Hepatic progenitors and strategies for liver cell therapies

Exploiting in silico modelling to enhance translation of liver cell therapies from bench to bedside

Cell therapies are emerging as promising treatments for a range of liver diseases but translational bottlenecks still remain including: securing and assessing the safe and effective delivery of cells to the disease site; ensuring successful cell engraftment and function; and preventing immunogenic responses. Here we highlight three therapies, each utilising a different cell type, at different stages in their clinical translation journey: transplantation of multipotent mesenchymal stromal/signalling cells, hepatocytes and macrophages. To overcome bottlenecks impeding clinical progression, we advocate for wider use of mechanistic in silico modelling approaches. We discuss how in silico approaches, alongside complementary experimental approaches, can enhance our understanding of the mechanisms underlying successful cell delivery and engraftment. Furthermore, such combined theoretical-experimental approaches can be exploited to develop novel therapies, address safety and efficacy challenges, bridge the gap between in vitro and in vivo model systems, and compensate for the inherent differences between animal model systems and humans. We also highlight how in silico model development can result in fewer and more targeted in vivo experiments, thereby reducing preclinical costs and experimental animal numbers and potentially accelerating translation to the clinic. The development of biologically-accurate in silico models that capture the mechanisms underpinning the behaviour of these complex systems must be reinforced by quantitative methods to assess cell survival post-transplant, and we argue that non-invasive in vivo imaging strategies should be routinely integrated into transplant studies.

Liver cell therapies: cellular sources and grafting strategies

The liver has a complex cellular composition and a remarkable regenerative capacity. The primary cell types in the liver are two parenchymal cell populations, hepatocytes and cholangiocytes, that perform most of the functions of the liver and that are helped through interactions with non-parenchymal cell types comprising stellate cells, endothelia and various hemopoietic cell populations. The regulation of the cells in the liver is mediated by an insoluble complex of proteins and carbohydrates, the extracellular matrix, working synergistically with soluble paracrine and systemic signals. In recent years, with the rapid development of genetic sequencing technologies, research on the liver's cellular composition and its regulatory mechanisms during various conditions has been extensively explored. Meanwhile breakthroughs in strategies for cell transplantation are enabling a future in which there can be a rescue of patients with end-stage liver diseases, offering potential solutions to the chronic shortage of livers and alternatives to liver transplantation. This review will focus on the cellular mechanisms of liver homeostasis and how to select ideal sources of cells to be transplanted to achieve liver regeneration and repair. Recent advances are summarized for promoting the treatment of end-stage liver diseases by forms of cell transplantation that now include grafting strategies.

Chemokine-mediated robust augmentation of liver engraftment: a novel approach

Effective repopulation of the liver is essential for successful clinical hepatocyte transplantation. The objective was to improve repopulation of the liver with human hepatocytes using chemokines. We used flow cytometry and immunohistochemistry assays to identify commonly expressed chemokine receptors on human fetal and adult hepatocytes. The migratory capacity of the cells to various chemokines was tested. For in vivo studies, we used a nude mouse model of partial hepatectomy followed by intraparenchymal injections of chemokine ligands at various concentrations. Human fetal liver cells transformed with human telomerase reverse transcriptase were used for intrasplenic cell transplantation. Repopulation and functionality were assessed 4 weeks after transplantation. The receptor CXCR3 was commonly expressed on both fetal and adult hepatocytes. Both cell types migrated efficiently toward corresponding CXC chemokine ligands 9, 10, and 11. In vivo, animals injected with recombinant chemokines showed the highest cell engraftment compared with controls (p<.05). The engrafted cells expressed several human hepatic markers such as cytokeratin 8 and 18 and albumin as well as transferrin, UGT1A1, hepatocyte nuclear factor (1α, 1β, and 4α), cytochrome CYP3A1, CCAAT/enhancer binding protein (α and β), and human albumin compared with controls. No inflammatory cells were detected in the livers at 4 weeks after transplantation. The improved repopulation of transplanted cells is likely a function of the chemokines to mediate cell homing and retention in the injured liver and might be an attractive strategy to augment repopulation of transplanted hepatocytes in vivo.

Liver bioengineering: from the stage of liver decellularized matrix to the multiple cellular actors and bioreactor special effects

Liver bioengineering has been a field of intense research and popular excitement in the past decades. It experiences great interest since the introduction of whole liver acellular scaffolds generated by perfusion decellularization (1-3). Nevertheless, the different strategies developed so far have failed to generate hepatic tissue in vitro bioequivalent to native liver tissue. Even notable novel strategies that rely on iPSC-derived liver progenitor cells potential to self-organize in association with endothelial cells in hepatic organoids are lacking critical components of the native tissue (e.g., bile ducts, functional vascular network, hepatic microarchitecture, etc) (4). Hence, it is vital to understand the strengths and short comes of our current strategies in this quest to re-create liver organogenesis in vitro. To shed some light into these issues, this review describes the different actors that play crucial roles in liver organogenesis and highlights the steps still missing to successfully generate whole livers and hepatic organoids in vitro for multiple applications.

