In vivo tracking in cardiac stem cell-based therapy

Repair of the Urethral Mucosa Defect Model Using Adipose-Derived Stem Cell Sheets and Monitoring the Fate of Indocyanine Green-Labeled Sheets by Near Infrared-II

Treatment of urethral mucosa defects is a major challenge in urology. Synthetic materials or autologous mucosa does not provide satisfactory treatment options for long-term or large urethral mucosa defects. In response to this problem, we used autologous adipose-derived stem cells (ADSCs) to synthesize cell sheets in vitro for repairing urethral mucosa defect models. In order to monitor the localization and distribution of cell sheets in vivo, cells and sheets were labeled with indocyanine green (ICG) and the second near-infrared (NIR-II) fluorescence imaging was performed. ICG-based NIR-II imaging can successfully track ADSCs and sheets in vivo up to 8 W. Then, rabbit urethral mucosa defect models were repaired with ICG-ADSCs sheets. At 3 months after operation, retrograde urethrography showed that ADSC sheets could effectively repair urethral mucosa defect and restore urethral patency. Histological analysis showed that in ADSC sheet groups, continuous epithelial cells covered the urethra at the transplantation site, and a large number of vascular endothelial cells could also be seen. In the cell-free sheet group, there was no continuous epithelial cell coverage at the repair site of the urethra, and the expression of pro-inflammatory factor TNF-α was increased. It shows that the extracellular matrix alone without cells is not suitable for repairing urethral defects. Surviving ADSCs in the sheets may play a key role in the repair process. This study provides a new tracing method for tissue engineering to dynamically track grafts using an NIR-II imaging system. The ADSC sheets can effectively restore the structure and function of the urethra. It provides a new option for the repair of urethral mucosa defects.

Differential Responses of Transplanted Stem Cells to Diseased Environment Unveiled by a Molecular NIR-II Cell Tracker

Stem cell therapy holds high promises in regenerative medicine. The major challenge of clinical translation is to precisely and quantitatively evaluate the in vivo cell distribution, migration, and engraftment, which cannot be easily achieved by current techniques. To address this issue, for the first time, we have developed a molecular cell tracker with a strong fluorescence signal in the second near-infrared (NIR-II) window (1,000-1,700 nm) for real-time monitoring of in vivo cell behaviors in both healthy and diseased animal models. The NIR-II tracker (CelTrac1000) has shown complete cell labeling with low cytotoxicity and profound long-term tracking ability for 30 days in high spatiotemporal resolution for semiquantification of the biodistribution of transplanted stem cells. Taking advantage of the unique merits of CelTrac1000, the responses of transplanted stem cells to different diseased environments have been discriminated and unveiled. Furthermore, we also demonstrate CelTrac1000 as a universal and effective technique for ultrafast real-time tracking of the cellular migration and distribution in a 100 μm single-cell cluster spatial resolution, along with the lung contraction and heart beating. As such, this NIR-II tracker will shift the optical cell tracking into a single-cell cluster and millisecond temporal resolution for better evaluating and understanding stem cell therapy, affording optimal doses and efficacy.

Noninvasive in vivo cell tracking using molecular imaging: A useful tool for developing mesenchymal stem cell-based cancer treatment

Mounting evidence has emphasized the potential of cell therapies in treating various diseases by restoring damaged tissues or replacing defective cells in the body. Cell therapies have become a strong therapeutic modality by applying noninvasive in vivo molecular imaging for examining complex cellular processes, understanding pathophysiological mechanisms of diseases, and evaluating the kinetics/dynamics of cell therapies. In particular, mesenchymal stem cells (MSCs) have shown promise in recent years as drug carriers for cancer treatment. They can also be labeled with different probes and tracked in vivo to assess the in vivo effect of administered cells, and to optimize therapy. The exact role of MSCs in oncologic diseases is not clear as MSCs have been shown to be involved in tumor progression and inhibition, and the exact interactions between MSCs and specific cancer microenvironments are not clear. In this review, a multitude of labeling approaches, imaging modalities, and the merits/demerits of each strategy are outlined. In addition, specific examples of the use of MSCs and in vivo imaging in cancer therapy are provided. Finally, present limitations and future outlooks in terms of the translation of different imaging approaches in clinics are discussed.

