A novel bioreactor design for in vitro reconstruction of in vivo liver characteristics

Kinetic Analysis of Lidocaine Elimination by Pig Liver Cells Cultured in 3D Multi-Compartment Hollow Fiber Membrane Network Perfusion Bioreactors

Liver cells cultured in 3D bioreactors is an interesting option for temporary extracorporeal liver support in the treatment of acute liver failure and for animal models for preclinical drug screening. Bioreactor capacity to eliminate drugs is generally used for assessing cell metabolic competence in different bioreactors or to scale-up bioreactor design and performance for clinical or preclinical applications. However, drug adsorption and physical transport often disguise the intrinsic drug biotransformation kinetics and cell metabolic state. In this study, we characterized the intrinsic kinetics of lidocaine elimination and adsorption by porcine liver cells cultured in 3D four-compartment hollow fiber membrane network perfusion bioreactors. Models of lidocaine transport and biotransformation were used to extract intrinsic kinetic information from response to lidocaine bolus of bioreactor versus adhesion cultures. Different from 2D adhesion cultures, cells in the bioreactors are organized in liver-like aggregates. Adsorption on bioreactor constituents significantly affected lidocaine elimination and was effectively accounted for in kinetic analysis. Lidocaine elimination and cellular monoethylglicinexylidide biotransformation featured first-order kinetics with near-to-in vivo cell-specific capacity that was retained for times suitable for clinical assist and drug screening. Different from 2D cultures, cells in the 3D bioreactors challenged with lidocaine were exposed to close-to-physiological lidocaine and monoethylglicinexylidide concentration profiles. Kinetic analysis suggests bioreactor technology feasibility for preclinical drug screening and patient assist and that drug adsorption should be accounted for to assess cell state in different cultures and when laboratory bioreactor design and performance is scaled-up to clinical use or toxicological drug screening.

Evaluation of the Function of Primary Human Hepatocytes Co-Cultured with the Human Hepatic Stellate Cell (HSC) Line LI90

Most bioartificial liver devices utilise primary hepatocytes alone although some have considered the use of non parenchymal cells in addition. However the effects of co-culture of human hepatocytes with different sinusoidal cell types has not been fully investigated. In this study we have examined the influence of co-culturing primary human hepatocytes with the human hepatic stellate cell (HSC) line, LI90. Cultures were monitored by light microscopy and on days 4, 8 and 14 urea synthesis and cytochrome P450 activity were measured. Morphologically LI90 cells proliferated to fill spaces between and into adjacent islands of hepatocytes. On day 14 cytochrome P450 activity in co-culture was significantly improved compared to hepatocytes cultured alone. By contrast, urea synthesis in hepatocytes was unaffected by single or co-culture. Therefore it can be concluded that a combination of primary human hepatocytes with LI90 cells is beneficial for growth and some stability of hepatocytes and may therefore be appropriate for seeding bioartificial liver devices.

Assessment of Liver Function in Primary Cultures of Hepatocytes Using Diethoxy (5,6) Chloromethylfluorescein and Confocal Laser Scanning Microscopy

A method is presented which can be used to assess the function of hepatocytes in complex culture configurations without disrupting the integrity of the cell environment. It utilises a fluorescent probe for cytochrome P450 dependent mixed function oxidase (MFO) activity, diethoxy (5,6) chloromethylfluorescein, and confocal laser scanning microscopy. The MFO activity of individual cells in primary cultures of intact hepatocytes can be detected in situ, and quantified by image analysis. This may be a valuable means of monitoring the effect of culture conditions on the function of bioartificial liver devices, and could be used to assess the need for effective oxygenation of cells, the influence of shear stress and of exposure to patient serum during clinical use.

Adhesion and Function of Rat Liver Cells Adherent to Silk Fibroin/Collagen Blend Films

Collagen is often used in bioartificial livers as a biomimetic coating to promote liver cell adhesion and differentiation. Animal proteins are expensive and expose the host to risks of cross-species infection due to contamination with prions. Silk fibroin (SF) is a biocompatible protein produced by Bombyx mori silk worms and possibly an alternative to collagen. We prepared SF-collagen blend films with different SF content adherent to the bottom of standard tissue culture dishes, and characterized their surface morphology by SEM, their wettability and examined them for their capacity to support rat liver cell adhesion and metabolism. Cell metabolism was characterized by estimating the rate at which cells eliminated ammonia and synthesized urea for up to 48h of culture. SF-containing films were smooth, clear and more wettable than collagen. Cells readily adhered, formed junctions and small size aggregates on all films. As many cells adhered on SF as on collagen films. Cell adhesion to high collagen content blend films could not be reliably estimated because cells dwelt in the large cavities in the film. The effect of SF on cell metabolism differed with the investigated metabolic pathway. However, cells on SF-containing films eliminated ammonia and synthesized urea at rates generally comparable to, for urea synthesis at times higher than, that of cells on collagen. These results suggest that silk fibroin is a suitable substratum for liver cell attachment and culture, and a potential alternative to collagen as a biomimetic coating.