Isolation of viable human hepatic progenitors from adult livers is possible even after 48 hours of cold ischemia

Liver transplantation, utilized routinely for end-stage liver disease, has been constrained by the paucity of organ donors, and is being complemented by alternative strategies such as liver cell transplantation. One of the most promising forms of liver cell transplantation is hepatic stem cell therapies, as the number of human hepatic stem cells (hHpSCs) and other early hepatic progenitor cells (HPCs) are sufficient to provide treatment for multiple patients from a single liver source. In the present study, human adult livers were exposed to cold ischemia and then processed after <24 or 48 h. Cells positive for epithelial cell adhesion molecule (EpCAM), a marker on early lineage stage HPCs, were immunoselected and counted. Approximately 100,000 EpCAM(+) cells/gram of tissue was obtained from surgical resection of livers subjected to cold ischemia up to 24 h and comparable numbers, albeit somewhat lower, were obtained from those exposed to 48 h of cold ischemia. The yields are similar to those reported from livers with minimal exposure to ischemia. When cultured on plastic dishes and in Kubota's Medium, a serum-free medium designed for early lineage stage HPCs, colonies of rapidly expanding cells formed. They were confirmed to be probable hHpSCs by their ability to survive and expand on plastic and in Kubota's Medium for months, by co-expression of EpCAM and neural cell adhesion molecule, minimal if any albumin expression, with EpCAM found throughout the cells, and no expression of alpha-fetoprotein. The yields of viable EpCAM(+) cells were surprisingly large, and the numbers from a single donor liver are sufficient to treat approximately 50-100 patients given the numbers of EpCAM(+) cells currently used in hepatic stem cell therapies. Thus, cold ischemic livers for up to 48 h are a new source of cells that might be used for liver cell therapies.

Long-term cryopreservation model of rat salivary gland stem cells for future therapy in irradiated head and neck cancer patients

Irradiated head and neck cancer patients suffer from irreversible loss of salivary gland (SG) function, along with significant morbidity and compromised quality of life. To date there is no biologically-based treatment for this distress. Adult salivary gland stem cells are promising candidates for autologous transplantation therapy in the context of tissue-engineered artificial SGs or direct cell therapy. The major restrictions in handling such cells are their limited lifespan during in vitro cultivation, resulting in a narrow time-window for implantation and a risk of tumorigenic changes during culture. To overcome these difficulties, we tested in a rat model the possibility of establishing a personal/autologous SG stem cell bank. SG's integrin-α6β1-expressing cells were shown to hold a subpopulation of SG-specific progenitor-cells. Explanted and cultured single cell-originated clones were cryopreserved for up to 3 years and shown to exhibit genetic and functional stability similar to noncryopreserved cells, as was emphasized by soft agar assay, division potential assessment, flow cytometric analysis, real-time reverse transcriptase-polymerase chain reaction, in vitro three-dimensional differentiation assay, and immunofluorescence confocal microscopy. Future integration of the novel strategies presented herein to a clinical therapeutic model will allow safe preservation until transplantation and repeated transplantation if needed. These tools open a new venue for adult autologous stem-cell transplantation-based SG regeneration.

Liver progenitor cell interactions with the extracellular matrix

Liver progenitor cells (LPCs) are a promising source of cells to treat liver disease by cell therapy, due to their capability for self-replication and bipotentiality. In order to establish useful culture systems of LPCs and apply them to future clinical therapies, it is necessary to understand their interactions with their microenvironment and especially with the extracellular matrix (ECM). There is considerable evidence from in vivo studies that matrix proteins affect the activation, expansion, migration and differentiation of LPCs, but the information on the role that specific ECMs play in regulating LPCs in vitro is more limited. Nevertheless, current studies suggest that laminin, collagen type III, collagen type IV and hyaluronic acid help to maintain the undifferentiated phenotype of LPCs and promote their proliferation when cultured in media supplemented with growth factors chosen for LPC expansion, whereas collagen type I and fibronectin are generally associated with a differentiated phenotype under the same conditions. Experimental evidence suggests that α6β1 and α5β1 integrins as well as CD44 on the surface of LPCs, and their related downstream signals, are important mediators of interactions between LPCs and the ECM. The interactions of LPCs with the ECM form the focus of this review and the contribution of ECM molecules to strategies for optimizing in vitro LPC cultures for therapeutic applications is discussed.

In situ labeling and magnetic resonance imaging of transplanted human hepatic stem cells

PURPOSE

The purpose is to address the problem in magnetic resonance imaging (MRI) of contrast agent dilution.

PROCEDURES

In situ magnetic labeling of cells and MRI were used to assess distribution and growth of human hepatic stem cells (hHpSCs) transplanted into severe combined immunodeficiency (SCID)/non-obese diabetic (NOD) mice. It was done with commercially available magnetic microbeads coupled to an antibody to a surface antigen, epithelial cell adhesion molecule (EpCAM), uniquely expressed in the liver by hepatic progenitors.

RESULTS

We validated the microbead connection to cells and related MRI data to optical microscopy observations in order to develop a means to quantitatively estimate cell numbers in the aggregates detected. Cell counts of hHpSCs at different times post-transplantation revealed quantifiable evidence of cell engraftment and expansion.

CONCLUSIONS

This magnetic labeling methodology can be used with any antibody coupled to a magnetic particle to target any surface antigen that distinguishes transplanted cells from host cells, thus facilitating studies that define methods and strategies for clinical cell therapy programs.

Notch signaling inhibits hepatocellular carcinoma following inactivation of the RB pathway

Mice lacking all three Rb genes in the liver develop tumors resembling specific subgroups of human hepatocellular carcinomas, and Notch activity appears to suppress the growth and progression of these tumors.

