Advancements in reprogramming strategies for the generation of induced pluripotent stem cells

Charting a Course toward a New Vaccine Strategy

There has been a long-simmering disquiet in Catholic circles regarding the use cell lines derived from aborted fetal tissue. This concern has exploded into the public consciousness given the use of questionable cell lines in the development and/or testing of the current COVID-19 vaccines, and the debate within the Church over the permissibility of using these vaccines. The history of cell culture and how the biological community came to rely on HEK293 fetal cell lines is explored, as is the way forward, moving the biotech industry away from ethically problematic cells and toward the development of more ethically sourced cell lines. The role of the Church in leading the way forward and the acceptable use of medicines utilizing HEK293 cell lines are all explored.

Urine-derived stem/progenitor cells: A focus on their characterization and potential

Cell therapy, i.e., the use of cells to repair an affected tissue or organ, is at the forefront of regenerative and personalized medicine. Among the multiple cell types that have been used for this purpose [including adult stem cells such as mesenchymal stem cells or pluripotent stem cells], urine-derived stem cells (USCs) have aroused interest in the past years. USCs display classical features of mesenchymal stem cells such as differentiation capacity and immunomodulation. Importantly, they have the main advantage of being isolable from one sample of voided urine with a cheap and unpainful procedure, which is broadly applicable, whereas most adult stem cell types require invasive procedure. Moreover, USCs can be differentiated into renal cell types. This is of high interest for renal cell therapy-based regenerative approaches. This review will firstly describe the isolation and characterization of USCs. We will specifically present USC phenotype, which is not an object of consensus in the literature, as well as detail their differentiation capacity. In the second part of this review, we will present and discuss the main applications of USCs. These include use as a substrate to generate human induced pluripotent stem cells, but we will deeply focus on the use of USCs for cell therapy approaches with a detailed analysis depending on the targeted organ or system. Importantly, we will also focus on the applications that rely on the use of USC-derived products such as microvesicles including exosomes, which is a strategy being increasingly employed. In the last section, we will discuss the remaining barriers and challenges in the field of USC-based regenerative medicine.

Cellular models for human cardiomyopathy: What is the best option?

The genetic cardiomyopathies are a group of disorders related by abnormal myocardial structure and function. Although individually rare, these diseases collectively represent a significant health burden since they usually develop early in life and are a major cause of morbidity and mortality amongst affected children. The heterogeneity and rarity of these disorders requires the use of an appropriate model system in order to characterize the mechanism of disease and develop useful therapeutics since standard drug trials are infeasible. A common approach to study human disease involves the use of animal models, especially rodents, but due to important biological and physiological differences, this model system may not recapitulate human disease. An alternative approach for studying the metabolic cardiomyopathies relies on the use of cellular models which have most frequently been immortalized cell lines or patient-derived fibroblasts. However, the recent introduction of induced pluripotent stem cells (iPSCs), which have the ability to differentiate into any cell type in the body, is of great interest and has the potential to revolutionize the study of rare diseases. In this paper we review the advantages and disadvantages of each model system by comparing their utility for the study of mitochondrial cardiomyopathy with a particular focus on the use of iPSCs in cardiovascular biology for the modeling of rare genetic or metabolic diseases.

Generation of defined neural populations from pluripotent stem cells

Effective and efficient generation of human neural stem cells and subsequently functional neural populations from pluripotent stem cells has facilitated advancements in the study of human development and disease modelling. This review will discuss the established protocols for the generation of defined neural populations including regionalized neurons and astrocytes, oligodendrocytes and microglia. Early protocols were established in embryonic stem cells (ESC) but the discovery of induced pluripotent stem cells (iPSC) in 2006 provided a new platform for modelling human disorders of the central nervous system (CNS). The ability to produce patient- and disease-specific iPSC lines has created a new age of disease modelling. Human iPSC may be derived from adult somatic cells and subsequently patterned into numerous distinct cell types. The ability to derive defined and regionalized neural populations from iPSC provides a powerful in vitro model of CNS disorders.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Urine-derived cells for human cell therapy