Visualizing the Fate of Intra-Articular Injected Mesenchymal Stem Cells In Vivo in the Second Near-Infrared Window for the Effective Treatment of Supraspinatus Tendon Tears

Mesenchymal stem cells (MSCs) are capable of exerting strong therapeutic potential for the treatment of supraspinatus tendon tear. However, MSC therapy remains underutilized and perhaps underrated due to the limited evidence of dynamic visualization of cellular behavior in vivo. Here, second near-infrared fluorescence imaging with biocompatible PbS quantum dots (QDs) provides a cellular migration map and information on the biodistribution and clearance processes of three densities of intra-articularly injected, labeled MSCs to treat supraspinatus tendon tear in mice. Intra-articular injection avoids entrapment of MSCs by filter organs and reduces the QD-induced organ toxicity. Notably, the MSCs share a similar migration direction, but the moderate density group is somewhat more efficient, showing the longest residence time and highest cell retention rate around the footprint during the repair stage. Furthermore, quantitative kinetic investigation demonstrates that labeled MSCs are cleared by feces and urine. Histomorphometric analysis demonstrates that the moderate density group achieves maximum therapeutic effect and labeled MSCs do not induce any injury or inflammation to major organs, which suggests that administration of too many or few MSCs may decrease their effectiveness. Such an imaging approach provides spatiotemporal evidence for response to MSC therapy in vivo, facilitating the optimization of MSC therapy.

Potential use of superparamagnetic iron oxide nanoparticles for in vitro and in vivo bioimaging of human myoblasts

Myocardial infarction (MI) is one of the most frequent causes of death in industrialized countries. Stem cells therapy seems to be very promising for regenerative medicine. Skeletal myoblasts transplantation into postinfarction scar has been shown to be effective in the failing heart but shows limitations such, e.g. cell retention and survival. We synthesized and investigated superparamagnetic iron oxide nanoparticles (SPIONs) as an agent for direct cell labeling, which can be used for stem cells imaging. High quality, monodisperse and biocompatible DMSA-coated SPIONs were obtained with thermal decomposition and subsequent ligand exchange reaction. SPIONs' presence within myoblasts was confirmed by Prussian Blue staining and inductively coupled plasma mass spectrometry (ICP-MS). SPIONs' influence on tested cells was studied by their proliferation, ageing, differentiation potential and ROS production. Cytotoxicity of obtained nanoparticles and myoblast associated apoptosis were also tested, as well as iron-related and coating-related genes expression. We examined SPIONs' impact on overexpression of two pro-angiogenic factors introduced via myoblast electroporation method. Proposed SPION-labeling was sufficient to visualize firefly luciferase-modified and SPION-labeled cells with magnetic resonance imaging (MRI) combined with bioluminescence imaging (BLI) in vivo. The obtained results demonstrated a limited SPIONs' influence on treated skeletal myoblasts, not interfering with basic cell functions.

Reversal of Experimental Liver Damage after Transplantation of Stem-Derived Cells Detected by FTIR Spectroscopy

The transplantation of autologous BM-MSCs holds great potential for treating end-stage liver diseases. The aim of this study was to compare the efficiency of transplanted rBM-MSCs and rBM-MSC-derived differentiated stem cells (rBM-MSC-DSCs) for suppression of dimethylnitrosamine-injured liver damage in rat model. Synchrotron radiation Fourier-transform infrared (SR-FTIR) microspectroscopy was applied to investigate changes in the macromolecular composition. Transplantation of rBM-MSC-DSCs into liver-injured rats restored their serum albumin level and significantly suppressed transaminase activity as well as the morphological manifestations of liver disease. The regenerative effects of rBM-MSC-DSCs were corroborated unequivocally by the phenotypic difference analysis between liver tissues revealed by infrared spectroscopy. Spectroscopic changes in the spectral region from 1190–970 cm⁻¹ (bands with absorbance maxima at 1150 cm⁻¹, 1081 cm⁻¹, and 1026 cm⁻¹) indicated decreased levels of carbohydrates, in rBM-MSC-DSC-transplanted livers, compared with untreated and rBM-MSC--transplanted animals. Principal component analysis (PCA) of spectra acquired from liver tissue could readily discriminate rBM-MSC-DSC-transplanted animals from the untreated and rBM-MSC-transplanted animals. We conclude that the transplantation of rBM-MSC-DSCs effectively treats liver disease in rats and SR-FTIR microspectroscopy provides important insights into the fundamental biochemical alterations induced by the stem-derived cell transplantation, including an objective “signature” of the regenerative effects of stem cell therapy upon liver injury.

MRI reveals slow clearance of dead cell transplants in mouse forelimb muscles

A small molecule tetraazacyclododecane-1,4,7,10-tetraacetic acid (Gd-DOTA)₄-TPP agent is used to label human mesenchymal stem cells (hMSCs) via electroporation (EP). The present study assessed the cytotoxicity of cell labeling, in addition to its effect on cell differentiation potential. There were no significant adverse effects on cell viability or differentiation induced by either EP or cellular uptake of (Gd-DOTA)₄-TPP. Labeled live and dead hMSCs were transplanted into mouse forelimb muscles. T₂-weighted magnetic resonance imaging (MRI) was used to track the in vivo fate of the cell transplants. The labeling and imaging strategy allowed long term tracking of the cell transplants and unambiguous distinguishing of the cell transplants from their surrounding tissues. Cell migration was observed for live hMSCs injected into subcutaneous tissues, however not for either live or dead hMSCS injected into limb muscles. A slow clearance process occurred of the dead cell transplants in the limb muscular tissue. The MRI results therefore reveal that the fate and physiological activities of cell transplants depend on the nature of their host tissue.

