Navigating the kidney organoid: insights into assessment and enhancement of nephron function.

Kidney organoids are three-dimensional structures generated from pluripotent stem cells (PSCs) that are capable of recapitulating the major structures of mammalian kidneys. As this technology is expected to be a promising tool for studying renal biology, drug discovery, and regenerative medicine, the functional capacity of kidney organoids has emerged as a critical question in the field. Kidney organoids produced using several protocols harbor key structures of native kidneys. Here, we review the current state, recent advances, and future challenges in the functional characterization of kidney organoids, strategies to accelerate and enhance kidney organoid functions, and access to PSC resources to advance organoid research. The strategies to construct physiologically relevant kidney organoids include the use of organ-on-a-chip technologies that integrate fluid circulation and improve organoid maturation. These approaches result in increased expression of the major tubular transporters and elements of mechanosensory signaling pathways suggestive of improved functionality. Nevertheless, continuous efforts remain crucial to create kidney tissue that more faithfully replicates physiological conditions for future applications in kidney regeneration medicine and their ethical use in patient care.NEW & NOTEWORTHY Kidney organoids are three-dimensional structures derived from stem cells, mimicking the major components of mammalian kidneys. Although they show great promise, their functional capacity has become a critical question. This review explores the advancements and challenges in evaluating and enhancing kidney organoid function, including the use of organ-on-chip technologies, multiomics data, and in vivo transplantation. Integrating these approaches to further enhance their physiological relevance will continue to advance disease modeling and regenerative medicine applications.

Rapid, Cost-Efficient, Enzyme-Free Passaging of Human Pluripotent Stem Cells on Feeder Cells by Ethylenediaminetetraacetic Acid-Mediated Dis-Adhesion.

Human pluripotent stem cells (human embryonic stem cells, hESCs, and human induced pluripotent stem cells, hiPSCs) were originally cultured on different types of feeder cells for maintenance in an undifferentiated state in long-term culture. This approach has been supplanted to a large extent by feeder-free culture protocols, but these involve more costly reagents and can promote a transition to a primed state, which restricts the cells' differentiation capacity. In both feeder and feeder-free conditions, the harvesting of hESC or hiPSC colonies for passaging is a necessary procedure for expanding the cultures. To provide an easy and high-yield procedure for passaging hESCs/hiPSCs cultured on feeder cells, we have established a harvesting method using dis-adhesion elicited by the calcium chelator ethylenediaminetetraacetic acid (EDTA). We have assessed the yield and quality of the resultant passaged cells by comparing this approach to the original mechanical harvesting approach, in which colonies are isolated with a scalpel under a microscope (mechanical harvesting was chosen as a comparator to avoid the reagent variability associated with enzymatic harvesting). In one set of experiments, two different hESC lines were maintained on a feeder cell layer of human foreskin fibroblasts. Each line was subjected to multiple passages using EDTA-based or mechanical harvesting and assessed for colony size and morphology, cell density, stemness marker expression, differentiation to the three germ layers in embryoid bodies, and genomic aberrations. In another set of experiments, we used EDTA-based harvesting on two different hiPSC lines and obtained similar results. EDTA-induced dis-adhesion saved time and gave a higher yield of colonies of a more favorable size and more uniform morphology compared to mechanical harvesting. It was also faster than enzymatic harvesting and not prone to enzyme batch variability. The EDTA-induced dis-adhesion method also facilitates the transfer of hESC/hiPSC lines from feeder cell-based culture to feeder-free conditions if desired for downstream use and analysis.

Tumorigenicity Assessment of Human Cancer Cell Lines Xenografted on Immunodeficient Mice as Positive Controls of Tumorigenicity Testing.

Recent advances in human pluripotent stem cell (hPSC)-derived cell therapies and genome editing technologies such as CRISPR/Cas9 make regenerative medicines promising for curing diseases previously thought to be incurable. However, the possibility of off-target effects during genome editing and the nature of hPSCs, which can differentiate into any cell type and infinitely proliferate, inevitably raises concerns about tumorigenicity. Tumorigenicity acts as a major obstacle to the application of hPSC-derived and gene therapy products in clinical practice. Thus, regulatory authorities demand mandatory tumorigenicity testing as a key pre-clinical safety step for the products. In the tumorigenicity testing, regulatory guidelines request to include human cancer cell line injected positive control group (PC) animals, which must form tumors. As the validity of the whole test is determined by the tumor-forming rates (typically above 90%) of PC animals, establishing the stable tumorigenic condition of PC animals is critical for successful testing. We conducted several studies to establish the proper positive control conditions, including dose, administration routes, and the selection of cell lines, in compliance with Good Laboratory Practice (GLP) regulations and/or guidelines, which are essential for pre-clinical safety tests of therapeutic materials. We expect that our findings provide insights and practical information to create a successful tumorigenicity test and its guidelines.

