Myocardial Homing and Coronary Endothelial Function After Autologous Blood CD34+ Progenitor Cells Intracoronary Injection in the Chronic Phase of Myocardial Infarction

Single cell and lineage tracing studies reveal the impact of CD34+ cells on myocardial fibrosis during heart failure

BACKGROUND

CD34⁺ cells have been used to treat the patients with heart failure, but the outcome is variable. It is of great significance to scrutinize the fate and the mechanism of CD34⁺ cell differentiation in vivo during heart failure and explore its intervention strategy.

METHODS

We performed single-cell RNA sequencing (scRNA-seq) of the total non-cardiomyocytes and enriched Cd34-tdTomato⁺ lineage cells in the murine (male Cd34-CreERT2; Rosa26-tdTomato mice) pressure overload model (transverse aortic constriction, TAC), and total non-cardiomyocytes from human adult hearts. Then, in order to determine the origin of CD34⁺ cell that plays a role in myocardial fibrosis, bone marrow transplantation model was performed. Furthermore, to further clarify the role of CD34 + cells in myocardial remodeling in response to TAC injury, we generated Cd34-CreERT2; Rosa26-eGFP-DTA (Cre/DTA) mice.

RESULTS

By analyzing the transcriptomes of 59,505 single cells from the mouse heart and 22,537 single cells from the human heart, we illustrated the dynamics of cell landscape during the progression of heart hypertrophy, including CD34⁺ cells, fibroblasts, endothelial and immune cells. By combining genetic lineage tracing and bone marrow transplantation models, we demonstrated that non-bone-marrow-derived CD34⁺ cells give rise to fibroblasts and endothelial cells, while bone-marrow-derived CD34⁺ cell turned into immune cells only in response to pressure overload. Interestingly, partial depletion of CD34⁺ cells alleviated the severity of myocardial fibrosis with a significant improvement of cardiac function in Cd34-CreERT2; Rosa26-eGFP-DTA model. Similar changes of non-cardiomyocyte composition and cellular heterogeneity of heart failure were also observed in human patient with heart failure. Furthermore, immunostaining showed a double labeling of CD34 and fibroblast markers in human heart tissue. Mechanistically, our single-cell pseudotime analysis of scRNA-seq data and in vitro cell culture study revealed that Wnt-β-catenin and TGFβ1/Smad pathways are critical in regulating CD34⁺ cell differentiation toward fibroblasts.

CONCLUSIONS

Our study provides a cellular landscape of CD34⁺ cell-derived cells in the hypertrophy heart of human and animal models, indicating that non-bone-marrow-derived CD34⁺ cells differentiating into fibroblasts largely account for cardiac fibrosis. These findings may provide novel insights for the pathogenesis of cardiac fibrosis and have further potential therapeutic implications for the heart failure.

Endothelial progenitor cells and coronary artery disease: Current concepts and future research directions

Vascular injury is a frequent pathology in coronary artery disease. To repair the vasculature, scientists have found that endothelial progenitor cells (EPCs) have excellent properties associated with angiogenesis. Over time, research on EPCs has made encouraging progress regardless of pathology or clinical technology. This review focuses on the origins and cell markers of EPCs, and the connection between EPCs and coronary artery disease. In addition, we summarized various studies of EPC-capturing stents and EPC infusion therapy, and aim to learn from past technology to predict the future.

Autologous CD34+ Cell Therapy for Ischemic Tissue Repair

In 1997, the seminal manuscript by Asahara, Murohara, Isner et al outlined the evidence for the existence of circulating, bone marrow-derived cells capable of stimulating and contributing to the formation of new blood vessels. Consistent with the paradigm shift that this work represented, it triggered much scientific debate and controversy, some of which persists 2 decades later. In contrast, the clinical application of autologous CD34 cell therapy has been marked by a track record of consistent safety and clinical benefit in multiple ischemic conditions. In this review, we summarize the preclinical and clinical evidence from over 700 patients in clinical trials of CD34 cell therapy.

