Repopulation efficiencies of adult hepatocytes, fetal liver progenitor cells, and embryonic stem cell-derived hepatic cells in albumin-promoter-enhancer urokinase-type plasminogen activator mice

Cell therapy in end-stage liver disease: replace and remodel

Liver disease is prevalent worldwide. When it reaches the end stage, mortality rises to 50% or more. Although liver transplantation has emerged as the most efficient treatment for end-stage liver disease, its application has been limited by the scarcity of donor livers. The lack of acceptable donor organs implies that patients are at high risk while waiting for suitable livers. In this scenario, cell therapy has emerged as a promising treatment approach. Most of the time, transplanted cells can replace host hepatocytes and remodel the hepatic microenvironment. For instance, hepatocytes derived from donor livers or stem cells colonize and proliferate in the liver, can replace host hepatocytes, and restore liver function. Other cellular therapy candidates, such as macrophages and mesenchymal stem cells, can remodel the hepatic microenvironment, thereby repairing the damaged liver. In recent years, cell therapy has transitioned from animal research to early human studies. In this review, we will discuss cell therapy in end-stage liver disease treatment, especially focusing on various cell types utilized for cell transplantation, and elucidate the processes involved. Furthermore, we will also summarize the practical obstacles of cell therapy and offer potential solutions.

Therapeutic Cell Repopulation of the Liver: From Fetal Rat Cells to Synthetic Human Tissues

Progenitor cells isolated from the fetal liver can provide a unique cell source to generate new healthy tissue mass. Almost 20 years ago, it was demonstrated that rat fetal liver cells repopulate the normal host liver environment via a mechanism akin to cell competition. Activin A, which is produced by hepatocytes, was identified as an important player during cell competition. Because of reduced activin receptor expression, highly proliferative fetal liver stem/progenitor cells are resistant to activin A and therefore exhibit a growth advantage compared to hepatocytes. As a result, transplanted fetal liver cells are capable of repopulating normal livers. Important for cell-based therapies, hepatic stem/progenitor cells containing repopulation potential can be separated from fetal hematopoietic cells using the cell surface marker δ-like 1 (Dlk-1). In livers with advanced fibrosis, fetal epithelial stem/progenitor cells differentiate into functional hepatic cells and out-compete injured endogenous hepatocytes, which cause anti-fibrotic effects. Although fetal liver cells efficiently repopulate the liver, they will likely not be used for human cell transplantation. Thus, utilizing the underlying mechanism of repopulation and developed methods to produce similar growth-advantaged cells in vitro, e.g., human induced pluripotent stem cells (iPSCs), this approach has great potential for developing novel cell-based therapies in patients with liver disease. The present review gives a brief overview of the classic cell transplantation models and various cell sources studied as donor cell candidates. The advantages of fetal liver-derived stem/progenitor cells are discussed, as well as the mechanism of liver repopulation. Moreover, this article reviews the potential of in vitro developed synthetic human fetal livers from iPSCs and their therapeutic benefits.

Preclinical-to-clinical innovations in stem cell therapies for liver regeneration

Acute and chronic liver diseases are the major cause of high morbidity and mortality globally. Liver transplantation is a widely used therapeutic option for liver failure. However, the shortage of availability of liver donors has encouraged research on the alternative approach to liver regeneration. Cell-based regenerative medicine is the best alternative therapy to cater to this need. To date, advanced preclinical approaches have been undertaken on stem cell differentiation and their use in liver tissue engineering for generating efficacious and promising regenerative therapies. Advancements in the bioengineering of stem cells, and organoid generation are the way forward to efficient therapies against liver injury. This review summarizes the recent approaches for stem cell therapy-based liver regeneration and their proof of concepts for clinical application, bioengineering liver organoids to alleviate the liver failure caused due to chronic liver diseases.

Induced Endothelial Cell-Integrated Liver Assembloids Promote Hepatic Maturation and Therapeutic Effect on Cholestatic Liver Fibrosis

The transplantation of pluripotent stem cell (PSC)-derived liver organoids has been studied to solve the current donor shortage. However, the differentiation of unintended cell populations, difficulty in generating multi-lineage organoids, and tumorigenicity of PSC-derived organoids are challenges. However, direct conversion technology has allowed for the generation lineage-restricted induced stem cells from somatic cells bypassing the pluripotent state, thereby eliminating tumorigenic risks. Here, liver assembloids (iHEAs) were generated by integrating induced endothelial cells (iECs) into the liver organoids (iHLOs) generated with induced hepatic stem cells (iHepSCs). Liver assembloids showed enhanced functional maturity compared to iHLOs in vitro and improved therapeutic effects on cholestatic liver fibrosis animals in vivo. Mechanistically, FN1 expressed from iECs led to the upregulation of Itgα5/β1 and Hnf4α in iHEAs and were correlated to the decreased expression of genes related to hepatic stellate cell activation such as Lox and Spp1 in the cholestatic liver fibrosis animals. In conclusion, our study demonstrates the possibility of generating transplantable iHEAs with directly converted cells, and our results evidence that integrating iECs allows iHEAs to have enhanced hepatic maturation compared to iHLOs.

Preparation of Functional Human Hepatocytes Ex Vivo

Hepatocytes are liver parenchymal cells involved in performing various metabolic reactions. During the development of therapeutic drugs, toxicological assays are conducted using hepatocyte cultures before clinical trials. However, since primary hepatocytes cannot proliferate and rapidly lose their functions in vitro, many efforts have been put into modifying culture conditions to expand primary hepatocytes and induce hepatocyte functions in intrinsic and extrinsic stem/progenitor cells. In this chapter, we summarize recent advances in preparing hepatocyte cultures and induction of hepatocytes from various cellular sources.

Induced Pluripotent Stem Cells as a Tool for Modeling Hematologic Disorders and as a Potential Source for Cell-Based Therapies

The breakthrough in human induced pluripotent stem cells (hiPSCs) has revolutionized the field of biomedical and pharmaceutical research and opened up vast opportunities for drug discovery and regenerative medicine, especially when combined with gene-editing technology. Numerous healthy and patient-derived hiPSCs for human disease modeling have been established, enabling mechanistic studies of pathogenesis, platforms for preclinical drug screening, and the development of novel therapeutic targets/approaches. Additionally, hiPSCs hold great promise for cell-based therapy, serving as an attractive cell source for generating stem/progenitor cells or functional differentiated cells for degenerative diseases, due to their unlimited proliferative capacity, pluripotency, and ethical acceptability. In this review, we provide an overview of hiPSCs and their utility in the study of hematologic disorders through hematopoietic differentiation. We highlight recent hereditary and acquired genetic hematologic disease modeling with patient-specific iPSCs, and discuss their applications as instrumental drug screening tools. The clinical applications of hiPSCs in cell-based therapy, including the next-generation cancer immunotherapy, are provided. Lastly, we discuss the current challenges that need to be addressed to fulfill the validity of hiPSC-based disease modeling and future perspectives of hiPSCs in the field of hematology.

Interleukin-17 in Liver Disease Pathogenesis

Interleukin 17A (IL-17A)-producing T helper 17 (Th17) cells were identified as a subset of T helper cells that play a critical role in host defense against bacterial and fungal pathogens. Th17 cells differentiate from Th0 naïve T-cells in response to transforming growth factor β1 (TGF-β1) and IL-6, the cytokines which also drive development of liver fibrosis, require activation of transcription factor retinoic acid receptor-related orphan nuclear receptor gamma t (RORγt). IL-17A signals through the ubiquitously expressed receptor IL-17RA. Expression of IL-17RA is upregulated in patients with hepatitis B virus/hepatitis C virus (HBV/HCV) infections, nonalcoholic steatohepatitis (NASH), alcohol-associated liver disease (AALD), hepatocellular carcinoma (HCC), and experimental models of chronic toxic liver injury. The role of IL-17 signaling in the pathogenesis of NASH- and AALD-induced metabolic liver injury and HCC will be the focus of this review. The role of IL-17A-IL-17RA axis in mediation of the cross-talk between metabolically injured hepatic macrophages, hepatocytes, and fibrogenic myofibroblasts will be discussed.

