the sites of the injured arteries compared with control arteries (P < 0.01). This study provides initial evidence that may support the potential use of MRI to detect homing of intravenously injected BM cells to injured arteries.

Iron-oxide labeling of hematogenous macrophages in a model of experimental autoimmune encephalomyelitis and the contribution to signal loss in fast imaging employing steady state acquisition (FIESTA) images

PURPOSE

To determine the contribution of blood-derived macrophages to the signal loss observed in MR images of inflammatory lesions in experimental autoimmune encephalomyelitis (EAE).

MATERIALS AND METHODS

A relapsing-remitting form of EAE was induced in transgenic mice that express enhanced green fluorescent protein (EGFP) specifically in hematopoietic cells of the myelomonocytic lineage. Animals were injected with Feridex, a superparamagnetic iron oxide (SPIO) nanoparticle, 24 hours prior to in vivo MRI. MRI was performed using a 1.5T whole-body scanner; a high-performance, custom-built gradient coil insert; and a 3D steady-state free precession (SSFP) imaging pulse sequence. Comparisons were made between MR images and corresponding anti-GFP and Perl's Prussian blue (PPB)-stained brain sections.

RESULTS

MR images revealed the presence of discrete regions of signal loss throughout the brains of EAE animals that were administered Feridex. Histological staining showed that regions of signal loss on MR images corresponded anatomically with regions of PPB- and GFP-positive cells.

CONCLUSION

This experiment provides the first direct evidence that macrophages of hematogenous origin are labeled with SPIO after intravenous administration of Feridex, and contribute to the regions of signal loss detected in MR images of EAE brain.

Magnetic resonance imaging as a tool for monitoring stem cell migration

Noninvasive monitoring of stem cells is an important step in developing stem-cell-based therapies. Among several imaging techniques available, magnetic resonance imaging (MRI) provides an effective way to detect implanted stem cells in live animals. In this mini-review, we discuss the available MRI contrast agents and different cell-labeling strategies used for detection of stem cell migration in the brain. The potential effects of MRI contrast agents on stem cell viability and differentiation are also discussed.

Molecular Neuroimaging: From Conventional to Emerging Techniques

The use of molecular imaging techniques in the central nervous system (CNS) has a rich history. Most of the important developments in imaging-such as computed tomography, magnetic resonance imaging, single photon emission computed tomography, and positron emission tomography-began with neuropsychiatric applications. These techniques and modalities were then found to be useful for imaging other organs involved with various disease processes. Molecular imaging of the CNS has enabled scientists and researchers to understand better the basic biology of brain function and the way in which various disease processes affect the brain. Unlike other organs, the brain is not easily accessible, and it has a highly selective barrier at the endothelial cell level known as the blood-brain barrier. Furthermore, the brain is the most complex cellular network known to exist. Various neurotransmitters act in either an excitatory or an inhibitory fashion on adjacent neurons through a multitude of mechanisms. The various neuronal systems and the myriad of neurotransmitter systems become altered in many diseases. Some of the most devastating diseases, including Alzheimer disease, Parkinson disease, brain tumors, psychiatric disease, and numerous degenerative neurologic diseases, affect only the brain. Molecular neuroimaging will be critical to the future understanding and treatment of these diseases. Molecular neuroimaging of the brain shows tremendous promise for clinical application. In this article, the current state and clinical applications of molecular neuroimaging will be reviewed.

Cell tracking using magnetic resonance imaging

Cell tracking by in vivo magnetic resonance imaging (MRI) requires strategies of labelling the cells with MRI contrast agents. The principal routes to achieve efficient cell labelling for neurological applications are discussed with methodological advantages and caveats. Beyond temporo-spatial localization of labelled cells, the investigation of functional cell status is of great interest to allow studies of functional cell dynamics. The two major approaches to reach this goal, use of responsive contrast agents and generation of transgenic cell lines, are discussed.

