Short-term immunosuppression promotes engraftment of embryonic and induced pluripotent stem cells

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Immunogenicity of in vitro maintained and matured populations: potential barriers to engraftment of human pluripotent stem cell derivatives

The potential to develop into any cell type makes human pluripotent stem cells (hPSCs) one of the most promising sources for regenerative treatments. Hurdles to their clinical applications include (1) formation of heterogeneously differentiated cultures, (2) the risk of teratoma formation from residual undifferentiated cells, and (3) immune rejection of engrafted cells. The recent production of human isogenic (genetically identical) induced PSCs (hiPSCs) has been proposed as a "solution" to the histocompatibility barrier. In theory, differentiated cells derived from patient-specific hiPSC lines should be histocompatible to their donor/recipient. However, propagation, maintenance, and non-physiologic differentiation of hPSCs in vitro may produce other, likely less powerful, immune responses. In light of recent progress towards the clinical application of hPSCs, this review focuses on two antigen presentation phenomena that may lead to rejection of isogenic hPSC derivates: namely, the expression of aberrant antigens as a result of long-term in vitro maintenance conditions or incomplete somatic cell reprogramming, and the unbalanced presentation of receptors and ligands involved in immune recognition due to accelerated differentiation. Finally, we discuss immunosuppressive approaches that could potentially address these immunological concerns.

Pluripotent stem cells: immune to the immune system?

Human embryonic stem cells (hESCs), initially thought to be immune privileged cells, are now known to be susceptible to immune recognition. Human induced pluripotent stem cells (iPSCs) have been proposed as a potential source of autologous stem cells for therapy, but even these autologous stem cells may be targets of immune rejection. With clinical trials on the horizon, it is imperative that the immunogenicity of hESCs and iPSCs be definitively understood.

Feasibility, safety, and therapeutic efficacy of human induced pluripotent stem cell-derived cardiomyocyte sheets in a porcine ischemic cardiomyopathy model

BACKGROUND

Human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) are a promising source of cells for regenerating myocardium. However, several issues, especially the large-scale preparation of hiPS-CMs and elimination of undifferentiated iPS cells, must be resolved before hiPS cells can be used clinically. The cell-sheet technique is one of the useful methods for transplanting large numbers of cells. We hypothesized that hiPS-CM-sheet transplantation would be feasible, safe, and therapeutically effective for the treatment of ischemic cardiomyopathy.

METHODS AND RESULTS

Human iPS cells were established by infecting human dermal fibroblasts with a retrovirus carrying Oct3/4, Sox2, Klf4, and c-Myc. Cardiomyogenic differentiation was induced by WNT signaling molecules, yielding hiPS-CMs that were almost 90% positive for α-actinin, Nkx2.5, and cardiac troponin T. hiPS-CM sheets were created using thermoresponsive dishes and transplanted over the myocardial infarcts in a porcine model of ischemic cardiomyopathy induced by ameroid constriction of the left anterior descending coronary artery (n=6 for the iPS group receiving sheet transplantation and the sham-operated group; both groups received tacrolimus daily). Transplantation significantly improved cardiac performance and attenuated left ventricular remodeling. hiPS-CMs were detectable 8 weeks after transplantation, but very few survived long term. No teratoma formation was observed in animals that received hiPS-CM sheets.

CONCLUSIONS

The culture system used yields a large number of highly pure hiPS-CMs, and hiPS-CM sheets could improve cardiac function after ischemic cardiomyopathy. This newly developed culture system and the hiPS-CM sheets may provide a basis for the clinical use of hiPS cells in cardiac regeneration therapy.

Challenges in allogeneic mesenchymal stem cell-mediated cardiac repair

Autologous mesenchymal stem cells (MSCs) have been proven safe in phase I and II clinical trials in patients who have suffered a myocardial infarction. However, their potential for proliferation and differentiation decreases with age, which limits their efficacy in elderly patients. Allogeneic MSCs offer several key advantages over autologous MSCs, including a high regenerative potential and availability for clinical use without the delay required for expansion. It was believed that allogeneic MSCs were immune privileged and thus able to escape the recipient's immune system. In several preclinical studies, allogeneic MSCs were successful in regenerating the myocardium, and the transplanted MSCs improved heart function early after implantation. However, the long-term ability of allogeneic MSCs to preserve heart function is limited because of a transition from an immune privileged to an immunogenic phenotype after the cells differentiate. The initial phase I/II clinical study using allogeneic MSCs in patients with acute myocardial infarction was safe, and no side effects were observed. However, the long-term safety and efficacy of allogeneic MSCs remain to be established. In this review, we discuss the challenges of using allogeneic MSCs for cardiac repair and present strategies to prevent the immune rejection of allogeneic MSCs to increase their potential for use in cardiac patients.