SiO2 Entrapment of Animal Cells for Hybrid Bioartificial Organs

Si-alkoxides in gas phase are reactive towards the surface of animal cells, depositing a homogeneous layer of porous silica. This encapsulation method preserves cell viability and does not alter the hindrance of the biological load.

In the prospective use for the design of a hybrid bioartificial liver, hepatocytes in a collagen matrix can be entrapped by the siliceous deposit which provides definite mechanical stability to the collagen matrix and molecular cutoff vs. high molecular weight proteins, including immunoglobulins. The functionality of the encapsulated cell load is maintained for the expressions of typical liver and pancreas metabolic activities.

Moving towards in situ tracheal regeneration: the bionic tissue engineered transplantation approach

In June 2008, the world's first whole tissue-engineered organ - the windpipe - was successfully transplanted into a 31-year-old lady, and about 18 months following surgery she is leading a near normal life without immunosuppression. This outcome has been achieved by employing three groundbreaking technologies of regenerative medicine: (i) a donor trachea first decellularized using a detergent (without denaturing the collagenous matrix), (ii) the two main autologous tracheal cells, namely mesenchymal stem cell derived cartilage-like cells and epithelial respiratory cells and (iii) a specifically designed bioreactor that reseed, before implantation, the in vitro pre-expanded and pre-differentiated autologous cells on the desired surfaces of the decellularized matrix. Given the long-term safety, efficacy and efforts using such a conventional approach and the potential advantages of regenerative implants to make them available for anyone, we have investigated a novel alternative concept how to fully avoid in vitro cell replication, expansion and differentiation, use the human native site as micro-niche, potentiate the human body's site-specific response by adding boosting, permissive and recruitment impulses in full respect of sociological and regulatory prerequisites. This tissue-engineered approach and ongoing research in airway transplantation is reviewed and presented here.

Cryopreservation of primary porcine liver cells in an organotypical sandwich model in a clinically relevant flat membrane bioreactor

To overcome the logistical difficulties of continuously supplying freshly-isolated, primary porcine liver cells to bioartificial liver support bioreactors, we developed a cryopreservation method using an organotypical sandwich model in a flat membrane bioreactor (FMB). We measured albumin secretion rate, urea synthesis rate and 7-ethoxy coumarin (ECOD) in long-term cultures of cryopreserved cells (up to 14 days). The albumin secretion rate was 62% that of non-cryopreserved cells at days 11 and 14. The ECOD activity was 54% that of fresh, control cells initially and increased up to 79% by the 14th day. The urea synthesis rate was stable at 60% that of the control. This study showed that cryopreserved cells can recover liver-specific functions. This result has the potential to dramatically expand the clinical application of bioartificial liver supports.

Preclinical characterization of primary porcine hepatocytes in a clinically relevant flat membrane bioreactor

Using primary porcine hepatocytes, artificial extracorporeal liver support (AEL) is a therapy that carries out the liver functions of liver failure patients until their own organs have been regenerated or until whole organ transplantation. Significant variation exists with regard to current bioreactor designs for AEL, and they may not reflect the in vivo architecture of the liver since each individual hepatocyte has its own direct contact with blood plasma for oxygen and nutrient supply and detoxification. The present study, based on our flat membrane bioreactor (FMB), aimed at in vivo liver architecture and to meet authentic clinical levels of human plasma exposure. Since many existing preclinical AELs are based on commercial culture medium with or without nonhuman serum, they may not authentically reflect the clinical situation in human patients, and little research has been done on human plasma exposure in in vitro culture-based bioreactors. To address this situation, herein we examined liver-specific functions such as albumin secretion, urea synthesis, glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), cell membrane stability by lactate dehydrogenase (LDH) test and ammonia clearance by using human plasma and serum-free medium in long-term culture of primary porcine hepatocytes to show the potential of our clinically relevant FMB. We observed that the organotypical double-gel (DG) culture is superior to conventional collagen-coated single-gel (SG) cultures. The performance of liver-specific functions by the FMB has long-term stability with intact cell morphology for up to 20 days under both plasma exposure and serum-free media. Our three focus points (long-term culture that correlates with the generation time of spontaneous regeneration, high-density culture, organotypical culture model using human plasma) may provide valuable clinical clues for AEL.

Development of a hybrid liver-support device

The development of an extracorporeal hybrid liver-support system using hepatocytes and an artificial device has been long awaited for the treatment of patients with hepatic failure. During the past decade important progress has been made in biotechnology and bioengineering, and a hybrid liver-support device using metabolically active hepatocytes may well become a reality in the near future. This paper outlines recent developments in bioreactor systems used as bioartificial liver-support devices, and focuses on critical issues for bioreactor design, main transport features and culture techniques for hepatocytes. We describe our bioreactor, which uses porcine hepatocytes, and the scaling-up of the device. The biochemical performance of such a device is comparable to that of those developed by other researchers, and we feel encouraged to perform in vivo experiments on animal models in order to evaluate the potential of the device as a bioartificial liver.

