Interferon gene transfer by a hepatitis B virus vector efficiently suppresses wild-type virus infection

Liver-specific transgenic expression of human NTCP in rhesus macaques confers HBV susceptibility on primary hepatocytes

Hepatitis B virus (HBV) poses a significant global health challenge, necessitating the urgent development of curative therapeutics. However, this progress is impeded by the lack of robust, immunocompetent preclinical animal models due to HBV's strict species specificity. We previously showed that vector-mediated expression of the HBV entry receptor, human sodium-taurocholate cotransporting polypeptide (hNTCP), renders macaques fully susceptible to HBV infection. In this study, we have generated transgenic macaques expressing hNTCP, marking the creation of the first transgenic nonhuman primate model for infectious disease research. We used PiggyBac (PB) transposon technology to insert a liver-specific hNTCP expression cassette into rhesus macaque zygotes and transferred the resulting embryos into surrogate females, resulting in two healthy transgenic offspring. In both animals, hNTCP is highly and selectively expressed in the liver. Most importantly, we show that isolated hepatocytes from these monkeys are susceptible to HBV infection. These findings lay the foundation for the development of a nonhuman primate HBV model, facilitating the advancement and validation of curative HBV therapies.

Interferon production by Viral, Bacterial & Yeast system: A comparative overview in 2023

Interferons play a critical role in the innate immune response against several infections and play a key role in the control of a variety of viral and bacterial infectious diseases such as hepatitis, covid-19, cancer, and multiple sclerosis. Therefore, natural or synthetic IFN production is important and had three common methods, including bacterial fermentation, animal cell culture, and recombinant nucleic acid technology. However, the safety, purity, and accuracy of the most preferred INF production systems have not been extensively studied. This study provides a comprehensive comparative overview of interferon production in various systems that include viral, bacterial, yeast, and mammalian. We aim to determine the most efficient, safe, and accurate interferon production system available in the year 2023. The mechanisms of artificial interferon production were reviewed in various organisms, and the types and subtypes of interferons produced by each system were compared. Our analysis provides a comprehensive overview of the similarities and differences in interferon production and highlights the potential for developing new therapeutic strategies to combat infectious diseases. This review article offers the diverse strategies used by different organisms in producing and utilizing interferons, providing a framework for future research into the evolution and function of this critical immune response pathway.

DNA Engineering and Hepatitis B Virus Replication

Recombinant DNA technology is a vital method in human hepatitis B virus (HBV), producing reporter viruses or vectors for gene transferring. Researchers have engineered several genes into the HBV genome for different purposes; however, a systematic analysis of recombinant strategy is lacking. Here, using a 500-bp deletion strategy, we scanned the HBV genome and identified two regions, region I (from nt 2,118 to 2,814) and region II (from nt 99 to 1,198), suitable for engineering. Ten exogenous genes, including puromycin N-acetyl transferase gene (Pac), blasticidin S deaminase gene (BSD), Neomycin-resistance gene (Neo), Gaussia luciferase (Gluc), NanoLuc (Nluc), copGFP, mCherry, UnaG, eGFP, and tTA1, were inserted into these two regions and fused into the open reading frames of hepatitis B core protein (HBC) and hepatitis B surface protein (HBS) via T2A peptide. Recombination of 9 of the 10 genes at region 99-1198 and 5 of the 10 genes at region 2118-2814 supported the formation of relaxed circular (RC) DNA. HBV DNA and HBV RNA assays implied that exogenous genes potentially abrogate RC DNA by inducing the formation of adverse secondary structures. This hypothesis was supported because sequence optimization of the UnaG gene based on HBC sequence rescued RC DNA formation. Findings from this study provide an informative basis and a valuable method for further constructing and optimizing recombinant HBV and imply that DNA sequence might be intrinsically a potential source of selective pressure in the evolution of HBV.