Characterization and therapeutic potential of induced pluripotent stem cell-derived cardiovascular progenitor cells

BACKGROUND

Cardiovascular progenitor cells (CPCs) have been identified within the developing mouse heart and differentiating pluripotent stem cells by intracellular transcription factors Nkx2.5 and Islet 1 (Isl1). Study of endogenous and induced pluripotent stem cell (iPSC)-derived CPCs has been limited due to the lack of specific cell surface markers to isolate them and conditions for their in vitro expansion that maintain their multipotency.

METHODOLOGY/PRINCIPAL FINDINGS

We sought to identify specific cell surface markers that label endogenous embryonic CPCs and validated these markers in iPSC-derived Isl1(+)/Nkx2.5(+) CPCs. We developed conditions that allow propagation and characterization of endogenous and iPSC-derived Isl1(+)/Nkx2.5(+) CPCs and protocols for their clonal expansion in vitro and transplantation in vivo. Transcriptome analysis of CPCs from differentiating mouse embryonic stem cells identified a panel of surface markers. Comparison of these markers as well as previously described surface markers revealed the combination of Flt1(+)/Flt4(+) best identified and facilitated enrichment for Isl1(+)/Nkx2.5(+) CPCs from embryonic hearts and differentiating iPSCs. Endogenous mouse and iPSC-derived Flt1(+)/Flt4(+) CPCs differentiated into all three cardiovascular lineages in vitro. Flt1(+)/Flt4(+) CPCs transplanted into left ventricles demonstrated robust engraftment and differentiation into mature cardiomyocytes (CMs).

CONCLUSION/SIGNIFICANCE

The cell surface marker combination of Flt1 and Flt4 specifically identify and enrich for an endogenous and iPSC-derived Isl1(+)/Nkx2.5(+) CPC with trilineage cardiovascular potential in vitro and robust ability for engraftment and differentiation into morphologically and electrophysiologically mature adult CMs in vivo post transplantation into adult hearts.

Induced pluripotent stem cells as a disease modeling and drug screening platform

Induced pluripotent stem cells (iPSCs) hold great hopes for therapeutic application in various diseases. Although ongoing research is dedicated to achieving clinical translation of iPSCs, further understanding of the mechanisms that underlie complex pathogenic conditions is required. Compared with other classical models for studying diseases, iPSCs provide considerable advantages. A newly emerging application of iPSCs is in vitro disease modeling, which can significantly improve the never-ending search for new pharmacological cures. Here, we will discuss current efforts to create iPSC-dependent patient-specific disease models. Furthermore, we will review the use of iPSCs for development and testing of new therapeutic agents and the implications for high-throughput drug screening.

Cardiac repair and restoration using human embryonic stem cells

Advances in directed differentiation of human embryonic stem cells (hESCs) toward cardiac lineages have generated much interest within the myocardial therapy field. Beyond the promise that hESCs would provide a supply of new cardiomyocytes to the damaged heart, recent studies have also shown that paracrine effects of stem cell therapy may facilitate myocardial healing. This review describes the advantages of hESCs for these purposes, current methods for directing differentiation of hESCs toward cardiac fates, approaches to purification and engineered selection of hESC-derived cardiomyocytes and cardiac precursors, as well as animal studies that have shed light on the therapeutic uses of hESCs in cardiac regenerative medicine.

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.

Microfluidic single-cell analysis shows that porcine induced pluripotent stem cell-derived endothelial cells improve myocardial function by paracrine activation

RATIONALE

Induced pluripotent stem cells (iPSCs) hold great promise for the development of patient-specific therapies for cardiovascular disease. However, clinical translation will require preclinical optimization and validation of large-animal iPSC models.