Optical spectroscopic imaging for cell therapy and tissue engineering

Cell-based therapies hold great potential to treat a wide range of human diseases, yet the mechanisms responsible for cell migration and homing are not fully understood. Emerging molecular imaging technology enables in vivo tracking of transplanted cells and their therapeutic efficacy, which together will improve the clinical outcome of cell-based therapy. Particularly, optical imaging provides highly sensitive, safe (non-radioactive), cost-effective, and fast solutions for real-time cellular trafficking compared to other conventional molecular imaging modalities. This review provides a comprehensive overview of current advances in optical imaging for cell-based therapy and tissue engineering. We discuss different types of fluorescent probes and their labeling methods with a special focus on cardiovascular disease, cancer immunotherapy, and tissue regeneration. In addition, advantages and limitations of optical imaging-based cell tracking strategies along with the future perspectives to translate this imaging technique for a clinical realm are discussed.

Nanodiamond-Gadolinium(III) Aggregates for Tracking Cancer Growth In Vivo at High Field

The ability to track labeled cancer cells in vivo would allow researchers to study their distribution, growth, and metastatic potential within the intact organism. Magnetic resonance (MR) imaging is invaluable for tracking cancer cells in vivo as it benefits from high spatial resolution and the absence of ionizing radiation. However, many MR contrast agents (CAs) required to label cells either do not significantly accumulate in cells or are not biologically compatible for translational studies. We have developed carbon-based nanodiamond-gadolinium(III) aggregates (NDG) for MR imaging that demonstrated remarkable properties for cell tracking in vivo. First, NDG had high relaxivity independent of field strength, a finding unprecedented for gadolinium(III) [Gd(III)]-nanoparticle conjugates. Second, NDG demonstrated a 300-fold increase in the cellular delivery of Gd(III) compared to that of clinical Gd(III) chelates without sacrificing biocompatibility. Further, we were able to monitor the tumor growth of NDG-labeled flank tumors by T1- and T2-weighted MR imaging for 26 days in vivo, longer than was reported for other MR CAs or nuclear agents. Finally, by utilizing quantitative maps of relaxation times, we were able to describe tumor morphology and heterogeneity (corroborated by histological analysis), which would not be possible with competing molecular imaging modalities.

Computer simulations of the interaction of human immunodeficiency virus (HIV) aspartic protease with spherical gold nanoparticles: implications in acquired immunodeficiency syndrome (AIDS)

The interaction of gold nanoparticles (AuNP) with human immune-deficiency virus aspartic protease (HIVPR) is modelled using a regime of molecular dynamics simulations. The simulations of the 'docking', first as a rigid-body complex, and eventually through flexible-fit analysis, creates 36 different complexes from four initial orientations of the nanoparticle strategically positioned around the surface of the enzyme. The structural deviations of the enzymes from the initial x-ray crystal structure during each docking simulation are assessed by comparative analysis of secondary structural elements, root mean square deviations, B-factors, interactive bonding energies, dihedral angles, radius of gyration (R g), circular dichroism (CD), volume occupied by C α , electrostatic potentials, solvation energies and hydrophobicities. Normalisation of the data narrows the selection from the initial 36 to one 'final' probable structure. It is concluded that, after computer simulations on each of the 36 initial complexes incorporating the 12 different biophysical techniques, the top five complexes are the same no matter which technique is explored. The significance of the present work is an expansion of an earlier study on the molecular dynamic simulation for the interaction of HIVPR with silver nanoparticles. This work is supported by experimental evidence since the initial 'orientation' of the AgNP with the enzyme is the same as the 'final' AuNP-HIVPR complex generated in the present study. The findings will provide insight into the forces of the binding of the HIVPR to AuNP. It is anticipated that the protocol developed in this study will act as a standard process for the interaction of any nanoparticle with any biomedical target.

Multipotent stem cells of the heart-do they have therapeutic promise?

The last decade has brought a comprehensive change in our view of cardiac remodeling processes under both physiological and pathological conditions, and cardiac stem cells have become important new players in the general mainframe of cardiac homeostasis. Different types of cardiac stem cells show different capacities for differentiation into the three major cardiac lineages: myocytes, endothelial cells and smooth muscle cells. Physiologically, cardiac stem cells contribute to cardiac homeostasis through continual cellular turnover. Pathologically, these cells exhibit a high level of proliferative activity in an apparent attempt to repair acute cardiac injury, indicating that these cells possess (albeit limited) regenerative potential. In addition to cardiac stem cells, mesenchymal stem cells represent another multipotent cell population in the heart; these cells are located in regions near pericytes and exhibit regenerative, angiogenic, antiapoptotic, and immunosuppressive properties. The discovery of these resident cardiac stem cells was followed by a number of experimental studies in animal models of cardiomyopathies, in which cardiac stem cells were tested as a therapeutic option to overcome the limited transdifferentiating potential of hematopoietic or mesenchymal stem cells derived from bone marrow. The promising results of these studies prompted clinical studies of the role of these cells, which have demonstrated the safety and practicability of cellular therapies for the treatment of heart disease. However, questions remain regarding this new therapeutic approach. Thus, the aim of the present review was to discuss the multitude of different cardiac stem cells that have been identified, their possible functional roles in the cardiac regenerative process, and their potential therapeutic uses in treating cardiac diseases.

