Efficient ex vivo gene transfer into non-human primate hepatocytes using HIV-1 derived lentiviral vectors

Short term but highly efficient Cas9 expression mediated by excisional system using adenovirus vector and Cre

Genome editing techniques such as CRISPR/Cas9 have both become common gene engineering technologies and have been applied to gene therapy. However, the problems of increasing the efficiency of genome editing and reducing off-target effects that induce double-stranded breaks at unexpected sites in the genome remain. In this study, we developed a novel Cas9 transduction system, Exci-Cas9, using an adenovirus vector (AdV). Cas9 was expressed on a circular molecule excised by the site-specific recombinase Cre and succeeded in shortening the expression period compared to AdV, which expresses the gene of interest for at least 6 months. As an example, we chose hepatitis B, which currently has more than 200 million carriers in the world and frequently progresses to liver cirrhosis or hepatocellular carcinoma. The efficiencies of hepatitis B virus genome disruption by Exci-Cas9 and Cas9 expression by AdV directly (Avec) were the same, about 80–90%. Furthermore, Exci-Cas9 enabled cell- or tissue-specific genome editing by expressing Cre from a cell- or tissue-specific promoter. We believe that Exci-Cas9 developed in this study is useful not only for resolving the persistent expression of Cas9, which has been a problem in genome editing, but also for eliminating long-term DNA viruses such as human papilloma virus.

Curative Ex Vivo Hepatocyte-Directed Gene Editing in a Mouse Model of Hereditary Tyrosinemia Type 1

Hereditary tyrosinemia type 1 (HT1) is an autosomal recessive disorder caused by deficiency of fumarylacetoacetate hydrolase (FAH). It has been previously shown that ex vivo hepatocyte-directed gene therapy using an integrating lentiviral vector to replace the defective Fah gene can cure liver disease in small- and large-animal models of HT1. This study hypothesized that ex vivo hepatocyte-directed gene editing using CRISPR/Cas9 could be used to correct a mouse model of HT1, in which a single point mutation results in loss of FAH function. To achieve high transduction efficiencies of primary hepatocytes, this study utilized a lentiviral vector (LV) to deliver both the Streptococcus pyogenes Cas9 nuclease and target guide RNA (LV-Cas9) and an adeno-associated virus (AAV) vector to deliver a 1.2 kb homology template (AAV-HT). Cells were isolated from Fah^-/- mice and cultured in the presence of LV and AAV vectors. Transduction of cells with LV-Cas9 induced significant indels at the target locus, and correction of the point mutation in Fah^-/- cells ex vivo using AAV-HT was completely dependent on LV-Cas9. Next, hepatocytes transduced ex vivo by LV-Cas9 and AAV-HT were transplanted into syngeneic Fah^-/- mice that had undergone a two-thirds partial hepatectomy or sham hepatectomy. Mice were cycled on/off the protective drug 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) to stimulate expansion of corrected cells. All transplanted mice became weight stable off NTBC. However, a significant improvement was observed in weight stability off NTBC in animals that received partial hepatectomy. After 6 months, mice were euthanized, and thorough biochemical and histological examinations were performed. Biochemical markers of liver injury were significantly improved over non-transplanted controls. Histological examination of mice revealed normal tissue architecture, while immunohistochemistry showed robust repopulation of recipient animals with FAH+ cells. In summary, this is the first report of ex vivo hepatocyte-directed gene repair using CRISPR/Cas9 to demonstrate curative therapy in an animal model of liver disease.

Transplantation of genetically modified hepatocytes after liver preconditioning in Watanabe heritable hyperlipidemic rabbit

BACKGROUND

Hepatocyte transplantation is a potentially less invasive alternative to liver transplantation for treating inherited metabolic liver diseases. We developed an autotransplantation protocol of ex vivo genetically modified hepatocytes combining lentiviral transduction and transplantation after liver preconditioning by partial portal vein embolization. We investigated the metabolic efficiency of this approach in Watanabe rabbits, animal model of familial hypercholesterolemia.

METHODS

Our autotransplantation experimental protocol was used in two groups of rabbits (n = 10), experimental and sham, receiving transduced and control hepatocytes, respectively. Isolated hepatocytes from left liver lobes were transduced using recombinant lentiviruses. Median lobe portal branches were embolized under fluoroscopic control. Functional measurement of low-density lipoprotein (LDL) receptor expression was assessed by LDL internalization assays. Cholesterol level evolution was monitored. Rabbits were killed 20 wk after the procedure.