OBJECTIVE

To successfully derive endothelial cells from porcine iPSCs and demonstrate their potential utility for the treatment of myocardial ischemia.

METHODS AND RESULTS

Porcine adipose stromal cells were reprogrammed to generate porcine iPSCs (piPSCs). Immunohistochemistry, quantitative PCR, microarray hybridization, and angiogenic assays confirmed that piPSC-derived endothelial cells (piPSC-ECs) shared similar morphological and functional properties as endothelial cells isolated from the autologous pig aorta. To demonstrate their therapeutic potential, piPSC-ECs were transplanted into mice with myocardial infarction. Compared with control, animals transplanted with piPSC-ECs showed significant functional improvement measured by echocardiography (fractional shortening at week 4: 27.2±1.3% versus 22.3±1.1%; P<0.001) and MRI (ejection fraction at week 4: 45.8±1.3% versus 42.3±0.9%; P<0.05). Quantitative protein assays and microfluidic single-cell PCR profiling showed that piPSC-ECs released proangiogenic and antiapoptotic factors in the ischemic microenvironment, which promoted neovascularization and cardiomyocyte survival, respectively. Release of paracrine factors varied significantly among subpopulations of transplanted cells, suggesting that transplantation of specific cell populations may result in greater functional recovery.

CONCLUSIONS

In summary, this is the first study to successfully differentiate piPSCs-ECs from piPSCs and demonstrate that transplantation of piPSC-ECs improved cardiac function after myocardial infarction via paracrine activation. Further development of these large animal iPSC models will yield significant insights into their therapeutic potential and accelerate the clinical translation of autologous iPSC-based therapy.

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.

Cell therapy for left ventricular dysfunction: an overview for cardiac clinicians

Cell therapies specifically targeting heart failure could greatly decrease morbidity and burgeoning health care costs worldwide. Due to the great number of cell types being investigated, navigating the cardiovascular regeneration field can be difficult. This brief review gives an overview of the main cell types being explored for cardiac cell therapy. These include populations from extra-cardiac sources (skeletal myoblasts, bone marrow derived mononuclear cells, endothelial progenitor cells, bone marrow or adipose derived mesenchymal stem cells and embryonic or induced pluripotent stem cells as well as newly discovered cardiac stem cell populations (isl1(+), c-kit(+), sca1(+), sca1(+)/pdgfrα(+), cardiosphere derived, cardiac side-population and epicardium derived cells). Although clinical trials using both groups of cell sources have been performed, the vast majority of studies have used bone marrow mononuclear cells. The current wave of clinical trials includes large studies refining specifics of bone marrow mononuclear cell therapy and early phase trials of mesenchymal stem cell and cardiac stem cell populations. Embryonic stem cell derived therapies are being studied in large animal models with the aim of swift progression to clinical trials. Lessons learnt from the intense investigation in this infant field have resulted in rapid translational progress and it is likely that several clinical products/protocols for cardiac repair will be available in the not too distant future.

Imaging stem cell therapy for the treatment of peripheral arterial disease

Arteriosclerotic cardiovascular diseases are among the leading causes of morbidity and mortality worldwide. Therapeutic angiogenesis aims to treat ischemic myocardial and peripheral tissues by delivery of recombinant proteins, genes, or cells to promote neoangiogenesis. Concerns regarding the safety, side effects, and efficacy of protein and gene transfer studies have led to the development of cell-based therapies as alternative approaches to induce vascular regeneration and to improve function of damaged tissue. Cell-based therapies may be improved by the application of imaging technologies that allow investigators to track the location, engraftment, and survival of the administered cell population. The past decade of investigations has produced promising clinical data regarding cell therapy, but design of trials and evaluation of treatments stand to be improved by emerging insight from imaging studies. Here, we provide an overview of pre-clinical and clinical experience using cell-based therapies to promote vascular regeneration in the treatment of peripheral arterial disease. We also review four major imaging modalities and underscore the importance of in vivo analysis of cell fate for a full understanding of functional outcomes.

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.