Single-cell tracking with PET using a novel trajectory reconstruction algorithm

Virtually all biomedical applications of positron emission tomography (PET) use images to represent the distribution of a radiotracer. However, PET is increasingly used in cell tracking applications, for which the "imaging" paradigm may not be optimal. Here, we investigate an alternative approach, which consists in reconstructing the time-varying position of individual radiolabeled cells directly from PET measurements. As a proof of concept, we formulate a new algorithm for reconstructing the trajectory of one single moving cell directly from list-mode PET data. We model the trajectory as a 3-D B-spline function of the temporal variable and use nonlinear optimization to minimize the mean-square distance between the trajectory and the recorded list-mode coincidence events. Using Monte Carlo simulations (GATE), we show that this new algorithm can track a single source moving within a small-animal PET system with 3 mm accuracy provided that the activity of the cell [Bq] is greater than four times its velocity [mm/s]. The algorithm outperforms conventional ML-EM as well as the "minimum distance" method used for positron emission particle tracking (PEPT). The new method was also successfully validated using experimentally acquired PET data. In conclusion, we demonstrated the feasibility of a new method for tracking a single moving cell directly from PET list-mode data, at the whole-body level, for physiologically relevant activities and velocities.

Adenovirus-mediated expression of human sodium-iodide symporter gene permits in vivo tracking of adipose tissue-derived stem cells in a canine myocardial infarction model

INTRODUCTION

In vivo tracking of the transplanted stem cells is important in pre-clinical research of stem cell therapy for myocardial infarction. We examined the feasibility of adenovirus-mediated sodium iodide symporter (NIS) gene to cell tracking imaging of transplanted stem cells in a canine infarcted myocardium by clinical single photon emission computed tomography (SPECT).

METHODS

Beagle dogs were injected intramyocardially with NIS-expressing adenovirus-transfected canine stem cells (Ad-hNIS-canine ADSCs) a week after myocardial infarction (MI) development. (99m)Tc-methoxyisobutylisonitrile ((99m)Tc-MIBI) and (99m)Tc-pertechnetate ((99m)TcO4(-)) SPECT imaging were performed for assessment of infarcted myocardium and viable stem cell tracking. Transthoracic echocardiography was performed to monitor any functional cardiac changes.

RESULTS

Left ventricular ejection fraction (LVEF) was decreased after LAD ligation. There was no significant difference in EF between the groups with the stem cell or saline injection. (125)I uptake was higher in Ad-hNIS-canine ADSCs than in non-transfected ADSCs. Cell proliferation and differentiation were not affected by hNIS-carrying adenovirus transfection. (99m)Tc-MIBI myocardial SPECT imaging showed decreased radiotracer uptake in the infarcted apex and mid-anterolateral regions. Ad-hNIS-canine ADSCs were identified as a region of focally increased (99m)TcO4(-) uptake at the lateral wall and around the apex of the left ventricle, peaked at 2 days and was observed until day 9.

CONCLUSIONS

Combination of adenovirus-mediated NIS gene transfection and clinical nuclear imaging modalities enables to trace the fate of transplanted stem cells in infarcted myocardium for translational in vivo cell tracking study for prolonged duration.

Tracking of stem cells in vivo for cardiovascular applications

In the past ten years, the concept of injecting stem and progenitor cells to assist with rebuilding damaged blood vessels and myocardial tissue after injury in the heart and peripheral vasculature has moved from bench to bedside. Non-invasive imaging can not only provide a means to assess cardiac repair and, thereby, cellular therapy efficacy but also a means to confirm cell delivery and engraftment after administration. In this first of a two-part review, we will review the different types of cellular labeling techniques and the application of these techniques in cardiovascular magnetic resonance and ultrasound. In addition, we provide a synopsis of the cardiac cellular clinical trials that have been performed to-date.