Desirable cells for human cell therapy would be ones that can be generated by simple isolation and culture techniques using a donor sample obtained by non-invasive methods. To date, the different donor-specific cells that can be isolated from blood, skin, and hair require invasive methods for sample isolation and incorporate complex and costly reagents to culture. These cells also take considerable time for their in-vitro isolation and expansion. Previous studies suggest that donor-derived cells, namely urine stem cells and renal cells, may be isolated from human urine samples using a cost-effective and simple method of isolation, incorporating not such complex reagents. Moreover, the isolated cells, particularly urine stem cells, are superior to conventional stem cell sources in terms of favourable gene profile and inherent multipotent potential. Transdifferentiation or differentiation of human urine-derived cells can generate desirable cells for regenerative therapy. In this review, we intended to discuss the characteristics and therapeutic applications of urine-derived cells for human cell therapy. Conclusively, with detailed study and optimisation, urine-derived cells have a prospective future to generate functional lineage-specific cells for patients from a clinical translation point of view.

Hematopoietic Developmental Potential of Human Pluripotent Stem Cell Lines Is Accompanied by the Morphology of Embryoid Bodies and the Expression of Endodermal and Hematopoietic Markers

The potential clinical applications of hematopoietic stem cells (HSCs) derived from human pluripotent stem cells (hPSCs) are limited by the difficulty of recapitulating embryoid hematopoiesis and by the unknown differentiation potential of hPSC lines. To evaluate their hematopoietic developmental potential, available hPSC lines were differentiated by an embryoid body (EB) suspension culture in serum-free medium supplemented with three different cytokine mixes (CMs). The hPSC differentiation status was investigated by the flow cytometry expression profiles of cell surface molecules, and the gene expression of pluripotency and differentiation markers over time was evaluated by real-time reverse transcription polymerase chain reaction (qRT-PCR). hPSC lines differed in several aspects of the differentiation process, including the absolute yield of hematopoietic progenitors, the proportion of hematopoietic progenitor populations, and the effect of various CMs. The ability to generate hematopoietic progenitors was then associated with the morphology of the developing EBs, the expression of the endodermal markers AFP and SOX17, and the hematopoietic transcription factor RUNX1. These findings deepen the knowledge about the hematopoietic propensity of hPSCs and identify its variability as an aspect that must be taken into account before the usage of hPSC-derived HSCs in downstream applications.

Highly efficient reprogramming and characterization of induced pluripotent stem cells by using a microwell array

Reprogramming of human somatic cells to induced pluripotent stem cells (iPSCs) enables the possibility of generating patient-specific cells. However, the low efficiency issue associated with iPSCs generation has limited iPSCs usage in research and clinical applications. In this study, we developed a high efficiency system to generate iPSCs by using a polydimethylsiloxane stencil. This device could be applied to the localization and reprogramming of human fibroblasts. Herein, a well-defined culture system based on a stencil, which supported efficient reprogramming of fibroblasts into iPSCs with 2-4 fold increase in efficacy over conventional methods, is presented. Subsequently, we prepared a multiple analysis system, which used a micro-patterned scissile microarray to characterize iPSCs. The results showed that iPSCs could be cultured into micro-patterns in a precisely controlled manner on the scissile poly(ethylene terephthalate) sheet, which was cut into pieces for subsequent analyses, indicating that this method allows multiple analyses to establish iPSC pluripotency in the same sample. Our approach provides a simple, cost-effective, but highly efficient system for the generation and characterization of iPSCs, and will serve as a powerful tool for establishing patient- and disease-specific pluripotent stem cells.

Electronic Supplementary Material

Supplementary material is available for this article at 10.1007/s13770-016-0015-0 and is accessible for authorized users.

The expanding horizon of MicroRNAs in cellular reprogramming

Research over the last few years in cellular reprogramming has enlightened the magical potential of microRNAs (miRNAs) in changing the cell fate from somatic to pluripotent. Recent investigations on exploring the role(s) of miRNAs in somatic cell reprogramming revealed that they target a wide range of molecules and refine their protein output. This leads to fine tuning of distinct cellular processes including cell cycle, signalling pathways, transcriptional activation/silencing and epigenetic modelling. The concerted actions of miRNA on different pathways simultaneously strengthen the transition from a differentiated to de-differentiated state. Despite the well characterized transcriptional and epigenetic machinery underlying somatic cell reprogramming, the molecular circuitry for miRNA mediated cellular reprogramming is rather fragmented. This review summarizes recent findings addressing the role of miRNAs in inducing or suppressing reprogramming thus uncovering novel potentials of miRNAs as regulators of induced pluripotency maintenance, establishment and associated signalling pathways. Our bioinformatic analysis sheds light on various unexplored biological processes and pathways associated with reprogramming inducing miRNAs, thus helps in identifying roadblocks to full reprogramming. Specifically, the biological significance of highly conserved and most studied miRNA cluster, i.e. miR-302-367, in reprogramming is also highlighted. Further, roles of miRNAs in the differentiation of neurons from iPSCs are discussed. A recent approach of direct conversion or transdifferentiation of differentiated cells into neurons by miRNAs is also elaborated. This approach is now widely gaining impetus for the generation of neurological patient's brain cells directly from his/her somatic cells in an efficient and safe manner. Thus, decoding the intricate circuitry between miRNAs and other gene regulatory networks will not only uncover novel pathways in the direct reprogramming of somatic cells but will also open new avenues in stem cell biology.