Magnetic-resonance-based tracking and quantification of intravenously injected neural stem cell accumulation in the brains of mice with experimental multiple sclerosis

Eliciting the in situ accumulation and persistence patterns of stem cells following transplantation would provide critical insight toward human translation of stem cell-based therapies. To this end, we have developed a strategy to track neural stem/precursor cells (NPCs) in vivo using magnetic resonance (MR) imaging. Initially, we evaluated three different human-grade superparamagnetic iron oxide particles for labeling NPCs and found the optimal labeling to be achieved with Resovist. Next, we carried out in vivo experiments to monitor the accumulation of Resovist-labeled NPCs following i.v. injection in mice with experimental autoimmune encephalomyelitis (EAE), the animal model of multiple sclerosis. With a human MR scanner, we were able to visualize transplanted cells as early as 24 hours post-transplantation in up to 80% of the brain demyelinating lesions. Interestingly, continued monitoring of transplanted mice indicated that labeled NPCs were still present 20 days postinjection. Neuropathological analysis confirmed the presence of transplanted NPCs exclusively in inflammatory demyelinating lesions and not in normal-appearing brain areas. Quantification of transplanted cells by means of MR-based ex vivo relaxometry (R2*) showed significantly higher R2* values in focal inflammatory brain lesions from EAE mice transplanted with labeled NPCs as compared with controls. Indeed, sensitive quantification of low numbers of NPCs accumulating into brain inflammatory lesions (33.3-164.4 cells per lesion; r(2) = .998) was also obtained. These studies provide evidence that clinical-grade human MR can be used for noninvasive monitoring and quantification of NPC accumulation in the central nervous system upon systemic cell injection. Disclosure of potential conflicts of interest is found at the end of this article.

Non-invasive Imaging in Gene Therapy

Several methods are available for non-invasive imaging of gene delivery and transgene expression, including magnetic resonance imaging (MRI), single photon emission tomography (SPECT)/positron emission tomography (PET), and fluorescence and bioluminescence imaging. However, these imaging modalities differ greatly in terms of their sensitivity, cost, and ability to measure the signal. Whereas MRI can produce a resolution of approximately 50 mum, optical imaging achieves only 3-5 mm but outperforms MRI in terms of the cost of the imaging device. Similarly, SPECT and PET give a resolution of only 1-2 mm but provide for relatively easy quantitation of the signal and need only nanograms of probe, compared with the microgram or milligram levels required for MRI and optical imaging. To develop safer and more efficient gene delivery vectors, it is essential to perform rigorous in vivo experiments, to image particle biodistribution and transduction patterns, and to quantify the transgene expression profile. Differences between modalities have a significant effect on the resultant imaging resolution for gene therapy. This review describes the methodologies in use and highlights recent key approaches using the latest imaging modalities in gene therapy. Future trends in gene therapy imaging are also discussed.

Cellular multiparametric MRI of neural stem cell therapy in a rat glioma model

Cellular multiparametric magnetic resonance imaging (MRI) provided an in vivo visualisation of neural stem cells' (NSCs) tropism for gliomas in the rat brain. NSCs were magnetically labelled in vitro with the bimodal gadolinium-based contrast agent, gadolinium rhodamine dextran (GRID), and injected into the contralateral hemisphere to the developing tumour. Contrast-to-noise measurements showed that GRID-labelled cells induced a signal attenuation on both T2-, T2(*)-weighted images, and a modest signal gain on T1-weighted images. Tumour development and progression were longitudinally monitored in vivo by serial MR scanning. Measurements of tumour volume and tumour progression over time in terms of tumour doubling time showed a tendency towards a reduced tumour growth in NSC-treated animals. MR findings of migration and infiltration of tumours by labelled NSCs were corroborated with immunohistopathology, where labelled cells were detected in the corpus callosum at the tumour border and dispersed in the solid tumour tissue. Immunohistopathology also revealed that macrophages invaded the tumour tissue and in some cases engulfed GRID-labelled stem cells. No significant difference in macrophage recruitment between NSC-treated and vehicle-treated animals were detected, indicating that magnetically labelled NSC do not increase macrophage invasion of tumour tissue. Our findings demonstrate that cellular multiparametric MRI provides a valuable tool for in vivo dynamic monitoring of tumour-directed neural stem cell migration as well as therapeutic efficacy.