Long-term maintenance of human hepatocytes in oxygen-permeable membrane bioreactor

An oxygen-permeable membrane bioreactor utilizing human hepatocytes has been tested in this study. In the bioreactor, human hepatocytes were cultured between flat-sheet gas-permeable polymeric membranes, which ensure the diffusion of O(2) and CO(2) providing a support for cell anchorage and growth and permit the online observation of the cells with an inverse microscope. This bioreactor allows a direct oxygenation of cells adhered on membranes and of the medium overlaying cells simulating in vivo sinusoidal organization. Human hepatocytes were cultured in the presence of some therapeutic molecules to assess the temporal liver-specific functions of the cells. Interleukin 6 (IL-6), which is a multifactorial proinflammatory cytokine involved in a variety of host defences and pathological processes, and diclofenac, an arylacetic non-steroidal anti-inflammatory drug, were used as therapeutic molecules. The aim of this study was to evaluate the in vitro performance of the small oxygen-permeable membrane bioreactor in the long-term maintenance and differentiation of human hepatocytes under in-vivo-like conditions. The fluid dynamics of the bioreactor were characterized before using it for human cell culture. The functional response to a step challenge in the medium of IL-6 (120 pg/ml), diclofenac (80 microm) and IL-6 and diclofenac together was investigated. The ability of hepatocytes to perform liver-specific functions in terms of urea and albumin synthesis, as well as secretion of total proteins, was maintained for 32 days. Also, the diclofenac biotransformation functions were sustained as the formation of the metabolites 4'-OH-diclofenac and 5-OH-diclofenac lactam demonstrated. This study attested the feasibility of the membrane bioreactor as an in vitro simple model system that allows human hepatocytes to be maintained in a differentiated state similar to that in vivo.

Influence of polymer membrane porosity on C3A hepatoblastoma cell adhesive interaction and function

The effect of the porosity of acrylonitrile-N-vinylpyrrolidone copolymer membranes on human C3A hepatoblastoma cell adhesive interaction and functioning is investigated on four membranes with an average pore size ranging between 6 and 12 nm. Adhesion of C3A cells was quantified and characterized by studying overall cell morphology and focal adhesion formation. Cell-cell interactions were characterized by E-cadherin expression and organization. Cell growth, fibronectin synthesis and cytochrome P450 activity were estimated as criteria of functional cell activity. The results suggest that membrane porosity influences the initial cell-surface interactions since an increasing pore size augmented cell adhesion and aggregate formation. Cell growth after 7 d was diminished on membranes with an average pore size of 12 nm. The activity of P450 measured by 7-ethoxycoumarin conversion at day 7 was influenced by membrane topography representing a clear optimum in the range of 7-10 nm pore size. These results indicate that membrane porosity is a determinant for the function of hepatocytes in extracorporal liver assist devices.

Formation of steady-state oxygen gradients in vitro: application to liver zonation

We have developed a perfusion bioreactor system that allows the formation of steady state oxygen gradients in cell culture. In this study, gradients were formed in cultures of rat hepatocytes to study the role of oxygen in modulating cellular functions. A model of oxygen transport in our flat-plate reactor was developed to estimate oxygen distribution at the cell surface. Experimental measurements of outlet oxygen concentration from various flow conditions were used to validate model predictions. We showed that cell viability was maintained over a 24-h period when operating with a physiologic oxygen gradient at the cell surface from 76 to 5 mmHg O(2) at the outlet. Oxygen gradients have been implicated in the maintenance of regional compartmentalized metabolic and detoxification functions in the liver, termed zonation. In this system, physiologic oxygen gradients in reactor cultures contributed to a heterogeneous distribution of phosphoenolpyruvate carboxykinase (predominantly localized upstream) and cytochrome p450 2B (predominantly localized downstream) that correlates with the distribution of these enzymes in vivo. The oxygen gradient chamber provides a means of probing the oxygen effects in vitro over a continuous range of O(2) tensions. In addition, this system serves as an in vitro model of zonation that could be further extended to study the role of gradients in ischemia-reperfusion injury, toxicity, and bioartificial liver design.

Engineering liver therapies for the future

Treatment of liver disease has been greatly improved by the advent and evolution of liver transplantation. However, as demand for donor organs continues to increase beyond their availability, the need for alternative liver therapies is clear. Several approaches including extracorporeal devices, cell transplantation, and tissue-engineered constructs have been proposed as potential adjuncts or even replacements for transplantation. Simultaneously, experience from the liver biology community have provided valuable insight into tissue morphogenesis and in vitro stabilization of the hepatocyte phenotype. The next generation of cellular therapies must therefore consider incorporating cell sources and cellular microenvironments that provide both a large population of cells and strategies to maintain liver-specific functions over extended time frames. As cell-based therapies evolve, their success will require contribution from many diverse disciplines including regenerative medicine, developmental biology, and transplant medicine.