Generation of induced pluripotent stem cells by using a mammalian artificial chromosome expression system

Direct reprogramming of mouse fibroblasts into induced pluripotent stem cells (iPS) was achieved recently by overexpression of four transcription factors encoded by retroviral vectors. Most of the virus vectors, however, may cause insertional mutagenesis in the host genome and may also induce tumor formation. Therefore, it is very important to discover novel and safer, non-viral reprogramming methods. Here we describe the reprogramming of somatic cells into iPS cells by a novel protein-based technique. Engineered Oct4, Sox2 and Klf4 transcription factors carrying an N-terminal Flag-tag and a C-terminal polyarginine tail were synthesized by a recently described mammalian artificial chromosome expression system (ACEs). This system is suitable for the high-level production of recombinant proteins in mammalian tissue culture cells. Recombinant proteins produced in this system contain all the post-translational modifications essential for the stability and the authentic function of the proteins. The engineered Oct4, Sox2 and Klf4 proteins efficiently induced the reprogramming of mouse embryonic fibroblasts by means of protein transduction. This novel method allows for the generation of iPS cells, which may be suitable for therapeutic applications in the future.

Modeling AEC-New approaches to study rare genetic disorders

Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome is a rare monogenetic disorder that is characterized by severe abnormalities in ectoderm-derived tissues, such as skin and its appendages. A major cause of morbidity among affected infants is severe and chronic skin erosions. Currently, supportive care is the only available treatment option for AEC patients. Mutations in TP63, a gene that encodes key regulators of epidermal development, are the genetic cause of AEC. However, it is currently not clear how mutations in TP63 lead to the various defects seen in the patients' skin. In this review, we will discuss current knowledge of the AEC disease mechanism obtained by studying patient tissue and genetically engineered mouse models designed to mimic aspects of the disorder. We will then focus on new approaches to model AEC, including the use of patient cells and stem cell technology to replicate the disease in a human tissue culture model. The latter approach will advance our understanding of the disease and will allow for the development of new in vitro systems to identify drugs for the treatment of skin erosions in AEC patients. Further, the use of stem cell technology, in particular induced pluripotent stem cells (iPSC), will enable researchers to develop new therapeutic approaches to treat the disease using the patient's own cells (autologous keratinocyte transplantation) after correction of the disease-causing mutations.

A defined xeno-free and feeder-free culture system for the derivation, expansion and direct differentiation of transgene-free patient-specific induced pluripotent stem cells

A defined xeno-free system for patient-specific iPSC derivation and differentiation is required for translation to clinical applications. However, standard somatic cell reprogramming protocols rely on using MEFs and xenogeneic medium, imposing a significant obstacle to clinical translation. Here, we describe a well-defined culture system based on xeno-free media and LN521 substrate which supported i) efficient reprogramming of normal or diseased skin fibroblasts from human of different ages into hiPSCs with a 15-30 fold increase in efficiency over conventional viral vector-based method; ii) long-term self-renewal of hiPSCs; and iii) direct hiPSC lineage-specific differentiation. Using an excisable polycistronic vector and optimized culture conditions, we achieved up to 0.15%-0.3% reprogramming efficiencies. Subsequently, transgene-free hiPSCs were obtained by Cre-mediated excision of the reprogramming factors. The derived iPSCs maintained long-term self-renewal, normal karyotype and pluripotency, as demonstrated by the expression of stem cell markers and ability to form derivatives of three germ layers both in vitro and in vivo. Importantly, we demonstrated that Parkinson's patient transgene-free iPSCs derived using the same system could be directed towards differentiation into dopaminergic neurons under xeno-free culture conditions. Our approach provides a safe and robust platform for the generation of patient-specific iPSCs and derivatives for clinical and translational applications.