RESULTS

Three rabbits of each group died several hours after hepatocyte transplantation; autopsy revealed portal vein thrombosis in two rabbits from each group. The protocol was therefore modified with hepatocytes being transplanted through splenic injection. Lentiviral hepatocyte transduction efficacy was 64.5%. Fluorescence microscopy revealed Dil-LDL internalization of transduced hepatocytes. Seven rabbits in each group were considered for lipid analysis. Four weeks after autotransplantation, median total cholesterol level decreased in the experimental group, without reaching statistical significance (8.9 [8.0-9.8] g/L versus 6.3 [0.5-8.3]; P = 0.171). In the experimental group, enzyme-linked immunosorbent assay detected significant antibody expression against human low-density lipoprotein receptor.

CONCLUSIONS

Autotransplantation protocol allowed a nonstatistically significant improvement of the lipid profile in Watanabe rabbits. Further experiments involving a larger number of animals are necessary to confirm or refute our findings.

Integration-deficient lentivectors: an effective strategy to purify and differentiate human embryonic stem cell-derived hepatic progenitors

Background

Human pluripotent stem cells (hPSCs) hold great promise for applications in regenerative medicine. However, the safety of cell therapy using differentiated hPSC derivatives must be improved through methods that will permit the transplantation of homogenous populations of a specific cell type. To date, purification of progenitors and mature cells generated from either embryonic or induced pluripotent stem cells remains challenging with use of conventional methods.

Results

We used lentivectors encoding green fluorescent protein (GFP) driven by the liver-specific apoliprotein A-II (APOA-II) promoter to purify human hepatic progenitors. We evaluated both integrating and integration-defective lentivectors in combination with an HIV integrase inhibitor. A human embryonic stem cell line was differentiated into hepatic progenitors using a chemically defined protocol. Subsequently, cells were transduced and sorted at day 16 of differentiation to obtain a cell population enriched in hepatic progenitor cells. After sorting, more than 99% of these APOA-II-GFP-positive cells expressed hepatoblast markers such as α-fetoprotein and cytokeratin 19. When further cultured for 16 days, these cells underwent differentiation into more mature cells and exhibited hepatocyte properties such as albumin secretion. Moreover, they were devoid of vector DNA integration.

Conclusions

We have developed an effective strategy to purify human hepatic cells from cultures of differentiating hPSCs, producing a novel tool that could be used not only for cell therapy but also for in vitro applications such as drug screening. The present strategy should also be suitable for the purification of a broad range of cell types derived from either pluripotent or adult stem cells.

Ex vivo gene delivery to hepatocytes: techniques, challenges, and underlying mechanisms

Gene delivery to primary hepatocytes is an important tool for a number of applications including the study of liver cell biology and pathology, drug screening, and gene therapy. Robust transfection of primary hepatocytes, however, is significantly more difficult to achieve than in cell lines or readily dividing primary cells. In this report, we investigated in vitro gene delivery to both primary rat hepatocytes and Huh7.5.1 cells (a hepatoma cell line) using a number of viral and non-viral methods, including Lipofectamine 2000, FuGene HD, Nucleofection, Magnetofection, and lentiviruses. Our results showed that Lipofectamine 2000 is the most efficient reagent for green fluorescent protein (GFP) gene delivery to primary rat hepatocytes (33.3 ± 1.8% transfection efficiency) with minimal adverse effect on several hepatic functions, such as urea and albumin secretion. The lentiviral vectors used in this study exhibited undetectable gene delivery to primary rat hepatocytes but significant delivery to Huh7.5.1 cells (>80% transfection efficiency). In addition, we demonstrated lentiviral-based and spatially defined delivery of the GFP gene to Huh7.5.1 cells for use in biological microelectromechanical systems.

Human foetal hepatocytes: isolation, characterization, and transplantation

Hepatocyte transplantation has become an alternative to orthotopic liver transplantation for the treatment of liver metabolic diseases. However, there is an increasing lack of donor organs and isolated mature hepatocytes are difficult to manipulate and cannot be expanded in vitro. It is therefore necessary to find alternative sources of hepatocytes, and different approaches to evaluate the therapeutic potential of stem cells of different origins are being developed. Hepatic progenitors (hepatoblasts) and/or foetal hepatocytes isolated from foetal livers may be one potential source to generate fully differentiated hepatocytes. We have reported that human foetal liver cells can be isolated and cultured. These cells also engraft and differentiate into mature hepatocytes in situ after transplantation into immunodeficient mice. Foetal cell populations could also be used as targets for gene therapy since efficient gene transfer is achieved with retroviral vectors. Use of such experimental approaches will help design strategies for clinical applications of liver cell therapy with hepatic progenitors.