Variability in contrast agent uptake by different but similar stem cell types

The need to track and evaluate the fate of transplanted cells is an important issue in regenerative medicine. In order to accomplish this, pre-labelling cells with magnetic resonance imaging (MRI) contrast agents is a well-established method. Uptake of MRI contrast agents by non-phagocytic stem cells, and factors such as cell homeostasis or the adverse effects of contrast agents on cell biology have been extensively studied, but in the context of nanoparticle (NP)-specific parameters. Here, we have studied three different types of NPs (Endorem®, magnetoliposomes [MLs], and citrate coated C-200) to label relatively larger, mesenchymal stem cells (MSCs) and, much smaller yet faster proliferating, multipotent adult progenitor cells (MAPCs). Both cell types are similar, as they are isolated from bone marrow and have substantial regenerative potential, which make them interesting candidates for comparative experiments. Using NPs with different surface coatings and sizes, we found that differences in the proliferative and morphological characteristics of the cells used in the study are mainly responsible for the fate of endocytosed iron, intracellular iron concentration, and cytotoxic responses. The quantitative analysis, using high-resolution electron microscopy images, demonstrated a strong relationship between cell volume/surface, uptake, and cytotoxicity. Interestingly, uptake and toxicity trends are reversed if intracellular concentrations, and not amounts, are considered. This indicates that more attention should be paid to cellular parameters such as cell size and proliferation rate in comparative cell-labeling studies.

Discrimination of functional hepatocytes derived from mesenchymal stem cells using FTIR microspectroscopy

Functional hepatocytes differentiated in vitro from mesenchymal stem cells (MSCs) need to be fully characterized before they could be applied as a therapy to treat liver disease. Here, we employed Fourier Transform Infrared (FTIR) microspectroscopy to investigate the characteristics of hepatocyte-like cells derived from rat bone marrow mesenchymal stem cells (rBM-MSCs) by detecting changes in macromolecular composition occurring during the hepatogenesis process. Partial Least Squares Discriminant Analysis (PLS-DA) enabled us to discriminate undifferentiated rBM-MSCs, and early, mid-stage and late stage rBM-MSCs derived hepatocytes by their characteristic FTIR "spectroscopic signatures". The predominant spectroscopic changes responsible for this discrimination were changes in FTIR absorbance bands at: 3012 cm(-1) (cis C[double bond, length as m-dash]C stretch from unsaturated lipids), 2952 cm(-1) (ν(as)CH(3) from lipids), 2854 cm(-1) (ν(s)CH(2) from lipids) and 1722 cm(-1) (C[double bond, length as m-dash]O stretching from lipids), which were associated with triglyceride and unsaturated fatty acid accumulation in the hepatocyte-like cells occurring during differentiation. Based on these findings, rBM-MSCs derived hepatocytes are characterized by high lipid content which facilitates a means of identifying hepatocytes from their stem cells progenitors by using FTIR microspectroscopy. Other complex changes in spectral bands assigned to proteins and nucleic acids were observed during hepatocyte differentiation indicating that mRNA translation was taking place producing proteins related to the formation of the new hepatocyte-like phenotype, which was corroborated by immunohistochemistry. The results show FTIR microspectroscopy combined with bioinformatic modeling constitutes a powerful new phenotypic-based methodology for monitoring and characterization of the process of stem cell differentiation leading to the formation of hepatocytes, providing complementary information to existing methodologies such as immunohistochemistry and gene analysis, but having advantages of being reagent-free and non-destructive of the sample.

An in vitro model of mesenchymal stem cell targeting using magnetic particle labelling

The specific targeting of cells to sites of tissue damage in vivo is a major challenge precluding the success of stem cell-based therapies. Magnetic particle-based targeting may provide a solution. Our aim was to provide a model system to study the trapping and potential targeting of human mesenchymal stem cells (MSCs) during in vitro fluid flow, which ultimately will inform cell targeting in vivo. In this system magnet arrays were used to trap superparamagnetic iron oxide particle-doped MSCs. The in vitro experiments demonstrated successful cell trapping, where the volume of cells trapped increased with magnetic particle concentration and decreased with increasing flow rate. Analysis of gene expression revealed significant increases in COL1A2 and SOX9. Using principles established in vitro, a proof-of-concept in vivo experiment demonstrated that magnetic particle-doped, luciferase-expressing MSCs were trapped by an implanted magnet in a subcutaneous wound model in nude mice. Our results demonstrate the effectiveness of using an in vitro model for testing superparamagnetic iron oxide particles to develop successful MSC targeting strategies during fluid flow, which ultimately can be translated to in vivo targeted delivery of cells via the circulation in a variety of tissue-repair models.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Towards whole-body imaging at the single cell level using ultra-sensitive stem cell labeling with oligo-arginine modified upconversion nanoparticles