SiO(2) entrapment of animal cells: liver-specific metabolic activities in silicaoOverlaid hepatocytes

Rat hepatocytes in a collagen-gel sandwich configuration were exposed to silicon alkoxides in a gas phase, yielding a 0.05 to 0.15 microm porous silica layer on the gel surface. Cell viability was unaffected by the procedure. After 24 h, bilirubin conjugation, ammonia removal, urea synthesis, and diazepam metabolism were unaffected by the procedure. However, both the ammonia removal rate and diazepam metabolism were increased after 48 hr, whereas urea synthesis was unaffected. These data indicate that silica overlay allows efficient metabolic activity of collagen-gel entrapped hepatocytes. The fact that the KM of bilirubin conjugation was unaffected by the presence of the silica membrane suggests that the transport of albumin-bound substrates is not decreased. The enhancement in some metabolic activities found 48 h after the entrapment procedure may be the result of favorable changes in the hepatocyte microenvironment. These characteristics might be useful for the development of organotypical bioartificial liver devices.

Functional behavior of primary rat liver cells in a three-dimensional perfused microarray bioreactor

We have previously described the design and operation of a microfabricated bioreactor that supports perfused 3D culture of liver cells and facilitates evolution of tissue-like morphological structures. Here, we describe the functional viability of cells maintained in this microarray bioreactor and examine the influence of different seeding protocols on the evolution of structure and function in comparison with static culture. Primary rat hepatocytes were seeded into the perfusion reactors either as single-cell suspensions immediately after isolation or as spheroidal aggregates formed over a 2- to 3-day period. Initial studies in which cells were cultured for 7 days postisolation revealed significantly greater functional activity and morphological stability of cells that were preaggregated for up to 3 days before seeding in the reactor, compared with direct seeding of single cells. Total albumin secretion and urea genesis rates in single-cell reactor cultures declined significantly during this initial culture period while remaining constant in preaggregated reactor cultures. Longer term studies indicate that rates of albumin secretion and urea genesis are maintained at constant levels through 15 days postisolation. These metabolic rates are an order of magnitude higher than observed for the same preaggregated structures cultured statically with comparable medium ratio and exchange conditions. The metabolic function data are supported by light microscopy images showing viable tissue structures, and electron microscopy images that reveal tight junctions, glycogen storage, and bile canaliculi.

Membranes for biohybrid liver support systems--investigations on hepatocyte attachment, morphology and growth

The biological properties of four different membranes were studied regarding their possible application in biohybrid liver support systems. Two of them, one made of polyetherimide (PEI), and a second based on polyacrylonitrile-N-vinylpyrollidone co-polymer (P(AN-NVP)), were recently developed in our lab and studied for the first time. Together with pure polyacrylonitrile (PAN) membranes, the three preparations were characterised as ultra-filtration membranes. Their ability to support cell attachment, morphology, proliferation and function of human hepatoblastoma C3A cells was studied. The role of surface morphology for the interaction with hepatocytes was highlighted using a commercial, moderately wettable polyvinylidendifluoride (PVDF) membrane with micro-filtration properties. Comparative investigations showed strongest interaction of C3A cells with PAN membranes, as the focal adhesion contacts were more expressed and cell growth was also high. However, the functional activity in terms of albumin synthesis was reduced. Very similar results were obtained with the most hydrophobic PEI membrane. In contrast, the most hydrophilic membrane P(AN-NVP) was found to provoke stronger homotypic adhesion (E-cadherin expression) of C3A cells and less substratum attachment (focal adhesions), but enhanced albumin secretion. However, proliferation of C3A cells was lowered. Micro-porous PVDF membrane showed very good initial attachment, but the resulting cell material and cell-cell interaction were relatively poor developed. Among four membranes tested, PEI seems to be the most attractive membrane for biohybrid liver devices, as it provides good surface properties for hepatocytes interaction, but in addition it is highly thermostable, which would permit steam sterilisation. No simple relationship, however, between the wettability of the membranes and their ability to support hepatocyte adhesion and function was found in this study.

Multi-layered microcapsules for cell encapsulation

Mechanical stability, complete encapsulation, selective permeability, and suitable extra-cellular microenvironment, are the major considerations in designing microcapsules for cell encapsulation. We have developed four types of multi-layered microcapsules that allow selective optimization of these parameters. Primary hepatocytes were used as model cells to test these different microcapsule configurations. Type-1 microcapsules with an average diameter of 400 microm were formed by complexing modified collagen with a ter-polymer shell of 2-hydroxyethyl methylacrylate (HEMA), methacrylic acid (MAA) and methyl methacrylate (MMA), resulting in a capsule thickness of 2-5 microm. Cells in these microcapsules exhibited improved cellular functions over those cultured on collagen monolayers. Type-II microcapsules were formed by encapsulating the Type-I microcapsules in another 2-5 microm ter-polymer shell and a approximately 5 microm collagen layer between the two ter-polymer shells to ensure complete cell encapsulation. Type-II microcapsules comprised of a macro-porous exoskeleton with materials such as alumina sol-gel coated on the Type-I microcapsules. Nano-indendation assay indicated an improved mechanical stability over the Type-I microcapsules. Type-IV microcapsules were created by encapsulating Type-III microcapsules in another 2-5 microm ter-polymer shell, with the aim of imparting a negatively charged smooth surface to minimize plasma protein absorption and ensure complete cell encapsulation. The permeability for nutrient exchange, cellular functions in terms of urea production and mechanical stability of the microcapsules were characterized. The advantages and limitations of these microcapsules for tissue engineering are discussed.