Biosafety in ex vivo gene therapy and conditional ablation of lentivirally transduced hepatocytes in nonhuman primates

Ex vivo gene therapy is an interesting alternative to orthotopic liver transplantation (OLT) for treating metabolic liver diseases. In this study, we investigated its efficacy and biosafety in nonhuman primates. Hepatocytes isolated from liver lobectomy were transduced in suspension with a bicistronic liver-specific lentiviral vector and immediately autotransplanted (SLIT) into three cynomolgus monkeys. The vector encoded cynomolgus erythropoietin (EPO) and the conditional suicide gene herpes simplex virus-thymidine kinase (HSV-TK). Survival of transduced hepatocytes and vector dissemination were evaluated by detecting transgene expression and vector DNA. SLIT was safely performed within a day in all three subjects. Serum EPO and hematocrit rapidly increased post-SLIT and their values returned to baseline within about 1 month. Isoforms of EPO detected in monkeys' sera differed from the physiological renal EPO. In liver biopsies at months 8 and 15, we detected EPO protein, vector mRNA and DNA, demonstrating long-term survival and functionality of transplanted lentivirally transduced hepatocytes. Valganciclovir administration resulted in complete ablation of the transduced hepatocytes. We demonstrated the feasibility and biosafety of SLIT, and the long term (>1 year) functionality of lentivirally transduced hepatocytes in nonhuman primates. The HSV-TK/valganciclovir suicide strategy can increase the biosafety of liver gene therapy protocols by safely and completely ablating transduced hepatocytes on demand.

Efficient hepatocyte engraftment and long-term transgene expression after reversible portal embolization in nonhuman primates

The feasibility of ex vivo gene therapy as an alternative to liver transplantation for the treatment of liver metabolic diseases needs to be analyzed in large animal models. This approach requires appropriate gene transfer vectors and effective hepatocyte engraftment. Lentiviral vectors have the ability to transduce nondividing differentiated cells, such as hepatocytes, and portal vein occlusion increases hepatocyte engraftment. We investigated whether reversible portal vein embolization combined with ex vivo lentivirus-mediated gene transfer is an effective approach for successful hepatocyte engraftment in nonhuman primates and whether the transgene remains expressed in the long term in transplanted hepatocytes in situ. Simian hepatocytes were isolated after left lobe resection, and the left and right anterior portal branches of animals were embolized with absorbable material. Isolated hepatocytes were labeled with Hoechst dye or transduced in suspension with lentiviruses expressing green fluorescent protein under the control of the human apolipoprotein A-II promoter and transplanted via the inferior mesenteric vein. The whole procedure was well tolerated. The embolized liver was revascularized within 2 weeks. The volume of nonembolized liver increased from 38.7% +/- 0.8% before embolization to 55.9% +/- 1% after embolization and hepatocytes significantly proliferated (10.5% +/- 0.4% on day 3 after embolization). Liver repopulation after transplantation with Hoechst-labeled hepatocytes was 7.4% +/- 1.2%. Liver repopulation was 2.1% +/- 0.2% with transduced hepatocytes, a proportion similar to that obtained with Hoechst-labeled cells, given that the mean transduction efficacy of simian hepatocyte population was 34%. Transgene expression persisted at 16 weeks after transplantation.

CONCLUSION

We have developed a new approach to improve hepatocyte engraftment and to express a transgene in the long term in nonhuman primates. This strategy could be suitable for clinical applications.

Hepatocyte transplantation in animal models

More than 30 years after the first hepatocyte transplant to treat the Gunn rat, the animal model for Crigler-Najjar syndrome, there are still a number of impediments to hepatocyte transplantation. Numerous animal models are still used in work aimed at improving hepatocyte engraftment and/or long-term function. Although other cell sources, particularly hepatic and extrahepatic stem cells, are being explored, adult hepatocytes remain the cells of choice for the treatment of liver diseases by cell therapy. In recent years, diverse approaches have been developed in various animal models to enhance hepatocyte transduction and amplification in vitro and cell engraftment and functionality in vivo. They have led to significant progress in hepatocyte transplantation for the treatment of patients with metabolic diseases and for bridging patients with acute injury until their own livers regenerate. This review presents and considers the results of this work with a special emphasis on procedures that might be clinically applicable.

Hepatocyte transplantation techniques: large animal models

The poor hepatocyte engraftment efficiency and the low level of their expansion in the host liver are a major limitation to cell therapy for the treatment of life-threatening liver diseases. Many rodent models have shown that liver repopulation via transplanted hepatocytes occurs only when liver growth capacity is impaired for an extended period of time. However, these models are not transposable to the clinics and to date there is no safe method to achieve this result in a clinical setting.Therefore, it is necessary to define on large animal models strategies that provide to transplanted hepatocytes sufficient proliferation stimuli to induce their division and that could permit a direct extrapolation to humans. Such procedures should be transposable to patients. We have defined a protocol of liver partial portal branch embolisation and shown that it induces the proliferation of transplanted hepatocytes in non-human primates (Macaca mulatta). This animal model is also appropriate to evaluate the lentiviral-mediated ex vivo gene therapy approach, since simian hepatocytes are efficiently transduced by HIV-1-derived lentivirus vectors.

