Embryonic stem cell immunogenicity increases upon differentiation after transplantation into ischemic myocardium

Stem cell therapy for cardiovascular disease: the demise of alchemy and rise of pharmacology

Regenerative medicine holds great promise as a way of addressing the limitations of current treatments of ischaemic disease. In preclinical models, transplantation of different types of stem cells or progenitor cells results in improved recovery from ischaemia. Furthermore, experimental studies indicate that cell therapy influences a spectrum of processes, including neovascularization and cardiomyogenesis as well as inflammation, apoptosis and interstitial fibrosis. Thus, distinct strategies might be required for specific regenerative needs. Nonetheless, clinical studies have so far investigated a relatively small number of options, focusing mainly on the use of bone marrow-derived cells. Rapid clinical translation resulted in a number of small clinical trials that do not have sufficient power to address the therapeutic potential of the new approach. Moreover, full exploitation has been hindered so far by the absence of a solid theoretical framework and inadequate development plans. This article reviews the current knowledge on cell therapy and proposes a model theory for interpretation of experimental and clinical outcomes from a pharmacological perspective. Eventually, with an increased association between cell therapy and traditional pharmacotherapy, we will soon need to adopt a unified theory for understanding how the two practices additively interact for a patient's benefit.

Cardiac stem cell therapy: review of the native cardiac progenitor cells and future direction

Various stem cell types have been tested for regenerating damaged myocardium after myocardial infarction. However, the results of clinical trials have not been consistent, with only some of the trials reporting small improvements in cardiac function. It seems that engraftment and survival of injected cells is limited and transplanted stem cells either do not differentiate into cardiac cells or differentiate into only limited number of cardiac cells. The exact mechanism(s) of cardiac functional improvement by cell therapy are unclear, but paracrine effect may play a central role. The resident cardiac progenitor cells identified within the adult myocardium have distinct advantages over other stem cell types for cardiac cell therapy, as they are likely precommitted to the cardiovascular fate. However, isolating and expanding these cells from cardiac biopsies is a challenge. More recently, direct reprogramming of fibroblasts into cardiomyocytes has given new hope for myocardial regeneration. Here we will review different stem cells used in cardiac cell therapy with a focus on the native cardiac progenitor cells and briefly outline future directions of cardiac cell therapy.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

Immunological barriers to stem cell therapy in the central nervous system

The central nervous system is vulnerable to many neurodegenerative disorders such as Alzheimer's disease that result in the extensive loss of neuronal cells. Stem cells have the ability to differentiate into many types of cells, which make them ideal for treating such disorders. Although stem cell therapy has shown some promising results in animal models for many brain disorders it has yet to translate into the clinic. A major hurdle to the translation of stem cell therapy into the clinic is the immune response faced by stem cell transplants. Here, we focus on immunological and related hurdles to stem cell therapies for central nervous system disorders.

Immunogenicity and tumorigenicity of pluripotent stem cells and their derivatives: genetic and epigenetic perspectives

One aim of stem cell-based therapy is to utilize pluripotent stem cells (PSCs) as a supplementary source of cells to repair or replace tissues or organs that have ceased to function due to severe tissue damage. However, PSC-based therapy requires extensive research to ascertain if PSC derivatives are functional without the risk of tumorigenicity, and also do not engender severe immune rejection that threatens graft survival and function. Recently, the suitability of induced pluripotent stem cells applied for patient-tailored cell therapy has been questioned since the discovery of several genetic and epigenetic aberrations during the reprogramming process. Hence, it is crucial to understand the effect of these abnormalities on the immunogenicity and survival of PSC grafts. As induced PSC-based therapy represents a hallmark for the potential solution to prevent and arrest immune rejection, this review also summarizes several up-to-date key findings in the field.

Embryonic Stem Cells Promoting Macrophage Survival and Function are Crucial for Teratoma Development

Stem cell therapies have had tremendous potential application for many diseases in recent years. However, the tumorigenic properties of stem cells restrict their potential clinical application; therefore, strategies for reducing the tumorigenic potential of stem cells must be established prior to transplantation. We have demonstrated that syngeneic transplantation of embryonic stem cells (ESCs) provokes an inflammatory response that involves the rapid recruitment of bone marrow-derived macrophages (BMDMs). ESCs are able to prevent mature macrophages from macrophage colony-stimulating factor (M-CSF) withdrawal-induced apoptosis, and thus prolong macrophage lifespan significantly by blocking various apoptotic pathways in an M-CSF-independent manner. ESCs express and secrete IL-34, which may be responsible for ESC-promoted macrophage survival. This anti-apoptotic effect of ESCs involves activation of extracellular signal-regulated kinase (ERK)1/2 and PI3K/Akt pathways and thus, inhibition of ERK1/2 and PI3K/AKT activation decreases ESC-induced macrophage survival. Functionally, ESC-treated macrophages also showed a higher level of phagocytic activity. ESCs further serve to polarize BMDMs into M2-like macrophages that exhibit most tumor-associated macrophage phenotypic and functional features. ESC-educated macrophages produce high levels of arginase-1, Tie-2, and TNF-α, which participate in angiogenesis and contribute to teratoma progression. Our study suggests that induction of M2-like macrophage activation is an important mechanism for teratoma development. Strategies targeting macrophages to inhibit teratoma development would increase the safety of ESC-based therapies, inasmuch as the depletion of macrophages completely inhibits ESC-induced angiogenesis and teratoma development.

Pluripotent stem cell derived cardiomyocytes for cardiac repair

OPINION STATEMENT

The adult mammalian heart has limited capacity for regeneration, and any major injury such as a myocardial infarction results in the permanent loss of up to 1 billion cardiomyocytes. The field of cardiac cell therapy aims to replace these lost contractile units with de novo cardiomyocytes to restore lost systolic function and prevent progression to heart failure. Arguably, the ideal cell for this application is the human cardiomyocyte itself, which can electromechanically couple with host myocardium and contribute active systolic force. Pluripotent stem cells from human embryonic or induced pluripotent lineages are attractive sources for cardiomyocytes, and preclinical investigation of these cells is in progress. Recent work has focused on the efficient generation and purification of cardiomyocytes, tissue engineering efforts, and examining the consequences of cell transplantation from mechanical, vascular, and electrical standpoints. Here we discuss historical and contemporary aspects of pluripotent stem cell-based cardiac cell therapy, with an emphasis on recent preclinical studies with translational goals.