Hepatocellular carcinoma (HCC) is the third cancer killer worldwide with >600,000 deaths every year. Although the major risk factors are known, therapeutic options in patients remain limited in part because of our incomplete understanding of the cellular and molecular mechanisms influencing HCC development. Evidence indicates that the retinoblastoma (RB) pathway is functionally inactivated in most cases of HCC by genetic, epigenetic, and/or viral mechanisms. To investigate the functional relevance of this observation, we inactivated the RB pathway in the liver of adult mice by deleting the three members of the Rb (Rb1) gene family: Rb, p107, and p130. Rb family triple knockout mice develop liver tumors with histopathological features and gene expression profiles similar to human HCC. In this mouse model, cancer initiation is associated with the specific expansion of populations of liver stem/progenitor cells, indicating that the RB pathway may prevent HCC development by maintaining the quiescence of adult liver progenitor cells. In addition, we show that during tumor progression, activation of the Notch pathway via E2F transcription factors serves as a negative feedback mechanism to slow HCC growth. The level of Notch activity is also able to predict survival of HCC patients, suggesting novel means to diagnose and treat HCC.

Three-dimensional perfusion bioreactor culture supports differentiation of human fetal liver cells

The ability of human fetal liver cells to survive, expand, and form functional tissue in vitro is of high interest for the development of bioartificial extracorporeal liver support systems, liver cell transplantation therapies, and pharmacologic models. Conventional static two-dimensional culture models seem to be inadequate tools. We focus on dynamic three-dimensional perfusion technologies and developed a scaled-down bioreactor, providing decentralized mass exchange with integral oxygenation. Human fetal liver cells were embedded in a hyaluronan hydrogel within the capillary system to mimic an in vivo matrix and perfusion environment. Metabolic performance was monitored daily, including glucose consumption, lactate dehydrogenase activity, and secretion of alpha-fetoprotein and albumin. At culture termination cells were analyzed for proliferation and liver-specific lineage-dependent cytochrome P450 (CYP3A4/3A7) gene expression. Occurrence of hepatic differentiation in bioreactor cultures was demonstrated by a strong increase in CYP3A4/3A7 gene expression ratio, lower alpha-fetoprotein, and higher albumin secretion than in conventional Petri dish controls. Cells in bioreactors formed three-dimensional structures. Viability of cells was higher in bioreactors than in control cultures. In conclusion, the culture model implementing three-dimensionality, constant perfusion, and integral oxygenation in combination with a hyaluronan hydrogel provides superior conditions for liver cell survival and differentiation compared to conventional culture.

Expedited growth factor-mediated specification of human embryonic stem cells toward the hepatic lineage

Human embryonic stem cells (hESCs) have the potential to be a promising source of liver cells, hepatocytes, for regenerative medicine given their unlimited proliferative and pluripotent differentiative capacity. However, the inefficient embryoid body process and limited understanding of molecular signals potentiating cell-specific differentiation plague the use of hESCs as a hepatic source. In this study, we describe an efficient growth factor-based process for directed differentiation of hESCs that bypasses embryoid body development. The system involves adherent hESC culture exposure to activin A treatment followed by incorporation of various growth factor combinations composed of dexamethasone, oncostatin M, hepatocyte growth factor, and Wnt3A. The hESC-derived hepatocyte-like cells resulting from optimal growth factor combinations exhibit characteristic hepatocyte morphology, express hepatocyte markers, and possess hepatospecific functional activity. The differentiated cultures express hepatic-related genes shown by reverse transcription-polymerase chain reaction and immunofluorescence analysis revealed binucleated cells with coexpression of albumin/cytokeratin 18. Furthermore, the hESC-derived hepatocyte-like cells exhibit functional hepatic characteristics, such as indocyanine green uptake and release, albumin secretion, and inducible cytochrome P450 activity. This directed differentiation of adherent hESCs offers an efficient process to produce hepatocyte-like cells in vitro for hepatocyte differentiation studies and organotypic cultures for diagnostic and therapeutic applications.

Acute liver failure: Summary of a workshop

Acute liver failure (ALF) is a rare but challenging clinical syndrome with multiple causes; a specific etiology cannot be identified in 15% of adult and 50% of pediatric cases. The course of ALF is variable and the mortality rate is high. Liver transplantation is the only therapy of proven benefit, but the rapidity of progression and the variable course of ALF limit its use. Currently in the United States, spontaneous survival occurs in approximately 45%, liver transplantation in 25%, and death without transplantation in 30% of adults with ALF. Higher rates of spontaneous recovery (56%) and transplantation (31%) with lower rates of death (13%) occur in children. The outcome of ALF varies by etiology, favorable prognoses being found with acetaminophen overdose, hepatitis A, and ischemia (approximately 60% spontaneous survival), and poor prognoses with drug-induced ALF, hepatitis B, and indeterminate cases (approximately 25% spontaneous survival). Excellent intensive care is critical in management of patients with ALF. Nonspecific therapies are of unproven benefit. Future possible therapeutic approaches include N-acetylcysteine, hypothermia, liver assist devices, and hepatocyte transplantation. Advances in stem cell research may allow provision of cells for bioartificial liver support. ALF presents many challenging opportunities in both clinical and basic research.

Improvement of the survival rate by fetal liver cell transplantation in a mice lethal liver failure model

BACKGROUND

The use of cell transplantation as an alternative therapy for orthotopic liver transplantation has been widely anticipated due to a chronic donor shortage. We previously reported the method used to enrich hepatic progenitor cells (HPCs) forming cell aggregations. In this study, we transplanted HPCs into the liver injury model mice to determine whether HPC transplantation may improve the liver dysfunction.

METHODS

We obtained donor cells from E13.5 fetal livers of green fluorescent protein (GFP) transgenic mice. We transplanted GFP-positive fetal liver cells into the transgenic mice which express diphtheria toxin (DT) receptors under the control of an albumin enhancer/promoter. Subsequently, we induced selective liver injury to recipient mice by DT administration. We then evaluated the engraftment of the transplanted cells and their effect on survivorship.