Mesenchymal stem cells (MSCs) have shown great potential in regenerative medicine. Sensitive and reliable methods for stem cell labeling and in vivo tracking are thus of great importance. Herein, we report the use of upconversion nanoparticles (UCNPs) as an exogenous contrast agent to track mouse MSCs (mMSCs) in vivo. To improve the labeling efficiency, oligo-arginine is conjugated to polyethylene glycol (PEG) coated UCNPs to enhance the nanoparticles uptake by mMSCs. Systematic in vitro tests reveal that the proliferation and differentiation of mMSCs are not notably affected by UCNP-labeling, suggesting that the labeled cells are able to maintain their stem cell potency. No apparent exocytosis is found in our in vitro labeling experiment by using a transwell culture system over a course of 10 days, indicating the potential capability of using our UCNP-labeling method for long-term stem cell tracking. To demonstrate the tracking sensitivity of our stem cell labeling approach, UCNP-labeled mMSCs are subcutaneously transplanted into mice and imaged using an in vivo upconversion luminescence (UCL) imaging system. As few as ~10 cells labeled with UCNPs are detected in vivo, evidencing a remarkable improvement in detection sensitivity of our UCNP-labeled hMSCs compared with other stem cell labeling techniques using conventional exogenous agents. We further track UCNP-labeled mMSCs after intravenous injection, and observe the translocation of mMSCs from lung where they initially accumulate, to liver, a phenomenon consistent to previous reports. Our results highlight the promise of using UCNPs as a new type of ultra-sensitive probes for labeling and in vivo tracking of stem cells at nearly the single cell level.

A photo-multiplier tube-based hybrid MRI and frequency domain fluorescence tomography system for small animal imaging

Fluorescence tomography (FT) is a promising molecular imaging technique that can spatially resolve both fluorophore concentration and lifetime parameters. However, recovered fluorophore parameters highly depend on the size and depth of the object due to the ill-posedness of the FT inverse problem. Structural a priori information from another high spatial resolution imaging modality has been demonstrated to significantly improve FT reconstruction accuracy. In this study, we have constructed a combined magnetic resonance imaging (MRI) and FT system for small animal imaging. A photo-multiplier tube is used as the detector to acquire frequency domain FT measurements. This is the first MR-compatible time-resolved FT system that can reconstruct both fluorescence concentration and lifetime maps simultaneously. The performance of the hybrid system is evaluated with phantom studies. Two different fluorophores, indocyanine green and 3-3' diethylthiatricarbocyanine iodide, which have similar excitation and emission spectra but different lifetimes, are utilized. The fluorescence concentration and lifetime maps are both reconstructed with and without the structural a priori information obtained from MRI for comparison. We show that the hybrid system can accurately recover both fluorescence intensity and lifetime within 10% error for two 4.2 mm-diameter cylindrical objects embedded in a 38 mm-diameter cylindrical phantom when MRI structural a priori information is utilized.

Tracking stem cells for cardiovascular applications in vivo: focus on imaging techniques

Despite rapid translation of stem cell therapy into clinical practice, the treatment of cardiovascular disease using embryonic stem cells, adult stem and progenitor cells or induced pluripotent stem cells has not yielded satisfactory results to date. Noninvasive stem cell imaging techniques could provide greater insight into not only the therapeutic benefit, but also the fundamental mechanisms underlying stem cell fate, migration, survival and engraftment in vivo. This information could also assist in the appropriate choice of stem cell type(s), delivery routes and dosing regimes in clinical cardiovascular stem cell trials. Multiple imaging modalities, such as MRI, PET, SPECT and CT, have emerged, offering the ability to localize, monitor and track stem cells in vivo. This article discusses stem cell labeling approaches and highlights the latest cardiac stem cell imaging techniques that may help clinicians, research scientists or other healthcare professionals select the best cellular therapeutics for cardiovascular disease management.

Dual-modal tracking of transplanted mesenchymal stem cells after myocardial infarction

PURPOSE

Results for implantation efficiency and effective improvement of cardiac function in the field of mesenchymal stem cells (MSCs) are controversial. To attempt to clarify this debate, we utilized magnetic resonance imaging (MRI) and near-infrared optical imaging (OI) to explore the effects of different delivery modes of mesenchymal stem cells on cell retention time and cardiac function after myocardial infarction (MI).

METHODS

Rat MSCs were labeled with superparamagnetic iron oxide nanoparticles and 1, 1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine, 4-chlorobenzenesulfonate salt (DiD) for noninvasive cell tracking in a rat MI model. Rats underwent coronary artery ligation and were randomized into three experimental groups: intravenous (IV), intramyocardial (IM), and a control group. The first two groups referred to the route of delivery of the transplanted dual-labeled MSCs; whereas the control group was given an IV injection of serum-free medium one day post-MI. Cellular engraftment was determined 1 day and 7 days post cell delivery by measuring the iron and optical signals in explanted organs. Prussian blue staining and fluorescent microscopy were performed on histological sections for iron and DiD, respectively. Cardiac function was measured by echocardiography on day 7.

RESULTS

The cardiac function of the IM group increased significantly compared to the IV and control groups at day 7. In the IM group, labeled cells were visualized in the infracted heart by serial MRI, and the intensity by OI was significantly higher on day 1. In the IV group, the heart signals were significantly attenuated by dual-modal tracking at two time points, but the lung signals in OI were significantly stronger than the IM group at both time points.