Collagen biomaterial stimulates the production of extracellular vesicles containing microRNA-21 and enhances the proangiogenic function of CD34+ cells

CD34⁺ cells are promising for revascularization therapy, but their clinical use is limited by low cell counts, poor engraftment, and reduced function after transplantation. In this study, a collagen type I biomaterial was used to expand and enhance the function of human peripheral blood CD34⁺ cells, and potential underlying mechanisms were examined. Compared to the fibronectin control substrate, biomaterial-cultured CD34⁺ cells from healthy donors had enhanced proliferation, migration toward VEGF, angiogenic potential, and increased secretion of CD63⁺CD81⁺ extracellular vesicles (EVs). In the biomaterial-derived EVs, greater levels of the angiogenic microRNAs (miRs), miR-21 and -210, were detected. Notably, biomaterial-cultured CD34⁺ cells had reduced mRNA and protein levels of Sprouty (Spry)1, which is an miR-21 target and negative regulator of endothelial cell proliferation and angiogenesis. Similar to the results of healthy donor cells, biomaterial culture increased miR-21 and -210 expression in CD34⁺ cells from patients who underwent coronary artery bypass surgery, which also exhibited improved VEGF-mediated migration and angiogenic capacity. Therefore, collagen biomaterial culture may be useful for expanding the number and enhancing the function of CD34⁺ cells in patients, possibly mediated through suppression of Spry1 activity by EV-derived miR-21. These results may provide a strategy to enhance the therapeutic potency of CD34⁺ cells for vascular regeneration.-McNeill, B., Ostojic, A., Rayner, K. J., Ruel, M., Suuronen, E. J. Collagen biomaterial stimulates the production of extracellular vesicles containing microRNA-21 and enhances the proangiogenic function of CD34⁺ cells.

Stem/Progenitor Cells and Their Therapeutic Application in Cardiovascular Disease

Cardiovascular disease is the leading cause of death in the world. The stem/progenitor cell-based therapy has emerged as a promising approach for the treatment of a variety of cardiovascular diseases including myocardial infarction, stroke, peripheral arterial disease, and diabetes. An increasing number of evidence has shown that stem/progenitor cell transplantation could replenish damaged cells, improve cardiac and vascular functions, and repair injured tissues in many pre-clinical studies and clinical trials. In this review, we have outlined the major types of stem/progenitor cells, and summarized the studies in applying these cells, especially endothelial stem/progenitor cells and their derivatives, in the treatment of cardiovascular disease. Here the strategies used to improve the stem/progenitor cell-based therapies in cardiovascular disease and the challenges with these therapies in clinical applications are also reviewed.

Protocol for MicroRNA Transfer into Adult Bone Marrow-derived Hematopoietic Stem Cells to Enable Cell Engineering Combined with Magnetic Targeting

While CD133⁺ hematopoietic stem cells (SCs) have been proven to provide high potential in the field of regenerative medicine, their low retention rates after injection into injured tissues as well as the observed massive cell death rates lead to very restricted therapeutic effects. To overcome these limitations, we sought to establish a non-viral based protocol for suitable cell engineering prior to their administration. The modification of human CD133⁺ expressing SCs using microRNA (miR) loaded magnetic polyplexes was addressed with respect to uptake efficiency and safety as well as the targeting potential of the cells. Relying on our protocol, we can achieve high miR uptake rates of 80-90% while the CD133⁺ stem cell properties remain unaffected. Moreover, these modified cells offer the option of magnetic targeting. We describe here a safe and highly efficient procedure for the modification of CD133⁺ SCs. We expect this approach to provide a standard technology for optimization of therapeutic stem cell effects and for monitoring of the administered cell product via magnetic resonance imaging (MRI).

Cellular therapies for chronic ischemic heart failure

The development of stem cell therapies for chronic ischemic heart failure is highly sought after to attempt to improve morbidity and mortality of this prevalent disease. This article reviews clinical trials that investigate stem cell therapy for chronic ischemic heart failure. To generate this review article, PubMed was searched using keywords "stem cell therapy heart failure" with the article type "Clinical Trial" selected on 10/04/2016. The raw search yielded 156 articles; 53 articles were selected for inclusion in the review between the original literature search and manual research/cross-referencing. Additional reviews and original articles were also manually researched and cross-referenced. Cellular-based therapies utilizing peripheral blood progenitor cells, bone marrow cells, mesenchymal stem cells, cells of cardiac origin, and embryonic stem cells have yielded mixed results, but some studies have shown modest efficacy. Skeletal myoblasts raised concerns about safety due to arrhythmias. Optimizing cell type and delivery method will be of critical importance in enhancing efficacy of therapy within various subsets of chronic ischemic heart failure patients. Although much more work needs to be done to optimize treatment strategies, developing stem cell therapies for chronic ischemic heart failure could be of critical importance to lessen the impactful health burden that heart failure has on patients and society.