Long-term monitoring of transplanted human neural stem cells in developmental and pathological contexts with MRI

Noninvasive monitoring of stem cells, using high-resolution molecular imaging, will be instrumental to improve clinical neural transplantation strategies. We show that labeling of human central nervous system stem cells grown as neurospheres with magnetic nanoparticles does not adversely affect survival, migration, and differentiation or alter neuronal electrophysiological characteristics. Using MRI, we show that human central nervous system stem cells transplanted either to the neonatal, the adult, or the injured rodent brain respond to cues characteristic for the ambient microenvironment resulting in distinct migration patterns. Nanoparticle-labeled human central nervous system stem cells survive long-term and differentiate in a site-specific manner identical to that seen for transplants of unlabeled cells. We also demonstrate the impact of graft location on cell migration and describe magnetic resonance characteristics of graft cell death and subsequent clearance. Knowledge of migration patterns and implementation of noninvasive stem cell tracking might help to improve the design of future clinical neural stem cell transplantation.

MRI of mouse models of neurological disorders

MRI has contributed to significant advances in the understanding of neurological diseases in humans. It has also been used to evaluate the spectrum of mouse models spanning from developmental abnormalities during embryogenesis, evaluation of transgenic and knockout models, through various neurological diseases such as stroke, tumors, degenerative and inflammatory diseases. The MRI techniques used clinically are technically more challenging in the mouse because of the size of the brain; however, mouse imaging provides researchers with the ability to explore cellular and molecular imaging that one day may translate into clinical practice. This article presents an overview of the use of MRI in mouse models of a variety of neurological disorders and a brief review of cellular imaging using magnetically tagged cells in the mouse central nervous system.

The in vitro effects of a bimodal contrast agent on cellular functions and relaxometry

The in vivo monitoring of cell survival and migration will be essential to the translation of cell-based therapies from the laboratory to clinical studies. The pre-labeling of cells with magnetic resonance imaging (MRI) contrast agents renders them visible in vivo for serial cellular imaging. However, little is known about the impact of the presence of these metal particles inside transplanted cells. The use of the bimodal contrast agent GRID made it possible to demonstrate by means of fluorescent microscopy and inductively coupled plasma mass spectrometry (ICP-MS) that, after 16 h of incubation (without the use of a transfection agent), neural stem cells (NSCs) were saturated and no longer incorporated particles. With this maximal uptake, no significant effect on cell viability was observed. However, a significant decrease in proliferation was evident in cells that underwent 24 h of labeling. A significant increase in reactive oxygen species was observed for all GRID labeling, with a very significant increase with 24 h of labeling. GRID labeling did not affect cell motility in comparison with PKH26-labeled NSCs in a glioma-based migration assay and also allowed differentiation into all major cell types of the brain. GRID-labeled cells induced a signal change of 47% on T(2) measurements and allows a detection of cell clusters of approximately 220 cells/microl. Further in vivo testing will be required to ensure that cell labeling with gadolinium-based MRI contrast agents does not impair their ability to repair.