CONCLUSION

IM injection of MSCs increased cell engraftment within infarcted hearts and improved cardiac function after MI. However, IV infusion has a low efficacy due to the cell trapping in the lung. Therefore, direct injection may provide an advantage over IV, with regard to retention of stem cells and protection of cardiac function.

Design and characterization of a polymeric MRI contrast agent based on PVA for in vivo living-cell tracking

A novel water-soluble MRI contrast agent for in vivo living cell tracking was developed. Unlike the conventional in vivo cell tracking system based on superparamagnetic iron oxide beads, the newly developed contrast agent is eliminated from the body when the contrast agent exits the cells upon cell death, which makes living cell tracking possible. The contrast agent is composed of gadolinium chelates (Gd-DOTA) and a water-soluble carrier, poly(vinyl alcohol) (PVA), which is known to interact with cells and tissues very weakly. Since the Gd-PVA was not taken up by cells spontaneously, the electroporation method was used for cell labeling. The delivered Gd-PVA was localized only in the cytosolic compartment of growing cells with low cytotoxicity and did not leak out of the living cells for long periods of time. This stability may be due to the weak cell-membrane affinity of Gd-PVA, and did not affect cell proliferation at all. After cell labeling, signal enhancement of cells was observed in vitro and in vivo. These results indicate that Gd-PVA can visualize only the living cells in vivo for a long period of time, even in areas deep within large animal bodies.

Multiwall carbon nanotubes as MRI contrast agents for tracking stem cells

In this study we investigate the potential of multiwall carbon nanotubes (MWCNTs) with low metal impurities (2.57% iron) as magnetic resonance imaging (MRI) contrast agents. Taking into account probable aggregation at high MWCNTs concentration analysis shows that the r(2) relaxivity of MWCNTs in 1% agarose gels at 19 °C is 564 ± 41 s(-1) mM(-1); this is attributed to both the presence of iron oxide impurities and also to the carbon MWCNT structure itself. Stem cells were labelled with MWCNTs to demonstrate the effectiveness of MWCNTs as MRI contrast agents for cellular MRI. The MWCNTs did not impair cell viability or proliferation. These results suggest that the MRI contrast agent properties of the MWCNTs could be used in vivo for stem cell tracking/imaging and during MWCNT-mediated targeted electro-chemotherapy of tumours.

Bone marrow mononuclear cell therapy for patients with cirrhosis: a Phase 1 study

BACKGROUND

Bone marrow-derived cell therapy has been investigated in patients with severe liver disease.

AIMS

To assess the feasibility, safety and cell kinetics of autologous bone marrow-derived mononuclear cells (BMMCs) infusion in cirrhotic patients.

METHODS

BMMCs were isolated from autologous bone marrow and 10% of the cells were labelled with (99m)Tc-SnCl₂. Whole body scintigraphy (WBS) was performed 3 and 24 h after infusion via the hepatic artery. Liver function and image were followed during 1 year.

RESULTS

Eight patients received 2.0-15.0 × 10⁸ cells. Three and 24-h WBS showed mean hepatic radiotracer retentions of 41 and 32% respectively. One case of dissection of the hepatic artery and one case of Tako-tsubo syndrome occurred as early complications. A patient developed a cutaneous immunomediated disorder and another patient developed hepatocellular carcinoma (HCC) 12 months after infusion. A reduction in bilirubin was shown at 1 week while serum albumin increased above baseline up to 1 month after infusion (P<0.05).

CONCLUSIONS

BMMCs infusion is feasible and practical in a clinical setting. In vivo tracking of labelled cells demonstrated that the hepatic artery route successfully delivered BMMCs to the liver. The early improvement of laboratory indices of liver function should be interpreted with caution, because this study was not designed to evaluate efficacy. The median Model for End-Stage Liver Disease score had not deteriorated 1 year later. The occurrence of a graft-versus-host disease-like phenomenon highlights the importance of sustained vigilance even when giving autologous cells. Controlled studies are needed to determine whether BMMCs infusion affects HCC development in cirrhosis.

Stem cell labeling for noninvasive delivery and tracking in cardiovascular regenerative therapy

Clinical and basic scientific studies of stem cell-based therapies have shown promising results for cardiovascular diseases. Despite a rapid transition from animal studies to clinical trials, the mechanisms by which stem cells improve heart function are yet to be fully elucidated. To optimize cell therapies in patients will require a noninvasive means to evaluate cell survival, biodistribution and fate in the same subject over time. Cell labeling offers the ability to image distinct cell lineages in vivo and investigate the efficacy of these therapies using standard noninvasive imaging techniques. In this article, we will discuss the most promising cell labeling techniques for translation to clinical cardiovascular applications.

Dual-modal nanoprobes for imaging of mesenchymal stem cell transplant by MRI and fluorescence imaging

Objective

To determine the feasibility of labeling human mesenchymal stem cells (hMSCs) with bifunctional nanoparticles and assessing their potential as imaging probes in the monitoring of hMSC transplantation.