Cell therapy for advanced liver diseases: Repair or rebuild

Advanced liver disease presents a significant worldwide health and economic burden and accounts for 3.5% of global mortality. When liver disease progresses to organ failure the only effective treatment is liver transplantation, which necessitates lifelong immunosuppression and carries associated risks. Furthermore, the shortage of suitable donor organs means patients may die waiting for a suitable transplant organ. Cell therapies have made their way from animal studies to a small number of early clinical trials. Herein, we review the current state of cell therapies for liver disease and the mechanisms underpinning their actions (to repair liver tissue or rebuild functional parenchyma). We also discuss cellular therapies that are on the clinical horizon and challenges that must be overcome before routine clinical use is a possibility.

Identification of hepatic progenitor cells in the canine fetal liver

The liver plays essential roles in human and animal organisms, such as the storage, release, metabolism, and elimination of various endogenous or exogenous substances. Although its vital importance, few treatments are yet available when a hepatic failure occurs, and hence, the use of stem cells has arisen as a possible solution for both human and veterinary medicines. Previous studies have shown the existence of hepatic progenitor cells in human fetuses that were positive for EpCAM and NCAM. There is limited evidence, however, further identification and characterization of these cells in other species. Considering the similarity between dogs and humans regarding physiology, and also the increasing importance of developing new treatments for both veterinary and translational medicine, this study attempted to identify hepatic progenitor cells in canine fetal liver. For that, livers from canine fetuses were collected, cells were isolated by enzymatic digestion and cultured. Cells were characterized regarding morphology and expression of EpCAM, NCAM, Nestin, and Thy-1/CD90 markers. Our results suggest that it is possible to identify hepatic progenitor cells in the canine fetal liver; however, for therapeutic use, further techniques for cellular isolation and culture are necessary to obtain enriched populations of hepatic progenitors from the canine fetal liver.

Sequentially induced motor neurons from human fibroblasts facilitate locomotor recovery in a rodent spinal cord injury model

Generation of autologous human motor neurons holds great promise for cell replacement therapy to treat spinal cord injury (SCI). Direct conversion allows generation of target cells from somatic cells, however, current protocols are not practicable for therapeutic purposes since converted cells are post-mitotic that are not scalable. Therefore, therapeutic effects of directly converted neurons have not been elucidated yet. Here, we show that human fibroblasts can be converted into induced motor neurons (iMNs) by sequentially inducing POU5F1(OCT4) and LHX3. Our strategy enables scalable production of pure iMNs because of the transient acquisition of proliferative iMN-intermediate cell stage which is distinct from neural progenitors. iMNs exhibited hallmarks of spinal motor neurons including transcriptional profiles, electrophysiological property, synaptic activity, and neuromuscular junction formation. Remarkably, transplantation of iMNs showed therapeutic effects, promoting locomotor functional recovery in rodent SCI model. Together, our advanced strategy will provide tools to acquire sufficient human iMNs that may represent a promising cell source for personalized cell therapy.

Improving the safety of human pluripotent stem cell therapies using genome-edited orthogonal safeguards

Despite their rapidly-expanding therapeutic potential, human pluripotent stem cell (hPSC)-derived cell therapies continue to have serious safety risks. Transplantation of hPSC-derived cell populations into preclinical models has generated teratomas (tumors arising from undifferentiated hPSCs), unwanted tissues, and other types of adverse events. Mitigating these risks is important to increase the safety of such therapies. Here we use genome editing to engineer a general platform to improve the safety of future hPSC-derived cell transplantation therapies. Specifically, we develop hPSC lines bearing two drug-inducible safeguards, which have distinct functionalities and address separate safety concerns. In vitro administration of one small molecule depletes undifferentiated hPSCs >10⁶-fold, thus preventing teratoma formation in vivo. Administration of a second small molecule kills all hPSC-derived cell-types, thus providing an option to eliminate the entire hPSC-derived cell product in vivo if adverse events arise. These orthogonal safety switches address major safety concerns with pluripotent cell-derived therapies.

Human pluripotent stem cell derived therapies can have serious safety risks. Here the authors design two drug inducible genetic safeguards to deplete undifferentiated hPSCs and hPSC-derived cell types.

Alternative Cell Sources for Liver Parenchyma Repopulation: Where Do We Stand?

Acute and chronic liver failure is a highly prevalent medical condition with high morbidity and mortality. Currently, the therapy is orthotopic liver transplantation. However, in some instances, chiefly in the setting of metabolic diseases, transplantation of individual cells, specifically functional hepatocytes, can be an acceptable alternative. The gold standard for this therapy is the use of primary human hepatocytes, isolated from livers that are not suitable for whole organ transplantations. Unfortunately, primary human hepatocytes are scarcely available, which has led to the evaluation of alternative sources of functional hepatocytes. In this review, we will compare the ability of most of these candidate alternative cell sources to engraft and repopulate the liver of preclinical animal models with the repopulation ability found with primary human hepatocytes. We will discuss the current shortcomings of the different cell types, and some of the next steps that we believe need to be taken to create alternative hepatocyte progeny capable of regenerating the failing liver.

A critical look: Challenges in differentiating human pluripotent stem cells into desired cell types and organoids

Too many choices can be problematic. This is certainly the case for human pluripotent stem cells (hPSCs): they harbor the potential to differentiate into hundreds of cell types; yet it is highly challenging to exclusively differentiate hPSCs into a single desired cell type. This review focuses on unresolved and fundamental questions regarding hPSC differentiation and critiquing the identity and purity of the resultant cell populations. These are timely issues in view of the fact that hPSC-derived cell populations have or are being transplanted into patients in over 30 ongoing clinical trials. While many in vitro differentiation protocols purport to "mimic development," the exact number and identity of intermediate steps that a pluripotent cell takes to differentiate into a given cell type in vivo remains largely unknown. Consequently, most differentiation efforts inevitably generate a heterogeneous cellular population, as revealed by single-cell RNA-sequencing and other analyses. The presence of unwanted cell types in differentiated hPSC populations does not portend well for transplantation therapies. This provides an impetus to precisely control differentiation to desired ends-for instance, by logically blocking the formation of unwanted cell types or by overexpressing lineage-specifying transcription factors-or by harnessing technologies to selectively purify desired cell types. Conversely, approaches to differentiate three-dimensional "organoids" from hPSCs intentionally generate heterogeneous cell populations. While this is intended to mimic the rich cellular diversity of developing tissues, whether all such organoids are spatially organized in a manner akin to native organs (and thus, whether they fully qualify as organoids) remains to be fully resolved. This article is categorized under: Adult Stem Cells > Tissue Renewal > Regeneration: Stem Cell Differentiation and Reversion Gene Expression > Transcriptional Hierarchies: Cellular Differentiation Early Embryonic Development: Gastrulation and Neurulation.

Glypican-3-Specific Antibody Drug Conjugates Targeting Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the second most common cause of cancer-related death in the world. Therapeutic outcomes of HCC remain unsatisfactory, and novel treatments are urgently needed. GPC3 (glypican-3) is an emerging target for HCC, given the findings that 1) GPC3 is highly expressed in more than 70% of HCC; (2) elevated GPC3 expression is linked with poor HCC prognosis; and (3) GPC3-specific therapeutics, including immunotoxin, bispecific antibody and chimeric antigen receptor T cells. have shown promising results. Here, we postulate that GPC3 is a potential target of antibody-drug conjugates (ADCs) for treating liver cancer. To determine the payload for ADCs against liver cancer, we screened three large drug libraries (> 9,000 compounds) against HCC cell lines and found that the most potent drugs are DNA-damaging agents. Duocarmycin SA and pyrrolobenzodiazepine dimer were chosen as the payloads to construct two GPC3-specific ADCs: hYP7-DC and hYP7-PC. Both ADCs showed potency at picomolar concentrations against a panel of GPC3-positive cancer cell lines, but not GPC3 negative cell lines. To improve potency, we investigated the synergetic effect of hYP7-DC with approved drugs. Gemcitabine showed a synergetic effect with hYP7-DC in vitro and in vivo. Furthermore, single treatment of hYP7-PC induced tumor regression in multiple mouse models. Conclusion: We provide an example of an ADC targeting GPC3, suggesting a strategy for liver cancer therapy.

