Advances and challenges in studying hepatitis C virus in its native environment

Conservation of cell-intrinsic immune responses in diverse nonhuman primate species

The transcriptomic response of diverse nonhuman primate (NHP) species to poly(I:C) is highly conserved, and this novel RNA sequencing dataset will help improve NHP genome annotations.

Differences in immune responses across species can contribute to the varying permissivity of species to the same viral pathogen. Understanding how our closest evolutionary relatives, nonhuman primates (NHPs), confront pathogens and how these responses have evolved over time could shed light on host range barriers, especially for zoonotic infections. Here, we analyzed cell-intrinsic immunity of primary cells from the broadest panel of NHP species interrogated to date, including humans, great apes, and Old and New World monkeys. Our analysis of their transcriptomes after poly(I:C) transfection revealed conservation in the functional consequences of their response. In mapping reads to either the human or the species-specific genomes, we observed that with the current state of NHP annotations, the percent of reads assigned to a genetic feature was largely similar regardless of the method. Together, these data provide a baseline for the cell-intrinsic responses elicited by a potent immune stimulus across multiple NHP donors, including endangered species, and serve as a resource for refining and furthering the existing annotations of NHP genomes.

hNTCP‑expressing primary pig hepatocytes are a valuable tool for investigating hepatitis B virus infection and antiviral drugs

Primary human hepatocytes (PHHs) are the 'gold standard' for investigating hepatitis B virus (HBV) infection and antiviral drugs. However, poor availability, variation between batches and ethical issues regarding PHHs limit their applications. The discovery of human sodium taurocholate co‑transporting polypeptide (hNTCP) as a functional HBV receptor has enabled the development of a surrogate model to supplement the use of PHHs. In the present study, the evolutionary distance of seven species was assessed based on single‑copy homologous genes. Based on the evolutionary distance and availability, PHHs and primary rabbit hepatocytes (PRHs) were isolated and infected with hNTCP‑recombinant lentivirus, and susceptibility to HBV infection in the two cell types was tested and compared. In addition, HBV infection efficiency of hNTCP‑expressing PPHs with pooled HBV‑positive serum and purified particles was determined. The potential use of HBV‑infected hNTCP‑expressing PPHs for drug screening was assessed. The results demonstrated that pigs and rabbits are closer to humans in the divergence tree compared with mice and rats, indicating that pigs and rabbits were more likely to facilitate the HBV post‑entry lifecycle. Following hNTCP complementation and HBV infection, PPHs and Huh7D human hepatocellular carcinoma cells, but not PRHs, exhibited increased hepatitis B surface antigen and hepatitis B e‑antigen secretion, covalently closed circular DNA formation and infectious particle secretion. hNTCP‑expressing PPHs were susceptible to infection with HBV particles purified from pooled HBV‑positive sera, but were poisoned by raw HBV‑positive sera. The use of HBV‑infected hNTCP‑expressing PPHs for viral entry inhibitor screening was revealed to be applicable and reproducible. In conclusion, hNTCP‑expressing PPHs may be valuable tool for investigating HBV infection and antiviral drugs.

Genetic Dissection of the Host Tropism of Human-Tropic Pathogens

Infectious diseases are the second leading cause of death worldwide. Although the host multitropism of some pathogens has rendered their manipulation possible in animal models, the human-restricted tropism of numerous viruses, bacteria, fungi, and parasites has seriously hampered our understanding of these pathogens. Hence, uncovering the genetic basis underlying the narrow tropism of such pathogens is critical for understanding their mechanisms of infection and pathogenesis. Moreover, such genetic dissection is essential for the generation of permissive animal models that can serve as critical tools for the development of therapeutics or vaccines against challenging human pathogens. In this review, we describe different experimental approaches utilized to uncover the genetic foundation regulating pathogen host tropism as well as their relevance for studying the tropism of several important human pathogens. Finally, we discuss the current and future uses of this knowledge for generating genetically modified animal models permissive for these pathogens.

