Determinants of hepatitis B and delta virus host tropism

Mechanisms of Hepatitis B Virus cccDNA and Minichromosome Formation and HBV Gene Transcription

Hepatitis B virus (HBV) is the etiologic agent of chronic hepatitis B, which puts at least 300 million patients at risk of developing fibrosis, cirrhosis, and hepatocellular carcinoma. HBV is a partially double-stranded DNA virus of the Hepadnaviridae family. While HBV was discovered more than 50 years ago, many aspects of its replicative cycle remain incompletely understood. Central to HBV persistence is the formation of covalently closed circular DNA (cccDNA) from the incoming relaxed circular DNA (rcDNA) genome. cccDNA persists as a chromatinized minichromosome and is the major template for HBV gene transcription. Here, we review how cccDNA and the viral minichromosome are formed and how viral gene transcription is regulated and highlight open questions in this area of research.

Structural basis of hepatitis B virus receptor binding

Hepatitis B virus (HBV), a leading cause of developing hepatocellular carcinoma affecting more than 290 million people worldwide, is an enveloped DNA virus specifically infecting hepatocytes. Myristoylated preS1 domain of the HBV large surface protein binds to the host receptor sodium-taurocholate cotransporting polypeptide (NTCP), a hepatocellular bile acid transporter, to initiate viral entry. Here, we report the cryogenic-electron microscopy structure of the myristoylated preS1 (residues 2-48) peptide bound to human NTCP. The unexpectedly folded N-terminal half of the peptide embeds deeply into the outward-facing tunnel of NTCP, whereas the C-terminal half formed extensive contacts on the extracellular surface. Our findings reveal an unprecedented induced-fit mechanism for establishing high-affinity virus-host attachment and provide a blueprint for the rational design of anti-HBV drugs targeting virus entry.

Long-Term Strategies for Poorly Water-Soluble Peptides: Combining Fatty Acid Modification with PAS Fusion

Bulevirtide, an entry inhibitor for the hepatitis B virus (HBV) and hepatitis D virus (HDV), is currently available on the European market. However, its clinical application is constrained by its short half-life and poor water solubility, rendering it unsuitable for fatty acid modification, aimed at achieving long-term effects. To address this limitation, we integrated a polypeptide chain consisting of Pro, Ala, and Ser at the C-terminus, which increased its hydrophilicity. To obtain the fusion sequence of A1 and A2, encompassing amino acids 1-47 of Bulevirtide and PAS, we used Escherichia coli fermentation expression. Subsequently, the N-terminal myristoyl groups of A1 and A2 were modified to yield Myr-A1 and Myr-A2, respectively. Five fatty acid moieties with the same hydrophilic spacers and different fatty acids were conjugated to analogs, generating 10 bioconjugations. The bioconjugates were then evaluated for their anti-HBV activity. Among them, HB-10 was selected for pharmacokinetic analysis and demonstrated a significantly prolonged half-life, with 5.88- and 13.18-fold increases in beagle dogs and rats, respectively. Additionally, higher drug doses resulted in substantially elevated liver concentrations. In conclusion, via fatty acid incorporation and PASylation, we successfully developed a novel Bulevirtide bioconjugate, HB-10, that exhibits an extended action duration. This compound holds substantial promise as a prospective long-acting entry inhibitor, warranting further investigation.

Conversion of hepatitis B virus relaxed circular to covalently closed circular DNA is supported in murine cells

Background & Aims

HBV has a narrow host restriction, with humans and chimpanzees representing the only known natural hosts. The molecular correlates of resistance in species that are commonly used in biomedical research, such as mice, are currently incompletely understood. Expression of human NTCP (hNTCP) in mouse hepatocytes enables HBV entry, but subsequently covalently closed circular (cccDNA) does not form in most murine cells. It is unknown if this blockade in cccDNA formation is due to deficiency in repair of relaxed circular DNA (rcDNA) to cccDNA.

Methods

Here, we deployed both in vivo and in vitro virological and biochemical approaches to investigate if murine cells contain a complete set of repair factors capable of converting HBV rcDNA to cccDNA.