RESULTS

The low dose of DT induced sublethal liver injury and the high dose of DT was lethal to the liver injury model mice. The transplanted GFP-positive cells were engrafted into the recipient livers and expressed albumin, resembling mature hepatocytes. They continued to proliferate, forming clusters. The survival rate at 25 days after transplantation of the cell-transplanted group (8 of 20; 40.0%) was improved significantly (P=0.0047) in comparison to that of the sham-operated group (0 of 20; 0%).

CONCLUSIONS

The transplanted cells were engrafted and repopulated the liver of recipient mice, resulting in the improvement of the survival rate of the liver injury model mice. We therefore propose that HPCs are a desirable cell source for cell transplantation.

Detection and characterization of hepatic engraftment of embryonic stem derived cells by fluorescent stereomicroscopy

BACKGROUND

Embryonic stem (ES) cells have been investigated as a potential replacement therapy for failed organs, such as the liver. However, detection of hepatic engraftment from candidate stem cells has been difficult due to low engraftment efficiency. Previous detection methods required that the graft be processed by molecular and/or immunohistochemical techniques, limiting further functional studies. This study evaluated the use of three-dimensional fluorescent stereomicroscopy for gross detection of ES cell derived hepatic engraftment.

MATERIAL AND METHODS

Murine ES cells expressing the enhanced green fluorescence protein (EGFP) underwent directed endodermal lineage differentiation. Three days after two thirds partial hepatectomy, cells were injected into the liver parenchyma, and livers were harvested at 10 to 20 d and examined by fluorescence stereomicroscopy with a GFP2 long pass filter (100447084; Leica Microsystems AG, Wetzlar, Germany). The sensitivity and reliability of the test was evaluated using quantitative polymerase chain reaction (q-PCR) to assay for the presence of EGFP mRNA in the tissue.

RESULTS

Fluorescent microscopy detected EGFP-positive cells engrafted with normal histology in 5 of 11 specimens. EGFP mRNA was confirmed in all five specimens by q-PCR. Only one of the 11 specimens was negative by fluorescence stereomicroscopy and positive by q-PCR, P < 0.02, Fisher's exact test.

CONCLUSION

Utilization of three-dimensional stereomicroscopy with a GFP2 long pass filter is a powerful and fast screening tool for GFP-ES derived hepatic engraftment.

Epithelial microchimerism: consistent finding in human liver transplants

BACKGROUND

Eleven liver biopsies from six male patients who received a liver transplant (LT) from female donors were examined in order to determine whether male host-derived hepatic cells were present in female grafts that exhibited minimal or important inflammatory damage.

METHODS

Immunohistochemistry for epithelial cell type differentiation (anticytokeratin monoclonal antibody) and fluorescence in situ hybridization for XY chromosomes identification were performed on each slide.

RESULTS

Host-derived hepatic cells were found in all except one transplant, with a frequency ranging from 2.3 to 25 per thousand of the total hepatocytes in the biopsy specimen. They were usually found as isolated cells scattered throughout the hepatic lobule; in one patient they were grouped into little clusters. Host-derived hepatic cells persisted throughout the histological follow up (up to 535 days after LT). Polyploidy for XY chromosome was observed.

CONCLUSION

Hepatocytes derived from extra-hepatic stem cells are frequently found in small numbers in human liver grafts and persist over time.

Bridging the regeneration gap: genetic insights from diverse animal models

Significant progress has recently been made in our understanding of animal regenerative biology, spurred on by the use of a wider range of model organisms and an increasing ability to use genetic tools in traditional models of regeneration. This progress has begun to delineate differences and similarities in the regenerative capabilities and mechanisms among diverse animal species, and to address some of the key questions about the molecular and cell biology of regeneration. Our expanding knowledge in these areas not only provides insights into animal biology in general, but also has important implications for regenerative medicine and stem-cell biology.

Functional analysis of encapsulated hepatic progenitor cells

A major challenge in developing therapies based on progenitor or stem cell populations (from sources other than bone marrow) involves developing a mode to deliver these cells in a manner that optimizes their viability, engraftment, proliferation, and differentiation. We have previously isolated a hepatic progenitor cell (HPC) population from adult liver tissue that differentiates into hepatic and biliary cell subtypes. We postulated that, using electrostatic encapsulation, we could reproducibly generate an ex vivo environment for the HPCs. We also theorized that this approach would foster cellular viability and function of the progenitor cell population. Using this encapsulation process, we consistently produced beads with uniform diameters between 200 and 700 microm. In vitro analysis of the encapsulated beads demonstrated extended periods of viability and function based on albumin production, urea metabolism, and glycogen storage. In conclusion, HPC encapsulation fosters the subsequent differentiation of HPCs into functional cells while maintaining their viability in long-term culture. These results demonstrate the efficacy of this method using somatic-derived progenitor cell populations and pave the way for clinical therapies.

Alginate-PLL microencapsulation: effect on the differentiation of embryonic stem cells into hepatocytes

The emergence of hepatocyte based clinical and pharmaceutical technologies, has been limited by the absence of a stable hepatocyte cell source. Embryonic stem cells may represent a potential solution to this cell source limitation problem since they are highly proliferative, renewable, and pluripotent. Although many investigators have described techniques to effectively differentiate stem cells into a variety of mature cell lineages, their practicality is limited by: (1) low yields of fully differentiated cells, (2) absence of large scale processing considerations, and (3) ineffective downstream enrichment protocols. Thus, a differentiation platform that may be modified to induce and sustain differentiated cell function and scaled to increase differentiated cell yield would improve current stem cell differentiation strategies. Microencapsulation provides a vehicle for the discrete control of key cell culture parameters such as the diffusion of growth factors, metabolites, and wastes. In addition, both cell seeding density and bead composition may be manipulated. In order to assess the feasibility of directing stem cell differentiation via microenvironment regulation, we have developed a murine embryonic stem cell (ES) alginate poly-l-lysine microencapsulation hepatocyte differentiation system. Our results indicate that the alginate microenvironment maintains cell viability, is conducive to ES cell differentiation, and maintains differentiated cellular function. This system may ultimately assist in developing scalable stem cell differentiation strategies.