A Roadmap for Human Liver Differentiation from Pluripotent Stem Cells

How are closely related lineages, including liver, pancreas, and intestines, diversified from a common endodermal origin? Here, we apply principles learned from developmental biology to rapidly reconstitute liver progenitors from human pluripotent stem cells (hPSCs). Mapping the formation of multiple endodermal lineages revealed how alternate endodermal fates (e.g., pancreas and intestines) are restricted during liver commitment. Human liver fate was encoded by combinations of inductive and repressive extracellular signals at different doses. However, these signaling combinations were temporally re-interpreted: cellular competence to respond to retinoid, WNT, TGF-β, and other signals sharply changed within 24 hr. Consequently, temporally dynamic manipulation of extracellular signals was imperative to suppress the production of unwanted cell fates across six consecutive developmental junctures. This efficiently generated 94.1% ± 7.35% TBX3⁺HNF4A⁺ human liver bud progenitors and 81.5% ± 3.2% FAH⁺ hepatocyte-like cells by days 6 and 18 of hPSC differentiation, respectively; the latter improved short-term survival in the Fah^-/-Rag2^-/-Il2rg^-/- mouse model of liver failure.

Biologically inspired approaches to enhance human organoid complexity

Organoids are complex three-dimensional in vitro organ-like model systems. Human organoids, which are derived from human pluripotent stem cells or primary human donor tissue, have been used to address fundamental questions about human development, stem cell biology and organ regeneration. Focus has now shifted towards implementation of organoids for biological discovery and advancing existing systems to more faithfully recapitulate the native organ. This work has highlighted significant unknowns in human biology and has invigorated new exploration into the cellular makeup of human organs during development and in the adult - work that is crucial for providing appropriate benchmarks for organoid systems. In this Review, we discuss efforts to characterize human organ cellular complexity and attempts to make organoid models more realistic through co-culture, transplantation and bioengineering approaches.

Hepatic differentiation of human pluripotent stem cells by developmental stage-related metabolomics products

Endogenous cell signals regulate tissue homeostasis and are significant for directing the fate of stem cells. During liver development, cytokines released from various cell types are critical for stem/progenitor cell differentiation and lineage expansions. To determine mechanisms in these stage-specific lineage interactions, we modeled potential effects of soluble signals derived from immortalized human fetal liver parenchymal cells on stem cells, including embryonic and induced pluripotent stem cells. For identifying lineage conversion and maturation, we utilized conventional assays of cell morphology, gene expression analysis and lineage markers. Molecular pathway analysis used functional genomics approaches. Metabolic properties were analyzed to determine the extent of hepatic differentiation. Cell transplantation studies were performed in mice with drug-induced acute liver failure to elicit benefits in hepatic support and tissue regeneration. These studies showed signals emanating from fetal liver cells induced hepatic differentiation in stem cells. Gene expression profiling and comparison of regulatory networks in immature and mature hepatocytes revealed stem cell-derived hepatocytes represented early fetal-like stage. Unexpectedly, differentiation-inducing soluble signals constituted metabolomics products and not proteins. In stem cells exposed to signals from fetal cells, mechanistic gene networks of upstream regulators decreased pluripotency, while simultaneously inducing mesenchymal and epithelial properties. The extent of metabolic and synthetic functions in stem cell-derived hepatocytes was sufficient for providing hepatic support along with promotion of tissue repair to rescue mice in acute liver failure. During this rescue, paracrine factors from transplanted cells contributed in stimulating liver regeneration. We concluded that hepatic differentiation of pluripotent stem cells with metabolomics products will be significant for developing therapies. The differentiation mechanisms involving metabolomics products could have an impact on advancing recruitment of stem/progenitor cells during tissue homeostasis.

Bioengineered functional humanized livers: An emerging supportive modality to bridge the gap of organ transplantation for management of end-stage liver diseases

End stage liver diseases (ESLD) represent a major, neglected global public health crisis which requires an urgent action towards finding a proper cure. Orthotropic liver transplantation has been the only definitive treatment modality for ESLD. However, shortage of donor organs, timely unavailability, post-surgery related complications and financial burden on the patients limits the number of patients receiving the transplants. Since last two decades cell-based therapies have revolutionized the field of organ/tissue regeneration. However providing an alternative organ source to address the donor liver shortage still poses potential challenges. The developments made in this direction provide useful futuristic approaches, which could be translated into pre-clinical and clinical settings targeting appropriate applications in specific disease conditions. Earlier studies have demonstrated the applicability of this particular approach to generate functional organ in rodent system by connecting them with portal and hepatic circulatory networks. However, such strategy requires very high level of surgical expertise and also poses the technical and financial questions towards its future applicability. Hence, alternative sites for generating secondary organs are being tested in several types of disease conditions. Among different sites, omentum has been proved to be more appropriate site for implanting several kinds of functional tissue constructs without eliciting much immunological response. Hence, omentum may be considered as better site for transplanting humanized bioengineered ex vivo generated livers, thereby creating a secondary organ at intra-omental site. However, the expertise for generating such bioengineered organs are limited and only very few centres are involved for investigating the potential use of such implants in clinical practice due to gap between the clinical transplant surgeons and basic scientists working on the concept evolution. Herein we discuss the recent advances and challenges to create functional secondary organs through intra-omental transplantation of ex vivo generated bioengineered humanized livers and their further application in the management of ESLD as a supportive bridge for organ transplantation.

SERS of cells: What can we learn from cell lysates?

Surface-enhanced Raman spectroscopy (SERS) is a promising and emerging technique to analyze the cellular environment. We developed an alternative, rapid and label-free SERS-based method to get information about the cellular environment by analyzing cells lysates, thus avoiding the need to incorporate nanoparticles into cells. Upon sonicating and filtrating cells, we obtained lysates which, mixed with Au or Ag nanoparticles, yield stable and repeatable SERS spectra, whose overall profile depends on the metal used as substrate, but not on the buffer used for the lysis process. Bands appearing in these spectra were shown to arise mostly from the cytosol and were assigned to adenine, guanine, adenosine and reduced glutathione (GSH). Spectral differences among various cell types also demonstrated that this approach is suitable for cell type identification.

Long-Term Fate of Human Fetal Liver Progenitor Cells Transplanted in Injured Mouse Livers

Liver progenitor cells have the potential to repair and regenerate a diseased liver. The success of any translational efforts, however, hinges on thorough understanding of the fate of these cells after transplant, especially in terms of long-term safety and efficacy. Here, we report transplantation of a liver progenitor population isolated from human fetal livers into immune-permissive mice with follow-up up to 36 weeks after transplant. We found that human progenitor cells engraft and differentiate into functional human hepatocytes in the mouse, producing albumin, alpha-1-antitrypsin, and glycogen. They create tight junctions with mouse hepatocytes, with no evidence of cell fusion. Interestingly, they also differentiate into functional endothelial cell and bile duct cells. Transplantation of progenitor cells abrogated carbon tetrachloride-induced fibrosis in recipient mice, with downregulation of procollagen and anti-smooth muscle actin. Paradoxically, the degree of engraftment of human hepatocytes correlated negatively with the anti-fibrotic effect. Progenitor cell expansion was most prominent in cirrhotic animals, and correlated with transcript levels of pro-fibrotic genes. Animals that had resolution of fibrosis had quiescent native progenitor cells in their livers. No evidence of neoplasia was observed, even up to 9 months after transplantation. Human fetal liver progenitor cells successfully attenuate liver fibrosis in mice. They are activated in the setting of liver injury, but become quiescent when injury resolves, mimicking the behavior of de novo progenitor cells. Our data suggest that liver progenitor cells transplanted into injured livers maintain a functional role in the repair and regeneration of the liver. Stem Cells 2018;36:103-113.