In vivo reprogramming in inflammatory bowel disease

The direct reprogramming of somatic cells has immense implications in various areas of medicine. Although remarkable progress has been made in developing novel reprogramming methods, the efficiency and fidelity of reprogramming still need to be improved. Inflammatory bowel disease (IBD) involves chronic inflammatory diseases of the gastrointestinal tract with a complex etiology caused by various genetic, immunological and environmental factors. Recently, the role of stem cells has been proposed in pathogenesis and therapy of IBD. However, the efficiency and the safety of the stem cell treatments depend on the origin of the stem cell and the administration method. We hypothesize that the reprogramming of the intestinal cells into a pluripotent state is of huge importance for IBD therapy and prevention. The vectors carrying reprogramming genes encoding pluripotency factors can be transferred to the damaged tissue, in this case the intestine, by means of invasive bacterial vectors able to colonize colon mucosa. Reconstruction of tissues in vivo might avoid problems encountered in tissue rebuilding in vitro because of lack of appropriate scaffolds and microenvironments. Herein we present a review of recent literature and a perspective of in vivo reprogramming in IBD using bacterial vectors and analyze the rationale for such approach.

Clonal genetic and hematopoietic heterogeneity among human-induced pluripotent stem cell lines

Induced pluripotent stem cells (iPSCs) hold great promise for modeling human hematopoietic diseases. However, intrinsic variability in the capacities of different iPSC lines for hematopoietic development complicates comparative studies and is currently unexplained. We created and analyzed 3 separate iPSC clones from fibroblasts of 3 different normal individuals using a standardized approach that included excision of integrated reprogramming genes by Cre-Lox mediated recombination. Gene expression profiling and hematopoietic differentiation assays showed that independent lines from the same individual were generally more similar to one another than those from different individuals. However, one iPSC line (WT2.1) exhibited a distinctly different gene expression, proliferation rate, and hematopoietic developmental potential relative to all other iPSC lines. This "outlier" clone also acquired extensive copy number variations (CNVs) during reprogramming, which may be responsible for its divergent properties. Our data indicate how inherent and acquired genetic differences can influence iPSC properties, including hematopoietic potential.

Learning the molecular mechanisms of the reprogramming factors: let's start from microRNAs

Induced reprogramming of somatic cells has had a great impact on stem cell research, and the reprogramming technologies have evolved from four transgenic factors (Oct4, Sox2, Klf4, and c-Myc; OSKM) to just a few microRNAs (mainly miR-290/302 seed family). Despite these advances, the molecular events occurring during various stages of reprogramming remain largely unknown. Here, we concisely review current knowledge of miRNA regulation from the initiation phase of OSKM-induced reprogramming, through the transitional stage, to final maturation. At the start of reprogramming, the microRNAs miR-21, miR-29a, let-7a, and miR-34 act as guards to secure the somatic identity and genomic integrity of the cell of origin. As reprogramming proceeds, miR-155, miR-10b, miR-205, and miR-429 modulate the epithelial-mesenchymal/mesenchymal-epithelial transition (EMT/MET), which is a critical step towards transformed pluripotent status. Finally, the pluripotency regulatory network is secured in the iPSCs and fine-tuned by a group of miRNAs belonging to the miR-290/302 seed family. Among the four reprogramming factors, c-Myc plays the dominant role in regulating the miRNAs under reprogramming-specific conditions. Accumulating evidence suggests that the reprogramming efficiency can be improved by either blocking barrier miRNAs or introducing helper miRNAs. Intriguingly, induced pluripotency can be obtained by introducing a single miR-302 cluster, although the supportive molecular mechanism is still lacking. In the near future, we may be able to realize the broad potential of miRNAs in the stem cell field, such as altering cell identities with high efficiency through the transient introduction of tissue-specific miRNAs.