Inducing iPSCs to escape the dish

Induced pluripotent stem cells (iPSCs) hold great promise for autologous cell therapies, but significant roadblocks remain to translating iPSCs to the bedside. For example, concerns about the presumed autologous transplantation potential of iPSCs have been raised by a recent paper demonstrating that iPSC-derived teratomas were rejected by syngeneic hosts. Additionally, the reprogramming process can alter genomic and epigenomic states, so a key goal at this point is to determine the clinical relevance of these changes and minimize those that prove to be deleterious. Finally, thus far few studies have examined the efficacy and tumorigenicity of iPSCs in clinically relevant transplantation scenarios, an essential requirement for the FDA. We discuss potential solutions to these hurdles to provide a roadmap for iPSCs to "jump the dish" and become useful therapies.

Stem cells and regenerative medicine - future perspectives

Stem cell research has expanded at an exponential rate, but its therapeutic applications have progressed much more slowly. Currently, the research focuses on understanding embryonic, adult, and inducible pluripotent stem cells. Translation of adult stem cell research has established a definitive benefit that is greater than that of the current standard of care in the field of cardiovascular medicine. The future of stem cell research and therapy will continue to provide novel avenues of diagnostics, therapeutics, and tissue regeneration. Here we discuss a brief history of stem cell research as it transitioned from the 20th to the 21st century. We address lessons learned in the first decade of the new millennium that could help guide others to translate research into therapy across disciplines. Finally, we highlight future goals and challenges that must be overcome and offer some perspective on the bright future of stem cell research and therapy.

Regenerating functional heart tissue for myocardial repair

Heart disease is one of the leading causes of death worldwide and the number of patients with the disease is likely to grow with the continual decline in health for most of the developed world. Heart transplantation is one of the only treatment options for heart failure due to an acute myocardial infarction, but limited donor supply and organ rejection limit its widespread use. Cellular cardiomyoplasty, or cellular implantation, combined with various tissue-engineering methods aims to regenerate functional heart tissue. This review highlights the numerous cell sources that have been used to regenerate the heart as well as cover the wide range of tissue-engineering strategies that have been devised to optimize the delivery of these cells. It will probably be a long time before an effective regenerative therapy can make a serious impact at the bedside.

Immunobiology of naïve and genetically modified HLA-class-I-knockdown human embryonic stem cells

Human embryonic stem cells (hESCs) can serve as a universal cell source for emerging cell or tissue replacement strategies, but immune rejection of hESC derivatives remains an unsolved problem. Here, we sought to describe the mechanisms of rejection for naïve hESCs and upon HLA class I (HLA I) knockdown (hESC(KD)). hESCs were HLA I-positive but negative for HLA II and co-stimulatory molecules. Transplantation of naïve hESC into immunocompetent Balb/c mice induced substantial T helper cell 1 and 2 (Th1 and Th2) responses with rapid cell death, but hESCs survived in immunodeficient SCID-beige recipients. Histology revealed mainly macrophages and T cells, but only scattered natural killer (NK) cells. A surge of hESC-specific antibodies against hESC class I, but not class II antigens, was observed. Using HLA I RNA interference and intrabody technology, HLA I surface expression of hESC(KD) was 88%-99% reduced. T cell activation after hESC(KD) transplantation into Balb/c was significantly diminished, antibody production was substantially alleviated, the levels of graft-infiltrating immune cells were reduced and the survival of hESC(KD) was prolonged. Because of their very low expression of stimulatory NK ligands, NK-susceptibility of naïve hESCs and hESC(KD) was negligible. Thus, HLA I recognition by T cells seems to be the primary mechanism of hESC recognition, and T cells, macrophages and hESC-specific antibodies participate in hESC killing.

Challenges for the Therapeutic use of Pluripotent Stem Derived Cells

Human embryonic stem cells (hESC) and induced pluripotent stem cells (hiPSC) are an attractive cell source for regenerative medicine. These cells can be expanded to vast numbers and can be differentiated to many desired pluripotent stem cells (PSC) derived therapeutic cells. Cell replacement bears promises, but also challenges. The introduction of exogenous cells in a recipient must address several different topics; its safety, the exclusion of tumor formation, the immunological response and possible rejection, the cells cleanliness and their biological quality, and quantity representing the functionality of the PSC derived therapeutic cells. Tumor formation requires the removal of any PSC remaining after differentiation. Immunological rejection can be addressed with immunomodulation of the cells and the recipient. Cleanliness can be optimized using good manufacturing practice quality systems. At last, the functionality of the cells must be tested in in vitro and in animal models. After addressing these challenges, precise strategies are developed to monitor the status of the cells at different times and in case of undesired results, corresponding counteracting strategies must exist before any clinical attempt.