Evaluating the regenerative potential and functionality of human liver cells in mice

Liver diseases afflict millions of patients worldwide. Currently, the only long-term treatment for liver failure is the transplantation of a new liver. However, intravenously transplanting a suspension of human hepatocytes might be a less-invasive approach to partially reconstitute lost liver functions in human patients as evinced by promising outcomes in clinical trials. The purpose of this essay is to emphasize outstanding questions that continue to surround hepatocyte transplantation. While adult primary human hepatocytes are the gold standard for transplantation, hepatocytes are heterogeneous. Whether all hepatocytes engraft equally and what specifically defines an "engraftable" hepatocyte capable of long-term liver reconstitution remains unclear. To this end, mouse models of liver injury enable the evaluation of human hepatocytes and their behavior upon transplantation into a complex injured liver environment. While mouse models may not be fully representative of the injured human liver and human hepatocytes tend to engraft mice less efficiently than mouse hepatocytes, valuable lessons have nonetheless been learned from transplanting human hepatocytes into mouse models. With an eye to the future, it will be crucial to eventually detail the optimal biological source (whether in vivo- or in vitro-derived) and presumptive heterogeneity of human hepatocytes and to understand the mechanisms through which they engraft and regenerate liver tissue in vivo.

Cholangiocytes: Cell transplantation

BACKGROUND

Due to significant limitations to the access to orthotropic liver transplantation, cell therapies for liver diseases have gained large interest worldwide.

SCOPE OF REVIEW

To revise current literature dealing with cell therapy for liver diseases. We discussed the advantages and pitfalls of the different cell sources tested so far in clinical trials and the rationale underlying the potential benefits of transplantation of human biliary tree stem cells (hBTSCs).

MAJOR CONCLUSIONS

Transplantation of adult hepatocytes showed transient benefits but requires immune-suppression that is a major pitfall in patients with advanced liver diseases. Mesenchymal stem cells and hematopoietic stem cells transplanted into patients with liver diseases are not able to replace resident hepatocytes but rather they target autoimmune or inflammatory processes into the liver. Stem cells isolated from fetal or adult liver have been recently proposed as alternative cell sources for advanced liver cirrhosis and metabolic liver disease. We demonstrated the presence of multipotent cells expressing a variety of endodermal stem cell markers in (peri)-biliary glands of bile ducts in fetal or adult human tissues, and in crypts of gallbladder epithelium. In the first cirrhotic patients treated in our center with biliary tree stem cell therapy, we registered no adverse event but significant benefits.

GENERAL SIGNIFICANCE

The biliary tree stem cell could represent the ideal cell source for the cell therapy of liver diseases. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Generation of a Humanized Mouse Liver Using Human Hepatic Stem Cells

A novel animal model involving chimeric mice with humanized livers established via human hepatocyte transplantation has been developed. These mice, in which the liver has been repopulated with functional human hepatocytes, could serve as a useful tool for investigating human hepatic cell biology, drug metabolism, and other preclinical applications. One of the key factors required for successful transplantation of human hepatocytes into mice is the elimination of the endogenous hepatocytes to prevent competition with the human cells and provide a suitable space and microenvironment for promoting human donor cell expansion and differentiation. To date, two major liver injury mouse models utilizing fumarylacetoacetate hydrolase (Fah) and uroplasminogen activator (uPA) mice have been established. However, Fah mice are used mainly with mature hepatocytes and the application of the uPA model is limited by decreased breeding. To overcome these limitations, Alb-toxin receptor mediated cell knockout (TRECK)/SCID mice were used for in vivo differentiation of immature human hepatocytes and humanized liver generation. Human hepatic stem cells (HpSCs) successfully repopulated the livers of Alb-TRECK/SCID mice that had developed lethal fulminant hepatic failure following diphtheria toxin (DT) treatment. This model of a humanized liver in Alb-TRECK/SCID mice will have functional applications in studies involving drug metabolism and drug-drug interactions and will promote other in vivo and in vitro studies.

Humanized mice efficiently engrafted with fetal hepatoblasts and syngeneic immune cells develop human monocytes and NK cells

BACKGROUND & AIMS

Human liver chimeric mice are useful models of human hepatitis virus infection, including hepatitis B and C virus infections. Independently, immunodeficient mice reconstituted with CD34(+) hematopoietic stem cells (HSC) derived from fetal liver reliably develop human T and B lymphocytes. Combining these systems has long been hampered by inefficient liver reconstitution of human fetal hepatoblasts. Our study aimed to enhance hepatoblast engraftment in order to create a mouse model with syngeneic human liver and immune cells.

METHODS

The effects of human oncostatin-M administration on fetal hepatoblast engraftment into immunodeficient fah(-/-) mice was tested. Mice were then transplanted with syngeneic human hepatoblasts and HSC after which human leukocyte chimerism and functionality were analyzed by flow cytometry, and mice were challenged with HBV.

RESULTS

Addition of human oncostatin-M enhanced human hepatoblast engraftment in immunodeficient fah(-/-) mice by 5-100 fold. In contrast to mice singly engrafted with HSC, which predominantly developed human T and B lymphocytes, mice co-transplanted with syngeneic hepatoblasts also contained physiological levels of human monocytes and natural killer cells. Upon infection with HBV, these mice displayed rapid and sustained viremia.

CONCLUSIONS

Our study provides a new mouse model with improved human fetal hepatoblast engraftment and an expanded human immune cell repertoire. With further improvements, this model may become useful for studying human immunity against viral hepatitis.

LAY SUMMARY

Important human pathogens such as hepatitis B virus, hepatitis C virus and human immunodeficiency virus only infect human cells which complicates the development of mouse models for the study of these pathogens. One way to make mice permissive for human pathogens is the transplantation of human cells into immune-compromised mice. For instance, the transplantation of human liver cells will allow the infection of these so-called "liver chimeric mice" with hepatitis B virus and hepatitis C virus. The co-transplantation of human immune cells into liver chimeric mice will further allow the study of human immune responses to hepatitis B virus or hepatitis C virus. However, for immunological studies it will be crucial that the transplanted human liver and immune cells are derived from the same human donor. In our study we describe the efficient engraftment of human fetal liver cells and immune cells derived from the same donor into mice. We show that liver co-engraftment resulted in an expanded human immune cell repertoire, including monocytes and natural killer cells in the liver. We further demonstrate that these mice could be infected with hepatitis B virus, which lead to an expansion of natural killer cells. In conclusion we have developed a new mouse model that could be useful to study human immune responses to human liver pathogens.

Hepatocyte cell therapy in liver disease

Liver disease is a leading cause of morbidity and mortality. Liver transplantation remains the only proven treatment for end-stage liver failure but is limited by the availability of donor organs. Hepatocyte cell therapy, either with bioartificial liver devices or hepatocyte transplantation, may help address this by delaying or preventing liver transplantation. Early clinical studies have shown promising results, however in most cases, the benefit has been short lived and so further research into these therapies is required. Alternative sources of hepatocytes, including stem cell-derived hepatocytes, are being investigated as the isolation of primary human hepatocytes is limited by the same shortage of donor organs. This review summarises the current clinical experience of hepatocyte cell therapy together with an overview of possible alternative sources of hepatocytes. Current and future areas for research that might lead towards the realisation of the full potential of hepatocyte cell therapy are discussed.

Allogeneic Liver Transplantation and Subsequent Syngeneic Hepatocyte Transplantation in a Rat Model: Proof of Concept for in vivo Tissue Engineering

Objectives: Stable long-term functioning of liver cells after transplantation in humans is still not achieved successfully. A new approach for successful engraftment of liver cells may be the transplantation of syngeneic cells into an allogeneic liver graft. We therefore developed a new rat model for combined liver and liver cell transplantation (cLCTx) under stable immunosuppression. Materials and Methods: After inducing a mitotic block, liver grafts from female donor rats (Dark Agouti) were transplanted into female recipients (Lewis). In male Lewis rats, liver cell proliferation was induced with subsequent cell isolation and transplantation into female recipients after organ transplantation. Y-chromosome detection of the transplanted male cells was performed by quantitative polymerase chain reaction (qPCR) and fluorescence in situ hybridization (FisH) with localization of transplanted cells by immunohistochemistry. Results: Immunohistochemistry demonstrated the engraftment of transplanted cells, as confirmed by FisH, showing repopulation of the liver graft with 15.6% male cells (± 1.8 SEM) at day 90. qPCR revealed 14.15% (± 5.09 SEM) male DNA at day 90. Conclusion: Engraftment of transplanted syngeneic cells after cLCTx was achieved for up to 90 days under immunosuppression. Immunohistochemistry indicated cell proliferation, and the FisH results were partly confirmed by qPCR. This new protocol in rats appears feasible for addressing long-term functioning and eventually the induction of operational tolerance in the future.