Results

We demonstrate that HBV cccDNA does form in murine cell culture or in mice when recombinant rcDNA without a protein adduct is directly introduced into cells. We further show that the murine orthologues of core components in DNA lagging strand synthesis, required for the repair of rcDNA to cccDNA in human cells, can support this crucial step in the HBV life cycle. It is worth noting that recombinant HBV rcDNA substrates, either without a protein adduct or containing neutravidin to mimic HBV polymerase, were used in our study; it remains unclear if the HBV polymerase removal processes are the same in mouse and human cells.

Conclusions

Collectively, our data suggest that the HBV life cycle is blocked post entry and likely before the repair stage in mouse cells, which yields critical insights that will aid in the construction of a mouse model with inbred susceptibility to HBV infection.

Lay summary

Hepatitis B virus (HBV) is only known to infect humans and chimpanzees in nature. Mouse models are often used in modeling disease pathogenesis and preclinical research to assess the efficacy and safety of interventions before they are then tested in human participants. However, because mice are not susceptible to HBV infection it is difficult to accurately model human infection (and test potential treatments) in mouse models. Herein, we have shown that mice are able to perform a key step in the HBV life cycle, tightening the net around the possible reason why HBV can not efficiently infect and replicate in mice.

Structure of the bile acid transporter and HBV receptor NTCP

Chronic infection with hepatitis B virus (HBV) affects more than 290 million people worldwide, is a major cause of cirrhosis and hepatocellular carcinoma, and results in an estimated 820,000 deaths annually^1,2. For HBV infection to be established, a molecular interaction is required between the large glycoproteins of the virus envelope (known as LHBs) and the host entry receptor sodium taurocholate co-transporting polypeptide (NTCP), a sodium-dependent bile acid transporter from the blood to hepatocytes³. However, the molecular basis for the virus-transporter interaction is poorly understood. Here we report the cryo-electron microscopy structures of human, bovine and rat NTCPs in the apo state, which reveal the presence of a tunnel across the membrane and a possible transport route for the substrate. Moreover, the cryo-electron microscopy structure of human NTCP in the presence of the myristoylated preS1 domain of LHBs, together with mutation and transport assays, suggest a binding mode in which preS1 and the substrate compete for the extracellular opening of the tunnel in NTCP. Our preS1 domain interaction analysis enables a mechanistic interpretation of naturally occurring HBV-insusceptible mutations in human NTCP. Together, our findings provide a structural framework for HBV recognition and a mechanistic understanding of sodium-dependent bile acid translocation by mammalian NTCPs.

Mechanism of Hepatitis B Virus cccDNA Formation

Hepatitis B virus (HBV) remains a major medical problem affecting at least 257 million chronically infected patients who are at risk of developing serious, frequently fatal liver diseases. HBV is a small, partially double-stranded DNA virus that goes through an intricate replication cycle in its native cellular environment: human hepatocytes. A critical step in the viral life-cycle is the conversion of relaxed circular DNA (rcDNA) into covalently closed circular DNA (cccDNA), the latter being the major template for HBV gene transcription. For this conversion, HBV relies on multiple host factors, as enzymes capable of catalyzing the relevant reactions are not encoded in the viral genome. Combinations of genetic and biochemical approaches have produced findings that provide a more holistic picture of the complex mechanism of HBV cccDNA formation. Here, we review some of these studies that have helped to provide a comprehensive picture of rcDNA to cccDNA conversion. Mechanistic insights into this critical step for HBV persistence hold the key for devising new therapies that will lead not only to viral suppression but to a cure.

Hepatitis B virus cccDNA is formed through distinct repair processes of each strand

Hepatitis B virus (HBV) is a highly contagious pathogen that afflicts over a third of the world’s population, resulting in close to a million deaths annually. The formation and persistence of the HBV covalently closed circular DNA (cccDNA) is the root cause of HBV chronicity. However, the detailed molecular mechanism of cccDNA formation from relaxed circular DNA (rcDNA) remains opaque. Here we show that the minus and plus-strand lesions of HBV rcDNA require different sets of human repair factors in biochemical repair systems. We demonstrate that the plus-strand repair resembles DNA lagging strand synthesis, and requires proliferating cell nuclear antigen (PCNA), the replication factor C (RFC) complex, DNA polymerase delta (POLδ), flap endonuclease 1 (FEN-1), and DNA ligase 1 (LIG1). Only FEN-1 and LIG1 are required for the repair of the minus strand. Our findings provide a detailed mechanistic view of how HBV rcDNA is repaired to form cccDNA in biochemical repair systems.

