Stem cell-derived hepatocytes: A promising novel tool to study hepatitis B virus infection

In vitro cell culture models to study hepatitis B and D virus infection

Chronic infection with the hepatitis B virus (HBV) and hepatitis D virus (HDV) can cause a major global health burden. Current medication regimens can repress viral replication and help to control disease progression, but a complete cure is hardly achieved due to the difficulties to eradicate viral templates (cccDNA and integrates). To develop novel curative antiviral therapies for HBV/HDV infection, it is vital to precisely understand the details of the molecular biology of both viruses and the virus-host interactions. One important prerequisite for gaining this aim is the availability of suitable in vitro models that support HBV/HDV infection, replicate both viruses via their authentic template and allow to adequately study host cell responses. The discovery of sodium taurocholate cotransporting polypeptide (NTCP) receptor as the most crucial host factor promoted HBV/HDV research to a new era. Recently, the structure of human NTCP was solved, gaining a deeper understanding of HBV recognition as the bona fide receptor. After decades of continuous efforts, new progress has been achieved in the development of cell culture models supporting HBV/HDV study. This review summarizes the cell culture models currently available, discusses the advantages and disadvantages of each model, and highlights their future applications in HBV and HDV research.

Na+-Taurocholate Co-Transporting Polypeptide (NTCP) in Livers, Function, Expression Regulation, and Potential in Hepatitis B Treatment

Chronic hepatitis B virus (HBV) infection has become one of the leading causes of liver cirrhosis and hepatocellular carcinoma globally. The discovery of sodium taurocholate co-transporting polypeptide (NTCP), a solute carrier, as a key receptor for HBV and hepatitis D virus (HDV) has opened new avenues for HBV treatment. Additionally, it has led researchers to generate hepatoma cell lines (including HepG2-NTCP and Huh-7-NTCP) susceptible to HBV infection in vitro, hence, paving the way to develop and efficiently screen new and novel anti-HBV drugs. This review summarizes the history, function and critical findings regarding NTCP as a viral receptor for HBV/HDV, and it also discusses recently developed drugs targeting NTCP.

Agonist c-Met Monoclonal Antibody Augments the Proliferation of hiPSC-derived Hepatocyte-Like Cells and Improves Cell Transplantation Therapy for Liver Failure in Mice

Rationale: Hepatocyte-like cells (HLCs) derived from human induced pluripotent stem cells (hiPSCs) have been developed to address the shortage of primary human hepatocytes (PHHs) for therapeutic applications. However, the in vivo repopulation capacity of HLCs remains limited. This study investigated the roles of agonist antibody activating the c-Met receptor in promoting the in vivo proliferation and repopulation of engrafted PHHs and/or HLCs in mice with liver injuries due to different causes.

Methods: An agonist c-Met receptor antibody (5D5) was used to treat PHHs and hiPSC-HLCs in both cell culture and hepatocyte-engrafted immunodeficient mice mimicking various inherited and acquired liver diseases. The promoting roles and potential influence on the hepatic phenotype of the 5D5 regimen in cell transplantation-based therapeutic applications were systematically evaluated.

Results: In hiPSC-HLC cell cultures, 5D5 treatment significantly stimulated c-Met receptor downstream signalling pathways and accelerated cell proliferation in dose-dependent and reversible manners. In contrast, only slight but nonsignificant promotion was observed in 5D5-treated PHHs. In vivo administration of 5D5 greatly promoted the expansion of implanted hiPSC-HLCs in fumarylacetoacetate hydrolase (Fah) deficient mice, resulting in significantly increased human albumin levels and high human liver chimerism (over 40%) in the transplanted mice at week 8 after transplantation. More importantly, transplantation of hiPSC-HLCs in combination with 5D5 significantly prolonged animal survival and ameliorated liver pathological changes in mice with acute and/or chronic liver injuries caused by Fas agonistic antibody treatment, carbon tetrachloride treatment and/or tyrosinemic stress.

Conclusion: Our results demonstrated that the proliferation of hiPSC-HLCs can be enhanced by antibody-mediated modulation of c-Met signalling and facilitate hiPSC-HLC-based therapeutic applications for life-threatening liver diseases.

Cell and Animal Models for Studying Hepatitis B Virus Infection and Drug Development

Many cell culture and animal models have been used to study hepatitis B virus (HBV) replication and its effects in the liver; these have facilitated development of strategies to control and clear chronic HBV infection. We discuss the advantages and limitations of systems for studying HBV and developing antiviral agents, along with recent advances. New and improved model systems are needed. Cell culture systems should be convenient, support efficient HBV infection, and reproduce responses of hepatocytes in the human body. We also need animals that are fully permissive to HBV infection, convenient for study, and recapitulate human immune responses to HBV and effects in the liver. High-throughput screening technologies could facilitate drug development based on findings from cell and animal models.