Migration, fate and in vivo imaging of adult stem cells in the CNS

Adult stem cells have been intensively studied for their potential use in cell therapies for neurodegenerative diseases, ischemia and traumatic injuries. One of the most promising cell sources for autologous cell transplantation is bone marrow, containing a heterogenous cell population that can be roughly divided into hematopoietic stem and progenitor cells and mesenchymal stem cells (MSCs). MSCs are multipotent progenitor cells that, in the case of severe tissue ischemia or damage, can be attracted to the lesion site, where they can secrete bioactive molecules, either naturally or through genetic engineering. They can also serve as vehicles for delivering therapeutic agents. Mobilized from the marrow, sorted or expanded in culture, MSCs can be delivered to the damaged site by direct or systemic application. In addition, MSCs can be labeled with superparamagnetic nanoparticles that allow in vivo cell imaging. Magnetic resonance imaging (MRI) is thus a suitable method for in vivo cell tracking of transplanted cells in the host organism. This review will focus on cell labeling for MRI and the use of MSCs in experimental and clinical studies for the treatment of brain and spinal cord injuries.

Implantation of pure cultured olfactory ensheathing cells in an animal model of parkinsonism

BACKGROUND

Implantation of neural cells has been proposed as a therapeutic strategy for repairing the injured or diseased brain. In the present study we have examined the potential of olfactory ensheathing cells (OEC) to promote brain repair after surgical implantation in a rodent model of parkinsonism.

METHODS

Neonatal OECs were implanted in the striatum after a 6-hydroxydopamine lesion of the ipsilateral substantia nigra. Amphetamine-induced rotational asymmetry scores were determined 48 hours before and 4, 6 and 8 weeks after OEC implantation. The density of immunostaining for tyrosine hydroxylase and synaptophysin in the striatum and the number of tyrosine hydroxylase-positive cells remaining in the substantia nigra were also determined.

RESULTS

Rotational asymmetry scores were similar in OEC-implanted and vehicle-treated groups at all time points examined, and at each time were similar to those observed prior to implantation. Levels of striatal tyrosine-hydroxylase and synaptophysin immunoreactivity were similar in OEC- and vehicle-treated groups. The number of tyrosine-hydroxylase-positive cells in the substantia nigra was similar in both groups indicating that severity of the lesion was similar. Visualisation of GFP-labelled OECs one week after implantation in a separate group of animals revealed the cells to be located in the area immediately surrounding the needle tract.

CONCLUSION

This study demonstrates that implantation of OECs alone is not sufficient to promote tissue repair and functional recovery in a rodent model of parkinsonism. The results add to a growing number of studies that propose a caveat for the use of pure OECs as a neurosurgical strategy for the treatment of brain disease or injury.

Cellular labeling with Gd(III) chelates: only high thermodynamic stabilities prevent the cells acting as 'sponges' of Gd3+ ions

MR-labeling of cells may be carried out by adding a Gd-based contrast agent to the incubation media. The amount of gadolinium internalized in HTC and C6 cells upon incubation with Gd-DTPA-BMA is circa one order of magnitude higher than those found with Gd-DTPA, Gd-DOTA and Gd-HPDO3A, respectively. The comparison of relaxometric and mass spectrometry determinations allows us to establish that only a minor fraction of intact Gd-DTPA-BMA is internalized into the cells. Moreover the binding/uptake behavior shown by Gd-DTPA-BMA resembles that found when GdCl(3) is added to the incubation medium. We suggest that the lower stability of Gd-DTPA-BMA is responsible for a shift in the dissociation equilibrium that results in the net transfer of Gd(3+) ions on the cell membrane followed by a slower internalization process. The transmetallation process is mediated by components of the incubation media, among which a dominant role is represented by phosphate anions. The uptake of Gd(3+) ions is clearly reflected in the drastic decrease of cell viability observed for cells labeled with Gd-DTPA-BMA.

Interferometer-based Phase-contrast X-ray Computed Tomography of Colon Cancer Specimens

OBJECTIVE

Phase-contrast X-ray computed tomography (PCCT) with an interferometer can reveal the inner soft tissue structures of biological objects without contrast agent, and the image quality is thought to resemble that of magnetic resonance imaging (MRI). Comparative study among PCCT, MRI, and optical microscopy was undertaken.