HBV covalently closed circular DNA (cccDNA) enables and persists in chronic infection, but the molecular mechanism of its formation is unclear. Here, Wei and Ploss elucidate the detailed kinetics and biochemical steps by which the relaxed circular DNA is converted into cccDNA.

Core components of DNA lagging strand synthesis machinery are essential for hepatitis B virus cccDNA formation

Chronic hepatitis B virus (HBV) infection results in 887,000 deaths annually. The central challenge in curing HBV is eradication of the stable covalently closed circular DNA (cccDNA) form of the viral genome, which is formed by the repair of lesion-bearing HBV relaxed circular DNA (rcDNA) delivered by the virions to hepatocytes. A complete and minimal set of host factors involved in cccDNA formation is unknown, largely due to the lack of a biochemical system that fully reconstitutes cccDNA formation. Here, we have developed experimental systems where various HBV rcDNA substrates are repaired to form cccDNA by both cell extracts and purified human proteins. Using yeast and human extract screenings, we identified five core components of lagging strand synthesis as essential for cccDNA formation: PCNA, the replication factor C (RFC) complex, DNA polymerase δ (POLδ), FEN-1, and DNA ligase 1 (LIG1). We reconstituted cccDNA formation with purified human homologs, establishing these as a minimal set of factors for cccDNA formation. We further demonstrated that treatment with DNA polymerase inhibitor aphidicolin diminishes cccDNA formation both in biochemical assays and in HBV-infected human cells. Altogether, our findings define key components in HBV cccDNA formation.

Woolly Monkey-HBV Infection in Squirrel Monkeys as a Surrogate Nonhuman Primate Model of HBV Infection

Development of curative therapies for chronic hepatitis B virus (HBV) infection will likely require new animal models. Here, we evaluate HBV infection in squirrel monkeys based on the high-sequence homology of the HBV receptor, Na+/taurocholate co-transporting peptide (NTCP), between humans and squirrel monkeys. HBV PreS1 peptide was examined for binding human and squirrel monkey NTCP. Immunodeficient Fah -/- , NOD, Rag1 -/- , Il2Rg null (FNRG) mice engrafted with human or squirrel monkey hepatocytes were challenged with HBV or Woolly Monkey HBV (WMHBV). In addition, adult squirrel monkeys were inoculated with HBV, WMHBV, adeno-associated virus containing an infectious genome of HBV (AAV-HBV), and AAV-WMHBV. Finally, neonate squirrel monkeys were assessed for the potential of chronic infection with WMHBV. PreS1 peptide efficiently bound to human and squirrel monkey NTCP but not to mouse or capuchin NTCP. FNRG mice engrafted with squirrel monkey hepatocytes were susceptible to infection by WMHBV but not human HBV. Similarly, adult squirrel monkeys could be infected with WMHBV but not human HBV, whereas chimeric mice engrafted with human hepatocytes were susceptible to HBV but not WMHBV. Infection of squirrel monkeys with AAV-WMHBV yielded maximum viremia of 108 genomes/mL with detectable virus for up to 8 months. Notably, covalently closed circular DNA was detected in the liver of these animals. Infection of neonates with WMHBV led to detectable viremia for up to 6 months. Conclusions: Adult and neonate squirrel monkeys exhibited prolonged WMHBV viremia lasting 6-8 months. This is greater than twice the duration of viremia achieved in other nonhuman primates and suggests that squirrel monkeys may be a suitable model for testing HBV therapeutics.