Endothelial Progenitor Cells for Ischemic Stroke: Update on Basic Research and Application

Ischemic stroke is one of the leading causes of human death and disability worldwide. So far, ultra-early thrombolytic therapy is the most effective treatment. However, most patients still live with varying degrees of neurological dysfunction due to its narrow therapeutic time window. It has been confirmed in many studies that endothelial progenitor cells (EPCs), as a kind of adult stem cells, can protect the neurovascular unit by repairing the vascular endothelium and its secretory function, which contribute to the recovery of neurological function after an ischemic stroke. This paper reviews the basic researches and clinical trials of EPCs especially in the field of ischemic stroke and addresses the combination of EPC application with new technologies, including neurovascular intervention, synthetic particles, cytokines, and EPC modification, with the aim of shedding some light on the application of EPCs in treating ischemic stroke in the future.

Cellular mechanisms underlying cardiac engraftment of stem cells

INTRODUCTION

Over the past decade, it has become clear that long-term engraftment of any ex vivo expanded cell product transplanted into injured myocardium is modest and all therapeutic regeneration is mediated by stimulation of endogenous repair rather than differentiation of transplanted cells into working myocardium. Given that increasing the retention of transplanted cells boosts myocardial function, focus on the fundamental mechanisms limiting retention and survival of transplanted cells may enable strategies to help to restore normal cardiac function. Areas covered: This review outlines the challenges confronting cardiac engraftment of ex vivo expanded cells and explores means of enhancing cell-mediated repair of injured myocardium. Expert opinion: Stem cell therapy has already come a long way in terms of regenerating damaged hearts though the poor retention of transplanted cells limits the full potential of truly cardiotrophic cell products. Multifaceted strategies directed towards fundamental mechanisms limiting the long-term survival of transplanted cells will be needed to enhance transplanted cell retention and cell-mediated repair of damaged myocardium for cardiac cell therapy to reach its full potential.

Time course of VCAM-1 expression in reperfused myocardial infarction in swine and its relation to retention of intracoronary administered bone marrow-derived mononuclear cells

Background

Intracoronary infusion of autologous bone marrow-derived mononuclear cells (BMMNC), after acute myocardial infarction (AMI), has been shown to improve myocardial function. However, therapeutic efficacy is limited, possibly because cell retention rates are low, suggesting that optimization of cell retention might increase therapeutic efficacy. Since retention of injected BMMNC is observed only within infarcted, but not remote, myocardium, we hypothesized that adhesion molecules on activated endothelium following reperfusion are essential. Consequently, we investigated the role of vascular cell adhesion molecule 1 (VCAM-1) in BMMNC retention in swine undergoing reperfused AMI produced by 120 min of percutaneous left circumflex coronary occlusion.

Methods and results

VCAM-1 expression in the infarct and remote region was quantified at 1, 3, 7, 14, and 35 days, post-reperfusion (n≥6 swine per group). Since expression levels were significantly higher at 3 days (2.41±0.62%) than at 7 days (0.98±0.28%; p<0.05), we compared the degree of cell retention at those time points in a follow-up study, in which an average of 43·10⁶ autologous BMMNCs were infused intracoronary at 3, or 7 days, post-reperfusion (n = 6 swine per group) and retention was histologically quantified one hour after intracoronary infusion of autologous BMMNCs. Although VCAM-1 expression correlated with retention of BMMNC within each time point, overall BMMNC retention was similar at day 3 and day 7 (2.3±1.3% vs. 3.1±1.4%, p = 0.72). This was not due to the composition of infused bone marrow cell fractions (analyzed with flow cytometry; n = 5 per group), as cell composition of the infused BMMNC fractions was similar.

Conclusion

These findings suggest that VCAM-1 expression influences to a small degree, but is not the principal determinant of, BMMNC retention.