Renovation of the injured heart with myocardial tissue engineering

Tissue engineering aims to create, repair and/or replace tissues and organs by using cells, scaffolds, biologically active molecules and physiologic signals. It is an interdisciplinary field that integrates aspects of engineering, chemistry, biology and medicine. One of the most challenging goals in the field of cardiovascular tissue engineering is the creation of a heart muscle patch. This review describes the principles, achievements and challenges of achieving this ambitious goal of creating contractile heart muscle. In addition, the new strategy of in situ and injectable tissue engineering for myocardial repair and regeneration is presented.

Costimulation-adhesion blockade is superior to cyclosporine A and prednisone immunosuppressive therapy for preventing rejection of differentiated human embryonic stem cells following transplantation

RATIONALE

Human embryonic stem cell (hESC) derivatives are attractive candidates for therapeutic use. The engraftment and survival of hESC derivatives as xenografts or allografts require effective immunosuppression to prevent immune cell infiltration and graft destruction.

OBJECTIVE

To test the hypothesis that a short-course, dual-agent regimen of two costimulation-adhesion blockade agents can induce better engraftment of hESC derivatives compared to current immunosuppressive agents.

METHODS AND RESULTS

We transduced hESCs with a double fusion reporter gene construct expressing firefly luciferase (Fluc) and enhanced green fluorescent protein, and differentiated these cells to endothelial cells (hESC-ECs). Reporter gene expression enabled longitudinal assessment of cell engraftment by bioluminescence imaging. Costimulation-adhesion therapy resulted in superior hESC-EC and mouse EC engraftment compared to cyclosporine therapy in a hind limb model. Costimulation-adhesion therapy also promoted robust hESC-EC and hESC-derived cardiomyocyte survival in an ischemic myocardial injury model. Improved hESC-EC engraftment had a cardioprotective effect after myocardial injury, as assessed by magnetic resonance imaging. Mechanistically, costimulation-adhesion therapy is associated with systemic and intragraft upregulation of T-cell immunoglobulin and mucin domain 3 (TIM3) and a reduced proinflammatory cytokine profile.

CONCLUSIONS

Costimulation-adhesion therapy is a superior alternative to current clinical immunosuppressive strategies for preventing the post-transplant rejection of hESC derivatives. By extending the window for cellular engraftment, costimulation-adhesion therapy enhances functional preservation following ischemic injury. This regimen may function through a TIM3-dependent mechanism.

Stem cell therapy to protect and repair the developing brain: a review of mechanisms of action of cord blood and amnion epithelial derived cells

In the research, clinical, and wider community there is great interest in the use of stem cells to reduce the progression, or indeed repair brain injury. Perinatal brain injury may result from acute or chronic insults sustained during fetal development, during the process of birth, or in the newborn period. The most readily identifiable outcome of perinatal brain injury is cerebral palsy, however, this is just one consequence in a spectrum of mild to severe neurological deficits. As we review, there are now clinical trials taking place worldwide targeting cerebral palsy with stem cell therapies. It will likely be many years before strong evidence-based results emerge from these trials. With such trials underway, it is both appropriate and timely to address the physiological basis for the efficacy of stem-like cells in preventing damage to, or regenerating, the newborn brain. Appropriate experimental animal models are best placed to deliver this information. Cell availability, the potential for immunological rejection, ethical, and logistical considerations, together with the propensity for native cells to form teratomas, make it unlikely that embryonic or fetal stem cells will be practical. Fortunately, these issues do not pertain to the use of human amnion epithelial cells (hAECs), or umbilical cord blood (UCB) stem cells that are readily and economically obtained from the placenta and umbilical cord discarded at birth. These cells have the potential for transplantation to the newborn where brain injury is diagnosed or even suspected. We will explore the novel characteristics of hAECs and undifferentiated UCB cells, as well as UCB-derived endothelial progenitor cells (EPCs) and mesenchymal stem cells (MSCs), and how immunomodulation and anti-inflammatory properties are principal mechanisms of action that are common to these cells, and which in turn may ameliorate the cerebral hypoxia and inflammation that are final pathways in the pathogenesis of perinatal brain injury.

Mechanistic molecular imaging of cardiac cell therapy for ischemic heart disease

Cell-based myocardial regeneration has emerged as a promising therapeutic option for ischemic heart disease, though not yet at the level of routine clinical utility. Despite the encouraging results from initial preclinical studies that have demonstrated improved function and reduced infarct size of the ischemic myocardium following several candidate cell transplantation, the beneficial effects and molecular mechanisms of cardiac cell therapy are still unclear in clinical applications to date, and much remains to be optimized. To improve engraftment, accurate methods are required for tracking cell fate and quantifying functional outcome. In the present review, we summarized the current status and challenges of cardiac cell therapy for ischemic heart disease and discussed the strengths and limitations of currently available in vivo imaging techniques with special focus on the newly developed multimodality approaches for assessing the efficacy of engrafted donor cells. We also addressed the hurdles these imaging modalities are facing, including issues regarding immunogenicity and tumorigenicity of transplanted stem cells, and provided some the future perspectives on stem cell imaging.

Immunogenicity of embryonic stem cell-derived progenitors after transplantation

PURPOSE OF REVIEW

This review focuses on the immunogenicity of embryonic stem cell (ESC)-derived progenitors and the impact of the immune response on applications of cell replacement therapy (CRT). Possible strategies to induce immunological tolerance to ESC-derived progenitor cells will also be discussed.

RECENT FINDINGS

Evidence for the differential epigenetic control of major histocompatibility (MHC) and antigen processing molecules in ESCs and differentiated ESCs has recently been described. The presence of T cells recognizing the pluripotency-associated transcription factor octamer-binding transcription factor 4 (OCT4) in healthy patient-derived peripheral blood mononuclear cells adds further complexity to the immune response against ESCs and ESC-derived progenitors.