Materials and Methods

The T1 and T2 relaxivities of the nanoparticles (MNP@SiO₂[RITC]-PEG) were measured at 1.5T and 3T magnetic resonance scanner. Using hMSCs and the nanoparticles, labeling efficiency, toxicity, and proliferation were assessed. Confocal laser scanning microscopy and transmission electron microscopy were used to specify the intracellular localization of the endocytosed iron nanoparticles. We also observed in vitro and in vivo visualization of the labeled hMSCs with a 3T MR scanner and optical imaging.

Results

MNP@SiO₂(RITC)-PEG showed both superparamagnetic and fluorescent properties. The r₁ and r₂ relaxivity values of the MNP@SiO₂(RITC)-PEG were 0.33 and 398 mM^-1 s^-1 at 1.5T, respectively, and 0.29 and 453 mM^-1 s^-1 at 3T, respectively. The effective internalization of MNP@SiO₂(RITC)-PEG into hMSCs was observed by confocal laser scanning fluorescence microscopy. The transmission electron microscopy images showed that MNP@SiO₂(RITC)-PEG was internalized into the cells and mainly resided in the cytoplasm. The viability and proliferation of MNP@SiO₂(RITC)-PEG-labeled hMSCs were not significantly different from the control cells. MNP@SiO₂(RITC)-PEG-labeled hMSCs were observed in vitro and in vivo with optical and MR imaging.

Conclusion

MNP@SiO₂(RITC)-PEG can be a useful contrast agent for stem cell imaging, which is suitable for a bimodal detection by MRI and optical imaging.

Tracking the migration of cardially delivered therapeutic stem cells in vivo: state of the art

Cell-based therapy is a promising, novel therapeutic strategy for cardiovascular disease. The rapid transition of this approach from the benchside to clinical trials has left a gap in the understanding of the mechanisms of cell therapy. Monitoring of cell homing and the fate of cardially delivered stem cells is fundamental for clarification of the myocardial regenerative process. Noninvasive imaging techniques allow an in vivo evaluation of the survival, migration and differentiation of implanted stem cells over time, and by this means, can help to answer unresolved questions. The most promising in vivo tracking methods involve the direct, nonspecific labeling of cells including MRI, radionuclide imaging and the use of reporter-gene imaging. This review summarizes the most important results of animal and human studies in which the fate and biodistribution of cardially delivered stem cells are assessed through different in vivo tracking methods.

Size, charge and concentration dependent uptake of iron oxide particles by non-phagocytic cells

A promising new direction for contrast-enhanced magnetic resonance (MR) imaging involves tracking the migration and biodistribution of superparamagnetic iron oxide (SPIO)-labeled cells in vivo. Despite the large number of cell labeling studies that have been performed with SPIO particles of differing size and surface charge, it remains unclear which SPIO configuration provides optimal contrast in non-phagocytic cells. This is largely because contradictory findings have stemmed from the variability and imprecise control over surface charge, the general need and complexity of transfection and/or targeting agents, and the limited number of particle configurations examined in any given study. In the present study, we systematically evaluated the cellular uptake of SPIO in non-phagocytic T cells over a continuum of particle sizes ranging from 33nm to nearly 1.5microm, with precisely controlled surface properties, and without the need for transfection agents. SPIO labeling of T cells was analyzed by flow cytometry and contrast enhancement was determined by relaxometry. SPIO uptake was dose-dependent and exhibited sigmoidal charge dependence, which was shown to saturate at different levels of functionalization. Efficient labeling of cells was observed for particles up to 300nm, however, micron-sized particle uptake was limited. Our results show that an unconventional highly cationic particle configuration at 107nm maximized MR contrast of T cells, outperforming the widely utilized USPIO (<50nm).

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.

Prowling wolves in sheep's clothing: the search for tumor stem cells

The importance of a subset of cells which have ‘stem like’ characteristics and are capable of tumor initiation has been reported for a range of tumors. Isolation of these tumor-initiating cells (TICs) has largely been based on differential cell surface protein expression. However, there is still much debate on the functional significance of these markers in initiating tumors, as many properties of tumor initiation are modified by cell-cell interactions. In particular, the relationship between TICs and their microenvironment is poorly understood but has therapeutic implications, as the microenvironment can maintain tumor cells in a prolonged period of quiescence. However, a major limitation in advancing our understanding of the crosstalk between TICs and their microenvironment is the lack of sensitive techniques which allow thein vivotracking and monitoring of TICs. Application of newin vivocellular and molecular imaging technologies holds much promise in uncovering the mysteries of TIC behavior at the three-dimensional level. This review will describe recent advances in our understanding of the TIC concept and how the application ofin vivoimaging techniques can advance our understanding of the biological fate of TICs. A supplementary resource guide describing TICs from different malignancies is also presented.