Magnet-Bead Based MicroRNA Delivery System to Modify CD133+ Stem Cells

Aim. CD133⁺ stem cells bear huge potential for regenerative medicine. However, low retention in the injured tissue and massive cell death reduce beneficial effects. In order to address these issues, we intended to develop a nonviral system for appropriate cell engineering. Materials and Methods. Modification of human CD133⁺ stem cells with magnetic polyplexes carrying microRNA was studied in terms of efficiency, safety, and targeting potential. Results. High microRNA uptake rates (~80-90%) were achieved without affecting CD133⁺ stem cell properties. Modified cells can be magnetically guided. Conclusion. We developed a safe and efficient protocol for CD133⁺ stem cell modification. Our work may become a basis to improve stem cell therapeutical effects as well as their monitoring with magnetic resonance imaging.

In Vivo Tracking of Cell Therapies for Cardiac Diseases with Nuclear Medicine

Even though heart diseases are amongst the main causes of mortality and morbidity in the world, existing treatments are limited in restoring cardiac lesions. Cell transplantations, originally developed for the treatment of hematologic ailments, are presently being explored in preclinical and clinical trials for cardiac diseases. Nonetheless, little is known about the possible efficacy and mechanisms for these therapies and they are the center of continuous investigation. In this scenario, noninvasive imaging techniques lead to greater comprehension of cell therapies. Radiopharmaceutical cell labeling, firstly developed to track leukocytes, has been used successfully to evaluate the migration of cell therapies for myocardial diseases. A substantial rise in the amount of reports employing this methodology has taken place in the previous years. We will review the diverse radiopharmaceuticals, imaging modalities, and results of experimental and clinical studies published until now. Also, we report on current limitations and potential advances of radiopharmaceutical labeling for cell therapies in cardiac diseases.

Translational applications of molecular imaging in cardiovascular disease and stem cell therapy

Cardiovascular disease (CVD) is the leading cause of mortality and morbidity worldwide. Molecular imaging techniques provide valuable information at cellular and molecular level, as opposed to anatomical and structural layers acquired from traditional imaging modalities. More specifically, molecular imaging employs imaging probes which interact with specific molecular targets and therefore makes it possible to visualize biological processes in vivo. Molecular imaging technology is now progressing towards preclinical and clinical application that gives an integral and comprehensive guidance for the investigation of cardiovascular disease. In addition, cardiac stem cell therapy holds great promise for clinical translation. Undoubtedly, combining stem cell therapy with molecular imaging technology will bring a broad prospect for the study and treatment of cardiac disease. This review will focus on the progresses of molecular imaging strategies in cardiovascular disease and cardiac stem cell therapy. Furthermore, the perspective on the future role of molecular imaging in clinical translation and potential strategies in defining safety and efficacy of cardiac stem cell therapies will be discussed.

Stem cell imaging: from bench to bedside

Although cellular therapies hold great promise for the treatment of human disease, results from several initial clinical trials have not shown a level of efficacy required for their use as a first line therapy. Here we discuss how in vivo molecular imaging has helped identify barriers to clinical translation and potential strategies that may contribute to successful transplantation and improved outcomes, with a focus on cardiovascular and neurological diseases. We conclude with a perspective on the future role of molecular imaging in defining safety and efficacy for clinical implementation of stem cell therapies.

Imaging cardiac stem cell transplantation using radionuclide labeling techniques: clinical applications and future directions

Stem cell therapy is emerging as a potential new therapy for patients with advanced heart failure. In recent years, advances in molecular imaging have allowed monitoring of stem cell homing and survival. In this review article, we will discuss the clinical application and future directions of stem cell imaging in advanced heart failure.

Enhanced potency of cell-based therapy for ischemic tissue repair using an injectable bioactive epitope presenting nanofiber support matrix

The translation of cell-based therapies for ischemic tissue repair remains limited by several factors, including poor cell survival and limited target site retention. Advances in nanotechnology enable the development of specifically designed delivery matrices to address these limitations and thereby improve the efficacy of cell-based therapies. Given the relevance of integrin signaling for cellular homeostasis, we developed an injectable, bioactive peptide-based nanofiber matrix that presents an integrin-binding epitope derived from fibronectin, and evaluated its feasibility as a supportive artificial matrix for bone marrow-derived pro-angiogenic cells (BMPACs) used as a therapy in ischemic tissue repair. Incubation of BMPACs with these peptide nanofibers in vitro significantly attenuated apoptosis while enhancing proliferation and adhesion. Pro-angiogenic function was enhanced, as cells readily formed tubes. These effects were, in part, mediated via p38, and p44/p42 MAP kinases, which are downstream pathways of focal adhesion kinase. In a murine model of hind limb ischemia, an intramuscular injection of BMPACs within this bioactive peptide nanofiber matrix resulted in greater retention of cells, enhanced capillary density, increased limb perfusion, reduced necrosis/amputation, and preserved function of the ischemic limb compared to treatment with cells alone. This self-assembling, bioactive peptide nanofiber matrix presenting an integrin-binding domain of fibronectin improves regenerative efficacy of cell-based strategies in ischemic tissue by enhancing cell survival, retention, and reparative functions.