Optimized HepaRG is a suitable cell source to generate the human liver chimeric mouse model for the chronic hepatitis B virus infection

The human liver chimeric mouse with primary human hepatocytes (PHHs) engraftment has been demonstrated to be a useful animal model to study hepatitis B virus (HBV) pathogenesis and evaluate anti-HBV drugs. However, the disadvantages of using PHHs include the inability for cellular expansion in vitro, limited donor availability, individual differences, and ethical issues, necessitating the development of alternatives. To obtain in vitro expandable hepatocytes, we optimized the hepatic differentiation procedure of the human liver progenitor cell line, HepaRG, using four functional small molecules (4SM) and enriched the precursor hepatocyte-like cells (HLCs). HepaRG cells of different hepatic differentiation states were engrafted to immunodeficient mice (FRGS) with weekly 4SM treatment. The HepaRG-engrafted mice were challenged with HBV and/or treated with several antivirals to evaluate their effects. We demonstrated that the 4SM treatment enhanced hepatic differentiation and promoted cell proliferation capacity both in vitro and in vivo. Mice engrafted with enriched HepaRG of prehepatic differentiation and treated with 4SM displayed approximately 10% liver chimerism at week 8 after engraftment and were maintained at this level for another 16 weeks. Therefore, we developed a HepaRG-based human liver chimeric mouse model: HepaRG-FRGS. Our experimental results showed that the liver chimerism of the mice was adequate to support chronic HBV infection for 24 weeks and to evaluate antivirals. We also demonstrated that HBV infection in HepaRG cells was dependent on their hepatic differentiation state and liver chimerism in vivo. Overall, HepaRG-FRGS mice provide a novel human liver chimeric mouse model to study chronic HBV infection and evaluate anti-HBV drugs.

A Chimeric Humanized Mouse Model by Engrafting the Human Induced Pluripotent Stem Cell-Derived Hepatocyte-Like Cell for the Chronic Hepatitis B Virus Infection

Humanized mouse model generated by grafting primary human hepatocytes (PHHs) to immunodeficient mouse has contributed invaluably to understanding the pathogenesis of hepatitis B virus (HBV). However, the source of PHHs is limited, which necessitates the search for alternatives. Recently, hepatocyte-like cells (HLCs) generated from human induced pluripotent stem cells (hiPSCs) have been used for in vitro HBV infection. Herein, we developed a robust human liver chimeric animal model to study in vivo HBV infection by engrafting the hiPSC-HLCs to Fah-/-Rag2-/-IL-2Rγc-/-SCID (FRGS) mice. After being optimized by a small molecule, XMU-MP-1, the hiPSC-HLCs engrafted FRGS (hHLC-FRGS) mice displayed approximately 40% liver chimerism at week 6 after engraftment and maintained at this level for at least 14 weeks. Viremia and HBV infection markers include antigens, RNA, DNA, and covalently closed circular DNA were detectable in HBV infected hHLC-FRGS mice. Furthermore, hiPSC-HLCs and hHLC-FRGS mice were successfully used to evaluate different antivirals. Therefore, we established a humanized mouse model for not only investigating HBV pathogenesis but also testing the effects of the anti-HBV drugs. Highlights:    (1) The implanted hiPSC-HLCs established a long-term chimerism in FRGS mice liver.    (2) hHLC-FRGS mice are adequate to support chronic HBV infection with a full viral life cycle.    (3) hiPSC-HLCs and hHLC-FRGS mice are useful tools for evaluation of antivirals against HBV infection in vitro and in vivo. Research in Context  To overcome the disadvantages of using primary human hepatocytes, we induced human pluripotent stem cells to hepatocyte-like cells (hiPSC-HLCs) that developed the capability to express important liver functional markers and critical host factors for HBV infection. The hiPSC-HLCs were permissive for the HBV infection and supported a full HBV replication. The hiPSC-HLCs were then engrafted to immunodeficient mouse to establish a chimeric liver mouse model, which was capable of supporting HBV infection in vivo and evaluating the effects of antiviral drugs. Our results shed light into improving the cellular and animal models for studying HBV and other hepatotropic viruses.

Hepatitis Delta Virus: Replication Strategy and Upcoming Therapeutic Options for a Neglected Human Pathogen

The human Hepatitis Delta Virus (HDV) is unique among all viral pathogens. Encoding only one protein (Hepatitis Delta Antigen; HDAg) within its viroid-like self-complementary RNA, HDV constitutes the smallest known virus in the animal kingdom. To disseminate in its host, HDV depends on a helper virus, the human Hepatitis B virus (HBV), which provides the envelope proteins required for HDV assembly. HDV affects an estimated 15-20 million out of the 240 million chronic HBV-carriers and disperses unequally in disparate geographical regions of the world. The disease it causes (chronic Hepatitis D) presents as the most severe form of viral hepatitis, leading to accelerated progression of liver dysfunction including cirrhosis and hepatocellular carcinoma and a high mortality rate. The lack of approved drugs interfering with specific steps of HDV replication poses a high burden for gaining insights into the molecular biology of the virus and, consequently, the development of specific novel medications that resiliently control HDV replication or, in the best case, functionally cure HDV infection or HBV/HDV co-infection. This review summarizes our current knowledge of HBV molecular biology, presents an update on novel cell culture and animal models to study the virus and provides updates on the clinical development of the three developmental drugs Lonafarnib, REP2139-Ca and Myrcludex B.