METHODS

Three formalin-fixed colon cancer specimens from nude mice were imaged both by PCCT with a reconstructed volumetric resolution of (0.018)3 mm3 and 4.74-T MRI with that of (0.075)3 mm3.

RESULTS

Phase-contrast X-ray computed tomography with an interferometer clearly demonstrated the inner structures of colon cancer masses, such as cancer, necrosis, surrounding tumor vessels, and skin, in a similar way to low-magnified optical microscopic images and had approximately 4.0-fold higher signal-to-noise ratio than MRI.

CONCLUSIONS

With formalin-fixed biological samples, PCCT exhibited higher image quality than MRI and was thought to be suitable for detailed imaging of soft tissue with high volumetric resolution.

Molecular imaging of novel cell- and viral-based therapies

Drugs, surgery, and radiation are the traditional modalities of therapy in medicine. To these are being added new therapies based on cells and viruses or their derivatives. In these novel therapies, a cell or viral vector acts as a drug in its own right, altering the host or a disease process to bring about healing. Most of these advances originate from the significant recent advances in molecular medicine, but some have been around for some time. Blood transfusions and cowpox vaccinations are part of the history of medicine...but nevertheless are examples of cell- and viral-based therapies. This article focuses on the modern molecular incarnations of these therapies, and specifically on how imaging is used to track and guide these novel agents. We survey the literature dealing with imaging these new cell and viral particle therapies and provide a framework for understanding publications in this area. Leading technology of gene modifications are the fundamental modifications applied to make these new therapies amenable to imaging.

Silica- and alkoxysilane-coated ultrasmall superparamagnetic iron oxide particles: a promising tool to label cells for magnetic resonance imaging

In this study silica- and alkoxysilane-coated ultrasmall superparamagnetic iron oxide (USPIO) particles were synthesized, and their ability to label immortalized progenitor cells for magnetic resonance imaging (MRI) was compared. USPIO particles were synthesized by coprecipitation of ferric and ferrous salts. Subsequently, the particles were coated with silica, (3-aminopropyl)trimethoxysilane (APTMS), and [N-(2-aminoethyl)-3-aminopropyl]trimethoxysilane (AEAPTMS). The size of the USPIO particles was about 10 nm without a significant increase in diameter after coating. The highest T2 relaxivity was achieved for silica-coated USPIO particles, 339.80 +/- 0.22 s-1 mM-1, as compared with APTMS- and AEAPTMS-coated ones, reaching 134.40 +/- 0.01 and 84.79 +/- 0.02 s-1 mM-1, respectively. No toxic effects on the cells could be detected by trypan blue, TUNEL, and MTS assays. Uptake of USPIO particles was evaluated by Prussian blue staining, transmission electron microscopy, T2-MR relaxometry, and mass spectrometry. It was found that cell uptake of the different USPIO particles increased for longer incubation times and higher doses. Maximum cellular iron concentrations of 42.1 +/- 4.0 pg/cell (silica-coated USPIO particles), 37.1 +/- 3.5 pg/cell (APTMS-coated USPIO particles), and 32.7 +/- 4.0 pg/cell (AEAPTMS-coated USPIO particles) were achieved after incubation of the cells with USPIO particles at a dose of 3 micromol/mL for 6 h. The decrease of the T2 relaxation time of the cell pellets was most pronounced for cells incubated with silica-coated USPIO particles followed by APTMS- and AEAPTMS-coated particles, respectively. In gelatin gels even small clusters of labeled cells were detected by 1.5 T MRI, and significant changes in the T2 relaxation times of the gels were determined for 10000 labeled cells/mL for all particles. In summary, as compared with APTMS- and AEAPTMS-coated particles, silica-coated USPIO particles provide the highest T2 relaxivity and most effectively reduce the T2 relaxation time of immortalized progenitor cells after internalization. This suggests silica-coated USPIO particles are most suited for cell labeling approaches in MRI.