SUMMARY

Although ESCs and ESC-derived progenitors appear to exert some level of immune privilege in specific circumstances, these allogeneic cells are indeed recognized by the immune system and can be subject to mechanisms of rejection. Herein, we discuss the importance of the recent reports describing an immunosuppressive capacity of ESCs, and the epigenetic control of MHC in ESCs and how these characteristics may be harnessed in the development of strategies to induce immunological tolerance.

Molecular imaging of induced pluripotent stem cell immunogenicity with in vivo development in ischemic myocardium

Whether differentiation of induced pluripotent stem cells (iPSCs) in ischemic myocardium enhances their immunogenicity, thereby increasing their chance for rejection, is unclear. Here, we dynamically demonstrated the immunogenicity and rejection of iPSCs in ischemic myocardium using bioluminescent imaging (BLI). Murine iPSCs were transduced with a tri-fusion (TF) reporter gene consisting of firefly luciferase-red fluorescent protein-truncated thymidine kinase (fluc-mrfp-tTK). Ascorbic acid (Vc) were used to induce iPSCs to differentiate into cardiomyocytes (CM). iPSCs and iPS-CMs were intramyocardially injected into immunocompetent or immunosuppressed allogenic murine with myocardial infarction. BLI was performed to track transplanted cells. Immune cell infiltration was evaluated by immunohistochemistry. Syngeneic iPSCs were also injected and evaluated. The results demonstrated that undifferentiated iPSCs survived and proliferated in allogenic immunocompetent recipients early post-transplantation, accompanying with mild immune cell infiltration. With in vivo differentiation, a progressive immune cell infiltration could be detected. While transplantation of allogenic iPSC-CMs were observed an acute rejection from receipts. In immune-suppressed recipients, the proliferation of iPSCs could be maintained and intramyocardial teratomas were formed. Transplantation of syngeneic iPSCs and iPSC-CMs were also observed progressive immune cell infiltration. This study demonstrated that iPSC immunogenicity increases with in vivo differentiation, which will increase their chance for rejection in iPSC-based therapy.

Evaluation of immunogenicity of rat ES-cell derived endothelial cells

Evaluation of the immunogenicity of embryonic stem cell derived differentiated cells is important for their potential application in cell replacement therapies and transplantations. Low immunogenicity or even an immune privileged status would enable their general use in allogeneic settings and therefore supply an unrestricted source. Based on conflicting data in terms of immunogenicity published for mouse and human ES-derived cells, the rat model was used to complement the knowledge in this specific area by a set of in vitro test systems using endothelial ES cell derivatives.This chapter describes the strategies and methods used to analyze immunogenicity of rat ES cell derived endothelial cells (RESC) in comparison to adult mature rat endothelial cells (EC). In a first characterization step, the endothelial nature of rat ES cell derived endothelial cells was proved by labelling with von Willebrand factor (vWF) as well as testing tube formation capacity on an extracellular matrix. The RESC can be characterized by their constitutive or cytokine-induced expression level of the Major Histocompatibility Complex (MHC) antigens class I and class II by Fluorescence Activated Cell Sorter (FACS) technology. Moreover, regulation of transcription factors involved in the IFNγ signaling pathway could be evaluated by detecting either the phosphorylation status by specific intracellular antibody staining followed by flow cytometric measurement or by analyzing the mRNA expression level by quantitative RT-PCR. By stimulating the RESC with IFNγ and coculturing with Carboxy-fluorescein diacetate succinimidyl ester (CFDA-SE)-labelled CD4(+) rat T cells, the ability of RESC to induce proliferation was analyzed by FACS technology. Allo-reactive cytotoxic T cells were generated in a mixed lymphocyte culture (MLC) with lymph node cells from two MHC-disparate rat strains and used to determine the susceptibility of RESC to lytic processes. Therefore, RESC were labelled with calcein and the release of this fluorochrome after coculture was measured. To analyze the response to humoral attacks, RESC were incubated with allo-antibody containing sera and rabbit complement and then cell damage was assessed by 7-actinomycin D (7-AAD) incorporation into the DNA using FACS analysis.

Immunogenicity of pluripotent stem cells and their derivatives

The ability of pluripotent stem cells to self-renew and differentiate into all somatic cell types brings great prospects to regenerative medicine and human health. However, before clinical applications, much translational research is necessary to ensure that their therapeutic progenies are functional and nontumorigenic, that they are stable and do not dedifferentiate, and that they do not elicit immune responses that could threaten their survival in vivo. For this, an in-depth understanding of their biology, genetic, and epigenetic make-up and of their antigenic repertoire is critical for predicting their immunogenicity and for developing strategies needed to assure successful long-term engraftment. Recently, the expectation that reprogrammed somatic cells would provide an autologous cell therapy for personalized medicine has been questioned. Induced pluripotent stem cells display several genetic and epigenetic abnormalities that could promote tumorigenicity and immunogenicity in vivo. Understanding the persistence and effects of these abnormalities in induced pluripotent stem cell derivatives is critical to allow clinicians to predict graft fate after transplantation, and to take requisite measures to prevent immune rejection. With clinical trials of pluripotent stem cell therapy on the horizon, the importance of understanding immunologic barriers and devising safe, effective strategies to bypass them is further underscored. This approach to overcome immunologic barriers to stem cell therapy can take advantage of the validated knowledge acquired from decades of hematopoietic stem cell transplantation.

Human iPSC-derived mesoangioblasts, like their tissue-derived counterparts, suppress T cell proliferation through IDO- and PGE-2-dependent pathways

Human mesoangioblasts are currently in a phase I/II clinical trial for the treatment of patients with Duchenne muscular dystrophy. However, limitations associated with the finite life span of these cells combined with the significant numbers of mesoangioblasts required to treat all of the skeletal muscles in these patients restricts their therapeutic potential. Induced pluripotent stem cell (iPSC)-derived mesoangioblasts may provide the solution to this problem. Although, the idea of using iPSC-derived cell therapies has been proposed for quite some time, our understanding of how the immune system interacts with these cells is inadequate. Herein, we show that iPSC-derived mesoangioblasts (HIDEMs) from healthy donors and, importantly, limb-girdle muscular dystrophy 2D patients exert immunosuppressive effects on T cell proliferation.  Interferon gamma (IFN-γ) and tumour necrosis factor alpha (TNF-α) play crucial roles in the initial activation of HIDEMs and importantly indoleamine 2,3 dioxygenase (IDO) and prostaglandin E2 (PGE-2) were identified as key mechanisms involved in HIDEM suppression of T cell proliferation. Together with recent studies confirming the myogenic function and regenerative potential of these cells, we suggest that HIDEMs could provide an unlimited alternative source for mesoangioblast-based therapies.