Primate hepatic foetal progenitor cells and their therapeutic potential

Transplantation of genetically modified or unmodified hepatocytes appears to be a less invasive alternative to liver transplantation. However, clinical trials performed for the treatment of metabolic deficiencies resulted in a partial and transitory correction due to an insufficient number of engrafted and functional hepatocytes. In vitro, adult hepatocytes do not proliferate and the lack of organ donors limits their availability. Concomitantly, numerous works on hepatocyte transplantation in rodents have shown that cell engraftment was inefficient in normal livers. It is therefore necessary to explore the therapeutic potential of new cell sources such as stem cells and to develop pre-clinical models of transplantation. Foetal liver progenitor cells (hepatoblasts) are bipotent and express markers of both foetal hepatocytes and cholangiocytes. We have immortalized one clone of primate hepatoblasts using a retroviral vector expressing SV40 Large T and have characterized the cells at different population doublings (PDs). After 500 days in culture, immortalized cells remained bipotent and kept contact inhibition, in spite of numerous chromosomal rearrangements. After transplantation into athymic mice, the cells expressed hepatocyte functions but did not proliferate. We isolated, phenotypically characterized, transduced and cryopreserved early human hepatoblasts. These cells repopulate up to 7% of recipient immunodeficient mouse livers. This indicates that early progenitor cells display molecular characteristics related to proliferation and migration that allow these cells to engraft within hepatic parenchyma more efficiently than adult hepatocytes.

Tissue distribution of fetal liver cells following in utero transplantation in mice

Transplantation of hepatic stem cells in utero has been advanced as a potential clinical approach to a variety of diseases, including deficiencies of coagulation factors. Although syngeneic transplantation has met with some success, consideration needs to be given to the potential for transplanted cells to colonize nontarget tissues. Liver cells were harvested from Rosa26 embyros at embryonic age 12.5 days postconception (pc) and transplanted into the peritoneal cavity of syngeneic recipients in utero. Tissues were harvested from tissue recipients at various time points ranging from 1 to 328 days pc, and tissues were stained for beta-galactosidase to identify the existence of cells derived from Rosa26 donors. Beta-galactosidase-positive cells were found in the lung, liver, and brain as early as 20 days pc and through 328 days pc. Positive cells in these tissues existed as islands of cells that were morphologically similar to hepatocytes. In the spleen, individual beta-galactosidase-positive cells of both leukocytic and erythrocytic lineages were present, and suggest that hematopoietic cells were transferred to recipients along with hepatocytes. The lack of an inflammatory response to the beta-galactosidase-positive cells suggests that the donor cells were immunologically tolerated. In summary, the possibility that cells administered in utero may inadvertently colonize nontarget tissues suggests that clinical application of this method will need to be approached with diligence.

Experimental models of acute and chronic liver failure in nude mice to study hepatocyte transplantation

Although hepatocyte transplantation is a promising therapy for acute liver failure in human, there is still a lack of animal models suffering from hepatic injury in which the benefits of hepatocyte transplantation could be evaluated solely, without the bias caused by immunosuppression. As a consequence, the aim of the study was first to develop reproducible models of partial hepatectomy and of thioacetamide (TA)- or Jo2-induced acute liver failure in nude mice. Chronic liver disease was also investigated by repeated injections of sublethal doses of thioacetamide. Survival rates, routine histologic observations, alanin aminotransferase sera content, Ki67, and caspase 3 immunodetection were investigated both after 40% partial hepatectomy and after toxic-induced damages. Liver injuries were more severe and/or precocious in nude mice than in Balb/c mice for a given treatment with a maximum of acute injury obtained 24 h after single toxic injection, and were found to be transitory and reversible within 10 days. Toxics induced apoptosis followed by necrosis, confirming recent published data. Onset of fibrosis leading to reproducible chronic cirrhosis in nude mice correlated with increasing number of Ki67-positive cells, indicating that high levels of cell proliferation occurred. Chronic cirrhosis progressively reversed to fibrosis when the treatment ceased. Preliminary results demonstrated that engrafted xenogeneic hepatocytes could be detected in the host liver by anti-MHC class I immunohistochemistry. Fractions enriched in 2n or 4n hepatocytes by cell sorting using a flow cytometer were equivalent to the unpurified fraction in terms of engraftment in control nude mice or in nude mice subjected to PH. However, in mice suffering from liver injury 24 h after Jo2 or TA treatment, the engraftment of 2n hepatocytes was about twice that of an unpurified hepatocyte population or of a population enriched in 4n hepatocytes.