Analysis of Host Responses to Hepatitis B and Delta Viral Infections in a Micro-scalable Hepatic Co-culture System

Hepatitis B virus (HBV) remains a major global health problem with 257 million chronically infected individuals worldwide, of whom approximately 20 million are co-infected with hepatitis delta virus (HDV). Progress toward a better understanding of the complex interplay between these two viruses and the development of novel therapies have been hampered by the scarcity of suitable cell culture models that mimic the natural environment of the liver. Here, we established HBV and HBV/HDV co-infections and super-infections in self-assembling co-cultured primary human hepatocytes (SACC-PHHs) for up to 28 days in a 384-well format and highlight the suitability of this platform for high-throughput drug testing. We performed RNA sequencing at days 8 and 28 on SACC-PHHs, either HBV mono-infected or HBV/HDV co-infected. Our transcriptomic analysis demonstrates that hepatocytes in SACC-PHHs maintain a mature hepatic phenotype over time, regardless of infection condition. We confirm that HBV is a stealth virus, as it does not induce a strong innate immune response; rather, oxidative phosphorylation and extracellular matrix-receptor interactions are dysregulated to create an environment that promotes persistence. Notably, HDV co-infection also did not lead to statistically significant transcriptional changes across multiple donors and replicates. The lack of innate immune activation is not due to SACC-PHHs being impaired in their ability to induce interferon stimulated genes (ISGs). Rather, polyinosinic:polycytidylic acid exposure activates ISGs, and this stimulation significantly inhibits HBV infection, yet only minimally affects the ability of HDV to infect and persist. Conclusion: These data demonstrate that the SACC-PHH system is a versatile platform for studying HBV/HDV co-infections and holds promise for performing chemical library screens and improving our understanding of the host response to such infections.

High prevalence and genetic diversity of hepatitis B viruses in insectivorous bats from China

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We found that 6.6% (13/197) bats from Shandong and Hubei provinces of China carried hepatis B virus (HBVs).

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HBVs from bats in the two places were phylogenetically in the same cluster, but distinct from bat HBVs from other places.

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HBVs were highly prevalent and genetic diversified in bats, supporting the hypothesis that bats may be the origin of primate hepadnaviruses.

Bats have been identified as the hosts of hepatitis B virus (HBV) in recent years and bats HBV can infect human hepatocyte. We investigated the prevalence and genetic diversity of HBV in bats in China. In this study, a total of 197 insectivorous bats belonging to 10 bat species were captured from karst caves in Mengyin County, Shandong Province and Xianning City, Hubei Province, China. PCR amplification indicated that in total 6.6% (13/197) bats were positive to HBVs. The HBV positive rate in bats was 7.1% (9/127) and 5.7% (4/70) in Shandong Province and Hubei Province, respectively. Phylogenetic analysis indicated that HBV from the two places were in the same cluster with 90.5%–99.5% homology, but distinct from bat HBVs from other places in China and other countries. We concluded that HBV was prevalent and genetic diversified in bats, supporting the hypothesis that bats may be the origin of primate hepadnaviruses.

Decellularized human liver scaffold-based three-dimensional culture system facilitate hepatitis B virus infection

Hepatitis B virus (HBV) study is hampered by lacking of idea cell model which support effective HBV infection and meanwhile recapitulate hepatocyte biology function in vivo. In this study, we developed decellularized human liver scaffolds for cell culture and further applied for HBV infection. As a result, primary human hepatocytes (PHHs) engrafted into liver scaffolds and maintained differentiation with stable albumin secretion and liver-specific gene expression. Comparing to mono-layer cell culture, scaffold-based three-dimensional (3D) culture system significantly augment HBV DNA (including cccDNA), RNA level as well as HBsAg secretion. Moreover, HepG2-NTCP cells cultured on 3D system exhibited higher infection efficiency and longer infection period in vitro. In addition, HBV DNA level was suppressed when anti-HBV medicine Entecavir (ETV) introduced into HepG2-NTCP 3D system. Herein, we evaluated the potential of decellularized human liver scaffold-based in 3D cell culture and disclosed that scaffold-based 3D culture system can facilitate HBV infection in vitro. This 3D culture system could be further applied in HBV-related study. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1744-1753, 2019.