Development of a hybrid artificial liver using polyurethane foam/hepatocyte spheroid culture in a preclinical pig experiment

We describe a preclinical study of our original hybrid artificial liver support system (HALSS) for a clinical trial. We designed a HALSS comprising a multi-capillary polyurethane foam packed-bed module (MC-PUF module) containing a total 200 g (2 x 10(10) cells) porcine hepatocytes, and an extracorporeal circulation device. Almost all porcine hepatocytes in the MC-PUF module formed many spherical multicellular aggregates (spheroids). This extracorporeal circulation device was improved to promote solute exchange between a living body and a MC-PUF module by including a plasma bypass line in the circulation loop. The efficacy of the HALSS was evaluated using a 25-kg pig with warm ischemic liver failure by portocaval shunt and ligation of hepatic artery (HALSS group, n=3). As a control experiment, the same system without hepatocytes in the module was used with the same kind of liver failure pig (Control group, n=3). The blood ammonia in the control group was 143 N-microg/dl at the start of circulation, and rapidly increased to 351 N-microg/dl at 2 hours and to 704 N-microg/dl at 6 hours. But the blood ammonia in the HALSS group was completely suppressed, and remained less than the hepatic coma level (over 200 N-microg/dl) during the circulation time. The blood glucose in the control group gradually decreased, and became less than 40 mg/dl within 6 hours of circulation. But the blood glucose in the HALSS group was maintained well, and remained the normal glucose level (50 - 105 mg/dl) for more than 20 hours of circulation. Improvement in blood creatinine and lactate, and the stabilization of vital signs and urinary excretion, were observed in the HALSS group. The survival time of the pigs in the HALSS group was 19.3 hours compared with 8.9 hours in the control group. In conclusion, our HALSS was effective to stabilize the general conditions of the body in addition to supporting various liver functions. These results suggest that our HALSS has a strong possibility to be used in treating liver failure patients. We have applied for approval of the clinical trial of our HALSS to our institutional ethics committee.

Longterm stability of phase I and phase II enzymes of porcine liver cells in flat membrane bioreactors

Recently, researchers have focused on the use of bioartificial liver devices to support patients with fulminant hepatic failure. Our team developed a cell-based flat membrane bioreactor (FMB). In this, porcine liver cells were maintained in 3D-coculture between two gel layers in a sandwich configuration for 3 weeks to study the influence of this bioreactor technique on the preservation of basic, not induced activities of phase I and phase II enzymes. First, the time and substrate dependencies of the following enzymes were measured: ethoxyresorufin-O-deethylase (EROD, CYP 1A1/1A2) and ethoxycoumarin-O-deethylase (ECOD, CYP 2B6) as phase I enzymes, and glutathione-S-transferase (GST), UDP-glucuronosyltransferase (UGT) and sulfotransferase (ST) as phase II enzymes. To find optimal test conditions Michaelis-Menten kinetics were calculated. Next, different potential inducers were tested to find out the most effective compounds. Based on these results, the basic, not induced levels of the different enzymes were determined in the flat membrane bioreactor. Furthermore, the response of these enzyme activities to the chosen inducers was investigated to examine whether the cells keep their ability for drug-drug interactions. Basic, not induced activities of both phase I enzymes and the phase II enzymes GST and UGT were maintained at nearly the initial levels during the complete period of study. In addition, it was possible to induce these enzymes twice or three times in a weekly interval. In contrast, the basic, not induced activity of ST increased during the first 10 days of culture. It stabilized then and was maintained steady. As in short-term investigations, no reaction of the ST-activity towards any inducer could be obtained. These results prove that porcine liver cells preserve their phase I and phase II activities and respond to inducing drugs over 3 weeks in culture. Therefore, the flat membrane bioreactor is not only suitable for investigating drug metabolism, drug-drug interactions, and enzyme induction but also for supporting liver functions.

Hybrid artificial liver using hepatocyte organoid culture

We developed 2 types of hybrid artificial liver modules using hepatocyte organoid culture. One was a polyurethane foam (PUF)/hepatocyte spheroid packed-bed module. Hepatocytes spontaneously formed spheroids in the PUF pores, and they maintained liver-specific functions well for at least 2 weeks in vitro. As a preclinical experiment, a hybrid artificial liver with 200 g porcine hepatocytes was applied to a pig (25 kg) with liver failure and showed that the hybrid artificial liver was effective in support of liver functions and stabilization of general conditions. We established a new technique of hepatocyte organoid formation using centrifugal force. A hepatocyte organoid formed by centrifugation in hollow fibers maintained functions for more than 4 months in vitro. We developed a new sinusoid-like structure module having hollow fibers arranged by spacers in a micro-regular arrangement. Inoculated hepatocytes in the extra-fiber space of the module formed the organoid by centrifugation, and they maintained the functions for at least 1 month in vitro. The results indicated that this module seems to be promising as a hybrid artificial liver.