The role of DNA repair in the pluripotency and differentiation of human stem cells

All living cells utilize intricate DNA repair mechanisms to address numerous types of DNA lesions and to preserve genomic integrity, and pluripotent stem cells have specific needs due to their remarkable ability of self-renewal and differentiation into different functional cell types. Not surprisingly, human stem cells possess a highly efficient DNA repair network that becomes less efficient upon differentiation. Moreover, these cells also have an anaerobic metabolism, which reduces the mitochondria number and the likelihood of oxidative stress, which is highly related to genomic instability. If DNA lesions are not repaired, human stem cells easily undergo senescence, cell death or differentiation, as part of their DNA damage response, avoiding the propagation of stem cells carrying mutations and genomic alterations. Interestingly, cancer stem cells and typical stem cells share not only the differentiation potential but also their capacity to respond to DNA damage, with important implications for cancer therapy using genotoxic agents. On the other hand, the preservation of the adult stem cell pool, and the ability of cells to deal with DNA damage, is essential for normal development, reducing processes of neurodegeneration and premature aging, as one can observe on clinical phenotypes of many human genetic diseases with defects in DNA repair processes. Finally, several recent findings suggest that DNA repair also plays a fundamental role in maintaining the pluripotency and differentiation potential of embryonic stem cells, as well as that of induced pluripotent stem (iPS) cells. DNA repair processes also seem to be necessary for the reprogramming of human cells when iPS cells are produced. Thus, the understanding of how cultured pluripotent stem cells ensure the genetic stability are highly relevant for their safe therapeutic application, at the same time that cellular therapy is a hope for DNA repair deficient patients.

Embryonic template-based generation and purification of pluripotent stem cell-derived cardiomyocytes for heart repair

Cardiovascular disease remains a leading cause of death in Western countries. Many types of cardiovascular diseases are due to a loss of functional cardiomyocytes, which can result in irreversible cardiac failure. Since the adult human heart has limited regenerative potential, cardiac transplantation is still the only effective therapy to address this cardiomyocyte loss. However, drawbacks, such as immune rejection and insufficient donor availability, are limiting this last-resort solution. Recent developments in the stem cell biology field have improved the potential of cardiac regeneration. Improvements in reprogramming strategies of differentiated adult cells into induced pluripotent stem cells, together with increased efficiency of directed differentiation of pluripotent stem cells toward cardiac myocytes, have brought cell-based heart muscle regeneration a few steps closer to the clinic. In this review, we outline the status of research on cardiac regeneration with a focus on directed differentiation of pluripotent stem cells toward the cardiac lineage.

Inducing pluripotency using in vivo gene therapy

Since the original study of Takahashi and Yamanaka in 2006 [1], the field of induced pluripotent stem (iPS) cells has made a great progress. Since then, a number of different cell types have been successfully brought to a state of pluripotency and a different set of transcription factors have been reported to be sufficient to reprogram mouse and human somatic cells. Although still with low efficiency of reprogramming, the patient- and disease-specific therapy represents the most valuable outcome of the whole area of iPS cells. Herein we hypothesize that inducing pluripotency in vivo might be an interesting alternative to the standard ex vivo methods. In vivo reprogramming would benefit from the direct administration of the DNA encoding the reprogramming factors into the target tissue/organ of an individual. The target cells that are to be reprogrammed would be transduced in their natural environment that can provide all the necessary molecular and spatial factors that could be missing during ex vivo reprogramming. However, since no available data exist on in vivo induced pluripotency, it is difficult to predict if testing the hypothesis will provide any promising results. On the way to this point, a number of pilot experiments have to be performed to overcome many limitations and pitfalls that are arising from such a risky concept. Safety issues, such as the risk of somatic tumor formation, will likely be the crucial point to focus on during the process of proving the validity of the hypothesis. However, initial data from the study on inflammatory bowel disease suggest that there might be some beneficial effect of in vivo gene therapy based on reprogramming the target cells.

MicroRNAs and induced pluripotent stem cells for human disease mouse modeling

Human disease animal models are absolutely invaluable tools for our understanding of mechanisms involved in both physiological and pathological processes. By studying various genetic abnormalities in these organisms we can get a better insight into potential candidate genes responsible for human disease development. To this point a mouse represents one of the most used and convenient species for human disease modeling. Hundreds if not thousands of inbred, congenic, and transgenic mouse models have been created and are now extensively utilized in the research labs worldwide. Importantly, pluripotent stem cells play a significant role in developing new genetically engineered mice with the desired human disease-like phenotype. Induced pluripotent stem (iPS) cells which represent reprogramming of somatic cells into pluripotent stem cells represent a significant advancement in research armament. The novel application of microRNA manipulation both in the generation of iPS cells and subsequent lineage-directed differentiation is discussed. Potential applications of induced pluripotent stem cell--a relatively new type of pluripotent stem cells--for human disease modeling by employing human iPS cells derived from normal and diseased somatic cells and iPS cells derived from mouse models of human disease may lead to uncovering of disease mechanisms and novel therapies.