Interferon mediated prophylactic protection against respiratory viruses conferred by a prototype live attenuated influenza virus vaccine lacking non-structural protein 1

The influenza A non-structural protein 1 (NS1) is known for its ability to hinder the synthesis of type I interferon (IFN) during viral infection. Influenza viruses lacking NS1 (ΔNS1) are under clinical development as live attenuated human influenza virus vaccines and induce potent influenza virus-specific humoral and cellular adaptive immune responses. Attenuation of ΔNS1 influenza viruses is due to their high IFN inducing properties, that limit their replication in vivo. This study demonstrates that pre-treatment with a ΔNS1 virus results in an antiviral state which prevents subsequent replication of homologous and heterologous viruses, preventing disease from virus respiratory pathogens, including SARS-CoV-2. Our studies suggest that ΔNS1 influenza viruses could be used for the prophylaxis of influenza, SARS-CoV-2 and other human respiratory viral infections, and that an influenza virus vaccine based on ΔNS1 live attenuated viruses would confer broad protection against influenza virus infection from the moment of administration, first by non-specific innate immune induction, followed by specific adaptive immunity.

Prophylactic protection against respiratory viruses conferred by a prototype live attenuated influenza virus vaccine

The influenza A non-structural protein 1 (NS1) is known for its ability to hinder the synthesis of type I interferon (IFN) during viral infection. Influenza viruses lacking NS1 (ΔNS1) are under clinical development as live attenuated human influenza virus vaccines and induce potent influenza virus-specific humoral and cellular adaptive immune responses. Attenuation of ΔNS1 influenza viruses is due to their high IFN inducing properties, that limit their replication in vivo. This study demonstrates that pre-treatment with a ΔNS1 virus results in an immediate antiviral state which prevents subsequent replication of homologous and heterologous viruses, preventing disease from virus respiratory pathogens, including SARS-CoV-2. Our studies suggest that ΔNS1 influenza viruses could be used for the prophylaxis of influenza, SARS-CoV-2 and other human respiratory viral infections, and that an influenza virus vaccine based on ΔNS1 live attenuated viruses would confer broad protection against influenza virus infection from the moment of administration, first by non-specific innate immune induction, followed by specific adaptive immunity.

Cre/LoxP-HBV plasmids generating recombinant covalently closed circular DNA genome upon transfection

New therapies against hepatitis B virus (HBV) require the elimination of covalently closed circular DNA (cccDNA), the episomal HBV genome. HBV plasmids containing an overlength 1.3-mer genome and bacterial backbone (pHBV1.3) are used in many different models, but do not replicate the unique features of cccDNA. Since the stable cccDNA pool is a barrier to HBV eradication in patients, we developed a recombinant circular HBV genome (rcccDNA) to mimic the cccDNA using Cre/LoxP technology. We validated four LoxP insertion sites into the HBV genome using hydrodynamic tail vein injection into murine liver, demonstrating high levels of HBV surface antigen (HBsAg) and HBV DNA expression with rcccDNA formation. HBsAg expression from rcccDNA was >30,000 ng/mL over 78 days, while HBsAg-expression from pHBV1.3 plasmid DNA declined from 2753 ng/mL to 131 ng/mL over that time in immunodeficient mice (P < 0.001), reflective of plasmid DNA silencing. We then cloned Cre-recombinase in cis on the LoxP-HBV plasmids, achieving plasmid stability in bacteria with intron insertion into Cre and demonstrating rcccDNA formation after transfection in vitro and in vivo. These cis-Cre/LoxP-HBV plasmids were then used to create HBx-mutant and GFP reporter plasmids to further probe cccDNA biology and antiviral strategies against cccDNA. Overall, we believe these auto-generating rcccDNA plasmids will be of great value to model cccDNA for testing new therapies against HBV infection.

Minicircle DNA vector expressing interferon-lambda-3 inhibits hepatitis B virus replication and expression in hepatocyte-derived cell line

BACKGROUND

Interferon-alpha (IFNα) is a first-line treatment option for chronic hepatitis B virus (HBV) infection, but the severe systemic side-effects limited its clinical application. Interferon-lambda (IFNλ) with comparable antiviral activity and less toxic side-effects is thought to be a good alternative interferon to IFNα. Additionally, the gene vector mediated sustainably expression of therapeutic product in the target cells/tissue may overcome the shortcomings resulted from the short half-life of IFNs.