Which hepatocyte will it be? Hepatocyte choice for bioartificial liver support systems

Liver failure, notwithstanding advances in medical management, remains a cause of considerable morbidity and mortality in the developed world. Although bioartificial liver (BAL) support systems offer the potential of significant therapeutic benefit for such patients, many issues relating to their use are still to be resolved. In this review, these issues are examined in terms of the functions required, the cells of choice in such a system, and the most appropriate environment to optimize the function of such cells. The major functions identified to date for a BAL are ammonia detoxification and biotransformation of toxic compounds, although this somewhat belies the complexity of the functions required. Two practical choices for cell type within such a system are xenogenic hepatocytes and immortalized human hepatocyte lines. Both these choices have drawbacks, such as the transmission of zoonoses and malignant infiltration, respectively. Finally, improvements in culture conditions, such as supplemented media, biodegradable scaffolds, and coculture, offer the possibility of prolonging the differentiated function of hepatocytes in a BAL.

Current status of liver support systems

The search for a support system for liver failure has been intensified. Methods currently being tested include those based on artificial support, on biological approaches (including extracorporeal liver perfusion and transplanted hepatocytes) as well as hybrid devices that combine artificial aspects with biological systems. Each of these three areas is undergoing fast technological and conceptual development. Controlled clinical trials are also under way.

Long-term experience after ex situ liver surgery

BACKGROUND

Ex situ liver surgery allows liver resection and vascular reconstruction in patients who have liver tumors located at critical sites. Only a small series of studies about ex situ liver surgery is available in the literature. No long-term results have been published.

METHODS

Twenty-four patients were considered for ex situ liver surgery because conventional liver surgery was considered impossible or too hazardous. The patient's ages were 51.3 +/- 7.5 years. Indications were various primary and secondary liver malignancies and benign liver tumors in 2 patients.

RESULTS

In 22 of 24 patients, the ex situ liver resection and subsequent autotransplantation were performed. The anhepatic periods in these patients lasted for 5.6 +/- 1.1 hours. In the remaining 2 patients, autotransplantation was not possible and allogenic liver transplantation was performed 17 and 19 hours after hepatectomy. In 4 patients, liver failure occurred after autotransplantation and required transplantation. The confluens between hepatic veins and the inferior vena cava was reconstructed in 5 patients. Fifteen patients survived the postoperative period and were discharged after 36.5 +/- 16 days. The median survival time of 6 patients who had metastases of colonic carcinoma was 21 months. The 2 patients with benign liver disease are alive 9 and 5 years after ex situ surgery.

CONCLUSIONS

Extended liver resections with difficult reconstructions of the hepatic venous confluens are feasible by ex situ liver surgery and subsequent autotransplantation. However, the early postoperative mortality rate is high, especially in patients with cholestatic livers. Early tumor recurrence remained the problem in these patients with extended local tumor spread. Ex situ liver surgery should only be performed in selected patients.

Acute liver failure: targeted artificial and hepatocyte-based support of liver regeneration and reversal of multiorgan failure

Acute liver failure (ALF) still represents a major therapeutic challenge for hepatologists due to its high mortality rate as a result of multiorgan failure. Although emergency orthotopic liver transplantation represents a major advance in the management of selected patients, it is not applicable to all candidates due to limited organ availability. Therefore, new therapeutic options should be developed to bridge selected patients to transplantation or to treat patients not candidates for liver transplantation. Although new techniques for cell culture and perfusion have resulted in a number of promising devices for the provision of temporary liver support in acute liver failure, their clinical efficacy is as yet uncertain. Controlled trials on a multi-centre basis in well-defined patient groups and with standardised outcome measures, including the extent to which treatment influences cell damage and regeneration and prevents or reverses multiorgan failure, will be essential to properly evaluate the clinical value of current and evolving artificial and bioartificial devices. The same considerations must also apply to the assessment of therapeutic efficacy of hepatocyte transplantation. A better understanding of mechanisms responsible for the development of liver cell death, along with cellular and molecular mechanisms allowing surviving cells to proliferate in a hostile environment, will be required if a more targeted therapeutic approach to decreasing hepatocellular injury and enhancing liver regeneration is to be achieved. Whether extracorporeal devices or the transplantation of primary hepatocytes, stem cells or cells genetically engineered to over-express key metabolic functions, a proliferative phenotype and/or cytoprotective pathways will be best suited to meeting these demanding challenges remains to be determined.

A new concept of bioartificial liver based on a fluidized bed bioreactor

Many bioartificial livers have been developed, but most of them suffer from difficulty when being scaled up and from poor efficiency of mass transfer between the plasma and the immobilized hepatocytes. We present a new concept of bioartificial liver based on the fluidized bed motion of hepatocytes entrapped in alginate beads. The bioreactor is designed to offer stable behavior. The maximum fluid perfusion velocity is determined to avoid any bead release from the bioreactor. The fluidized bed height depends on the amount of beads and the velocity employed. Under the optimized operating conditions, the mass transfer between perfusion fluid and beads is very efficient; only 10 min are necessary to reach concentration equilibrium. Hence, this fluidized bed bioartificial liver appears to be a promising tool for a liver support system in the treatment of acute liver failure.