Production of de novo cardiomyocytes: human pluripotent stem cell differentiation and direct reprogramming

Cardiovascular disease is a leading cause of death worldwide. The limited capability of heart tissue to regenerate has prompted methodological developments for creating de novo cardiomyocytes, both in vitro and in vivo. Beyond uses in cell replacement therapy, patient-specific cardiomyocytes may find applications in drug testing, drug discovery, and disease modeling. Recently, approaches for generating cardiomyocytes have expanded to encompass three major sources of starting cells: human pluripotent stem cells (hPSCs), adult heart-derived cardiac progenitor cells (CPCs), and reprogrammed fibroblasts. We discuss state-of-the-art methods for generating de novo cardiomyocytes from hPSCs and reprogrammed fibroblasts, highlighting potential applications and future challenges.

Promoting long-term survival of insulin-producing cell grafts that differentiate from adipose tissue-derived stem cells to cure type 1 diabetes

BACKGROUND

Insulin-producing cell clusters (IPCCs) have recently been generated in vitro from adipose tissue-derived stem cells (ASCs) to circumvent islet shortage. However, it is unknown how long they can survive upon transplantation, whether they are eventually rejected by recipients, and how their long-term survival can be induced to permanently cure type 1 diabetes. IPCC graft survival is critical for their clinical application and this issue must be systematically addressed prior to their in-depth clinical trials.

METHODOLOGY/PRINCIPAL FINDINGS

Here we found that IPCC grafts that differentiated from murine ASCs in vitro, unlike their freshly isolated islet counterparts, did not survive long-term in syngeneic mice, suggesting that ASC-derived IPCCs have intrinsic survival disadvantage over freshly isolated islets. Indeed, β cells retrieved from IPCC syngrafts underwent faster apoptosis than their islet counterparts. However, blocking both Fas and TNF receptor death pathways inhibited their apoptosis and restored their long-term survival in syngeneic recipients. Furthermore, blocking CD40-CD154 costimulation and Fas/TNF signaling induced long-term IPCC allograft survival in overwhelming majority of recipients. Importantly, Fas-deficient IPCC allografts exhibited certain immune privilege and enjoyed long-term survival in diabetic NOD mice in the presence of CD28/CD40 joint blockade while their islet counterparts failed to do so.

CONCLUSIONS/SIGNIFICANCE

Long-term survival of ASC-derived IPCC syngeneic grafts requires blocking Fas and TNF death pathways, whereas blocking both death pathways and CD28/CD40 costimulation is needed for long-term IPCC allograft survival in diabetic NOD mice. Our studies have important clinical implications for treating type 1 diabetes via ASC-derived IPCC transplantation.

Imaging: guiding the clinical translation of cardiac stem cell therapy

Stem cells have been touted as the holy grail of medical therapy, with promises to regenerate cardiac tissue, but it appears the jury is still out on this novel therapy. Using advanced imaging technology, scientists have discovered that these cells do not survive nor engraft long-term. In addition, only marginal benefit has been observed in large-animal studies and human trials. However, all is not lost. Further application of advanced imaging technology will help scientists unravel the mysteries of stem cell therapy and address the clinical hurdles facing its routine implementation. In this review, we will discuss how advanced imaging technology will help investigators better define the optimal delivery method, improve survival and engraftment, and evaluate efficacy and safety. Insights gained from this review may direct the development of future preclinical investigations and clinical trials.

Distinct strategies are required to suppress antigen-specific responses to genetically modified keratinocytes and fibroblasts

Keratinocytes and fibroblasts are potential targets of gene/cell therapy for genodermatoses. Immune elimination of genetically modified cells, however, presents a major impediment to effective therapy. Using ex vivo approaches to gene transfer, we have previously shown that expression of an antigen by either cell type in skin induces immune rejection of transplanted cells, although the nature of immune responses induced by these two cell types are distinct. In this study, we explore the efficacy of local immunosuppressive strategies to divert destructive immune responses from genetically modified fibroblast and keratinocytes. Expression of CTLA4Ig and, to a lesser extent, PDL1, by antigenic fibroblasts protected them from immune rejection resulting in long-term graft survival (>18 weeks). Similar treatment was not effective for antigenic keratinocytes. Long-term protection of transgenic keratinocytes was achieved through transient blockade of CD40/CD154 interactions during the first 2 weeks of cell transplantation. Although neither of these strategies induced antigen-specific tolerance, they were sufficient to prevent rejection of genetically modified cells. These results indicate that different strategies are required to protect antigenic cell types even within the same tissue. Moreover, induction of antigen-specific tolerance is not a necessary requirement for long-term survival of genetically modified skin cells.