Minimally invasive cell-seeded biomaterial systems for injectable/epicardial implantation in ischemic heart disease

Myocardial infarction (MI) is characterized by heart-wall thinning, myocyte slippage, and ventricular dilation. The injury to the heart-wall muscle after MI is permanent, as after an abundant cell loss the myocardial tissue lacks the intrinsic capability to regenerate. New therapeutics are required for functional improvement and regeneration of the infarcted myocardium, to overcome harmful diagnosis of patients with heart failure, and to overcome the shortage of heart donors. In the past few years, myocardial tissue engineering has emerged as a new and ambitious approach for treating MI. Several left ventricular assist devices and epicardial patches have been developed for MI. These devices and acellular/cellular cardiac patches are employed surgically and sutured to the epicardial surface of the heart, limiting the region of therapeutic benefit. An injectable system offers the potential benefit of minimally invasive release into the myocardium either to restore the injured extracellular matrix or to act as a scaffold for cell delivery. Furthermore, intramyocardial injection of biomaterials and cells has opened new opportunities to explore and also to augment the potentials of this technique to ease morbidity and mortality rates owing to heart failure. This review summarizes the growing body of literature in the field of myocardial tissue engineering, where biomaterial injection, with or without simultaneous cellular delivery, has been pursued to enhance functional and structural outcomes following MI. Additionally, this review also provides a complete outlook on the tissue-engineering therapies presently being used for myocardial regeneration, as well as some perceptivity into the possible issues that may hinder its progress in the future.

Generation of skeletal muscle cells from embryonic and induced pluripotent stem cells as an in vitro model and for therapy of muscular dystrophies

Muscular dystrophies (MDs) are a heterogeneous group of inherited disorders characterized by progressive muscle wasting and weakness likely associated with exhaustion of muscle regeneration potential. At present, no cures or efficacious treatments are available for these diseases, but cell transplantation could be a potential therapeutic strategy. Transplantation of myoblasts using satellite cells or other myogenic cell populations has been attempted to promote muscle regeneration, based on the hypothesis that the donor cells repopulate the muscle and contribute to its regeneration. Embryonic stem cells (ESCs) and more recently induced pluripotent stem cells (iPSCs) could generate an unlimited source of differentiated cell types, including myogenic cells. Here we review the literature regarding the generation of myogenic cells considering the main techniques employed to date to elicit efficient differentiation of human and murine ESCs or iPSCs into skeletal muscle. We also critically analyse the possibility of using these cellular populations as an alternative source of myogenic cells for cell therapy of MDs.

Immune response to human embryonic stem cell-derived cardiac progenitors and adipose-derived stromal cells

Transplantation of allogeneic human embryonic stem cell-derived cardiac progenitors triggers an immune response. We assessed whether this response could be modulated by the concomitant use of adipose-derived stromal cells (ADSC). Peripheral blood mononuclear cells were collected from 40 patients with coronary artery disease (CAD) and nine healthy controls. Cardiac progenitors (CD15⁺ Mesp1⁺) were generated as already reported from the I6 cell line treated with bone morphogenetic protein (BMP)-2. Adipose-derived stromal cells were obtained from abdominal dermolipectomies. We assessed the proliferative response of peripheral lymphocytes from patients and controls to cardiac progenitors cultured on a monolayer of ADSC, to allogeneic lymphocytes in mixed lymphocyte culture and to the T cell mitogen phytohemaglutin A in presence or absence of ADSC. Cardiac progenitors cultured on a monolayer of ADSC triggered a proliferation of lymphocytes from both patients and controls albeit lower than that induced by allogeneic lymphocytes. When cultured alone, ADSC did not induce any proliferation of allogeneic lymphocytes. When added to cultures of lymphocytes, ADSC significantly inhibited the alloantigen or mitogen-induced proliferative response. Compared to healthy controls, lymphocytes from patients presenting CAD expressed a decreased proliferative capacity, in particular to mitogen-induced stimulation. Adipose-derived stromal cells express an immunomodulatory effect that limits both alloantigen and mitogen-induced lymphocyte responses. Furthermore, lymphocytes from patients with CAD are low responders to conventional stimuli, possibly because of their age and disease-associated treatment regimens. We propose that, in combination, these factors may limit the in vivo immunogenicity of cardiac progenitors co-implanted with ADSC in patients with CAD.

From pluripotency to distinct cardiomyocyte subtypes

Differentiated adult cardiomyocytes (CMs) lack significant regenerative potential, which is one reason why degenerative heart diseases are the leading cause of death in the western world. For future cardiac repair, stem cell-based therapeutic strategies may become alternatives to donor heart transplantation. The principle of reprogramming adult terminally differentiated cells (iPSC) had a major impact on stem cell biology. One can now generate autologous pluripotent cells that highly resemble embryonic stem cells (ESC) and that are ethically inoffensive as opposed to human ESC. Yet, due to genetic and epigenetic aberrations arising during the full reprogramming process, it is questionable whether iPSC will enter the clinic in the near future. Therefore, the recent achievement of directly reprogramming fibroblasts into cardiomyocytes via a milder approach, thereby avoiding an initial pluripotent state, may become of great importance. In addition, various clinical scenarios will depend on the availability of specific cardiac cellular subtypes, for which a first step was achieved via our own programming approach to achieve cardiovascular cell subtypes. In this review, we discuss recent progress in the cardiovascular stem cell field addressing the above mentioned aspects.