Animal models for the study of HCV

The development and evaluation of effective therapies and vaccines for the hepatitis C virus (HCV) and the study of its interactions with the mammalian host have been hindered for a long time by the absence of suitable small animal models. Immune compromised mouse models that recapitulate the complete HCV life cycle have been useful to investigate many aspects of the HCV life cycle including antiviral interventions. However, HCV has a high propensity to establish persistence and associated histopathological manifestations such as steatosis, fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Better understanding of these processes requires the development of a permissive and fully immunocompetent small animal model. In this review we summarize the in vivo models that are available for the study of HCV.

HCV animal models and liver disease

The development and evaluation of effective therapies and vaccines for the hepatitis C virus (HCV) and the study of its interactions with the mammalian host have been hindered for a long time by the absence of suitable small animal models. Due to the narrow host tropism of HCV, the development of mice that can be robustly engrafted with human hepatocytes was a major breakthrough since they recapitulate the complete HCV life cycle. This model has been useful to investigate many aspects of the HCV life cycle, including antiviral interventions. However, studies of cellular immunity, immunopathogenesis and resulting liver diseases have been hampered by the lack of a small animal model with a functional immune system. In this review, we summarize the evolution of in vivo models for the study of HCV.

Establishment of a novel triple-transgenic mouse: conditionally and liver-specifically expressing hepatitis C virus NS3/4A protease

It is well known that NS3/4A protein plays crucial roles in the hepatitis C virus (HCV) replication. NS3/4A protein also results to virus-mediated immune evasion and persistence of infection through the interaction with host proteins. However, the lack of a suitable animal model hampers studies of HCV NS3/4A protein interaction with host proteins, which impacts immunopathology due to infection. Here, transgenic vector containing transcriptional regulation and Fluc reporter gene was constructed to conditionally express NS3/4A protein under the dual control of Tet-On regulatory system and Cre/LoxP gene-knockout system. NS3/4A transgenic founder mice were continuously crossed with Lap transgenic mice expressing reverse tetracycline-controlled transcriptional activator (rtTA), the NS3/4A/Lap double transgenic mouse lines with liver-specifically and conditionally expressing reporter (luciferase Fluc) under control of Tet-On system were established. The NS3/4A/Lap double transgenic mouse are mated with Lap/LC-1 double transgenic mouse with liver-specifically and conditionally expressing Cre recombinase under control of Tet-On system, NS3/4A/Lap/LC-1 triple transgenic mouse were generated. In vivo bioluminescent imaging, western blotting and immunohistochemical staining (IHS) was used to confirm that NS3/4A protein was strictly expressed in the liver of Doxycycline-induced triple transgenic mice. The results show that we established a triple-transgenic mouse model conditionally expressing the HCV NS3/4A protein under strict control of the Tet-On regulatory system and Cre/loxP system. This novel transgenic mouse model expressing NS3/4A in a temporally and spatially-specific manner will be useful for studying interactions between HCV NS3/4A protein and the host, also for evaluating NS3/4A protease inhibitors.

Revival, characterization, and hepatitis B virus infection of cryopreserved human fetal hepatocytes

Primary human hepatocytes are considered the ideal cellular model for in-vitro studies of liver-specific pathology, such as hepatitis B virus (HBV) infection. However, poor accessibility, limited cell numbers, and lot-to-lot variation of primary human hepatocytes limit their broad application. Human fetal hepatocytes were isolated from postmortem embryonic liver tissues by two-step collagenase perfusion and cryopreserved. A monolayer of cryopreserved human fetal hepatocytes was established by optimizing such conditions as cell density and viability and purification of viable cells by Percoll. Finally, revived human fetal hepatocytes were characterized and infected with HBV. A large number of viable human fetal hepatocytes could be isolated and cryopreserved, with seeding density and viability being critical for the establishment of a compact monolayer culture. Using low-viability cryopreserved human fetal hepatocytes, a typical monolayer was established by purification with Percoll. The revived cells were actively proliferative, showed identical morphologic characteristics to non-cryopreserved cells, and had a typical hepatic gene expression profile. Moreover, this optimized model was susceptible to HBV infection and could be used to screen entry inhibitors against HBV infection. In conclusion, these methods can be used on human fetal hepatocytes to provide a cell bank for studies of the early stages of HBV infection.