Preclinical assessment of antiviral combination therapy in a genetically humanized mouse model for hepatitis delta virus infection

Chronic delta hepatitis, caused by hepatitis delta virus (HDV), is the most severe form of viral hepatitis, affecting at least 20 million hepatitis B virus (HBV)-infected patients worldwide. HDV/HBV co- or superinfections are major drivers for hepatocarcinogenesis. Antiviral treatments exist only for HBV and can only suppress but not cure infection. Development of more effective therapies has been impeded by the scarcity of suitable small-animal models. We created a transgenic (tg) mouse model for HDV expressing the functional receptor for HBV and HDV, the human sodium taurocholate cotransporting peptide NTCP. Both HBV and HDV entered hepatocytes in these mice in a glycoprotein-dependent manner, but one or more postentry blocks prevented HBV replication. In contrast, HDV persistently infected hNTCP tg mice coexpressing the HBV envelope, consistent with HDV dependency on the HBV surface antigen (HBsAg) for packaging and spread. In immunocompromised mice lacking functional B, T, and natural killer cells, viremia lasted at least 80 days but resolved within 14 days in immunocompetent animals, demonstrating that lymphocytes are critical for controlling HDV infection. Although acute HDV infection did not cause overt liver damage in this model, cell-intrinsic and cellular innate immune responses were induced. We further demonstrated that single and dual treatment with myrcludex B and lonafarnib efficiently suppressed viremia but failed to cure HDV infection at the doses tested. This small-animal model with inheritable susceptibility to HDV opens opportunities for studying viral pathogenesis and immune responses and for testing novel HDV therapeutics.

A new HDV mouse model identifies mitochondrial antiviral signaling protein (MAVS) as a key player in IFN-β induction

BACKGROUND & AIMS

Studying hepatitis delta virus (HDV) and developing new treatments is hampered by the limited availability of small animal models. Herein, a description of a robust mouse model of HDV infection that mimics several important characteristics of the human disease is presented.

METHODS

HDV and hepatitis B virus (HBV) replication competent genomes were delivered to the mouse liver using adeno-associated viruses (AAV; AAV-HDV and AAV-HBV). Viral load, antigen expression and genomes were quantified at different time points after AAV injection. Furthermore, liver pathology, genome editing, and the activation of the innate immune response were evaluated.

RESULTS

AAV-HDV infection initiated HDV replication in mouse hepatocytes. Genome editing was confirmed by the presence of small and large HDV antigens and sequencing. Viral replication was detected for 45days, even after the AAV-HDV vector had almost disappeared. In the presence of HBV, HDV infectious particles were detected in serum. Furthermore, as observed in patients, co-infection was associated with the reduction of HBV antigen expression and the onset of liver damage that included the alteration of genes involved in the development of liver pathologies. HDV replication induced a sustained type I interferon response, which was significantly reduced in immunodeficient mice and almost absent in mitochondrial antiviral signaling protein (MAVS)-deficient mice.

CONCLUSION

The animal model described here reproduces important characteristics of human HDV infection and provides a valuable tool for characterizing the viral infection and for developing new treatments. Furthermore, MAVS was identified as a main player in HDV detection and adaptive immunity was found to be involved in the amplification of the innate immune response. Lay summary: Co-infection with hepatitis B and D virus (HBV and HDV, respectively) often causes a more severe disease condition than HBV alone. Gaining more insight into HDV and developing new treatments is hampered by limited availability of adequate immune competent small animal models and new ones are needed. Here, a mouse model of HDV infection is described, which mimics several important characteristics of the human disease, such as the initiation and maintenance of replication in murine hepatocytes, genome editing and, in the presence of HBV, generation of infectious particles. Lastly, the involvement of an adaptive immunity and the intracellular signaling molecule MAVS in mounting a strong and lasting innate response was shown. Thus, our model serves as a useful tool for the investigation of HDV biology and new treatments.

Long-term hepatitis B infection in a scalable hepatic co-culture system

Hepatitis B virus causes chronic infections in 250 million people worldwide. Chronic hepatitis B virus carriers are at risk of developing fibrosis, cirrhosis, and hepatocellular carcinoma. A prophylactic vaccine exists and currently available antivirals can suppress but rarely cure chronic infections. The study of hepatitis B virus and development of curative antivirals are hampered by a scarcity of models that mimic infection in a physiologically relevant, cellular context. Here, we show that cell-culture and patient-derived hepatitis B virus can establish persistent infection for over 30 days in a self-assembling, primary hepatocyte co-culture system. Importantly, infection can be established without antiviral immune suppression, and susceptibility is not donor dependent. The platform is scalable to microwell formats, and we provide proof-of-concept for its use in testing entry inhibitors and antiviral compounds.