Future of liver transplantation: Non-human primates for patient-specific organs from induced pluripotent stem cells

Strategies to fill the huge gap in supply versus demand of human organs include bioartificial organs, growing humanized organs in animals, cell therapy, and implantable bioengineered constructs. Reproducing the complex relations between different cell types, generation of adequate vasculature, and immunological complications are road blocks in generation of bioengineered organs, while immunological complications limit the use of humanized organs produced in animals. Recent developments in induced pluripotent stem cell (iPSC) biology offer a possibility of generating human, patient-specific organs in non-human primates (NHP) using patient-derived iPSC and NHP-derived iPSC lacking the critical developmental genes for the organ of interest complementing a NHP tetraploid embryo. The organ derived in this way will have the same human leukocyte antigen (HLA) profile as the patient. This approach can be curative in genetic disorders as this offers the possibility of gene manipulation and correction of the patient’s genome at the iPSC stage before tetraploid complementation. The process of generation of patient-specific organs such as the liver in this way has the great advantage of making use of the natural signaling cascades in the natural milieu probably resulting in organs of great quality for transplantation. However, the inexorable scientific developments in this direction involve several social issues and hence we need to educate and prepare society in advance to accept the revolutionary consequences, good, bad and ugly.

Safely modulating the immune system in regenerative medicine

The destruction of pluripotent stem cell-derived grafts by the host immune system presents a significant barrier to clinical translation of cell therapies. Pearl et al. (2011) report in this issue of Cell Stem Cell that a brief, nontoxic immunosuppressive regimen, achieved by blockade of leucocyte costimulatory pathways, may overcome this problem.

Imaging of Induced Pluripotent Stem Cells: From Cellular Reprogramming to Transplantation

Successful reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) ushered in a new era of regenerative medicine. Human iPSCs provide powerful new approaches for disease modeling, drug testing, developmental studies, and therapeutic applications. Investigating iPSC behavior in vivo and the ultimate feasibility of cell transplantation therapy necessitates the development of novel imaging techniques to longitudinally monitor iPSC localization, proliferation, integration, and differentiation in living subjects. At this five year mark of initial iPSC discovery, we review the current status of imaging iPSCs which ranges from in vitro studies, where imaging was used to study the processes/mechanisms of cellular reprogramming, to in vivo imaging of the survival of transplanted cells. To date, most imaging studies of iPSCs have been based on optical techniques, which include fluorescence and bioluminescence imaging. Since each imaging technique has its advantages and limitations, a combination of multiple imaging modalities may provide complementary information. The ideal imaging approach for tracking iPSCs or their derivatives in patients requires the imaging tag to be non-toxic, biocompatible, and highly specific to reduce perturbation of these cells. In few other scenarios can "personalized medicine" be better illustrated than the use of individual patient-specific iPSCs. Much future effort will be required before this can become a reality and clinical routine, where imaging will play an indispensible role in many facets of iPSC-based research and therapies.

Human embryonic stem cell-derived vascular smooth muscle cells in therapeutic neovascularisation