Extracorporeal support and hepatocyte transplantation in acute liver failure and cirrhosis

The relative shortage of donor organs and lack of immediate availability mean that many patients with acute liver failure die before orthotopic liver transplantation can be performed. An effective temporary liver support system could improve the chance of survival with or without a transplant being ultimately carried out. Recent technological advances resulting in improved maintenance of hepatocyte viability and function in culture and bioreactor designs which facilitate adequate perfusion of the cellular component and removal of products of cellular metabolism have led to the development of a number of bioartificial devices for liver support. Three such devices have undergone preliminary clinical evaluation in the setting of acute liver failure, with a statistically significant reduction in raised intracerebral pressure along with improvements in consciousness level and some biochemical parameters associated with treatment with one of these. Several other devices with different characteristics have shown promise in vitro and/or in animal models but await clinical evaluation. Several new totally artificial systems have also been described, along with the emergence of isolated hepatocyte transplantation, with reports of successful 'bridging' to liver transplantation. Controlled trials on a multicentre basis in well-defined patient groups and with standardized outcome measures will be required to properly evaluate the clinical value of each of these approaches to providing liver support in acute liver failure and cirrhosis. A better understanding of mechanisms underlying multiorgan failure and of factors inhibiting liver regeneration, thereby allowing a more targeted approach, will be essential to the further development of effective liver support strategies in these settings.

Numerical model of fluid flow and oxygen transport in a radial-flow microchannel containing hepatocytes

The incorporation of monolayers of cultured hepatocytes into an extracorporeal perfusion system has become a promising approach for the development of a temporary bioartificial liver (BAL) support system. In this paper we present a numerical investigation of the oxygen tension, shear stress, and pressure drop in a bioreactor for a BAL composed of plasma-perfused chambers containing monolayers of porcine hepatocytes. The chambers consist of microfabricated parallel disks with center-to-edge radial flow. The oxygen uptake rate (OUR), measured in vitro for porcine hepatocytes, was curve-fitted using Michaelis-Menten kinetics for simulation of the oxygen concentration profile. The effect of different parameters that may influence the oxygen transport inside the chambers, such as the plasma flow rate, the chamber height, the initial oxygen tension in the perfused plasma, the OUR, and K(m) was investigated. We found that both the plasma flow rate and the initial oxygen tension may have an important effect upon oxygen transport. Increasing the flow rate and/or the inlet oxygen tension resulted in improved oxygen transport to cells in the radial-flow microchannels, and allowed significantly greater diameter reactor without oxygen limitation to the hepatocytes. In the range investigated in this paper (10 microns < H < 100 microns), and for a constant plasma flow rate, the chamber height, H, had a negligible effect on the oxygen transport to hepatocytes. On the contrary, it strongly affected the mechanical stress on the cells that is also crucial for the successful design of the BAL reactors. A twofold decrease in chamber height from 50 to 25 microns produced approximately a fivefold increase in maximal shear stress at the inlet of the reactor from 2 to 10 dyn/cm2. Further decrease in chamber height resulted in shear stress values that are physiologically unrealistic. Therefore, the channel height needs to be carefully chosen in a BAL design to avoid deleterious hydrodynamic effects on hepatocytes.

Evaluation of a novel bioartificial liver in rats with complete liver ischemia: treatment efficacy and species-specific alpha-GST detection to monitor hepatocyte viability

BACKGROUND/AIMS

There is an urgent need for an effective bioartificial liver system to bridge patients with fulminant hepatic failure to liver transplantation or to regeneration of their own liver. Recently, we proposed a bioreactor with a novel design for use as a bioartificial liver (BAL). The reactor comprises a spirally wound nonwoven polyester fabric in which hepatocytes are cultured (40 x 10(6) cells/ml) as small aggregates and homogeneously distributed oxygenation tubing for decentralized oxygen supply and CO2 removal. The aims of this study were to evaluate the treatment efficacy of our original porcine hepatocyte-based BAL in rats with fulminant hepatic failure due to liver ischemia (LIS) and to monitor the viability of the porcine hepatocytes in the bioreactor during treatment. The latter aim is novel and was accomplished by applying a new species-specific enzyme immunoassay (EIA) for the determination of porcine alpha-glutathione S-transferase (alpha-GST), a marker for hepatocellular damage.

METHODS

Three experimental groups were studied: the first control group (LIS Control, n = 13) received a glucose infusion only; a second control group (LIS No-Cell-BAL, n = 8) received BAL treatment without cells; and the treated group (LIS Cell-BAL, n = 8) was connected to our BAL which had been seeded with 4.4 x 10(8) viable primary porcine hepatocytes.

RESULTS/CONCLUSIONS

In contrast to previous comparable studies, BAL treatment significantly improved survival time in recipients with LIS. In addition, the onset of hepatic encephalopathy was significantly delayed and the mean arterial blood pressure significantly improved. Significantly lower levels of ammonia and lactate in the LIS Cell-BAL group indicated that the porcine hepatocytes in the bioreactor were metabolically activity. Low pig alpha-GST levels suggested that our bioreactor was capable of maintaining hepatocyte viability during treatment. These results provide a rationale for a comparable study in LIS-pigs as a next step towards potential clinical application.