Mesoangioblasts suppress T cell proliferation through IDO and PGE-2-dependent pathways

Human mesoangioblasts are vessel-associated stem cells that are currently in phase I/II clinical trials for the treatment of patients with Duchenne muscular dystrophy. To date, little is known about the effect of mesoangioblasts on human immune cells and vice versa. We hypothesized that mesoangioblasts could modulate the function of immune cells in a similar manner to mesenchymal stromal cells. Human mesoangioblasts did not evoke, but rather potently suppressed human T-cell proliferation and effector function in vitro in a dose- and time-dependent manner. Furthermore, mesoangioblasts exert these inhibitory effects uniformly on human CD4+ and CD8+ T cells in a reversible manner without inducing a state of anergy. Interferon (IFN)-γ and tumor necrosis factor (TNF)-α play crucial roles in the initial activation of mesoangioblasts. Indoleamine 2,3-dioxygenase (IDO) and prostaglandin E-2 (PGE) were identified as key mechanisms of action involved in the mesoangioblast suppression of T-cell proliferation. Together, these data demonstrate a previously unrecognized capacity of mesoangioblasts to modulate immune responses.

Concise review: Immune recognition of induced pluripotent stem cells

Autologous-induced pluripotent stem cells (iPSCs) may eventually be used in cell replacement therapies to treat a wide range of diseases and have been touted as a solution to the vexing problem of immune rejection in this context. Emerging evidence suggests, however, that ostensibly histocompatible iPSCs may be rejected following transplantation. Here, we review the mechanisms that contribute to immunogenicity in iPSCs and forward approaches to permit their acceptance in potential cell replacement therapies.

Anti-HLA antibodies in regenerative medicine stem cell therapy

Research on stem cell therapies for regenerative medicine is progressing rapidly. Although the use of autologous stem cells is a tempting choice, there are several instances in which they are either defective or not available in due time. Allogenic stem cells derived from healthy donors presents a promising alternative. Whether autologous or allogenic, recent advances have proven that stem cells are not as immune privileged as they were thought. Therefore understanding the interactions of these cells with the recipient immune system is paramount to their clinical application. Transplantation of stem cells induces humoral as well as cellular immune response. This review focuses on the humoral response elicited by stem cells upon their administration and consequences on the survival and maintenance of the graft. Current transplantation identifies pre- and post-transplantation anti-HLA antibodies as immune rejection and cell signaling effectors. These two mechanisms are likely to operate similarly in the context of SC therapeutics. Ultimately this knowledge will help to propose novel strategies to mitigate the allogenic barriers. Immunogenetics selection of the donor cell and immunomonitoring are key factors to allow the implementation of regenerative stem cell in the clinics.

Challenges to the clinical application of pluripotent stem cells: towards genomic and functional stability

Human embryonic stem cells (hESCs) can undergo unlimited self-renewal and are pluripotent, retaining the ability to differentiate into all cell types in the body. As a renewable source of various types of human cells, hESCs hold great therapeutic potential. Although significant advances have been achieved in defining the conditions needed to differentiate hESCs into various types of biologically active cells, many challenges remain in the clinical development of hESC-based cell therapy, such as the immune rejection of allogeneic hESC-derived cells by recipients. Breakthroughs in the generation of induced pluripotent stem cells (iPSCs), which are reprogrammed from somatic cells with defined factors, raise the hope that autologous cells derived from patient-specific iPSCs can be transplanted without immune rejection. However, recent genomic studies have revealed epigenetic and genetic abnormalities associated with induced pluripotency, a risk of teratomas, and immunogenicity of some iPSC derivatives. These findings have raised safety concerns for iPSC-based therapy. Here, we review recent advances in understanding the genomic and functional stability of human pluripotent stem cells, current challenges to their clinical application and the progress that has been made to overcome these challenges.

Nonclinical safety strategies for stem cell therapies

Recent breakthroughs in stem cell biology, especially the development of the induced pluripotent stem cell techniques, have generated tremendous enthusiasm and efforts to explore the therapeutic potential of stem cells in regenerative medicine. Stem cell therapies are being considered for the treatment of degenerative diseases, inflammatory conditions, cancer and repair of damaged tissue. The safety of a stem cell therapy depends on many factors including the type of cell therapy, the differentiation status and proliferation capacity of the cells, the route of administration, the intended clinical location, long term survival of the product and/or engraftment, the need for repeated administration, the disease to be treated and the age of the population. Understanding the product profile of the intended therapy is crucial to the development of the nonclinical safety study design.

Autologous stem cells for personalised medicine

Increasing understanding of stem cell biology, the ability to reprogramme differentiated cells to a pluripotent state and evidence of multipotency in certain adult somatic stem cells has opened the door to exciting therapeutic advances as well as a great deal of regulatory and ethical issues. Benefits will come from the possibility of modelling human diseases and develop individualised therapies, and from their use in transplantation and bioengineering. The use of autologous stem cells is highly desirable, as it avoids the problem of tissue rejection, and also reduces ethical and regulatory issues. Identification of the most appropriate cell sources for different potential applications, development of appropriate clinical grade methodologies and large scale well controlled clinical trials will be essential to assess safety and value of cell based therapies, which have been generating much hope, but are by and large not yet close to becoming standard clinical practice. We briefly discuss stem cells in the context of tissue repair and regenerative medicine, with a focus on individualised clinical approaches, and give examples of sources of autologous cells with potential for clinical intervention.

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.

Embryonic stem cell-derived factors inhibit T effector activation and induce T regulatory cells by suppressing PKC-θ activation

Embryonic stem cells (ESCs) possess immune privileged properties and have the capacity to modulate immune activation. However, the mechanisms by which ESCs inhibit immune activation remain mostly unknown. We have previously shown that ESC-derived factors block dendritic cell maturation, thereby indirectly affecting T cell activation. Here, we show that ESC-derived factors also directly affect T cell activation. We provide the first demonstration that ESC-derived factors significantly down-regulated the expressions of IL-2 and IFN-γ, while markedly up-regulating the expression of IL-10, TGF-β, and Treg transcription factor Foxp3 in CD4+ CD25+ T cells. Furthermore, ESC-derived factors robustly suppressed T cell proliferation in response to the protein kinase C-θ (PKC-θ) activator phorbol 12-myristate 13-acetate (PMA). Western blot analysis indicated that ESC-derived factors prevented PKC-θ phosphorylation without influencing total PKC-θ levels. Moreover, IκB-α degradation was abrogated, confirming absence of PKC-θ activity. The impact of ESC-derived factors on PKC-θ activation appeared to be specific since other upstream T cell signaling components were not affected. In conclusion, ESCs appear to directly impact T cell activation and polarization by negatively regulating the PKC-θ pathway.