Antiviral gene therapy

This chapter describes the major gene therapeutic approaches for viral infections. The vast majority of published approaches target severe chronic viral infections such as hepatitis B or C and HIV infection. Two basic gene therapy strategies are introduced here. The first involves the expression of a protein or an RNA that inhibits viral replication by targeting crucial steps of the viral life cycle or by interfering with a cellular factor required for virus replication. The major limitation of this approach is that primary levels of gene modification have generally not been sufficient to reduce the availability of target cells permissive for virus replication to a level that significantly decreases overall viral load. Thus, investigators have banked on the expectation that gene-protected cells have a sufficient selective advantage to accumulate and gain prevalence over time, a prediction that so far could not be confirmed in clinical trials. In vivo levels of gene modification can be improved, however, by introducing an additional selectable marker. In addition, a secreted antiviral gene product that exerts a bystander effect could significantly reduce overall virus replication despite relatively low levels of gene modification. In addition to these direct antiviral approaches, several strategies have been developed that employ or aim to enhance host immune responses. The innate immune response has been enhanced, for example, by the in vivo expression of interferons. Alternatively, T cells can be grafted with recombinant receptors to boost adaptive virus-specific immunity. These approaches are especially promising for chronic virus infection, where natural immune responses are evidently not sufficient to effectively control virus replication.

Gene therapy for viral hepatitis

Hepatitis B and C infections are two of the most prevalent viral diseases in the world. Existing therapies against chronic viral hepatitis are far from satisfactory due to low response rates, undesirable side effects and selection of resistant viral strains. Therefore, new therapeutic approaches are urgently needed. This review, after briefly summarising the in vitro and in vivo systems for the study of both diseases and the genetic vehicles commonly used for liver gene transfer, examines the existing status of gene therapy-based antiviral strategies that have been employed to prevent, eliminate or reduce viral infection. In particular, the authors focus on the results obtained in clinical trials and experimental clinically relevant animal models.

Efficient Hepatocyte Engraftment in a Nonhuman Primate Model After Partial Portal Vein Embolization

BACKGROUND

Hepatocyte transplantation could be an alternative to whole liver transplantation for the treatment of metabolic liver diseases. However, the results of clinical investigations suggest that the number of engrafted hepatocytes was insufficient to correct metabolic disorders. This may partly result from a lack of proliferation of transplanted hepatocytes. In rodents, portal ligation enhances hepatocyte engraftment after transplantation. We investigated the effects of partial portal ligation and embolization on engraftment and proliferation of transplanted hepatocytes in primates.

METHODS

Hepatocyte autotransplantation was performed in Macaca monkeys. The left lateral lobe was resected for hepatocyte isolation. The first group of monkeys underwent surgical ligation of the left and right anterior portal branches; in the second group, the same portal territories were obstructed by embolization with biological glue. To evaluate the proportion of cell engraftment hepatocytes were Hoechst-labeled and transplanted via the portal vein. Cell proliferation was measured by BrdU incorporation.

RESULTS

Hepatocyte proliferation was induced by both procedures but it was significantly higher after partial portal embolization (23.5% and 11.2% of dividing hepatocytes on days 3 and 7) than after ligation (3% and 0.8%). Hepatocytes engrafted more efficiently after embolization than after ligation. They proliferated and participated to liver regeneration representing 10% of the liver mass on day seven and their number remained constant on day 15.

CONCLUSIONS

These data suggest that partial portal embolization of the recipient liver improves engraftment of transplanted hepatocytes in a primate preclinical model providing a new strategy for hepatocyte transplantation.

Hepatocyte transplantation: studies in preclinical models

Transplantation of allogeneic or genetically modified autologous hepatocytes may be an alternative to whole-liver transplantation for the treatment of hereditary metabolic liver diseases. Human hepatocytes have already been transplanted in patients, demonstrating the safety and feasibility of both approaches. Although a few cases of allogeneic transplantation have resulted in long-term engraftment and function, only a partial and transient correction of the disease was achieved. This may partly result from a lack of proliferation of transplanted cells. In rodents, transplanted hepatocytes do not proliferate in adult quiescent livers and repopulate recipient livers only when they display a proliferative advantage over resident hepatocytes. Most of these models are not transposable to humans, however. Our aim is to develop preclinical approaches to hepatocyte transplantation in nonhuman primates. We have defined a strategy that increases the engraftment efficiency of transplanted hepatocytes by inducing their proliferation together with that of resident hepatocytes. We have also immortalized simian fetal hepatic progenitor cells and shown that these cells do not proliferate in situ after transplantation into the livers of immunodeficient mice. By contrast early human hepatoblasts repopulate mouse livers more efficiently. However, if we consider the number of cells to be transplanted (one to several billion), the means of expanding and differentiating stem or progenitor cells other than hepatocytes will have to be determined prior to envisaging treating patients.