Enhanced oxygen delivery reverses anaerobic metabolic states in prolonged sandwich rat hepatocyte culture

It must be assumed that current petri dish primary hepatocyte culture models do not supply sufficient amounts of oxygen and thus cause anaerobic metabolism of the cells. This is contrary to the physiologic state of the cells. In vivo the liver is a highly vascularized organ with a rather high blood flow rate of a mixture of arterial and venous blood. The aim of the present study was to show the oxygen dependence of primary rat hepatocytes in long-term culture and to define appropriate conditions that could allow hepatocytes to maintain tissue specific functions in an aerobic environment. To this purpose matrix overlaid hepatocytes were either cultured on gas-permeable (fluorinated hydrocarbon films) or gas-impermeable (polystyrene) supports at 10% and 20% ambient oxygen concentration (v/v), respectively. Tissue-specific functions were assessed by studying albumin and urea secretion as well as xenobiotic metabolism. The mRNA expression and catalytic activities of the cytoprotective antioxidant enzymes mitochondrial manganese superoxide dismutase (MnSOD), cytosolic copper and zinc superoxide dismutase, peroxisomal catalase, and cytosolic glutathione peroxidase were investigated to assess intracellular responses to the defined variations in oxygen supply. Hepatocytes could successfully be maintained at aerobic conditions in long-term culture on gas-permeable PTFE films. At 50% (10%, v/v) of currently used oxygen levels lactate accumulation was prevented, a plateau-like albumin secretion reestablished, urea secretion improved, and xenobiotic metabolism proceeded at physiological rates. mRNA expression of cytoprotective enzymes responded to the pericellular availability of oxygen and was most pronounced in the case of MnSOD. However, the biggest stress factor for the hepatocytes still appeared to be the isolation procedure, as mRNA expression and catalytic activities were most elevated shortly thereafter. In conclusion, this study clearly shows the oxygen dependence of primary rat hepatocytes in long-term culture and indicates means to establish appropriate conditions for the aerobic culture of primary rat sandwich hepatocytes with full maintenance of function. The long-term culture of hepatocytes on oxygenating supports at in vivo-like oxygen tensions therefore appears to be more physiologic and beneficial for the cells.

Semipermeable hollow fiber membranes in hepatocyte bioreactors: a prerequisite for a successful bioartificial liver?

Recent studies have shown that liver support systems based on viable hepatocytes can prolong life in animal models of acute liver failure. Now the time has come to elucidate the design characteristics that are essential to construct an efficient bioreactor. The gold standard remains the intact liver. Despite the very high cell density in this organ, individual cell perfusion is guaranteed resulting in low diffusional gradients which are essential for optimal mass transfer. These conditions are not met in bioreactors based on hollow fiber membranes. Moreover, the semipermeable membranes can foul and act as a diffusional barrier between the hepatocytes and the blood or plasma of the recipient. We devised a novel bioreactor for use as a bioartificial liver that does not include hollow fiber membranes for blood or plasma perfusion. The device is based on an integral oxygenator and a nonwoven polyester matrix material for hepatocyte culture as small aggregates. The efficacy of this original design was tested in rats with liver ischemia. Preliminary results show statistically significantly improved survival; life was prolonged 100% compared to the control experiments.

Reconstruction of liver tissue in vitro: geometry of characteristic flat bed, hollow fiber, and spouted bed bioreactors with reference to the in vivo liver

Bioreactors currently being developed for hybrid artificial livers vary greatly with respect to their microenvironment. The specific architecture modifies the relationship parenchymal and nonparenchymal cells have with the exchange surfaces of the bioreactor. Most designs are either based on hollow fiber, spouted bed, or flat bed devices. This diversity is contrasted by the uniform and unique organization of the in vivo liver. The liver cells are arranged as plates and both sinusoidal surfaces of the hepatocytes are enclosed within the matrix of the space of Disse. In this study we intended to define the in vivo liver tissue characteristics in a manner useful for an organotypical approach to hepatic tissue engineering. Transmission electron microscopy of an in vivo liver was utilized to describe these ratios. The ratios defined in this study are based on the constant hepatocellular expression of two sinusoidal surfaces. A relationship is established between the expression of the sinusoidal surfaces and their use as attachment and exchange surfaces inside a bioreactor. The presence of biliary surfaces and nonparenchymal cell surfaces is compared. The functional relevance of an in vivo like extracellular matrix geometry for oxidative biotransformation of primary hepatocytes in vitro was studied using the two model drugs cyclosporin and rapamycin. The generation of the hydroxylated cyclosporin metabolites AM 9 and AM 1 and four rapamycin metabolites was analyzed by high performance liquid chromatography (HPLC). It is shown that the cell-specific biotransformation rates at 1 week in culture in matrix overlayed hepatocytes was 5-10 times that of hepatocytes without matrix overlay. Bilaminar membrane (BLM) bioreactors were used to reconstruct extracellular matrix geometry, three-dimensional cell plates, and sinusoidal analogs in between cell plates.