Development of human pluripotent stem cell-derived hepatic organoids as an alternative model for drug safety assessment.

Human in vitro hepatic models that faithfully recapitulate liver function are essential for successful basic and translational research. A limitation of current in vitro models, which are extensively used for drug discovery and toxicity testing, is the loss of drug metabolic function due to the low expression and activity of cytochrome P450 (CYP450) enzymes. Here, we aimed to generate human pluripotent stem cell-derived hepatic organoids (hHOs) with a high drug metabolic ability. We established a two-step protocol to produce hHOs from human pluripotent stem cells for long-term expansion and drug testing. Fully differentiated hHOs had multicellular composition and exhibited cellular polarity and hepatobiliary structures. They also displayed remarkable CYP450 activity and recapitulated the metabolic clearance, CYP450-mediated drug toxicity, and metabolism. Furthermore, hHOs successfully modeled Wilson's disease in terms of Cu metabolism, drug responses, and diagnostic marker expression and secretion. In conclusion, hHOs exhibit high capacity for drug testing and disease modeling. Hence, this hepatic model system provides an advanced tool for studying hepatic drug metabolism and diseases.

Assessment of Endothelial-to-Hematopoietic Transition of Individual Hemogenic Endothelium and Bulk Populations in Defined Conditions.

Endothelial-to-hematopoietic transition (EHT) is a unique morphogenic event in which flat, adherent hemogenic endothelial (HE) cells acquire round, non-adherent blood cell morphology. Investigating the mechanisms of EHT is critical for understanding the development of hematopoietic stem cells (HSCs) and the entirety of the adult immune system, and advancing technologies for manufacturing blood cells from human pluripotent stem cells (hPSCs). Here we describe a protocol to (a) generate and isolate subsets of HE from hPSCs, (b) assess EHT and hematopoietic potential of HE subsets in bulk cultures and at the single-cell level, and (c) evaluate the role of NOTCH signaling during HE specification and EHT. The generation of HE from hPSCs and EHT bulk cultures are performed in xenogen- and feeder-free system, providing the unique advantage of being able to investigate the role of individual signaling factors during EHT and the definitive lympho-myeloid cell specification from hPSCs.

Assessment of arrhythmia mechanism and burden of the infarcted ventricles following remuscularization with pluripotent stem cell-derived cardiomyocyte patches using patient-derived models.

AIMS: Direct remuscularization with pluripotent stem cell-derived cardiomyocytes (PSC-CMs) seeks to address the onset of heart failure post-myocardial infarction (MI) by treating the persistent muscle deficiency that underlies it. However, direct remuscularization with PSC-CMs could potentially be arrhythmogenic. We investigated two possible mechanisms of arrhythmogenesis-focal vs. re-entrant-arising from direct remuscularization with PSC-CM patches in two personalized, human ventricular computer models of post-MI. Moreover, we developed a principled approach for evaluating arrhythmogenicity of direct remuscularization that factors in the VT propensity of the patient-specific post-MI fibrotic substrate and use it to investigate different conditions of patch remuscularization. METHODS AND RESULTS: Two personalized, human ventricular models of post-MI (P1 and P2) were constructed from late gadolinium enhanced (LGE)-magnetic resonance images (MRIs). In each model, remuscularization with PSC-CM patches was simulated under different treatment conditions that included patch engraftment, patch myofibril orientation, remuscularization site, patch size (thickness and diameter), and patch maturation. To determine arrhythmogenicity of treatment conditions, VT burden of heart models was quantified prior to and after simulated remuscularization and compared. VT burden was quantified based on inducibility (i.e. weighted sum of pacing sites that induced) and severity (i.e. the number of distinct VT morphologies induced). Prior to remuscularization, VT burden was significant in P1 (0.275) and not in P2 (0.0, not VT inducible). We highlight that re-entrant VT mechanisms would dominate over focal mechanisms; spontaneous beats emerging from PSC-CM grafts were always a fraction of resting sinus rate. Moreover, incomplete patch engraftment can be particularly arrhythmogenic, giving rise to particularly aberrant electrical activation and conduction slowing across the PSC-CM patches along with elevated VT burden when compared with complete engraftment. Under conditions of complete patch engraftment, remuscularization was almost always arrhythmogenic in P2 but certain treatment conditions could be anti-arrhythmogenic in P1. Moreover, the remuscularization site was the most important factor affecting VT burden in both P1 and P2. Complete maturation of PSC-CM patches, both ionically and electrotonically, at the appropriate site could completely alleviate VT burden. CONCLUSION: We identified that re-entrant VT would be the primary VT mechanism in patch remuscularization. To evaluate the arrhythmogenicity of remuscularization, we developed a principled approach that factors in the propensity of the patient-specific fibrotic substrate for VT. We showed that arrhythmogenicity is sensitive to the patient-specific fibrotic substrate and remuscularization site. We demonstrate that targeted remuscularization can be safe in the appropriate individual and holds the potential to non-destructively eliminate VT post-MI in addition to addressing muscle deficiency underlying heart failure progression.