In vivo models of hepatitis B and C virus infection

Globally, more than 500 million individuals are chronically infected with hepatitis B (HBV), delta (HDV), and/or C (HCV) viruses, which can result in severe liver disease. Mechanistic studies of viral persistence and pathogenesis have been hampered by the scarcity of animal models. The limited species and cellular host range of HBV, HDV, and HCV, which robustly infect only humans and chimpanzees, have posed challenges for creating such animal models. In this review, we will discuss the barriers to interspecies transmission and the progress that has been made in our understanding of the HBV, HDV, and HCV life cycles. Additionally, we will highlight a variety of approaches that overcome these barriers and thus facilitate in vivo studies of these hepatotropic viruses.

A Distinct Subpopulation of Bone Marrow Mesenchymal Stem Cells, Muse Cells, Directly Commit to the Replacement of Liver Components

Genotyping graft livers by short tandem repeats after human living-donor liver transplantation (n = 20) revealed the presence of recipient or chimeric genotype cases in hepatocytes (6 of 17, 35.3%), sinusoidal cells (18 of 18, 100%), cholangiocytes (15 of 17, 88.2%) and cells in the periportal areas (7 of 8, 87.5%), suggesting extrahepatic cell involvement in liver regeneration. Regarding extrahepatic origin, bone marrow mesenchymal stem cells (BM-MSCs) have been suggested to contribute to liver regeneration but compose a heterogeneous population. We focused on a more specific subpopulation (1-2% of BM-MSCs), called multilineage-differentiating stress-enduring (Muse) cells, for their ability to differentiate into liver-lineage cells and repair tissue. We generated a physical partial hepatectomy model in immunodeficient mice and injected green fluorescent protein (GFP)-labeled human BM-MSC Muse cells intravenously (n = 20). Immunohistochemistry, fluorescence in situ hybridization and species-specific polymerase chain reaction revealed that they integrated into regenerating areas and expressed liver progenitor markers during the early phase and then differentiated spontaneously into major liver components, including hepatocytes (≈74.3% of GFP-positive integrated Muse cells), cholangiocytes (≈17.7%), sinusoidal endothelial cells (≈2.0%), and Kupffer cells (≈6.0%). In contrast, the remaining cells in the BM-MSCs were not detected in the liver for up to 4 weeks. These results suggest that Muse cells are the predominant population of BM-MSCs that are capable of replacing major liver components during liver regeneration.

Concise review: cell therapies for hereditary metabolic liver diseases-concepts, clinical results, and future developments

The concept of cell-based therapies for inherited metabolic liver diseases has been introduced for now more than 40 years in animal experiments, but controlled clinical data in humans are still not available. In the era of dynamic developments in stem cell science, the "right" cell for transplantation is considered as an important key for successful treatment. Do we aim to transplant mature hepatocytes or do we consider the liver as a stem/progenitor-driven organ and replenish the diseased liver with genetically normal stem/progenitor cells? Although conflicting results from cell tracing and transplantation experiments have recently emerged about the existence and role of stem/progenitor cells in the liver, their overall contribution to parenchymal cell homeostasis and tissue repair is limited. Accordingly, engraftment and repopulation efficacies of extrahepatic and liver-derived stem/progenitor cell types are considered to be lower compared to mature hepatocytes. On the basis of these results, we will discuss the current clinical cell transplantation programs for inherited metabolic liver diseases and future developments in liver cell therapy.

Stem Cell Therapies for Treatment of Liver Disease

Cell therapy is an emerging form of treatment for several liver diseases, but is limited by the availability of donor livers. Stem cells hold promise as an alternative to the use of primary hepatocytes. We performed an exhaustive review of the literature, with a focus on the latest studies involving the use of stem cells for the treatment of liver disease. Stem cells can be harvested from a number of sources, or can be generated from somatic cells to create induced pluripotent stem cells (iPSCs). Different cell lines have been used experimentally to support liver function and treat inherited metabolic disorders, acute liver failure, cirrhosis, liver cancer, and small-for-size liver transplantations. Cell-based therapeutics may involve gene therapy, cell transplantation, bioartificial liver devices, or bioengineered organs. Research in this field is still very active. Stem cell therapy may, in the future, be used as a bridge to either liver transplantation or endogenous liver regeneration, but efficient differentiation and production protocols must be developed and safety must be demonstrated before it can be applied to clinical practice.

Transplantation of hESC-derived hepatocytes protects mice from liver injury

Background

Hepatic cell therapy has become a viable alternative to liver transplantation for life-threatening liver diseases. However, the supply of human hepatocytes is limited due to the shortage of suitable donor organs required to isolate high-quality cells. Human pluripotent stem cells reflect a potential renewable source for generating functional hepatocytes. However, most differentiation protocols use undefined matrices or factors of animal origin; as such, the resulting hepatocytes are not Good Manufacturing Practice compliant. Moreover, the preclinical studies employed to assess safety and function of human embryonic stem cell (hESC)-derived hepatocytes are generally limited to immunodeficient mice. In the present study, we evaluate the generation of hepatocytes under defined conditions using a European hESC line (VAL9) which was derived under animal-free conditions. The function capacity of VAL9-derived hepatocytes was assessed by transplantation into mice with acetaminophen-induced acute liver failure, a clinically relevant model.

Methods

We developed a protocol that successfully differentiates hESCs into bipotent hepatic progenitors under defined conditions, without the use of chromatin modifiers such as dimethyl sulphoxide. These progenitors can be cryopreserved and are able to generate both committed precursors of cholangiocytes and neonate-like hepatocytes.

Results

Thirty days post-differentiation, hESCs expressed hepatocyte-specific markers such as asialoglycoprotein receptor and hepatic nuclear factors including HNF4α. The cells exhibited properties of mature hepatocytes such as urea secretion and UGT1A1 and cytochrome P450 activities. When transplanted into mice with acetaminophen-induced acute liver failure, a model of liver damage, the VAL9-derived hepatocytes efficiently engrafted and proliferated, repopulating up to 10 % of the liver. In these transplanted livers, we observed a significant decrease of liver transaminases and found no evidence of tumourigenicity. Thus, VAL9-derived hepatocytes were able to rescue hepatic function in acetaminophen-treated animals.

Conclusions

Our study reveals an efficient protocol for differentiating VAL9 hESCs to neonatal hepatocytes which are then able to repopulate livers in vivo without tumour induction. The human hepatocytes are able to rescue liver function in mice with acetaminophen-induced acute toxicity. These results provide proof-of-concept that replacement therapies using hESC-derived hepatocytes are effective for treating liver diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0227-6) contains supplementary material, which is available to authorized users.

Determinants of hepatitis B and delta virus host tropism

Hepatitis B virus (HBV) infections are a global health problem afflicting approximately 360 million patients. Of these individuals, 15-20 million are co-infected with hepatitis delta virus (HDV). Progress toward curative therapies has been impeded by the highly restricted host tropism of HBV, which is limited to productive infections in humans and chimpanzees. Here, we will discuss different approaches that have been taken to study HBV and HDV infections in vivo. The development of transgenic and humanized mice has lead to deeper insights into HBV pathogenesis. An improved understanding of the determinants governing HBV and HDV species tropism will aid in the construction of a small animal model with inheritable susceptible to HBV/HDV.