The lack of models that mimic hepatitis B virus (HBV) infection in a physiologically relevant context has hampered drug development. Here, Winer et al. establish a self-assembling, primary hepatocyte co-culture system that can be infected with patient-derived HBV without further modifications.

Hepatitis delta virus: insights into a peculiar pathogen and novel treatment options

Chronic hepatitis D is the most severe form of viral hepatitis, affecting ∼20 million HBV-infected people worldwide. The causative agent, hepatitis delta virus (HDV), is a unique human pathogen: it is the smallest known virus; it depends on HBV to disseminate its viroid-like RNA; it encodes only one protein (HDAg), which has both structural and regulatory functions; and it replicates using predominantly host proteins. The failure of HBV-specific nucleoside analogues to suppress the HBV helper function, and the limitations of experimental systems to study the HDV life cycle, have impeded the development of HDV-specific drugs. Thus, the only clinical regimen for HDV is IFNα, which shows some efficacy but long-term virological responses are rare. Insights into the receptor-mediated entry of HDV, and the observation that HDV assembly requires farnesyltransferase, have enabled novel therapeutic strategies to be developed. Interference with entry, for example through blockade of the HBV-HDV-specific receptor sodium/taurocholate cotransporting polypeptide NTCP by Myrcludex B, and inhibition of assembly by blockade of farnesyltransferase using lonafarnib or nucleic acid polymers such as REP 2139-Ca, have shown promising results in phase II studies. In this Review, we summarize our knowledge of HDV epidemiology, pathogenesis and molecular biology, with a particular emphasis on possible future developments.

Suppression of hepatitis B virus antigen production and replication by wild-type HBV dependently replicating HBV shRNA vectors in vitro and in vivo

Chronic infection with hepatitis B virus (HBV), a small DNA virus that replicates by reverse transcription of a pregenomic (pg) RNA precursor, greatly increases the risk for terminal liver disease. RNA interference (RNAi) based therapy approaches have shown potential to overcome the limited efficacy of current treatments. However, synthetic siRNAs as well as small hairpin (sh) RNAs expressed from non-integrating vectors require repeated applications; integrating vectors suffer from safety concerns. We pursue a new concept by which HBV itself is engineered into a conditionally replicating, wild-type HBV dependent anti-HBV shRNA vector. Beyond sharing HBV's hepatocyte tropism, such a vector would be self-renewing, but only as long as wild-type HBV is present. Here, we realized several important aspects of this concept. We identified two distinct regions in the 3.2 kb HBV genome which tolerate replacement by shRNA expression cassettes without compromising reverse transcription when complemented in vitro by HBV helper constructs or by wild-type HBV; a representative HBV shRNA vector was infectious in cell culture. The vector-encoded shRNAs were active, including on HBV as target. A dual anti-HBV shRNA vector delivered into HBV transgenic mice, which are not susceptible to HBV infection, by a chimeric adenovirus-HBV shuttle reduced serum hepatitis B surface antigen (HBsAg) up to ∼4-fold, and virus particles up to ∼20-fold. Importantly, a fraction of the circulating particles contained vector-derived DNA, indicating successful complementation in vivo. These data encourage further investigations to prove antiviral efficacy and the predicted self-limiting vector spread in a small animal HBV infection model.

Hepatocarcinogenesis associated with hepatitis B, delta and C viruses

Globally, over half a billion people are persistently infected with hepatitis B (HBV) and/or hepatitis C viruses. Chronic HBV and HCV infection frequently lead to fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Co-infections with hepatitis delta virus (HDV), a subviral satellite requiring HBV for its propagation, accelerates the progression of liver disease toward HCC. The mechanisms by which these viruses cause malignant transformation, culminating in HCC, remain incompletely understood, partially due to the lack of adequate experimental models for dissecting these complex disease processes in vivo.