A Simple, Rapid, and Cost-Effective Method for Loss-of-Function Assays in Pluripotent Cells.

Embryonic stem cells (ESCs) are derived from the inner cell mass of the preimplantation blastocyst and can be maintained indefinitely in vitro without losing their properties. Given their self-renewal and pluripotency, ESCs not only represent a key tool to study early embryonic development in a dish, but also an unlimited source of material for tissue replacement in regenerative medicine. Loss-of-function assays using RNA interference are a powerful tool to understand the roles of specific genes and are facilitated by lentiviral-mediated delivery of vector-encoded shRNAs which allows long-term silencing of single or multiple genes. Here, we describe the steps for rapid and cost-effective production and testing of lentiviral particles with vector-encoded shRNAs for gene silencing in ESCs. This protocol can be easily adapted for loss-of-function assays in other pluripotent cells or culture conditions of interest.

Neuronal cell-based medicines from pluripotent stem cells: Development, production, and preclinical assessment.

Brain degeneration and damage is difficult to cure due to the limited endogenous repair capability of the central nervous system. Furthermore, drug development for treatment of diseases of the central nervous system remains a major challenge. However, it now appears that using human pluripotent stem cell-derived neural cells to replace degenerating cells provides a promising cell-based medicine for rejuvenation of brain function. Accordingly, a large number of studies have carried out preclinical assessments, which have involved different neural cell types in several neurological diseases. Recent advances in animal models identify the transplantation of neural derivatives from pluripotent stem cells as a promising path toward the clinical application of cell therapies [Stem Cells Transl Med 2019;8:681-693; Drug Discov Today 2019;24:992-999; Nat Med 2019;25:1045-1053]. Some groups are moving toward clinical testing in humans. However, the difficulty in selection of valuable critical quality criteria for cell products and the lack of functional assays that could indicate suitability for clinical effect continue to hinder neural cell-based medicine development [Biologicals 2019;59:68-71]. In this review, we summarize the current status of preclinical studies progress in this area and outline the biological characteristics of neural cells that have been used in new developing clinical studies. We also discuss the requirements for translation of stem cell-derived neural cells in examples of stem cell-based clinical therapy.

Assessment of the Hematopoietic Differentiation Potential of Human Pluripotent Stem Cells in 2D and 3D Culture Systems.

BACKGROUND: In vitro methods for hematopoietic differentiation of human pluripotent stem cells (hPSC) are a matter of priority for the in-depth research into the mechanisms of early embryogenesis. So-far, published results regarding the generation of hematopoietic cells come from studies using either 2D or 3D culture formats, hence, it is difficult to discern their particular contribution to the development of the concept of a unique in vitro model in close resemblance to in vivo hematopoiesis. AIM OF THE STUDY: To assess using the same culture conditions and the same time course, the potential of each of these two formats to support differentiation of human pluripotent stem cells to primitive hematopoiesis without exogenous activation of Wnt signaling. METHODS: We used in parallel 2D and 3D formats, the same culture environment and assay methods (flow cytometry, IF, qPCR) to investigate stages of commitment and specification of mesodermal, and hemogenic endothelial cells to CD34 hematopoietic cells and evaluated their clonogenic capacity in a CFU system. RESULTS: We show an adequate formation of mesoderm, an efficient commitment to hemogenic endothelium, a higher number of CD34 hematopoietic cells, and colony-forming capacity potential only in the 3D format-supported differentiation. CONCLUSIONS: This study shows that the 3D but not the 2D format ensures the induction and realization by endogenous mechanisms of human pluripotent stem cells' intrinsic differentiation program to primitive hematopoietic cells. We propose that the 3D format provides an adequate level of upregulation of the endogenous Wnt/ß-catenin signaling.