Cell therapy for liver disease: From liver transplantation to cell factory

Work over several decades has laid solid foundations for the advancement of liver cell therapy. To date liver cell therapy in people has taken the form of hepatocyte transplantation for metabolic disorders with a hepatic basis, and for acute or chronic liver failure. Although clinical trials using various types of autologous cells have been implemented to promote liver regeneration or reduce liver fibrosis, clear evidence of therapeutic benefits have so far been lacking. Cell types that have shown efficacy in preclinical models include hepatocytes, liver sinusoidal endothelial cells, mesenchymal stem cells, endothelial progenitor cells, and macrophages. However, positive results in animal models have not always translated through to successful clinical therapies and more realistic preclinical models need to be developed. Studies defining the optimal repopulation by transplanted cells, including routes of cell transplantation, superior engraftment and proliferation of transplanted cells, as well as optimal immunosuppression regimens are required. Tissue engineering approaches to transplant cells in extrahepatic locations have also been proposed. The derivation of hepatocytes from pluripotent or reprogrammed cells raises hope that donor organ and cell shortages could be overcome in the future. Critical hurdles to be overcome include the production of hepatocytes from pluripotent cells with equal functional capacity to primary hepatocytes and long-term phenotypic stability in vivo.

Human hepatic stem cells transplanted into a fulminant hepatic failure Alb-TRECK/SCID mouse model exhibit liver reconstitution and drug metabolism capabilities

INTRODUCTION

Chimeric mice with humanized livers were recently established by transplanting human hepatocytes. This mouse model that is repopulated with functional human hepatocytes could be a useful tool for investigating human hepatic cell biology and drug metabolism and for other preclinical applications. Successfully transplanting human hepatocytes into mice requires that recipient mice with liver failure do not reject these human cells and provide a suitable microenvironment (supportive niche) to promote human donor cell expansion and differentiation. To overcome the limitations of current mouse models, we used Alb-TRECK/SCID mice for in vivo human immature hepatocyte differentiation and humanized liver generation.

METHODS

1.5 μg/kg diphtheria toxin was administrated into 8-week-old Alb-TRECK/SCID mice, and the degree of liver damage was assessed by serum aspartate aminotransferase activity levels. Forty-eight hours later, mice livers were sampled for histological analyses, and the human donor cells were then transplanted into mice livers on the same day. Chimeric rate and survival rate after cell transplantation was evaluated. Expressions of human hepatic-related genes were detected. A human albumin enzyme-linked immunosorbent assay was performed after 50 days of transplantation. On day 60 after transplantation, drug metabolism was examined in mice.

RESULTS

Both human primary fetal liver cells and hepatic stem cells were successfully repopulated in the livers of Alb-TRECK/SCID mice that developed lethal fulminant hepatic failure after administering diphtheria toxin; the repopulation rate in some mice was nearly 100%. Compared with human primary fetal liver cells, human hepatic stem cell transplantation rescued Alb-TRECK/SCID mice with lethal fulminant hepatic failure, and human hepatic stem cell-derived humanized livers secreted more human albumin into mouse sera and also functioned as a "human liver" that could metabolize the drugs ketoprofen and debrisoquine.

CONCLUSION

Our model of a humanized liver in Alb-TRECK/SCID mice may provide for functional applications such as drug metabolism, drug to drug interactions, and promote other in vivo and in vitro studies.

Generation of improved mouse models for the study of hepatitis C virus

Approximately 3% of the world׳s population suffers from chronic infections with hepatitis C virus (HCV). Although current treatment regimes are capable of effectively eradicating HCV infection from these patients, the cost of these combinations of direct-acting antivirals are prohibitive. Approximately 80% of untreated chronic HCV carriers will be at high risk for developing severe liver disease, including fibrosis, cirrhosis, and hepatocellular carcinoma. A vaccine is urgently needed to lessen this global burden. Besides humans, HCV infection can be experimentally transmitted to chimpanzees, and this is the best model for studies of HCV infection and related innate and adaptive immune responses. Although the chimpanzee model yielded valuable insight, limited availability, high cost and ethical considerations limit their utility. The only small animal models of robust HCV infection are highly immunodeficient mice with human chimeric livers. However, these mice cannot be used to study adaptive immune responses and therefore a more relevant animal model is needed to assist in vaccine development. Novel strains of immunodeficient mice have been developed that allow for the engraftment of human hepatopoietic stem cells, as well as functional human lymphoid cells and tissues, effectively creating human immune systems in otherwise immunodeficient mice. These humanized mice are rapidly emerging as pre-clinical bridges for numerous pathogens that, like HCV, only cause infectious disease in humans. This review highlights the potential these new models have for changing the current landscape for HCV research and vaccine development.

A novel mouse model for stable engraftment of a human immune system and human hepatocytes

Hepatic infections by hepatitis B virus (HBV), hepatitis C virus (HCV) and Plasmodium parasites leading to acute or chronic diseases constitute a global health challenge. The species tropism of these hepatotropic pathogens is restricted to chimpanzees and humans, thus model systems to study their pathological mechanisms are severely limited. Although these pathogens infect hepatocytes, disease pathology is intimately related to the degree and quality of the immune response. As a first step to decipher the immune response to infected hepatocytes, we developed an animal model harboring both a human immune system (HIS) and human hepatocytes (HUHEP) in BALB/c Rag2-/- IL-2Rγc-/- NOD.sirpa uPAtg/tg mice. The extent and kinetics of human hepatocyte engraftment were similar between HUHEP and HIS-HUHEP mice. Transplanted human hepatocytes were polarized and mature in vivo, resulting in 20-50% liver chimerism in these models. Human myeloid and lymphoid cell lineages developed at similar frequencies in HIS and HIS-HUHEP mice, and splenic and hepatic compartments were humanized with mature B cells, NK cells and naïve T cells, as well as monocytes and dendritic cells. Taken together, these results demonstrate that HIS-HUHEP mice can be stably (> 5 months) and robustly engrafted with a humanized immune system and chimeric human liver. This novel HIS-HUHEP model provides a platform to investigate human immune responses against hepatotropic pathogens and to test novel drug strategies or vaccine candidates.

MicroRNA-199a-5p inhibition enhances the liver repopulation ability of human embryonic stem cell-derived hepatic cells

BACKGROUND & AIMS

Current hepatic differentiation protocols for human embryonic stem cells (ESCs) require substantial improvements. MicroRNAs (miRNAs) have been reported to regulate hepatocyte cell fate during liver development, but their utility to improve hepatocyte differentiation from ESCs remains to be investigated. Therefore, our aim was to identify and to analyse hepatogenic miRNAs for their potential to improve hepatocyte differentiation from ESCs.

METHODS

By miRNA profiling and in vitro screening, we identified miR-199a-5p among several potential hepatogenic miRNAs. Transplantation studies of miR-199a-5p-inhibited hepatocyte-like cells (HLCs) in the liver of immunodeficient fumarylacetoacetate hydrolase knockout mice (Fah(-/-)/Rag2(-/-)/Il2rg(-/-)) were performed to assess their in vivo liver repopulation potential. For target determination, western blot and luciferase reporter assay were carried out.

RESULTS

miRNA profiling revealed 20 conserved candidate hepatogenic miRNAs. By miRNA screening, only miR-199a-5p inhibition in HLCs was found to be able to enhance the in vitro hepatic differentiation of mouse as well as human ESCs. miR-199a-5p inhibition in human ESCs-derived HLCs enhanced their engraftment and repopulation capacity in the liver of Fah(-/-)/Rag2(-/-)/Il2rg(-/-) mice. Furthermore, we identified SMARCA4 and MST1 as novel targets of miR-199a-5p that may contribute to the improved hepatocyte generation and in vivo liver repopulation.

CONCLUSIONS

Our findings demonstrate that miR-199a-5p inhibition in ES-derived HLCs leads to improved hepatocyte differentiation. Upon transplantation, HLCs were able to engraft and repopulate the liver of Fah(-/-)/Rag2(-/-)/Il2rg(-/-) mice. Thus, our findings suggest that miRNA modulation may serve as a promising approach to generate more mature HLCs from stem cell sources for the treatment of liver diseases.