Visualizing hepatitis C virus infection in humanized mice

Hepatitis C virus (HCV) establishes frequently persistent infections. Chronic carriers can develop severe liver disease. HCV has been intensely studied in a variety of cell culture systems. However, commonly used cell lines and primary hepatocyte cultures do not or only in part recapitulate the intricate host environment HCV faces in the liver. HCV infects readily only humans and chimpanzees, which poses challenges in studying HCV infection in vivo. Consequently, tractable small animal models are needed that are not only suitable for analyzing HCV infection but also for testing novel therapeutics. Here, we will focus our discussion on humanized mice, i.e. mice engrafted with human tissues or expressing human genes, which support HCV infection. We will further highlight novel methods that can be used to unambiguously detect HCV infected cells in situ, thereby facilitating a spatio-temporal dissection of HCV infection in the three dimensional context of the liver.

Complete replication of hepatitis B virus and hepatitis C virus in a newly developed hepatoma cell line

The absence of a robust cell culture system for hepatitis B virus (HBV) and hepatitis C virus (HCV) infection has limited the analysis of the virus lifecycle and drug discovery. We have established a hepatoma cell line, HLCZ01, the first cell line, to the authors' knowledge, supporting the entire lifecycle of both HBV and HCV. HBV surface antigen (HBsAg)-positive particles can be observed in the supernatant and the lumen of the endoplasmic reticulum of the cells via electron microscopy. Interestingly, HBV and HCV clinical isolates propagate in HLCZ01 cells. Both viruses replicate in the cells without evidence of overt interference. HBV and HCV entry are blocked by antibodies against HBsAg and human CD81, respectively, and the replication of HBV and HCV is inhibited by antivirals. HLCZ01 cells mount an innate immune response to virus infection. The cell line provides a powerful tool for exploring the mechanisms of virus entry and replication and the interaction between host and virus, facilitating the development of novel antiviral agents and vaccines.

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

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

Usage of adenovirus expressing thymidine kinase mediated hepatocellular damage for enabling mouse liver repopulation with allogenic or xenogenic hepatocytes

It has been shown that the liver of immunodeficient mice can be efficiently repopulated with human hepatocytes when subjected to chronic hepatocellular damage. Mice with such chimeric livers represent useful reagents for medical and clinical studies. However all previously reported models of humanized livers are difficult to implement as they involve cross-breeding of immunodeficient mice with mice exhibiting genetic alterations causing sustained hepatic injury. In this paper we attempted to create chimeric livers by inducing persistent hepatocellular damage in immunodeficient Rag2(-/-) γc(-/-) mice using an adenovirus encoding herpes virus thymidine kinase (AdTk) and two consecutive doses of ganciclovir (GCV). We found that this treatment resulted in hepatocellular damage persisting for at least 10 weeks and enabled efficient engraftment and proliferation within the liver of either human or allogenic hepatocytes. Interestingly, while the nodules generated from the transplanted mouse hepatocytes were well vascularized, the human hepatocytes experienced progressive depolarization and exhibited reduced numbers of murine endothelial cells inside the nodules. In conclusion, AdTk/GCV-induced liver damage licenses the liver of immunodeficient mice for allogenic and xenogenic hepatocyte repopulation. This approach represents a simple alternative strategy for chimeric liver generation using immunodeficient mice without additional genetic manipulation of the germ line.