Bench-to-Bedside in Vascular Medicine: Optimizing the Translational Pipeline for Patients With Peripheral Artery Disease.

Peripheral arterial disease is a growing worldwide problem with a wide spectrum of clinical severity and is projected to consume >$21 billion per year in the United States alone. While vascular researchers have brought several therapies to the clinic in recent years, few of these approaches have leveraged advances in high-throughput discovery screens, novel translational models, or innovative trial designs. In the following review, we discuss recent advances in unbiased genomics and broader omics technology platforms, along with preclinical vascular models designed to enhance our understanding of disease pathobiology and prioritize targets for additional investigation. Furthermore, we summarize novel approaches to clinical studies in subjects with claudication and ischemic ulceration, with an emphasis on streamlining and accelerating bench-to-bedside translation. By providing a framework designed to enhance each aspect of future clinical development programs, we hope to enrich the pipeline of therapies that may prevent loss of life and limb for those with peripheral arterial disease.

A Comparative Assessment of Marker Expression Between Cardiomyocyte Differentiation of Human Induced Pluripotent Stem Cells and the Developing Pig Heart.

The course of differentiation of pluripotent stem cells into cardiomyocytes and the intermediate cell types are characterized using molecular markers for different stages of development. These markers have been selected primarily from studies in the mouse and from a limited number of human studies. However, it is not clear how well mouse cardiogenesis compares with human cardiogenesis at the molecular level. We tackle this issue by analyzing and comparing the expression of common cardiomyogenesis markers [platelet-derived growth factor receptor, alpha polypeptide (PDGFR-α), fetal liver kinase 1 (FLK1), ISL1, NK2 homeobox 5 (NKX2.5), cardiac troponin T (CTNT), connexin43 (CX43), and myosin heavy chain 7 (MYHC-B)] in the developing pig heart at embryonic day (E)15, E16, E18, E20, E22, and E24 and in differentiating cardiomyocytes from human induced pluripotent stem cells (hiPSCs). We found that porcine expression of the mesoderm marker FLK1 and the cardiac progenitor marker ISL1 was in line with our differentiating hiPSC and reported murine expression. The cardiac lineage marker NKX2.5 was expressed at almost all stages in the pig and hiPSC, with an earlier onset in the hiPSC compared with reported murine expression. Markers of immature cardiomyocytes, CTNT, and MYHC-B were consistently expressed throughout E16-E70 in the pig, which is comparable with mouse development, whereas the markers increased over time in the hiPSC. However, the commonly used mature cardiomyocyte marker, CX43, should be used with caution, as it was also expressed in the pig mesoderm, as well as hiPSC immature cardiomyocytes, while this has not been reported in mice. Based on our observations in the various species, we suggest to use FLK1/PDGFR-α for identifying cardiac mesoderm and ISL1/NKX2.5 for cardiac progenitors. Furthermore, a combination of two or more of the following, CTNT+/MYHC-B+/ISL1+ could mark immature cardiomyocytes and CTNT+/ISL1- mature cardiomyocytes. CX43 should be used together with sarcomeric proteins. This knowledge may help improving differentiation of hiPSC into more in vivo-like cardiac tissue in the future.

An automated and high-throughput-screening compatible pluripotent stem cell-based test platform for developmental and reproductive toxicity assessment of small molecule compounds.

The embryonic stem cell test (EST) represents the only validated and accepted in vitro system for the detection and classification of compounds according to their developmental and reproductive teratogenic potency. The widespread implementation of the EST, however, in particular for routine application in pharmaceutical development, has not been achieved so far. Several drawbacks still limit the high-throughput screening of potential drug candidates in this format: The long assay period, the use of non-homogeneous viability assays, the low throughput analysis of marker protein expression and the compatibility of the assay procedures to automation. We have therefore introduced several advancements into the EST workflow: A reduction of the assay period, an introduction of homogeneous viability assays, and a straightforward analysis of marker proteins by flow cytometry and high content imaging to assess the impact of small molecules on differentiation capacity. Most importantly, essential parts of the assay procedure have been adapted to lab automation in 96-well format, thus enabling the interrogation of several compounds in parallel. In addition, extensive investigations were performed to explore the predictive capacity of this next-generation EST, by testing a set of well-known embryotoxicants that encompasses the full range of chemical-inherent embryotoxic potencies possible. Due to these significant improvements, the augmented workflow provides a basis for a sensitive, more rapid, and reproducible high throughput screening compatible platform to predict in vivo developmental toxicity from in vitro data which paves the road towards application in an industrial setting. Graphical abstract •The embryonic stem cell test to predict teratogenicity was made automation-compatible. •Several key improvements to the assay procedure have been introduced to increase performance. •The workflow was adapted to human iPS cells and isogenic fibroblast donor cells.