Engrafted human stem cell-derived hepatocytes establish an infectious HCV murine model

The demonstrated ability to differentiate both human embryonic stem cells (hESCs) and patient-derived induced pluripotent stem cells (hiPSCs) into hepatocyte-like cells (HLCs) holds great promise for both regenerative medicine and liver disease research. Here, we determined that, despite an immature phenotype, differentiated HLCs are permissive to hepatitis C virus (HCV) infection and mount an interferon response to HCV infection in vitro. HLCs differentiated from hESCs and hiPSCs could be engrafted in the liver parenchyma of immune-deficient transgenic mice carrying the urokinase-type plasminogen activator gene driven by the major urinary protein promoter. The HLCs were maintained for more than 3 months in the livers of chimeric mice, in which they underwent further maturation and proliferation. These engrafted and expanded human HLCs were permissive to in vivo infection with HCV-positive sera and supported long-term infection of multiple HCV genotypes. Our study demonstrates efficient engraftment and in vivo HCV infection of human stem cell-derived hepatocytes and provides a model to study chronic HCV infection in patient-derived hepatocytes, action of antiviral therapies, and the biology of HCV infection.

Cell-based therapy for acute and chronic liver failures: distinct diseases, different choices

Cell-based therapies (CBTs) are considered the effective approaches to treat liver failure. However, which cell type is the most suitable source of CBTs for acute liver failure (ALF) or chronic liver failure (CLF) remains unclear. To investigate this, mature hepatocytes in adult liver (adult HCs), fetal liver cells (FLCs), induced hepatic stem cells (iHepSCs) and bone marrow derived mesenchymal stromal cells (BMSCs) were used to CBTs for ConA-induced ALF and Fah-deficient induced CLF in mice. The results showed that only BMSCs remitted liver damage and rescued ALF in ConA-treated mice. In this process, BMSCs inhibited ConA-induced inflammatory response by decreasing the mRNA expressions of TNF-α, IFN-γ and FasL and increasing IL-10 mRNA expression. However, in the CLF model, not BMSCs but adult HCs transplantation lessened liver injury, recovered liver function and rescued the life of Fah-/- mice after NTBC withdrawal. Further study showed that adult HCs offered more effective liver regeneration compared to other cells in Fah-/- mice without NTBC. These results demonstrated that BMSCs and adult HCs are the optimal sources of CBTs for ConA-induced ALF and Fah-deficient induced CLF in mice, respectively. This finding deepens our understanding about how to select a proper CBT for different liver failure.

Fumarylacetoacetate hydrolase deficient pigs are a novel large animal model of metabolic liver disease

Hereditary tyrosinemia type I (HT1) is caused by deficiency in fumarylacetoacetate hydrolase (FAH), an enzyme that catalyzes the last step of tyrosine metabolism. The most severe form of the disease presents acutely during infancy, and is characterized by severe liver involvement, most commonly resulting in death if untreated. Generation of FAH(+/-) pigs was previously accomplished by adeno-associated virus-mediated gene knockout in fibroblasts and somatic cell nuclear transfer. Subsequently, these animals were outbred and crossed to produce the first FAH(-/-) pigs. FAH-deficiency produced a lethal defect in utero that was corrected by administration of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3 cyclohexanedione (NTBC) throughout pregnancy. Animals on NTBC were phenotypically normal at birth; however, the animals were euthanized approximately four weeks after withdrawal of NTBC due to clinical decline and physical examination findings of severe liver injury and encephalopathy consistent with acute liver failure. Biochemical and histological analyses, characterized by diffuse and severe hepatocellular damage, confirmed the diagnosis of severe liver injury. FAH(-/-) pigs provide the first genetically engineered large animal model of a metabolic liver disorder. Future applications of FAH(-/-) pigs include discovery research as a large animal model of HT1 and spontaneous acute liver failure, and preclinical testing of the efficacy of liver cell therapies, including transplantation of hepatocytes, liver stem cells, and pluripotent stem cell-derived hepatocytes.

Hepatitis B virus infection and immunopathogenesis in a humanized mouse model: induction of human-specific liver fibrosis and M2-like macrophages

The mechanisms of chronic HBV infection and immunopathogenesis are poorly understood due to a lack of a robust small animal model. Here we report the development of a humanized mouse model with both human immune system and human liver cells by reconstituting the immunodeficient A2/NSG (NOD.Cg-Prkdc(scid) Il2rg(tm1Wjl)/SzJ mice with human HLA-A2 transgene) with human hematopoietic stem cells and liver progenitor cells (A2/NSG-hu HSC/Hep mice). The A2/NSG-hu HSC/Hep mouse supported HBV infection and approximately 75% of HBV infected mice established persistent infection for at least 4 months. We detected human immune responses, albeit impaired in the liver, chronic liver inflammation and liver fibrosis in infected animals. An HBV neutralizing antibody efficiently inhibited HBV infection and associated liver diseases in humanized mice. In addition, we found that the HBV mediated liver disease was associated with high level of infiltrated human macrophages with M2-like activation phenotype. Importantly, similar M2-like macrophage accumulation was confirmed in chronic hepatitis B patients with liver diseases. Furthermore, gene expression analysis showed that induction of M2-like macrophage in the liver is associated with accelerated liver fibrosis and necrosis in patients with acute HBV-induced liver failure. Lastly, we demonstrate that HBV promotes M2-like activation in both M1 and M2 macrophages in cell culture studies. Our study demonstrates that the A2/NSG-hu HSC/Hep mouse model is valuable in studying HBV infection, human immune responses and associated liver diseases. Furthermore, results from this study suggest a critical role for macrophage polarization in hepatitis B virus-induced immune impairment and liver pathology.

Human fetal liver cells for regulated ex vivo erythropoietin gene therapy

Possible risks and lack of donor livers limit application of liver transplantation. Liver cell transplantation is, at this moment, not a feasible alternative because engraftment in the liver is poor. Furthermore, there is also shortage of cells suitable for transplantation. Fetal liver cells are able to proliferate in cell culture and could therefore present an alternative source of cells for transplantation. In this study, we investigated the utility of human fetal liver cells for therapeutic protein delivery. We transplanted human fetal liver cells in immunodeficient mice but were not able to detect engraftment of human hepatocytes. In contrast, transplantation of human adult hepatocytes led to detectable engraftment of hepatocytes in murine liver. Transplantation of fetal liver cells did lead to abundant reconstitution of murine liver with human endothelium, indicating that endothelial cells are the most promising cell type for ex vivo liver cell gene therapy. Human liver endothelial cells were subsequently transduced with a lentiviral autoregulatory erythropoietin expression vector. After transplantation in immunodeficient mice, these cells mediated long-term regulation of murine hematocrits. Our study shows the potential of human liver endothelial cells for long-term regulated gene therapy.

Murine models of hepatitis C: what can we look forward to?

The study of interactions between hepatitis C virus (HCV) with its mammalian host, along with the development of more effective therapeutics and vaccines has been delayed by the lack of a suitable small animal model. HCV readily infects only humans and chimpanzees, which poses logistic, economic and ethical challenges with analyzing HCV infection in vivo. Progress has been made in understanding the determinants that dictate HCV's narrow host range providing a blueprint for constructing a mouse model with inheritable susceptibility to HCV infection. Indeed, genetically humanized mice were generated that support viral uptake, replication and production of infectious virions--albeit at low levels. These efforts are complemented with attempts to select for viral variants that are inherently more capable of replicating in non-human species. In parallel, engraftment of relevant human tissues into improved xenorecipients is being continuously refined. Incorporating advances in stem-cell-biology and tissue engineering may allow the generation of patient-specific humanized mice. Construction of such mouse "avatars" may allow analyzing functionally patient-specific differences with respect to susceptibility to infection, disease progression and responses to treatment. In this review, we discuss the three, before mentioned approaches to overcome current species barriers and generate a small animal model for HCV infection, i.e. genetic modification of mice to increase their susceptibility to the virus; genetic modification of HCV, to increase its pathogenicity for mice; and the introduction of human liver and immune cells into immunodeficient mice, to create "humanized" mice. Although in the foreseeable future there will not be a single model that perfectly mimics the natural course of HCV in humans there is reason for optimism. The spectrum of murine animal models for hepatitis C provides a broad arsenal for analyzing the disease. These models may play an important role by prioritizing vaccine candidates and possibly refining combination anti-viral drug therapies. This article forms part of a symposium in Anti-viral Research on "Hepatitis C: next steps toward global eradication."