Approved Antiviral Drugs over the Past 50 Years

Since the first antiviral drug, idoxuridine, was approved in 1963, 90 antiviral drugs categorized into 13 functional groups have been formally approved for the treatment of the following 9 human infectious diseases: (i) HIV infections (protease inhibitors, integrase inhibitors, entry inhibitors, nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and acyclic nucleoside phosphonate analogues), (ii) hepatitis B virus (HBV) infections (lamivudine, interferons, nucleoside analogues, and acyclic nucleoside phosphonate analogues), (iii) hepatitis C virus (HCV) infections (ribavirin, interferons, NS3/4A protease inhibitors, NS5A inhibitors, and NS5B polymerase inhibitors), (iv) herpesvirus infections (5-substituted 2'-deoxyuridine analogues, entry inhibitors, nucleoside analogues, pyrophosphate analogues, and acyclic guanosine analogues), (v) influenza virus infections (ribavirin, matrix 2 protein inhibitors, RNA polymerase inhibitors, and neuraminidase inhibitors), (vi) human cytomegalovirus infections (acyclic guanosine analogues, acyclic nucleoside phosphonate analogues, pyrophosphate analogues, and oligonucleotides), (vii) varicella-zoster virus infections (acyclic guanosine analogues, nucleoside analogues, 5-substituted 2'-deoxyuridine analogues, and antibodies), (viii) respiratory syncytial virus infections (ribavirin and antibodies), and (ix) external anogenital warts caused by human papillomavirus infections (imiquimod, sinecatechins, and podofilox). Here, we present for the first time a comprehensive overview of antiviral drugs approved over the past 50 years, shedding light on the development of effective antiviral treatments against current and emerging infectious diseases worldwide.

Engineering Hepadnaviruses as Reporter-Expressing Vectors: Recent Progress and Future Perspectives

The Hepadnaviridae family of small, enveloped DNA viruses are characterized by a strict host range and hepatocyte tropism. The prototype hepatitis B virus (HBV) is a major human pathogen and constitutes a public health problem, especially in high-incidence areas. Reporter-expressing recombinant viruses are powerful tools in both studies of basic virology and development of antiviral therapeutics. In addition, the highly restricted tropism of HBV for human hepatocytes makes it an ideal tool for hepatocyte-targeting in vivo applications such as liver-specific gene delivery. However, compact genome organization and complex replication mechanisms of hepadnaviruses have made it difficult to engineer replication-competent recombinant viruses that express biologically-relevant cargo genes. This review analyzes difficulties associated with recombinant hepadnavirus vector development, summarizes and compares the progress made in this field both historically and recently, and discusses future perspectives regarding both vector design and application.

Hepatitis B Virus Infection of a Mouse Hepatic Cell Line Reconstituted with Human Sodium Taurocholate Cotransporting Polypeptide

Hepatitis B virus (HBV) enters hepatocytes via its receptor, human sodium taurocholate cotransporting polypeptide (hNTCP). So far, HBV infection has been achieved only in human hepatic cells reconstituted with hNTCP and not in cells of mouse origin. Here, the first mouse liver cell line (AML12) which gains susceptibility to HBV upon hNTCP expression is described. Thus, HBV infection of receptor-expressing mouse hepatocytes does not principally require a human cofactor but can be triggered by endogenous murine determinants.

Advances and Challenges in Studying Hepatitis B Virus In Vitro

Hepatitis B virus (HBV) is a small DNA virus that infects the liver. Current anti-HBV drugs efficiently suppress viral replication but do not eradicate the virus due to the persistence of its episomal DNA. Efforts to develop reliable in vitro systems to model HBV infection, an imperative tool for studying HBV biology and its interactions with the host, have been hampered by major limitations at the level of the virus, the host and infection readouts. This review summarizes major milestones in the development of in vitro systems to study HBV. Recent advances in our understanding of HBV biology, such as the discovery of the bile-acid pump sodium-taurocholate cotransporting polypeptide (NTCP) as a receptor for HBV, enabled the establishment of NTCP expressing hepatoma cell lines permissive for HBV infection. Furthermore, advanced tissue engineering techniques facilitate now the establishment of HBV infection systems based on primary human hepatocytes that maintain their phenotype and permissiveness for infection over time. The ability to differentiate inducible pluripotent stem cells into hepatocyte-like cells opens the door for studying HBV in a more isogenic background, as well. Thus, the recent advances in in vitro models for HBV infection holds promise for a better understanding of virus-host interactions and for future development of more definitive anti-viral drugs.