The potential of human fetal mesenchymal stem cells for off-the-shelf bone tissue engineering application

Mesenchymal stem cells (MSCs) have become one of the most promising cell sources for bone tissue engineering (BTE) applications. In this review, we first highlight recent progress in the understanding of MSC biology, their in vivo niche, multi-faceted contribution to fracture healing and bone re-modelling, and their role in BTE. A literature review from clinicaltrials.gov and Pubmed on clinical usage of MSC for both orthopedic and non-orthopedic indications suggests that translational use of MSC for BTE indications is likely to bear fruit in the ensuing decade. Last, we disscuss the profound influence of ontological and antomical origins of MSC on their proliferation and osteogenesis and demonstrated human fetal MSC (hfMSC) as a superior cellular candidate for off-the-shelf BTE applications. This relates to their superior proliferation capacity, more robust osteogenic potential and lower immunogenecity, as compared to MSC from perinatal and postnatal sources. Furthermore, we discuss our experience in developing a hfMSC based BTE strategy with the integrated use of bioreactor-based dynamic priming within macroporous scaffolds, now ready for evaluation in clinical trials. In conclusion, hfMSC is likely the most promising cell source for allogeneic based BTE application, with proven advantages compared to other MSC based ones.

Ceramide and sphingosine-1-phosphate signaling in embryonic stem cell differentiation

Recent studies show that bioactive lipids are important regulators for stem cell survival and differentiation. The sphingolipid ceramide and its derivative, sphingosine-1-phosphate (S1P), can act synergistically on embryonic stem (ES) cell differentiation. We show here simple methods to analyze sphingolipids in differentiating ES cells and to use ceramide and S1P analogs for the guided differentiation of mouse ES cells toward neuronal and glial lineage.

Mouse and human pluripotent stem cells and the means of their myogenic differentiation

Pluripotent stem cells, such as embryonic stem cells and induced pluripotent stem cells, are an important tool in the studies focusing at the differentiation of various cell types, including skeletal myoblasts. They are also considered as a source of the cells that due to their pluripotent character and availability could be turned into any required tissue and then used in future in regenerative medicine. However, the methods of the derivation of some of cell types from pluripotent cells still need to be perfected. This chapter summarizes the history and current advancements in the derivation and testing of pluripotent stem cells-derived skeletal myoblasts. It focuses at the in vitro methods allowing the differentiation of stem cells grown in monolayer or propagated as embryoid bodies, and also at in vivo tests allowing the verification of the functionality of obtained skeletal myoblasts.

Osteoblasts derived from induced pluripotent stem cells form calcified structures in scaffolds both in vitro and in vivo

Reprogramming somatic cells into an ESC-like state, or induced pluripotent stem (iPS) cells, has emerged as a promising new venue for customized cell therapies. In this study, we performed directed differentiation to assess the ability of murine iPS cells to differentiate into bone, cartilage, and fat in vitro and to maintain an osteoblast phenotype on a scaffold in vitro and in vivo. Embryoid bodies derived from murine iPS cells were cultured in differentiation medium for 8–12 weeks. Differentiation was assessed by lineage-specific morphology, gene expression, histological stain, and immunostaining to detect matrix deposition. After 12 weeks of expansion, iPS-derived osteoblasts were seeded in a gelfoam matrix followed by subcutaneous implantation in syngenic imprinting control region (ICR) mice. Implants were harvested at 12 weeks, histological analyses of cell and mineral and matrix content were performed. Differentiation of iPS cells into mesenchymal lineages of bone, cartilage, and fat was confirmed by morphology and expression of lineage-specific genes. Isolated implants of iPS cell-derived osteoblasts expressed matrices characteristic of bone, including osteocalcin and bone sialoprotein. Implants were also stained with alizarin red and von Kossa, demonstrating mineralization and persistence of an osteoblast phenotype. Recruitment of vasculature and microvascularization of the implant was also detected. Taken together, these data demonstrate functional osteoblast differentiation from iPS cells both in vitro and in vivo and reveal a source of cells, which merit evaluation for their potential uses in orthopedic medicine and understanding of molecular mechanisms of orthopedic disease.

Methods for the derivation and use of cardiomyocytes from human pluripotent stem cells

The availability of human cardiomyocytes derived from embryonic stem cells (ESCs) has generated -considerable excitement, as these cells are an excellent model system for studying myocardial development and may have eventual application in cell-based cardiac repair. Cardiomyocytes derived from the related induced pluripotent stem cells (iPSCs) have similar properties, but also offer the prospects of patient-specific disease modeling and cell therapies. Unfortunately, the methods by which cardiomyocytes have been historically generated from pluripotent stem cells are unreliable and typically result in preparations of low cardiac purity (typically <1% cardiomyocytes). We detail here the methods for a recently reported directed cardiac differentiation protocol, which involves the serial application of two growth factors known to be involved in early embryonic heart development, activin A, and bone morphogenetic protein-4 (BMP-4). This protocol reliably yields preparations of 30-60% cardiomyocytes, which can then be further enriched to >90% cardiomyocytes using straightforward physical methods.

Immunological considerations for embryonic and induced pluripotent stem cell banking

Recent advances in stem cell technology have generated enthusiasm for their potential to study and treat a diverse range of human disease. Pluripotent human stem cells for therapeutic use may, in principle, be obtained from two sources: embryonic stem cells (hESCs), which are capable of extensive self-renewal and expansion and have the potential to differentiate into any somatic tissue, and induced pluripotent stem cells (iPSCs), which are derived from differentiated tissue such as adult skin fibroblasts and appear to have the same properties and potential, but their generation is not dependent upon a source of embryos. The likelihood that clinical transplantation of hESC- or iPSC-derived tissues from an unrelated (allogeneic) donor that express foreign human leucocyte antigens (HLA) may undergo immunological rejection requires the formulation of strategies to attenuate the host immune response to transplanted tissue. In clinical practice, individualized iPSC tissue derived from the intended recipient offers the possibility of personalized stem cell therapy in which graft rejection would not occur, but the logistics of achieving this on a large scale are problematic owing to relatively inefficient reprogramming techniques and high costs. The creation of stem cell banks comprising HLA-typed hESCs and iPSCs is a strategy that is proposed to overcome the immunological barrier by providing HLA-matched (histocompatible) tissue for the target population. Estimates have shown that a stem cell bank containing around 10 highly selected cell lines with conserved homozygous HLA haplotypes would provide matched tissue for the majority of the UK population. These simulations have practical, financial, political and ethical implications for the establishment and design of stem cell banks incorporating cell lines with HLA types that are compatible with different ethnic populations throughout the world.