Ischemic diseases remain one of the major causes of morbidity and mortality throughout the world. In recent clinical trials on cell-based therapies, the use of adult stem and progenitor cells only elicited marginal benefits. Therapeutic neovascularisation is the Holy Grail for ischemic tissue recovery. There is compelling evidence from animal transplantation studies that the inclusion of mural cells in addition to endothelial cells (ECs) can enhance the formation of functional blood vessels. Vascular smooth muscle cells (SMCs) and pericytes are essential for the stabilisation of nascent immature endothelial tubes. Despite the intense interest in the utility of human embryonic stem cells (ESCs) for vascular regenerative medicine, ESC-derived vascular SMCs have received much less attention than ECs. This review begins with developmental insights into a range of smooth muscle progenitors from studies on embryos and ESC differentiation systems. We then summarise the methods of derivation of smooth muscle progenitors and cells from human ESCs. The primary emphasis is on the inherent heterogeneity of smooth muscle progenitors and cells and the limitations of current in vitro characterisation. Essential transplantation issues such as the type and source of therapeutic cells, mode of cell delivery, measures to enhance cell viability, putative mechanisms of benefit and long-term tracking of cell fate are also discussed. Finally, we highlight the challenges of clinical compatibility and scaling up for medical use in order to eventually realise the goal of human ESC-based vascular regenerative medicine.

Harnessing the potential of induced pluripotent stem cells for regenerative medicine

The discovery of methods to convert somatic cells into induced pluripotent stem cells (iPSCs) through expression of a small combination of transcription factors has raised the possibility of producing custom-tailored cells for the study and treatment of numerous diseases. Indeed, iPSCs have already been derived from patients suffering from a large variety of disorders. Here we review recent progress that has been made in establishing iPSC-based disease models, discuss associated technical and biological challenges, and highlight possible solutions to overcome these barriers. We believe that a better understanding of the molecular basis of pluripotency, cellular reprogramming and lineage-specific differentiation of iPSCs is necessary for progress in regenerative medicine.

Derivation of human induced pluripotent stem cells for cardiovascular disease modeling

The successful derivation of human induced pluripotent stem cells (hiPSCs) by dedifferentiation of somatic cells offers significant potential to overcome obstacles in the field of cardiovascular disease. hiPSC derivatives offer incredible potential for new disease models and regenerative medicine therapies. However, many questions remain regarding the optimal starting materials and methods to enable safe, efficient derivation of hiPSCs suitable for clinical applications. Initial reprogramming experiments were carried out using lentiviral or retroviral gene delivery methods. More recently, various nonviral methods that avoid permanent and random transgene insertion have emerged as alternatives. These include transient DNA transfection using plasmids or minicircles, protein transduction, or RNA transfection. In addition, several small molecules have been found to significantly augment hiPSC derivation efficiency, allowing the use of a fewer number of genes during pluripotency induction. We review these various methods for the derivation of hiPSCs, focusing on their ultimate clinical applicability, with an emphasis on their potential for use as cardiovascular therapies and disease-modeling platforms.

In vivo bioluminescence for tracking cell fate and function

Tracking the fate and function of cells in vivo is paramount for the development of rational therapies for cardiac injury. Bioluminescence imaging (BLI) provides a means for monitoring physiological processes in real time, ranging from cell survival to gene expression to complex molecular processes. In mice and rats, BLI provides unmatched sensitivity because of the absence of endogenous luciferase expression in mammalian cells and the low background luminescence emanating from animals. In the field of stem cell therapy, BLI provides an unprecedented means to monitor the biology of these cells in vivo, giving researchers a greater understanding of their survival, migration, immunogenicity, and potential tumorigenicity in a living animal. In addition to longitudinal monitoring of cell survival, BLI is a useful tool for semiquantitative measurements of gene expression in vivo, allowing a better optimization of drug and gene therapies. Overall, this technology not only enables rapid, reproducible, and quantitative monitoring of physiological processes in vivo but also can measure the influences of therapeutic interventions on the outcome of cardiac injuries.

Stem cell therapy for chronic heart failure: lessons from a 15-year experience

Although cell therapy has entered the clinical arena since 2000, its benefits are still controversial. This is partly due to a shift of the whole paradigm from the mere provision of new cells intended to replenish the pool of dead cardiomyocytes to the exploitation of the cell's paracrine effects to activate host-associated cytoprotective signalling pathways, particularly those involved in angiogenesis, prevention of apoptosis and possibly recruitment of endogenous cells capable to mature into functional cardiomyocytes. This review will discuss how these two basic mechanisms (direct donor cell-derived myocardial regeneration versus paracrine signalling) underlie the rational selection of cells in light of the target clinical indication, with a particular focus on chronic heart failure, and will emphasize the importance of optimizing cell delivery and survival to fully exploit the potential benefits of this novel approach to acute and chronic heart diseases.