Viral pandemic preparedness: A pluripotent stem cell-based machine-learning platform for simulating SARS-CoV-2 infection to enable drug discovery and repurposing.

Infection with the SARS-CoV-2 virus has rapidly become a global pandemic for which we were not prepared. Several clinical trials using previously approved drugs and drug combinations are urgently under way to improve the current situation. A vaccine option has only recently become available, but worldwide distribution is still a challenge. It is imperative that, for future viral pandemic preparedness, we have a rapid screening technology for drug discovery and repurposing. The primary purpose of this research project was to evaluate the DeepNEU stem-cell based platform by creating and validating computer simulations of artificial lung cells infected with SARS-CoV-2 to enable the rapid identification of antiviral therapeutic targets and drug repurposing. The data generated from this project indicate that (a) human alveolar type lung cells can be simulated by DeepNEU (v5.0), (b) these simulated cells can then be infected with simulated SARS-CoV-2 virus, (c) the unsupervised learning system performed well in all simulations based on available published wet lab data, and (d) the platform identified potentially effective anti-SARS-CoV2 combinations of known drugs for urgent clinical study. The data also suggest that DeepNEU can identify potential therapeutic targets for expedited vaccine development. We conclude that based on published data plus current DeepNEU results, continued development of the DeepNEU platform will improve our preparedness for and response to future viral outbreaks. This can be achieved through rapid identification of potential therapeutic options for clinical testing as soon as the viral genome has been confirmed.

Deep-learning-based multi-class segmentation for automated, non-invasive routine assessment of human pluripotent stem cell culture status.

Human induced pluripotent stem cells (hiPSCs) are capable of differentiating into a variety of human tissue cells. They offer new opportunities for personalized medicine and drug screening. This requires large quantities of high quality hiPSCs, obtainable only via automated cultivation. One of the major requirements of an automated cultivation is a regular, non-invasive analysis of the cell condition, e.g. by whole-well microscopy. However, despite the urgency of this requirement, there are currently no automatic, image-processing-based solutions for multi-class routine quantification of this nature. This paper describes a method to fully automate the cell state recognition based on phase contrast microscopy and deep-learning. This approach can be used for in process control during an automated hiPSC cultivation. The U-Net based algorithm is capable of segmenting important parameters of hiPSC colony formation and can discriminate between the classes hiPSC colony, single cells, differentiated cells and dead cells. The model achieves more accurate results for the classes hiPSC colonies, differentiated cells, single hiPSCs and dead cells than visual estimation by a skilled expert. Furthermore, parameters for each hiPSC colony are derived directly from the classification result such as roundness, size, center of gravity and inclusions of other cells. These parameters provide localized information about the cell state and enable well based treatment of the cell culture in automated processes. Thus, the model can be exploited for routine, non-invasive image analysis during an automated hiPSC cultivation. This facilitates the generation of high quality hiPSC derived products for biomedical purposes.

Development of a cost-effective automated platform to produce human liver spheroids for basic and applied research.

Liver disease represents an increasing cause of global morbidity and mortality. Currently, liver transplant is the only treatment curative for end-stage liver disease. Donor organs cannot meet the demand and therefore scalable treatments and new disease models are required to improve clinical intervention. Pluripotent stem cells represent a renewable source of human tissue. Recent advances in three-dimensional cell culture have provided the field with more complex systems that better mimic liver physiology and function. Despite these improvements, current cell-based models are variable in performance and expensive to manufacture at scale. This is due, in part, to the use of poorly defined or cross-species materials within the process, severely affecting technology translation. To address this issue, we have developed an automated and economical platform to produce liver tissue at scale for modelling disease and small molecule screening. Stem cell derived liver spheres were formed by combining hepatic progenitors with endothelial cells and stellate cells, in the ratios found within the liver. The resulting tissue permitted the study of human liver biology 'in the dish' and could be scaled for screening. In summary, we have developed an automated differentiation system that permits reliable self-assembly of human liver tissue for biomedical application. Going forward we believe that this technology will not only serve as anin vitroresource, and may have an important role to play in supporting failing liver function in humans.