Differentiation of nonhuman primate embryonic stem cells into hepatocyte-like cells

OBJECTIVE

To investigate whether cells derived from rhesus monkey embryonic stem cells (ESC) had hepatocyte characteristics after the differentiation.

METHODS

Rhesus monkey ESC were induced towards hepatocyte-like cells via a four-step differentiation process: the formation of embryoid bodies (EB), EB in activin A and insulin-transferrin-selenium medium for 4 days, in fibroblast growth factor (FGF)-4 and bone morphogenetic protein-2 (BMP2) medium for 8 days, in hepatocyte culture medium containing hepatocyte growth factor for 3 days and then with oncostatin M and dexamethasone for another 5 days. Expression of albumin (ALB), glucose-6-phosphatase, α-fetoprotein (AFP) and α-1 antitrypsin (α1-AT) at the mRNA level in differentiated cells were detected by reverse transcription-polymerase chain reaction. The expression of hepatocyte markers AFP, ALB, hepatocyte nuclear factor 4 (HNF4), cytokeratin 8 (CK8), CK19 and cell proliferation marker, Ki67, in the differentiated cells were determined by immunocytochemistry. The ultrastructure of the differentiated cells was examined by electron microscopy. Indocyanine green (ICG) uptake was also explored.

RESULTS

After induction, some differentiated cells were binucleate, which is typical of hepatocytes. Hepatocyte-specific genes ALB, glucose-6-phosphatase, AFP and α1-AT were expressed in the differentiated cells. The differentiated cells expressed hepatocyte markers AFP, ALB, HNF4, CK8 and CK19 at the protein level. The cells also expressed cell proliferation marker Ki67. Under electron microscopy, the ultrastructures of hepatocyte-like cells, such as mitochondrion and catalase-containing peroxisomes, were observed in the differentiated cells. ICG uptake test was positive in differentiated cells.

CONCLUSIONS

With cytokine induction, rhesus monkey ESC differentiated into cells displaying morphological features, gene expression patterns and metabolic activities characteristic of hepatocytes.

Human hepatocytes and hematolymphoid dual reconstitution in treosulfan-conditioned uPA-NOG mice

Human-specific HIV-1 and hepatitis co-infections significantly affect patient management and call for new therapeutic options. Small xenotransplantation models with human hepatocytes and hematolymphoid tissue should facilitate antiviral/antiretroviral drug trials. However, experience with mouse strains tested for dual reconstitution is limited, with technical difficulties such as risky manipulations with newborns and high mortality rates due to metabolic abnormalities. The best animal strains for hepatocyte transplantation are not optimal for human hematopoietic stem cell (HSC) engraftment, and vice versa. We evaluated a new strain of highly immunodeficient nonobese diabetic/Shi-scid (severe combined immunodeficiency)/IL-2Rγc(null) (NOG) mice that carry two copies of the mouse albumin promoter-driven urokinase-type plasminogen activator transgene for dual reconstitution with human liver and immune cells. Three approaches for dual reconstitution were evaluated: i) freshly isolated fetal hepatoblasts were injected intrasplenically, followed by transplantation of cryopreserved HSCs obtained from the same tissue samples 1 month later after treosulfan conditioning; ii) treosulfan conditioning is followed by intrasplenic simultaneous transplantation of fetal hepatoblasts and HSCs; and iii) transplantation of mature hepatocytes is followed by mismatched HSCs. The long-term dual reconstitution was achieved on urokinase-type plasminogen activator-NOG mice with mature hepatocytes (not fetal hepatoblasts) and HSCs. Even major histocompatibility complex mismatched transplantation was sustained without any evidence of hepatocyte rejection by the human immune system.

Production of factor VIII by human liver sinusoidal endothelial cells transplanted in immunodeficient uPA mice

Liver sinusoidal endothelial cells (LSECs) form a semi-permeable barrier between parenchymal hepatocytes and the blood. LSECs participate in liver metabolism, clearance of pathological agents, immunological responses, architectural maintenance of the liver and synthesis of growth factors and cytokines. LSECs also play an important role in coagulation through the synthesis of Factor VIII (FVIII). Herein, we phenotypically define human LSECs isolated from fetal liver using flow cytometry and immunofluorescence microscopy. Isolated LSECs were cultured and shown to express endothelial markers and markers specific for the LSEC lineage. LSECs were also shown to engraft the liver when human fetal liver cells were transplanted into immunodeficient mice with liver specific expression of the urokinase-type plasminogen activator (uPA) transgene (uPA-NOG mice). Engrafted cells expressed human Factor VIII at levels approaching those found in human plasma. We also demonstrate engraftment of adult LSECs, as well as hepatocytes, transplanted into uPA-NOG mice. We propose that overexpression of uPA provides beneficial conditions for LSEC engraftment due to elevated expression of the angiogenic cytokine, vascular endothelial growth factor. This work provides a detailed characterization of human midgestation LSECs, thereby providing the means for their purification and culture based on their expression of CD14 and CD32 as well as a lack of CD45 expression. The uPA-NOG mouse is shown to be a permissive host for human LSECs and adult hepatocytes, but not fetal hepatoblasts. Thus, these mice provide a useful model system to study these cell types in vivo. Demonstration of human FVIII production by transplanted LSECs encourages further pursuit of LSEC transplantation as a cellular therapy for the treatment of hemophilia A.

Identification of liver cancer progenitors whose malignant progression depends on autocrine IL-6 signaling

Hepatocellular carcinoma (HCC) is a slowly developing malignancy postulated to evolve from premalignant lesions in chronically damaged livers. However, it was never established that premalignant lesions actually contain tumor progenitors that give rise to cancer. Here, we describe isolation and characterization of HCC progenitor cells (HcPCs) from different mouse HCC models. Unlike fully malignant HCC, HcPCs give rise to cancer only when introduced into a liver undergoing chronic damage and compensatory proliferation. Although HcPCs exhibit a similar transcriptomic profile to bipotential hepatobiliary progenitors, the latter do not give rise to tumors. Cells resembling HcPCs reside within dysplastic lesions that appear several months before HCC nodules. Unlike early hepatocarcinogenesis, which depends on paracrine IL-6 production by inflammatory cells, due to upregulation of LIN28 expression, HcPCs had acquired autocrine IL-6 signaling that stimulates their in vivo growth and malignant progression. This may be a general mechanism that drives other IL-6-producing malignancies.

Potential barriers to human hepatocyte transplantation in MUP-uPAtg(⁺/⁺)Rag2⁻/⁻γC⁻/⁻ mice

Primary human fetal and adult hepatocytes have been considered feasible donor cell sources for cell transplantation. We compared the engraftment efficiencies between adult human, fetal human, and adult porcine hepatocytes after transplantation into MUP-uPA(tg(+/+))Rag2(-/-)γC(-/-)mice. Transplantation of adult human hepatocytes yielded a 1,000-fold higher serum albumin level compared to transplantation of fetal human hepatocytes, while transplantation of adult porcine hepatocytes resulted in a 100-fold higher serum albumin level than adult human hepatocytes. These results suggest that adult liver cells are superior to fetal liver cells for transplantation, and caution should be applied if porcine hepatocytes are used for preclinical studies as a proof of concept for human hepatocytes.

Human Liver Progenitor Cells for Liver Repair

Because of their high proliferative capacity, resistance to cryopreservation, and ability to differentiate into hepatocyte-like cells, stem and progenitor cells have recently emerged as attractive cell sources for liver cell therapy, a technique used as an alternative to orthotopic liver transplantation in the treatment of various hepatic ailments ranging from metabolic disorders to end-stage liver disease. Although stem and progenitor cells have been isolated from various tissues, obtaining them from the liver could be an advantage for the treatment of hepatic disorders. However, the techniques available to isolate these stem/progenitor cells are numerous and give rise to cell populations with different morphological and functional characteristics. In addition, there is currently no established consensus on the tests that need to be performed to ensure the quality and safety of these cells when used clinically. The purpose of this review is to describe the different types of liver stem/progenitor cells currently reported in the literature, discuss their suitability and limitations in terms of clinical applications, and examine how the culture and transplantation techniques can potentially be improved to achieve a better clinical outcome.