RESULTS

We constructed a liver-specific IFNλ3-expressing minicircle (MC) vector under the control of a hepatocyte-specific ApoE promoter (MC.IFNλ3) and investigated its anti-HBV activity in a HBV-expressing hepatocyte-derived cell model (HepG2.2.15). As expected, the MC.IFNλ3 vector capable of expressing IFNλ3 in the recipient hepatocytes has demonstrated robust anti-HBV activity, in terms of suppressing viral antigen expression and viral DNA replication, via activation the interferon-stimulated gene (ISG) expression in HepG2.2.15 cells.

CONCLUSIONS

Given the MC vector can be easily delivered into liver, the liver-targeted IFN gene-transfer (MC.IFNλ3), instead of systemic administrating IFN repeatedly, provides a promising concept for the treatment of chronic HBV infection.

Construction of a replication-competent hepatitis B virus vector carrying secreted luciferase transgene and establishment of new hepatitis B virus replication and expression cell lines

BACKGROUND

Previously, we have successfully constructed replication-competent hepatitis B virus (HBV) vectors by uncoupling the P open reading frame (ORF) from the preC/C ORF to carefully design the transgene insertion site to overcome the compact organization of the HBV genome and maintain HBV replication competence. Consequently, the replication-competent HBV vectors carrying foreign genes, including pCH-BsdR, carrying blasticidin resistance gene (399 bp), and pCH-hrGFP, carrying humanized renilla green fluorescent protein gene (720 bp), were successfully obtained. However, the replication efficiency of the former is higher but it is tedious to use, while that of the latter is poor and cannot be quantified. Hence, we need to search for a new reporter gene that is convenient and quantifiable for further research.

AIM

To establish a helpful tool for intracellular HBV replication and anti-viral drugs screening studies.

METHODS

We utilized the replication-competent HBV viral vectors constructed by our laboratory, combined with the secreted luciferase reporter gene, to construct replication-competent HBV vectors expressing the reporter gene secretory Nanoluc Luciferase (SecNluc). HepG2.TA2-7 cells were transfected with this vector to obtain cell lines with stably secreted HBV particles carrying secNluc reporter gene.

RESULTS

The replication-competent HBV vector carrying the SecNluc reporter gene pCH-sNLuc could produce all major viral RNAs and a full set of envelope proteins and achieve high-level secreted luciferase expression. HBV replication intermediates could be produced from this vector. Via transfection with pTRE-sNLuc and selection by hygromycin, we obtained isolated cell clones, named HBV-NLuc-35 cells, which could secrete secNLuc recombinant viruses, and were sensitive to existing anti-HBV drugs. Using differentiated HepaRG cells, it was verified that recombinant HBV possessed infectivity.

CONCLUSION

Our research demonstrated that a replication-competent HBV vector carrying a secreted luciferase transgene possesses replication and expression ability, and the established HBV replication and expression cell lines could stably secrete viral particles carrying secNluc reporter gene. More importantly, the cell line and the secreted recombinant viral particles could be used to trace HBV replication or infection.

A dual role for SAMHD1 in regulating HBV cccDNA and RT-dependent particle genesis

Chronic hepatitis B is one of the world's unconquered diseases with more than 240 million infected subjects at risk of developing liver disease and hepatocellular carcinoma. Hepatitis B virus reverse transcribes pre-genomic RNA to relaxed circular DNA (rcDNA) that comprises the infectious particle. To establish infection of a naïve target cell, the newly imported rcDNA is repaired by host enzymes to generate covalently closed circular DNA (cccDNA), which forms the transcriptional template for viral replication. SAMHD1 is a component of the innate immune system that regulates deoxyribonucleoside triphosphate levels required for host and viral DNA synthesis. Here, we show a positive role for SAMHD1 in regulating cccDNA formation, where KO of SAMHD1 significantly reduces cccDNA levels that was reversed by expressing wild-type but not a mutated SAMHD1 lacking the nuclear localization signal. The limited pool of cccDNA in infected Samhd1 KO cells is transcriptionally active, and we observed a 10-fold increase in newly synthesized rcDNA-containing particles, demonstrating a dual role for SAMHD1 to both facilitate cccDNA genesis and to restrict reverse transcriptase-dependent particle genesis.