Transplantation of cultured salivary gland cells into an atrophic salivary gland

Patients with dry mouth have been treated with salivary substitutes and/or medications such as pilocarpine or cevimeline hydrochloride. These treatments temporarily relieve their symptoms and induce salivation from residual tissue. However, no treatment is available for the purpose of regenerating an atrophic gland. In this study, the feasibility of a cell transplantation therapy for the atrophic submandibular glands was investigated in rats. Further, the potential of cell differentiation into a useful phenotype was assessed by immunohistochemistry together with cell tracking with the fluorescent dye PKH 26. Rat submandibular glands were excised, and the salivary gland epithelial cells were cultured for 3 weeks with 3T3 cells as a feeder layer. Ductal ligation of the submandibular gland was employed to generate an atrophic gland. One week after the operation, the ligation was removed, and the cultured cells labeled with PKH 26 were injected into the atrophic submandibular glands. As a control, the cultured cells were also injected into normal submandibular glands. Two weeks after cell transplantation, the transplanted cells were detectable in both the experimental and control groups. The cells were clustered in the connective tissue between the lobules. Four weeks after transplantation, the labeled cells were detectable in the experimental group but not in the control group. In the atrophic glands, the scattered transplanted cells were observed over a broad area of the gland but localized mainly around the acini and ductal region. Immunostaining results showed a possible involvement of the transplanted cells in ductal regeneration, while neither myoepithelial nor acinar differentiations were observed within the 4 weeks since transplantation. This study demonstrated that cell transplantation to the salivary gland is feasible, and that the transplanted cells were selectively attracted to and remained in the damaged area without affecting normal tissue.

Expression of an Olfactomedin-Related Gene in Rat Hair Follicular Papilla Cells

Follicular papilla (FP) cells, but not their closely related dermal fibroblasts, can maintain hair growth suggesting cell type-specific molecular signals. To define the molecular differences between these two cell types, we generated a subtraction complementary DNA (cDNA) library highly enriched in FP-specific cDNA. Differential screening identified FP-1 as the most abundant cDNA sequence in this subtraction library. FP-1 message RNA is highly abundant in cultured rat vibrissa FP cells, can be detected at very low levels in the stomach and the ovary, and is undetectable in cultured dermal fibroblasts and in 16 rat non-follicular tissues. The full-length, 2.3 kb FP-1 cDNA encodes a protein of 549 amino acids harboring a signal peptide, collagen triple helix repeats, and an olfactomedin-like domain. Monospecific rabbit antibodies to FP-1 recognize in cultured FP cells a single approximately 72 kDa glycoprotein with a approximately 60 kDa protein core. FP-1 protein is expressed in vivo in a hair cycle-dependent manner, as it can be detected in FP during anagen, but not in catagen and telogen phases of the hair cycle. FP-1 is presumably a highly specific extracellular matrix protein synthesized by FP cells and may be involved in the organization of FP during certain phases of normal or pathological hair growth.

Stem-cell therapy for hepatobiliary pancreatic disease

The transplantation of pancreatic beta cells or hepatocytes represents a potential therapeutic approach for type I diabetes and inherited liver diseases, respectively. Furthermore, acquired liver diseases, particularly acute hepatic failure due to toxic or viral injury, have been treated in limited clinical trials with fetal and adult hepatocytes. However, a major limitation is the insufficient amount of beta cells and hepatocytes available for grafts. Alternative sources of these cells have yet to be determined. During the past few years, progress has been made in the development of new strategies to produce mature beta cells and hepatocytes. In this review, we outline the current state of scientific understanding and controversy regarding the properties of embryonic and adult stem cells in the field of hepatobiliary and pancreatic diseases. Our objective is to provide a framework of understanding for the challenges behind translating fundamental stem cell biology into clinical therapies.

MELD Scores Do Not Predict Patient Morbidity While on the Liver Transplant Waiting List

The goals of this study were to assess waitlist morbidity in terms of the frequency of health care services utilized by patients while on the liver transplant (LTX) waiting list and to determine whether that utilization can be predicted by the Model for End-Stage Liver Disease (MELD). Sixty-three noncomatose subjects were followed from waitlist placement until death, change in status, LTX, or study discontinuance. Health care events included doctor/clinic visits, labs, outpatient/inpatient tests and procedures, and hospital/intensive care unit days. Listing MELD scores and LTX MELD scores were examined against the number of health care event occurrences within 60 days of listing and 60 days of LTX, respectively, as were changes in MELD scores between listing and LTX and differences in the number of occurrences between the two time points. The only significant correlations noted were between LTX MELD scores and number of hospital days near LTX (r = .360, P = .046) and between LTX MELD scores and the sum total number of occurrences near LTX (r = .370, P = .044). These results suggest that MELD scores do not appear to predict morbidity in terms of health care utilization in patients awaiting LTX. Developing a system capable of predicting waitlist morbidity may lead to the implementation of medical interventions aimed at circumventing foreseeable complications and/or crises in patients awaiting LTX.

Thy1-positive mesenchymal cells promote the maturation of CD49f-positive hepatic progenitor cells in the mouse fetal liver

Previously, we reported a system to enrich mouse fetal hepatic progenitor cells (HPCs) by forming cell aggregates. In this study, we sorted two cell populations, CD49f(+)Thy1(-)CD45(-) cells (CD49f-positive cells) and CD49f(+/-)Thy1(+)CD45(-) cells (Thy1-positive cells), from the cell aggregates using a flow cytometer. CD49f-positive cells stained positive for endodermal specific markers such as alpha-fetoprotein (AFP), albumin (ALB), and cytokeratin 19 (CK19), and are thus thought to be HPCs. However, Thy1-positive cells were a morphologically heterogeneous population; reverse-transcription polymerase chain reaction (RT-PCR) and immunocytochemical analyses revealed the expression of mesenchymal cell markers such as alpha-smooth muscle actin, desmin, and vimentin, but not of AFP, ALB, or CK19. Therefore, Thy1-positive cells were thought to be of a mesenchymal lineage. When these two cell populations were co-cultured, the CD49f-positive colonies matured morphologically and stored a significant amount of glycogen. Furthermore, real-time RT-PCR demonstrated an increased expression of tyrosine amino transferase and tryptophan oxygenase mRNA, and transmission electron microscopy confirmed that co-cultured cells produced mature hepatocytes. However, when CD49f-positive cells were cultured alone or when the two populations were cultured separately, the CD49f-positive cells did not mature. These results indicate that CD49f-positive cells are primitive hepatic endodermal cells with the capacity to differentiate into hepatocytes, and that Thy1-positive cells promote the maturation of CD49f-positive cells by direct cell-to-cell contact. In conclusion, we were able to isolate CD49f-positive primitive hepatic endodermal cells and Thy1-positive mesenchymal cells and to demonstrate the requirement of cell-to-cell contact between these cell types for the maturation of the hepatic precursors.