Improving cardiovascular outcomes in rheumatic diseases: therapeutic potential of circulating endothelial progenitor cells

Patients with Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE) have a significantly increased risk of cardiovascular disease (CVD). The reason for this is unclear but may be due, at least in part, to the failure of endothelial repair mechanisms. Over the last 15 years there has been much interest in the mechanisms of endothelial renewal and its potential as a therapy for CVD. In the circulation there are two distinct populations of cells; myeloid angiogenic cells (MACs) which augment repair by the paracrine secretion of angiogenic factors, and outgrowth endothelial cells (OECs) which are true endothelial progenitor cells (EPCs) and promote vasculogenesis by differentiating into mature endothelium. There are marked abnormalities in the number and function of these cells in patients with RA and SLE. Inflammatory cytokines including interferon-alpha (IFNα) and tumour-necrosis factor alpha (TNFα) both impair MAC and OEC function ex vivo and may therefore contribute to the CVD risk in these patients. Whilst administration of mononuclear cells, MACs and other progenitors has improved cardiovascular outcomes in the acute setting, this is not a viable option in chronic disease. The pharmacological manipulation of MAC/OEC function in vivo however has the potential to significantly improve endothelial repair and thus reduce CVD in this high risk population.

Molecular imaging: the key to advancing cardiac stem cell therapy

Cardiac stem cell therapy continues to hold promise for the treatment of ischemic heart disease despite the fact that early promising pre-clinical findings have yet to be translated into consistent clinical success. The latest human studies have collectively identified a pressing need to better understand stem cell behavior in humans and called for more incorporation of noninvasive imaging techniques into the design and evaluation of human stem cell therapy trials. This review discusses the various molecular imaging techniques validated to date for studying stem cells in living subjects, with a particular emphasis on their utilities in assessing the acute retention and the long-term survival of transplanted stem cells. These imaging techniques will be essential for advancing cardiac stem cell therapy by providing the means to both guide ongoing optimization and predict treatment response in humans.

Stem cells as a tool for breast imaging

Stem cells are a scientific field of interest due to their therapeutic potential. There are different groups, depending on the differentiation state. We can find lonely stem cells, but generally they distribute in niches. Stem cells don't survive forever. They are affected for senescence. Cancer stem cells are best defined functionally, as a subpopulation of tumor cells that can enrich for tumorigenic property and can regenerate heterogeneity of the original tumor. Circulating tumor cells are cells that have detached from a primary tumor and circulate in the bloodstream. They may constitute seeds for subsequent growth of additional tumors (metastasis) in different tissues. Advances in molecular imaging have allowed a deeper understanding of the in vivo behavior of stem cells and have proven to be indispensable in preclinical and clinical studies. One of the first imaging modalities for monitoring pluripotent stem cells in vivo, magnetic resonance imaging (MRI) offers high spatial and temporal resolution to obtain detailed morphological and functional information. Advantages of radioscintigraphic techniques include their picomolar sensitivity, good tissue penetration, and translation to clinical applications. Radionuclide imaging is the sole direct labeling technique used thus far in human studies, involving both autologous bone marrow derived and peripheral stem cells.

Molecular imaging of stem cells: tracking survival, biodistribution, tumorigenicity, and immunogenicity

Being able to self-renew and differentiate into virtually all cell types, both human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have exciting therapeutic implications for myocardial infarction, neurodegenerative disease, diabetes, and other disorders involving irreversible cell loss. However, stem cell biology remains incompletely understood despite significant advances in the field. Inefficient stem cell differentiation, difficulty in verifying successful delivery to the target organ, and problems with engraftment all hamper the transition from laboratory animal studies to human clinical trials. Although traditional histopathological techniques have been the primary approach for ex vivo analysis of stem cell behavior, these postmortem examinations are unable to further elucidate the underlying mechanisms in real time and in vivo. Fortunately, the advent of molecular imaging has led to unprecedented progress in understanding the fundamental behavior of stem cells, including their survival, biodistribution, immunogenicity, and tumorigenicity in the targeted tissues of interest. This review summarizes various molecular imaging technologies and how they have advanced the current understanding of stem cell survival, biodistribution, immunogenicity, and tumorigenicity.