Episomal HBV persistence within transcribed host nuclear chromatin compartments involves HBx

Background

In hepatocyte nuclei, hepatitis B virus (HBV) genomes occur episomally as covalently closed circular DNA (cccDNA). The HBV X protein (HBx) is required to initiate and maintain HBV replication. The functional nuclear localization of cccDNA and HBx remains unexplored.

Results

To identify virus–host genome interactions and the underlying nuclear landscape for the first time, we combined circular chromosome conformation capture (4C) with RNA-seq and ChIP-seq. Moreover, we studied HBx-binding to HBV episomes. In HBV-positive HepaRG hepatocytes, we observed preferential association of HBV episomes and HBx with actively transcribed nuclear domains on the host genome correlating in size with constrained topological units of chromatin. Interestingly, HBx alone occupied transcribed chromatin domains. Silencing of native HBx caused reduced episomal HBV stability.

Conclusions

As part of the HBV episome, HBx might stabilize HBV episomal nuclear localization. Our observations may contribute to the understanding of long-term episomal stability and the facilitation of viral persistence. The exact mechanism by which HBx contributes to HBV nuclear persistence warrants further investigations.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0204-2) contains supplementary material, which is available to authorized users.

Investigating the hepatitis B virus life cycle using engineered reporter hepatitis B viruses

Chronic infection with hepatitis B virus (HBV) increases the risk of developing fibrosis, cirrhosis or hepatocellular carcinoma. Current therapies are limited to type-I interferons and/or nucleos(t)ide analogues; however, these are only partially effective. The development of novel anti-HBV agents for new treatment strategies has been hampered by the lack of a suitable system that allows the in vitro replication of HBV. Studies of virus infection/replication at the molecular level using wild-type HBV are labor-intensive and time-consuming. To overcome these problems, we previously constructed a recombinant reporter HBV bearing the NanoLuc gene and showed its usefulness in identifying factors that affect HBV proliferation. Because this system mimics the early stage of the HBV life cycle faithfully, we conducted a quantitative analysis of HBV infectivity to several human hepatocyte cell lines as well as the effect of dimethyl sulfoxide and HBV protein X on the early stage of HBV proliferation using this system. Furthermore, we developed a system to produce a reporter HBV expressing a pol gene. These reporter HBV may provide an opportunity to enhance our understanding of the HBV life cycle and aid strategies for the development of new anti-HBV agents.

A Role for the Host DNA Damage Response in Hepatitis B Virus cccDNA Formation-and Beyond?

Chronic hepatitis B virus (HBV) infection puts more than 250 million people at a greatly increased risk to develop end-stage liver disease. Like all hepadnaviruses, HBV replicates via protein-primed reverse transcription of a pregenomic (pg) RNA, yielding an unusually structured, viral polymerase-linked relaxed-circular (RC) DNA as genome in infectious particles. Upon infection, RC-DNA is converted into nuclear covalently closed circular (ccc) DNA. Associating with cellular proteins into an episomal minichromosome, cccDNA acts as template for new viral RNAs, ensuring formation of progeny virions. Hence, cccDNA represents the viral persistence reservoir that is not directly targeted by current anti-HBV therapeutics. Eliminating cccDNA will thus be at the heart of a cure for chronic hepatitis B. The low production of HBV cccDNA in most experimental models and the associated problems in reliable cccDNA quantitation have long hampered a deeper understanding of cccDNA molecular biology. Recent advancements including cccDNA-dependent cell culture systems have begun to identify select host DNA repair enzymes that HBV usurps for RC-DNA to cccDNA conversion. While this list is bound to grow, it may represent just one facet of a broader interaction with the cellular DNA damage response (DDR), a network of pathways that sense and repair aberrant DNA structures and in the process profoundly affect the cell cycle, up to inducing cell death if repair fails. Given the divergent interactions between other viruses and the DDR it will be intriguing to see how HBV copes with this multipronged host system.