In vitro and in vivo cardiomyogenic differentiation of amniotic fluid stem cells

Cell therapy has developed as a complementary treatment for myocardial regeneration. While both autologous and allogeneic uses have been advocated, the ideal candidate has not been identified yet. Amniotic fluid-derived stem (AFS) cells are potentially a promising resource for cell therapy and tissue engineering of myocardial injuries. However, no information is available regarding their use in an allogeneic context. c-kit-sorted, GFP-positive rat AFS (GFP-rAFS) cells and neonatal rat cardiomyocytes (rCMs) were characterized by cytocentrifugation and flow cytometry for the expression of mesenchymal, embryonic and cell lineage-specific antigens. The activation of the myocardial gene program in GFP-rAFS cells was induced by co-culture with rCMs. The stem cell differentiation was evaluated using immunofluorescence, RT-PCR and single cell electrophysiology. The in vivo potential of Endorem-labeled GFP-rAFS cells for myocardial repair was studied by transplantation in the heart of animals with ischemia/reperfusion injury (I/R), monitored by magnetic resonance imaging (MRI). Three weeks after injection a small number of GFP-rAFS cells acquired an endothelial or smooth muscle phenotype and to a lesser extent CMs. Despite the low GFP-rAFS cells count in the heart, there was still an improvement of ejection fraction as measured by MRI. rAFS cells have the in vitro propensity to acquire a cardiomyogenic phenotype and to preserve cardiac function, even if their potential may be limited by poor survival in an allogeneic setting.

Immunological barriers to stem-cell based cardiac repair

Repair of damaged myocardium with pluripotent stem cell derived cardiomyocytes is becoming increasingly more feasible. Developments in stem cell research emphasize the need to address the foreseeable problem of immune rejection following transplantation. Pluripotent stem cell (PSC) derived cardiomyocytes have unique immune characteristics, some of which are not advantageous for transplantation. Here we review the possible mechanisms of PSC-derived cardiomyocytes rejection, summarize the current knowledge pertaining to immunogenicity of such cells and describe the existing controversies. Myocardial graft rejection can be reduced by modifying PSCs prior to their differentiation into cardiomyocytes. Overall, this approach facilitates the development of universal donor stem cells suitable for the regeneration of many different tissue types.

Human pluripotent stem cell-based approaches for myocardial repair: from the electrophysiological perspective

Heart diseases are a leading cause of mortality worldwide. Terminally differentiated adult cardiomyocytes (CMs) lack the innate ability to regenerate. Their malfunction or significant loss can lead to conditions from cardiac arrhythmias to heart failure. For myocardial repair, cell- and gene-based therapies offer promising alternatives to donor organ transplantation. Human embryonic stem cells (hESCs) can self-renew while maintaining their pluripotency. Direct reprogramming of adult somatic cells to become pluripotent hES-like cells (also known as induced pluripotent stem cells or iPSCs) has been achieved. Both hESCs and iPSCs have been successfully differentiated into genuine human CMs. In this review, we describe our current knowledge of the structure-function properties of hESC/iPSC-CMs, with an emphasis on their electrophysiology and Ca(2+) handling, along with the hurdles faced and potential solutions for translating into clinical and other applications (e.g., disease modeling, cardiotoxicity and drug screening).

MicroRNA-1 transfected embryonic stem cells enhance cardiac myocyte differentiation and inhibit apoptosis by modulating the PTEN/Akt pathway in the infarcted heart

microRNAs (miRs) have emerged as critical modulators of various physiological processes including stem cell differentiation. Indeed, miR-1 has been reported to play an integral role in the regulation of cardiac muscle progenitor cell differentiation. However, whether overexpression of miR-1 in embryonic stem (ES) cells (miR-1-ES cells) will enhance cardiac myocyte differentiation following transplantation into the infarcted myocardium is unknown. In the present study, myocardial infarction (MI) was produced in C57BL/6 mice by left anterior descending artery ligation. miR-1-ES cells, ES cells, or culture medium (control) was transplanted into the border zone of the infarcted heart, and 2 wk post-MI, cardiac myocyte differentiation, adverse ventricular remodeling, and cardiac function were assessed. We provide evidence demonstrating enhanced cardiac myocyte commitment of transplanted miR-1-ES cells in the mouse infarcted heart as compared with ES cells. Assessment of apoptosis revealed that overexpression of miR-1 in transplanted ES cells protected host myocardium from MI-induced apoptosis through activation of p-AKT and inhibition of caspase-3, phosphatase and tensin homolog, and superoxide production. A significant reduction in interstitial and vascular fibrosis was quantified in miR-1-ES cell and ES cell transplanted groups compared with control MI. However, no statistical significance between miR-1-ES cell and ES cell groups was observed. Finally, mice receiving miR-1-ES cell transplantation post-MI had significantly improved heart function compared with respective controls (P < 0.05). Our data suggest miR-1 drives cardiac myocyte differentiation from transplanted ES cells and inhibits apoptosis post-MI, ultimately giving rise to enhanced cardiac repair, regeneration, and function.

Induced pluripotent stem cells for regenerative cardiovascular therapies and biomedical discovery

The discovery of induced pluripotent stem cells (iPSC) has, in the short time since their discovery, revolutionized the field of stem cell biology. This technology allows the generation of a virtually unlimited supply of cells with pluripotent potential similar to that of embryonic stem cells (ESC). However, in contrast to ESC, iPSC are not subject to the same ethical concerns and can be easily generated from living individuals. For the first time, patient-specific iPSC can be generated and offer a supply of genetically identical cells that can be differentiated into all somatic cell types for potential use in regenerative therapies or drug screening and testing. As the techniques for generation of iPSC lines are constantly evolving, new uses for human iPSC are emerging from in-vitro disease modeling to high throughput drug discovery and screening. This technology promises to revolutionize the field of medicine and offers new hope for understanding and treatment of numerous diseases.