Emerging roles for integrated imaging modalities in cardiovascular cell-based therapeutics: a clinical perspective

Despite preclinical promise, the progress of cell-based therapy to clinical cardiovascular practice has been slowed by several challenges and uncertainties that have been highlighted by the conflicting results of human trials. Most telling has been the revelation that current strategies fall short of achieving sufficient retention and engraftment of cells to meet the ambitious objective of myocardial regeneration. This has sparked novel research into the refinement of cell biology and delivery to overcome these shortcomings. Within this context, molecular imaging has emerged as a valuable tool for providing noninvasive surveillance of cell fate in vivo. Direct and indirect labelling of cells can be coupled with clinically relevant imaging modalities, such as radionuclide single photon emission computed tomography and positron emission tomography, and magnetic resonance imaging, to assess their short- and long-term distributions, along with their viability, proliferation and functional interaction with the host myocardium. This review details the strengths and limitations of the different cell labelling and imaging techniques and their potential application to the clinical realm. We also consider the broader, multifaceted utility of imaging throughout the cell therapy process, providing a discussion of its considerable value during cell delivery and its importance during the evaluation of cardiac outcomes in clinical studies.

Imaging cardiac stem cell therapy: translations to human clinical studies

Stem cell therapy promises to open exciting new options in the treatment of cardiovascular diseases. Although feasible and clinically safe, the in vivo behavior and integration of stem cell transplants still remain largely unknown. Thus, the development of innovative non-invasive imaging techniques capable of effectively tracking such therapy in vivo is vital for a more in-depth investigation into future clinical applications. Such imaging modalities will not only generate further insight into the mechanisms behind stem cell-based therapy, but also address some major concerns associated with translational cardiovascular stem cell therapy. In the present review, we summarize the principles underlying three major stem cell tracking methods: (1) radioactive labeling for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, (2) iron particle labeling for magnetic resonance imaging (MRI), and (3) reporter gene labeling for bioluminescence, fluorescence, MRI, SPECT, and PET imaging. We then discuss recent clinical studies that have utilized these modalities to gain biological insights into stem cell fate.

Endothelial progenitor cells as therapeutic agents in the microcirculation: an update

This review evaluates novel beneficial effects of circulating endothelial progenitor cells (EPCs) as shown by several preclinical studies and clinical trials carried out to test the safety and feasibility of using EPCs. There are 31 registered clinical trials (and many others still ongoing) and 19 published studies. EPCs originate in the bone marrow and migrate into the bloodstream where they undergo a differentiation program leading to major changes in their antigenic characteristics. EPCs lose typical progenitor markers and acquire endothelial markers, and two important receptors, (VEGFR and CXCR-4), which recruit circulating EPCs to damaged or ischemic microcirculatory (homing to damaged tissues) beds. Overall, therapeutic angiogenesis will likely change the face of regenerative medicine in the next decade with many patients worldwide predicted to benefit from these treatments.

Imaging of cellular therapies

Cellular therapy promises to revolutionize medicine, by restoring tissue and organ function, and combating key disorders including cancer. As with all major developments, new tools must be introduced to allow optimization. For cell therapy, the key tool is in vivo imaging for real time assessment of parameters such as cell localization, numbers and viability. Such data is critical to modulate and tailor the therapy for each patient. In this review, we discuss recent work in the field of imaging cell therapies in the clinic, including preclinical work where clinical examples are not yet available. Clinical trials in which transferred cells were imaged using magnetic resonance imaging (MRI), nuclear scintigraphy, single photon emission computed tomography (SPECT), and positron emission tomography (PET) are evaluated from an imaging perspective. Preclinical cell tracking studies that focus on fluorescence and bioluminescence imaging are excluded, as these modalities are generally not applicable to clinical cell tracking. In this review, we assess the advantages and drawbacks of the various imaging techniques available, focusing on immune cells, particularly dendritic cells. Both strategies of prelabeling cells before transplant and the use of an injectable label to target cells in situ are covered. Finally, we discuss future developments, including the emergence of multimodal imaging technology for cell tracking from the preclinical to the clinical realm.