Human hepatocyte isolation and relationship of cell viability to early graft function

Hepatocyte transplantation is emerging as an additional modality of treatment for patients with acute liver failure or liver-based metabolic disorders. The procedure requires isolation of high-quality hepatocytes from unused donor livers. Hepatocytes were isolated from 20 donor livers (11 right lobes, 3 left lateral segments, 6 whole livers) using a collagenase perfusion technique. Cell viability (median 56%, range 13-95%) and yield (median 1.4 x 10(9) cells, range 2.0 x 10(6)-1.8 x 10(10) cells) varied according to the tissue available. Fatty livers rejected for transplantation gave lower cell viability (median 45%, range 25-59%). There was a significant correlation between age of donor (median 21 years, range 7-66 years) and viability of isolated hepatocytes in vitro (r = -0.683, p = 0.001). The 13 segments of livers were from reduced/split grafts used for clinical transplantation in 9 children and 4 adults. There was no significant correlation between in vitro cell viability and clinical parameters including intensive care stay, serum aspartate aminotransferase,and international normalized ratio (in the first 7 days), and allograft rejection or other early posttransplant complications, in patients transplanted with the corresponding tissue.

Induction of hepatic differentiation in embryonic stem cells by co-culture with embryonic cardiac mesoderm

BACKGROUND

Modifications in vitro have been used to direct embryonic stem (ES) cells toward endodermal phenotypes including hepatocytes; however, developmental correlates and evidence of biologic activity is lacking, and critical cell-cell interactions have not been investigated. In this study, we hypothesized that cardiac mesoderm (CM) signals ES cells in co-culture to undergo differentiation toward early hepatocyte lineage as determined by morphology and induction of genes essential for endodermal competence and hepatocyte development.

METHODS

Green fluorescent protein ES derived from A129 mice were cultured with or without embryonic chick cardiac mesoderm. Cultures from day 1, 2, and 4 were analyzed for colony formation and ES morphology and 10(6) ES-derived cells were isolated for mRNA analysis.

RESULTS

ES in co-culture with CM displayed colony formation, polymorphic appearance, and definitive interface with CM. In addition, ES + CM co-culture activated crucial transcription factors (sox 17alpha, HNF3beta, and GATA 4) required for hepatocyte development by day 1. mRNA for albumin and especially a-fetoprotein were also increased by culture days 2 and 4.

CONCLUSIONS

ES cells co-cultured with CM display morphology and gene expression pattern required for hepatocyte differentiation and appear to recapitulate the molecular events of hepatogenesis.

Liver stem cells: prospects for treatment of inherited and acquired liver diseases

It is now understood that there are three cell compartments which physiologically contribute to vertebrate liver parenchymal maintenance and regeneration after injury: mature liver cells (hepatocytes, cholangiocytes), intraorgan stem/progenitor cells (cells of the proximal biliary tree, periductal cells) and extraorgan stem cells (from the circulation and the bone marrow). All of these cell populations, as well as other, non-physiologic stem cells (e.g., mesenchymal stromal cells from the bone marrow, fetal hepatoblasts, embryonic stem [ES] cells), may be used therapeutically for treatment of inherited and acquired liver diseases. This article will summarise our current understanding of these various cell populations, and review possible approaches to their therapeutic use, including cell transplantation, bioartificial liver devices (BLDs), gene therapy and administration of exogenous factors to stimulate normal physiological responses to repair.

Enrichment of hepatic progenitor cells from adult mouse liver

Hepatic progenitor cells (HPCs) have been characterized in several drug-treated rodent models and in the fetal liver; however, their properties have not been fully clarified in the normal adult liver, presumably because of their relatively small population and the existence of mature hepatocytes. In an attempt to resolve this issue, we developed a new enrichment system for HPCs using their cell aggregate formation properties. Nonparenchymal cells (NPCs) derived from enzymatically digested liver cells in normal adult mouse liver were treated in a hypoxic 2-hour suspension culture under constant shaking. This procedure resulted in cell aggregate formation and almost complete elimination of mature hepatocytes. Cell aggregates were formed only in Ca(2+)-containing medium, suggesting cadherin-dependent cell-cell adhesion. In these cell aggregates, 95% consisted of vascular endothelial cells that expressed VE-cadherin. The remaining 5% consisted of rapidly proliferating, small epithelial cells that expressed alpha-fetoprotein (AFP), E-cadherin, and albumin but not cytokeratin 19 (CK19), alpha-smooth muscle actin, or VE-cadherin. These results are consistent with an immature hepatic cell phenotype. When these immature hepatic cells were cultured with 10(-7) mol/L dexamethasone and 1% dimethyl sulfoxide, the de novo expression of mature hepatocyte markers such as tryptophan-2,3-dioxygenase (TO) was induced concomitantly with the induction of morphologic characteristics such as mitochondria- and peroxisome-rich cytoplasm and bile canaliculi formation. In conclusion, our methodology allows the enrichment of immature hepatic cells from the normal adult mouse. These cells are capable of growth and maturation along the hepatocyte lineage, indicating that these cells are HPCs.