Intramyocardial transplantation of undifferentiated rat induced pluripotent stem cells causes tumorigenesis in the heart

Background

Induced pluripotent stem cells (iPSCs) are a novel candidate for use in cardiac stem cell therapy. However, their intrinsic tumorigenicity requires further investigation prior to use in a clinical setting. In this study we investigated whether undifferentiated iPSCs are tumorigenic after intramyocardial transplantation into immunocompetent allogeneic recipients.

Methodology/Principal Findings

We transplanted 2×10⁴, 2×10⁵, or 2×10⁶ cells from the established rat iPSC line M13 intramyocardially into intact or infarcted hearts of immunocompetent allogeneic rats. Transplant duration was 2, 4, or 6 weeks. Histological examination with hematoxylin-eosin staining confirmed that undifferentiated rat iPSCs could generate heterogeneous tumors in both intracardiac and extracardiac sites. Furthermore, tumor incidence was independent of cell dose, transplant duration, and the presence or absence of myocardial infarction.

Conclusions/Significance

Our study demonstrates that allogeneic iPSC transplantation in the heart will likely result in in situ tumorigenesis, and that cells leaked from the beating heart are a potential source of tumor spread, underscoring the importance of evaluating the safety of future iPSC therapy for cardiac disease.

Effects of histocompatibility and host immune responses on the tumorigenicity of pluripotent stem cells

Pluripotent stem cells hold great promises for regenerative medicine. They might become useful as a universal source for a battery of new cell replacement therapies. Among the major concerns for the clinical application of stem cell-derived grafts are the risks of immune rejection and tumor formation. Pluripotency and tumorigenicity are closely linked features of pluripotent stem cells. However, the capacity to form teratomas or other tumors is not sufficiently described by inherited features of a stem cell line or a stem cell-derived graft. The tumorigenicity always depends on the inability of the recipient to reject the tumorigenic cells. This review summarizes recent data on the tumorigenicity of pluripotent stem cells in immunodeficient, syngeneic, allogeneic, and xenogeneic hosts. The effects of immunosuppressive treatment and cell differentiation are discussed. Different immune effector mechanisms appear to be involved in the rejection of undifferentiated and differentiated cell populations. Elements of the innate immune system, such as natural killer cells and the complement system, which are active also in syngeneic recipients, appear to preferentially reject undifferentiated cells. This effect could reduce the risk of tumor formation in immunocompetent recipients. Cell differentiation apparently increases susceptibility to rejection by the adaptive immune system in allogeneic hosts. The current data suggest that the immune system of the recipient has a major impact on the outcome of pluripotent stem cell transplantation, whether it is rejection, engraftment, or tumor development. This has to be considered when the results of experimental transplantation models are interpreted and even more when translation into clinics is planned.

Postinfarction healing dynamics in the mechanically unloaded rat left ventricle

The healing process is a key determinant for postinfarction left ventricular (LV) remodeling and the development of heart failure, which could be influenced by mechanical (pressure and/or volume) load. So far, limited information exists regarding an indepth characterization of the postinfarct healing process in the mechanically unloaded state. In the present work, we performed isogenic Lewis-to-Lewis rat abdominal heterotopic heart transplantation, which is characterized by hemodynamic unloading in the left ventricle, and simultaneously ligated the left anterior descending coronary artery (T-infarct group). Pathological evolution was dynamically compared with that of in situ infarcted Lewis hearts (I-infarct group) on days 3, 7, 14, and 35. There was a remarkable myocardial salvage in the unloaded heart, as shown by the improvement in infarct size (T-infarct group: 25.47% ± 4.31% vs. I-infarct group: 38.46% ± 4.82%, P < 0.01) and the smaller fraction of fibrosis in infarct segments (T-infarct group: 42.12% ± 8.40% vs. I-infarct group: 75.65% ± 10.51%, P < 0.01). In addition, there was a progressive disorganization of the two-dimensional collagen fiber alignment as well as retarded collagen fiber maturation in the T-infarct group. We also observed enhanced angiogenesis, lymphangiogenesis, and inflammatory cell retention in the infarct region during mechanical unloading. Moreover, capillary density and collagen deposition were significantly increased in the noninfarcted area of the unloaded heart compared with the same region in the in situ infarcted heart. In conclusion, ischemic insult in the mechanically unloaded heart elicits an altered inflammatory and healing response, which is characterized by myocardial salvage, delayed resolution of inflammation, and disorganization of the collagen orientation in the infarcted region. These findings could provide novel insights into the contribution of hemodynamic load in the postinfarction healing process. Further studies are warranted to elucidate its potential mechanism.

Embryonic stem cells, derived either after in vitro fertilization or nuclear transfer, prolong survival of semiallogeneic heart transplants

Tolerance induction toward allogeneic organ grafts represents one of the major aims of transplantation medicine. Stem cells are promising candidates for promoting donor-specific tolerance. In this study, we investigated the immunomodulatory properties of murine embryonic stem cells (ESCs), obtained either by in vitro fertilization (IVF-ESCs) or by nuclear transfer (NT-ESCs), in heart transplant mouse models. IVF-ESCs did not prolong the survival of fully allogeneic cardiac transplants but significantly prolonged the survival of semiallogeneic hearts from the same ESC donor strain for >100 d in 44% of the animals. However, 28% of transplanted animals infused with IVF-ESCs experienced development of a teratoma. NT-ESCs similarly prolonged semiallogeneic heart graft survival (>100 d in 40% of the animals) but were less teratogenic. By in vitro studies, IVF-ESC and NT-ESC immunoregulation was mediated both by cell contact-dependent mechanisms and by the release of soluble factors. By adding specific inhibitors, we identified PGE(2) as a soluble mediator of ESC immunoregulation. Expansion of regulatory T cells was found in lymphoid organs and in the grafts of IVF-ESC- and NT-ESC-tolerized mice. Our study demonstrates that both IVF-ESCs and NT-ESCs modulate recipient immune response toward tolerance to solid organ transplantation, and that NT-ESCs exhibit a lower tendency for teratoma formation. Because NT-ESCs are obtained by NT of a somatic cell from living individuals into an enucleated oocyte, they could represent a source of donor-derived stem cells to induce tolerance to solid organ allograft.