Molecular Imaging of Brain Tumors: A Bridge Between Clinical and Molecular Medicine?

As the research on cellular changes has shed invaluable light on the pathophysiology and biochemistry of brain tumors, clinical and experimental use of molecular imaging methods is expanding and allows quantitative assessment. The term molecular imaging is defined as the in vivo characterization and measurement of biologic processes at the cellular and molecular level. Molecular imaging sets forth to probe the molecular abnormalities that are the basis of disease rather than to visualize the end effects of these molecular alterations and, therefore, provides different additional biochemical or molecular information about primary brain tumors compared to histological methods "classical" neuroradiological diagnostic studies. Common clinical indications for molecular imaging contain primary brain tumor diagnosis and identification of the metabolically most active brain tumor reactions (differentiation of viable tumor tissue from necrosis), prediction of treatment response by measurement of tumor perfusion, or ischemia. The interesting key question remains not only whether the magnitude of biochemical alterations demonstrated by molecular imaging reveals prognostic value with respect to survival, but also whether it identifies early disease and differentiates benign from malignant lesions. Moreover, an early identification of treatment success or failure by molecular imaging could significantly influence patient management by providing more objective decision criteria for evaluation of specific therapeutic strategies. Specially, as molecular imaging represents a novel technology for visualizing metabolism and signal transduction to gene expression, reporter gene assays are used to trace the location and temporal level of expression of therapeutic and endogenous genes. Molecular imaging probes and drugs are being developed to image the function of targets without disturbing them and in mass amounts to modify the target's function as a drug. Molecular imaging helps to close the gap between in vitro and in vivo integrative biology of disease.

In vivo MRI tracking of exogenous monocytes/macrophages targeting brain tumors in a rat model of glioma

This study has shown that murine monocytes/macrophages (Mo/Ma) can be labeled simply and efficiently with large, green-fluorescent, micrometer-sized particles of iron-oxide (MPIO). Neither size nor proliferation rate of the Mo/Ma is significantly affected by this labeling. The labeled Mo/Ma have been administered intravenously to rats that had developed a glioma following stereotactic injection of C6 cells. The labeled Mo/Ma were shown to target the brain tumors, a process that could be monitored non-invasively using T2*-weighted MRI. MRI observations were confirmed by Prussian blue staining, lectin staining and fluorescence histology. Overall, the results of this study suggest that the use of Mo/Ma may be envisaged in the clinic for vectorizing therapeutic agents towards gliomas.

Sensitivity of magnetic resonance imaging of dendritic cells for in vivo tracking of cellular cancer vaccines

Success of immunotherapy with dendritic cells (DC) to treat cancer is highly dependent on their interaction with and activation of antigen specific T cells. To maximize DC-T cell contact accurate delivery of the therapeutic cells into the lymph node, or efficient trafficking of DC to the lymph nodes of the patient is essential. Since responses are seen in some patients but not in others, monitoring of the injected cells may be of major importance. Tracking of cells with magnetic resonance (MR) imaging is a non-invasive method that provides detailed anatomical information and is therefore more informative for the evaluation of the localization of therapeutic cells after injection than e.g. scintigraphic imaging. To challenge the sensitivity of this novel technique, we investigated the minimum amount of label and the number of cells required for MR imaging and the effect of labeling on DC function. DC were labeled with different concentrations of a clinically approved MR contrast agent consisting of superparamagnetic iron oxide particles and were imaged at both 3 and 7 T. Our results demonstrate the following: (i) When loaded with 30 (+/-4) pg Fe/cell, cell numbers as low as 1,000 cells/mm3 at 3 T and 500 cells/mm3 at 7 T could be readily imaged; (ii) Labeling does not affect cell viability and function; (iii) Because of its high spatial resolution and sensitivity, MRI is ideally suited to track therapeutic cells in vivo.