Isolation of human hepatocytes from livers rejected for liver transplantation on a national basis: results of a 2-year experience

The offer of liver transplantation to many patients affected by liver failure is limited by organ shortage. Clinical application of human-based liver cell therapies, such as bioartificial liver and hepatocyte transplantation, might support liver transplantation, allowing more patients to be treated and decreasing mortality in the waiting list. The development of a standardized method of hepatocyte isolation is a mainstay for large-scale application of liver cell therapy. The aim of this study is to analyze retrospectively a 2-year experience of human hepatocyte isolation from livers rejected from transplantation at organ harvesting, performed on a national basis in Italy. All the livers judged unsuitable for transplantation were considered for hepatocyte isolation. Macrosteatosis greater than 60% was the most common reason of refusal, followed by nonviral cirrhosis. Fifty-four organs were used. Human hepatocyte isolation resulted in more that 7 million liver cells/g of tissue digested with 73% +/- 14% viability. Steatotic organs gave better results in terms of cell yield than cirrhotic livers. Isolated hepatocytes were able to perform specific liver functions, and evidence of factor IX and albumin messenger RNA (mRNA) production was reported when cells were plated in culture. Modifications of the traditional method of hepatocyte isolation, aimed at reducing ischemia-reperfusion damage and improving post-isolation cell conditions, showed improvements in post-isolation viability. In conclusion, we show that it is possible to use the vast majority of livers not suitable for transplantation on a national basis for human hepatocyte isolation, obtaining a large amount of viable functioning human hepatocytes that might be used for cell transplantation and therapy.

Hepatocyte ploidy in normal young rat

The aim of the present study was to examine the relation between hepatocyte size and ploidy in Sprague-Dawley rat liver. Therefore, subpopulations of hepatocytes of various sizes were separated from the isolated crude hepatocyte population either mechanically or by using centrifugal elutriation. Hepatocyte size was determined on scanning electron microscopy photographs. Ploidy of hepatocytes was assessed by flow cytometry. The crude hepatocyte population was very heterogeneous in sizes, with diameters ranging from 8 to 39 microm. Hepatocyte ultrastructure was well preserved as demonstrated by transmission electron microscopy. The distribution of hepatocytes within the ploidy classes was the following: 19.6+/-3.6% diploid, 56.2+/-3.2% tetraploid and 3.4+/-0.6% octoploid mononucleated cells. Thus approximately 79% of hepatocytes appeared mononucleated. The binucleated hepatocytes (21%) had two diploid nuclei (18.7+/-2.9%) or two tetraploid nuclei (2.1+/-0.6%). A similar distribution of hepatocytes into ploidy classes was obtained in subpopulations of hepatocytes of various sizes. Our findings suggest that distribution into ploidy classes is not strictly correlated with hepatocyte size. In accordance with previous observations, our results on hepatocyte ploidy from periportal or perivenous origin using digitonin perfusion, is in favour of the existence of ploidy zonation within the rat hepatic lobule.

In utero transplantation of wild-type fetal liver cells rescues factor X-deficient mice from fatal neonatal bleeding diatheses

Factor X (FX)-deficient embryos suffer partial embryonic lethality with approximately 30% of the embryos arresting at midgestation. The remaining animals survive to term but die perinatally mainly from abdominal or intracranial hemorrhage. We have rescued FX-deficient mice by transplanting fetal liver cells from FX+/+, Rosa26 fetuses into midgestation embryos derived from FX+/- heterozygous crosses. FX-/- embryos were born at the expected frequency and approximately 50% of the FX-/- neonates survived longer than 4 months. FX-/- embryos receiving saline injections that survived to term died perinatally similar to untreated FX-deficient mice. The plasma levels of FX in the rescued 16-week-old FX-/- mice were approximately 1-6% of wild-type levels. beta-Galactosidase-staining cells derived from the donor Rosa26 fetal liver cells were detected in 47% of the livers of adult mice. In addition, donor-derived cells were also recovered in the bone marrow, spleen, lung, and occasionally in the brain and testis. These results suggest that in utero cell transplantation could be an effective therapeutic strategy to treat pathologies resulting from the deficiency of hepatic-expressed factors.

The role of hepatocytes and oval cells in liver regeneration and repopulation

The liver has the unique capacity to regulate its growth and mass. In rodents and humans, it grows rapidly after resection of more than 50% of its mass. This growth process, as well as that following acute chemical injury is known as liver regeneration, although growth takes place by compensatory hyperplasia rather than true regeneration. In addition to hepatocytes and non-parenchymal cells, the liver contains intra-hepatic "stem" cells which can generate a transit compartment of precursors named oval cells. Liver regeneration after partial hepatectomy does not involve intra or extra-hepatic (hemopoietic) stem cells but depends on the proliferation of hepatocytes. Transplantation and repopulation experiments have demonstrated that hepatocytes, which are highly differentiated and long-lived cells, have a remarkable capacity for multiple rounds of replication. In this article, we review some aspects of the regulation of hepatocyte proliferation as well as the interrelationships between hepatocytes and oval cells in different liver growth processes. We conclude that in the liver, normally quiescent differentiated cells replicate rapidly after tissue resection, while intra-hepatic precursor cells (oval cells) proliferate and generate lineage only in situations in which hepatocyte proliferation is blocked or delayed. Although bone marrow stem cells can generate oval cells and hepatocytes, transdifferentiation is very rare and inefficient.