Cost-effective culture of human induced pluripotent stem cells using UV/ozone-modified culture plastics with reduction of cell-adhesive matrix coating.

Human induced pluripotent stem cells (hiPSCs) are considered to be one of the most promising cell resources for regenerative medicine. HiPSCs usually maintain their pluripotency when they are cultured on feeder cell layers or are attached to a cell-adhesive extracellular matrix. In this study, we developed a culture system based on UV/ozone modification for conventional cell culture plastics to generate a suitable surface condition for hiPSCs. Time of flight secondary ion mass spectrometry (ToF-SIMS) was carried out to elucidate the relationship between hiPSC adhesion and UV/ozone irradiation-induced changes to surface chemistry of cell culture plastics. Cell culture plastics with modified surfaces enabled growth of a feeder-free hiPSC culture with markedly reduced cell-adhesive matrix coating. Our cell culture system using UV/ozone-modified cell culture plastics may produce clinically relevant hiPSCs at low costs, and can be easily scaled up in culture systems to produce a large number of hiPSCs.

Developmental IL-6 Exposure Favors Production of PDGF-Responsive Multipotential Progenitors at the Expense of Neural Stem Cells and Other Progenitors.

Interleukin-6 (IL-6) is increased in maternal serum and amniotic fluid of children subsequently diagnosed with autism spectrum disorders. However, it is not clear how increased IL-6 alters brain development. Here, we show that IL-6 increases the prevalence of a specific platelet-derived growth factor (PDGF)-responsive multipotent progenitor, with opposite effects on neural stem cells and on subsets of bipotential glial progenitors. Acutely, increasing circulating IL-6 levels 2-fold above baseline in neonatal mice specifically stimulated the proliferation of a PDGF-responsive multipotential progenitor accompanied by increased phosphorylated STAT3, increased Fbxo15 expression, and decreased Dnmt1 and Tlx expression. Fate mapping studies using a Nestin-CreERT2 driver revealed decreased astrogliogenesis in the frontal cortex. IL-6-treated mice were hyposmic; however, olfactory bulb neuronogenesis was unaffected. Altogether, these studies provide important insights into how inflammation alters neural stem cells and progenitors and provide new insights into the molecular and cellular underpinnings of neurodevelopmental disorders associated with maternal infections.

Longitudinal assessment of tumor development using cancer avatars derived from genetically engineered pluripotent stem cells.

Many cellular models aimed at elucidating cancer biology do not recapitulate pathobiology including tumor heterogeneity, an inherent feature of cancer that underlies treatment resistance. Here we introduce a cancer modeling paradigm using genetically engineered human pluripotent stem cells (hiPSCs) that captures authentic cancer pathobiology. Orthotopic engraftment of the neural progenitor cells derived from hiPSCs that have been genome-edited to contain tumor-associated genetic driver mutations revealed by The Cancer Genome Atlas project for glioblastoma (GBM) results in formation of high-grade gliomas. Similar to patient-derived GBM, these models harbor inter-tumor heterogeneity resembling different GBM molecular subtypes, intra-tumor heterogeneity, and extrachromosomal DNA amplification. Re-engraftment of these primary tumor neurospheres generates secondary tumors with features characteristic of patient samples and present mutation-dependent patterns of tumor evolution. These cancer avatar models provide a platform for comprehensive longitudinal assessment of human tumor development as governed by molecular subtype mutations and lineage-restricted differentiation.

Tumorigenicity assessment of cell therapy products: The need for global consensus and points to consider.

Pluripotent stem cells offer the potential for an unlimited source for cell therapy products. However, there is concern regarding the tumorigenicity of these products in humans, mainly due to the possible unintended contamination of undifferentiated cells or transformed cells. Because of the complex nature of these new therapies and the lack of a globally accepted consensus on the strategy for tumorigenicity evaluation, a case-by-case approach is recommended for the risk assessment of each cell therapy product. In general, therapeutic products need to be qualified using available technologies, which ideally should be fully validated. In such circumstances, the developers of cell therapy products may have conducted various tumorigenicity tests and consulted with regulators in respective countries. Here, we critically review currently available in vivo and in vitro testing methods for tumorigenicity evaluation against expectations in international regulatory guidelines. We discuss the value of those approaches, in particular the limitations of in vivo methods, and comment on challenges and future directions. In addition, we note the need for an internationally harmonized procedure for tumorigenicity assessment of cell therapy products from both regulatory and technological perspectives.