Concise review: Cord blood banking, transplantation and induced pluripotent stem cell: success and opportunities

Hematopoietic cell transplantation (HCT) has become a standard practice to treat a number of malignant and nonmalignant hematologic diseases. Bone marrow, mobilized peripheral blood, and umbilical cord blood can all serve as primary sources of cells for HCT. The number of cord blood units currently stored is large, although it represents only a fraction of potential collections. With much of the collection being sequestered in private banks for possible autologous use, there is a reason to expect that public banks may not be able to provide for the demand in coming years as use of cord blood for treatment of patients with diseases such as leukemia and lymphoma continues to increase. We suggest that a possible solution to encourage private banks to share their valuable units is to apply recent methodologies to generate induced pluripotent stem cells from cord cells and to optimize techniques to generate hematopoietic lineages from them. This strategy would allow us to take advantage of the units already collected under appropriate regulatory guidelines, to access a pristine cell that can be converted to a pluripotent cell at a much higher efficiency and in a shorter time period than other cells. The ability to potentially replenish a used cord unit with new cells, as well as extend the potential utility of cord blood for additional therapeutic applications, should allow banks to develop an appropriate business model for both private and public cord blood banks to flourish.

SOX2 hypomorphism disrupts development of the prechordal floor and optic cup

Haploinsufficiency for the HMG-box transcription factor SOX2 results in abnormalities of the human ventral forebrain and its derivative structures. These defects include anophthalmia (absence of eye), microphthalmia (small eye) and hypothalamic hamartoma (HH), an overgrowth of the ventral hypothalamus. To determine how Sox2 deficiency affects the morphogenesis of the ventral diencephalon and eye, we generated a Sox2 allelic series (Sox2(IR), Sox2(LP), and Sox2(EGFP)), allowing for the generation of mice that express germline hypomorphic levels (<40%) of SOX2 protein and that faithfully recapitulate SOX2 haploinsufficient human phenotypes. We find that Sox2 hypomorphism significantly disrupts the development of the posterior hypothalamus, resulting in an ectopic protuberance of the prechordal floor, an upregulation of Shh signaling, and abnormal hypothalamic patterning. In the anterior diencephalon, both the optic stalks and optic cups (OC) of Sox2 hypomorphic (Sox2(HYP)) embryos are malformed. Furthermore, Sox2(HYP) eyes exhibit a loss of neural potential and coloboma, a common phenotype in SOX2 haploinsufficient humans that has not been described in a mouse model of SOX2 deficiency. These results establish for the first time that germline Sox2 hypomorphism disrupts the morphogenesis and patterning of the hypothalamus, optic stalk, and the early OC, establishing a model of the development of the abnormalities that are observed in SOX2 haploinsufficient humans.

Advances and applications of induced pluripotent stem cells

Direct reprogramming of somatic cells into pluripotent cells is an emerging technology for creating patient-specific cells, and potentially opens new scenarios in medical and pharmacological fields. From the discovery of Shinya Yamanaka, who first obtained pluripotent cells from fibroblasts by retrovirus-derived ectopic expression of defined embryonic transcription factors, new methods have been developed to generate safe induced pluripotent stem (iPS) cells without genomic manipulations. This review will focus on the recent advances in iPS technology and their application in pharmacology and medicine.

Reprogramming of somatic cells

Reprogramming of adult somatic cells into pluripotent stem cells may provide an attractive source of stem cells for regenerative medicine. It has emerged as an invaluable method for generating patient-specific stem cells of any cell lineage without the use of embryonic stem cells. A revolutionary study in 2006 showed that it is possible to convert adult somatic cells directly into pluripotent stem cells by using a limited number of pluripotent transcription factors and is called as iPS cells. Currently, both genomic integrating viral and nonintegrating nonviral methods are used to generate iPS cells. However, the viral-based technology poses increased risk of safety, and more studies are now focused on nonviral-based technology to obtain autologous stem cells for clinical therapy. In this review, the pros and cons of the present iPS cell technology and the future direction for the successful translation of this technology into the clinic are discussed.