Animal models for hepatitis C

Hepatitis C remains a global epidemic. Approximately 3 % of the world's population suffers from chronic hepatitis C, which is caused by hepatitis C virus (HCV)-a positive sense, single-stranded RNA virus of the Flaviviridae family. HCV has a high propensity for establishing a chronic infection. If untreated chronic HCV carriers can develop severe liver disease including fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Antiviral treatment is only partially effective, costly, and poorly tolerated. A prophylactic or therapeutic vaccine for HCV does not exist. Mechanistic studies of virus-host interactions, HCV immunity, and pathogenesis as well as the development of more effective therapies have been hampered by the lack of a suitable small animal model. Besides humans, chimpanzees are the only species that is naturally susceptible to HCV infection. While experimentation in these large primates has yielded valuable insights, ethical considerations, limited availability, genetic heterogeneity, and cost limit their utility. In search for more tractable small animal models, numerous experimental approaches have been taken to recapitulate parts of the viral life cycle and/or aspects of viral pathogenesis that will be discussed in this review. Exciting new models and improvements in established models hold promise to further elucidate our understanding of chronic HCV infection.

Expression of heterologous proteins flanked by NS3-4A cleavage sites within the hepatitis C virus polyprotein

Hepatitis C virus (HCV) contributes substantially to human morbidity and mortality world-wide. The development of HCV genomes expressing heterologous proteins has enhanced the ability to study viral infection, but existing systems have drawbacks. Recombinant viruses often require adaptive mutations to compensate for reduced viral titers, or rely on an artificial genomic organization that uncouples viral protein expression from recombinant gene expression. Here, we sought to exploit the viral polyprotein processing machinery to express heterologous proteins within the context of the HCV polyprotein. We show that HCV genotypes 2a and 1b permit insertion of reporter proteins between NS5A and NS5B with minimal impact on viral fitness. Using this strategy we constructed reporter genomes exhibiting a wide dynamic range, simplifying analysis of HCV infection in primary hepatocytes. Expression of heterologous proteins within the HCV genome offers new opportunities to analyze HCV infection in experimental systems without perturbing functions of individual viral proteins.

New hepatitis C virus drug discovery strategies and model systems

INTRODUCTION

Hepatitis C virus (HCV) is a major cause of liver disease worldwide and the leading indication for liver transplantation in the United States. Current treatment options are expensive, not effective in all patients and are associated with serious side effects. Although preclinical, anti-HCV drug screening is still hampered by the lack of readily infectable small animal models, the development of cell culture HCV experimental model systems has driven a promising new wave of HCV antiviral drug discovery.

AREAS COVERED

This review contains a concise overview of current HCV treatment options and limitations with a subsequent in-depth focus on the available experimental models and novel strategies that have, and continue to enable, important advances in HCV drug development.

EXPERT OPINION

With a large cohort of chronically HCV-infected patients progressively developing liver disease that puts them at risk for hepatocellular carcinoma and hepatic decompensation, there is an urgent need to develop effective therapeutics that are well tolerated and effective in all patients and against all HCV genotypes. Significant advances in HCV experimental model development have expedited drug discovery; however, additional progress is needed. Importantly, the current trends and momentum in the field suggests that we will continue to overcome critical experimental challenges to reach this end goal.

Study of hepatitis C virus entry in genetically humanized mice

Approximately 2% of the world's population is chronically infected with hepatitis C virus (HCV). Chronic hepatitis C can culminate in end stage liver disease and liver cancer if the infection is untreated. Current therapy is only partially effective and a vaccine for HCV does not exist. Since the discovery of HCV as the etiologic agent causing hepatitis C several experimental tools have been developed which have improved our understanding of the viral life cycle and the interaction of HCV with human cells. However, it remains challenging to study HCV infection in its native liver environment given its narrow species tropism, limited to humans and chimpanzees. Mice can be rendered susceptible to HCV infection by transplanting human hepatocytes into immunocompromized liver injury strains. Such human liver chimeric mice are useful as a challenge model for human hepatotropic pathogens but their utility is hampered by their inability to mount functional immune responses and practical aspects including high costs, low throughput, and donor-to-donor variability. The barriers that restrict HCV species tropism are incompletely understood. We have previously shown that expression of human CD81 and human OCLN is required for HCV uptake into mouse cells. This led to the construction of a genetically humanized mouse model for HCV infection. Here, we provide a detailed protocol for the generation of these animals and highlight some of its applications for studying HCV biology and preclinical testing of drug and vaccine candidates.