Assessing responses to cancer therapy using molecular imaging

Tumor responses to therapy in the clinic are still evaluated primarily from non-invasive imaging measurements of reductions in tumor size. This approach, however, lacks sensitivity and can only give a delayed indication of a positive response to treatment. Major advances in our understanding of the molecular mechanisms responsible for cancer, combined with new targeted clinical imaging technologies designed to detect the molecular correlates of disease progression and response to treatment, are set to revolutionize our approach to the detection and treatment of the disease. We describe here the imaging technologies available to image tumor cell proliferation and migration, metabolism, receptor and gene expression, apoptosis and tumor angiogenesis and vascular function, and show how measurements of these parameters can be used to give early indications of positive responses to treatment or to detect drug resistance and/or disease recurrence. Special emphasis has been placed on those applications that are already used in the clinic and those that are likely to translate into clinical application in the near future or whose use in preclinical studies is likely to facilitate translation of new treatments into the clinic.

Non-invasive imaging of dendritic cell migration in vivo

Dendritic cells (DCs) play important roles in the initiation of adaptive immune responses. The transport of antigen from the infection site to the draining lymph node by DCs is a crucial component in this process. Accordingly, immunotherapeutic applications of in vitro-generated DCs require reliable methods experimentally in mice and clinically in patients to monitor the efficiency of their successful lymph node homing after injection. Recent developments of new methods to follow DC migration by non-invasive imaging modalities such as scintigraphy, PET, MRI, or bioluminescence imaging, have gained attraction because of their potential clinical applicability. The current state of the literature and a comparative evaluation of the methods are reported in this review.

Radiopaque Alginate Microcapsules for X-ray Visualization and Immunoprotection of Cellular Therapeutics

Alginate-poly-L-lysine-alginate (APA) microcapsules have been explored as vehicles for therapeutic drug and cell delivery. The permselectivity of these capsules provides a unique means of controlled drug release and immunoisolation of encapsulated cells. Immunoisolation is especially attractive as it abrogates the need for chronic immunosuppressive therapy and opens up the possibility for the delivery of numerous cell sources including xenogeneic grafts. APA microcapsules containing cellular therapeutics have proven effective in the short-term treatment of a wide range of diseases requiring enzyme or endocrine replacement therapy, including type I diabetes. If these microcapsules could be noninvasively monitored with X-ray imaging modalities (i.e., fluoroscopy, CT, and digital subtraction angiography), questions such as the ideal transplantation site, the best means of delivery, and the long-term survival of grafts could be better addressed. We have developed two novel alginate-based radiopaque microcapsule formulations containing either barium sulfate (Ba X-Caps) or bismuth sulfate (Bi X-Caps). As compared to conventional, nonradiopaque APA capsules, Ba X-Caps and Bi X-Caps containing human cadaveric islets resulted in a decrease in cellular viability of less than 5% up to 14 days after encapsulation. Both radiopaque capsules were found to be permeable to lectins < or =75 kDa, but were impermeable to lectins > or =120 kDa, thus ensuring the blockage of the penetration of antibodies while allowing free diffusion of insulin and nutrients. The glucose-responsive insulin secretion of the radiopaque encapsulated human islets was found to be unaltered compared to that of unlabeled controls, with human C-peptide levels ranging from 3.21 to 2.87 (Ba X-Caps) and 3.23 to 2.87 (Bi X-Caps) ng/islet at 7 and 14 days postencapsulation, respectively. Using fluoroscopy, both Ba X-Caps and Bi X-Caps could be readily visualized as single radiopaque entities in vitro. Furthermore, following transplantation in vivo in mice and rabbits, single capsules could be identified with no significant change in contrast for at least 2 weeks. This study represents the first attempt at making radiopaque microcapsules for X-ray guided delivery and imaging of cellular therapeutics. While human cadaveric islets were used as a proof-of-principle, these radiopaque capsules may have wide ranging therapeutic applications for a variety of cell types.