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.

Visualization of hepatitis B virus entry - novel tools and approaches to directly follow virus entry into hepatocytes

Hepatitis B virus (HBV) is a widespread human pathogen, responsible for chronic infections of ca. 240 million people worldwide. Until recently, the entry pathway of HBV into hepatocytes was only partially understood. The identification of human sodium taurocholate cotransporting polypeptide (NTCP) as a bona fide receptor of HBV has provided us with new tools to investigate this pathway in more details. Combined with advances in virus visualization techniques, approaches to directly visualize HBV cell attachment, NTCP interaction, virion internalization and intracellular transport are now becoming feasible. This review summarizes our current understanding of how HBV specifically enters hepatocytes, and describes possible visualization strategies applicable for a deeper understanding of the underlying cell biological processes.

Establishment of an inducible HBV stable cell line that expresses cccDNA-dependent epitope-tagged HBeAg for screening of cccDNA modulators

Hepatitis B virus (HBV) covalently closed circular (ccc) DNA is essential to the virus life cycle, its elimination during chronic infection is considered critical to a durable therapy but has not been achieved by current antivirals. Despite being essential, cccDNA has not been the major target of high throughput screening (HTS), largely because of the limitations of current HBV tissue culture systems, including the impracticality of detecting cccDNA itself. In response to this need, we have previously developed a proof-of-concept HepDE19 cell line in which the production of wildtype e antigen (HBeAg) is dependent upon cccDNA. However, the existing assay system is not ideal for HTS because the HBeAg ELISA cross reacts with a viral HBeAg homologue, which is the core antigen (HBcAg) expressed largely in a cccDNA-independent fashion in HepDE19 cells. To further improve the assay specificity, we report herein a "second-generation" cccDNA reporter cell line, termed HepBHAe82. In the similar principle of HepDE19 line, an in-frame HA epitope tag was introduced into the precore domain of HBeAg open reading frame in the transgene of HepBHAe82 cells without disrupting any cis-element critical for HBV replication and HBeAg secretion. A chemiluminescence ELISA assay (CLIA) for the detection of HA-tagged HBeAg with HA antibody serving as capture antibody and HBeAb serving as detection antibody has been developed to eliminate the confounding signal from HBcAg. The miniaturized HepBHAe82 cell based assay system exhibits high level of cccDNA-dependent HA-HBeAg production and high specific readout signals with low background. We have also established a HepHA-HBe4 cell line expressing transgene-dependent HA-HBeAg as a counter screen to identify HBeAg inhibitors. The HepBHAe82 system is amenable to antiviral HTS development, and can be used to identify host factors that regulate cccDNA metabolism and transcription.

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.

Re-Designed Recombinant Hepatitis B Virus Vectors Enable Efficient Delivery of Versatile Cargo Genes to Hepatocytes with Improved Safety

Hepatitis B virus (HBV) takes humans as its sole natural host, and productive infection in vivo is restricted exclusively to hepatocytes in the liver. Consequently, HBV-derived viral vectors are attractive candidates for liver-targeting gene therapies. Previously, we developed a novel recombinant HBV vector, designated 5c3c, from a highly replicative clinical isolate. 5c3c was demonstrated to be capable of efficiently delivering protein or RNA expression into infected primary tupaia hepatocytes (PTH), but the design of 5c3c imposes stringent restrictions on inserted sequences, which have limited its wider adoption. In this work, we addressed issues with 5c3c by re-designing the insertion strategy. The resultant vector, designated 5dCG, was more replicative than parental 5c3c, imposed no specific restrictions on inserted sequences, and allowed insertion of a variety of cargo genes without significant loss of replication efficiency. 5dCG-based recombinant HBV effectively delivered protein and RNA expression into infected PTH. Furthermore, due to the loss of functional core ORF, 5dCG vectors depend on co-infecting wild type HBV for replication and efficient expression of cargo genes. Development of the improved 5dCG vector makes wider applications of recombinant HBV possible, while dependence on co-infecting wild type HBV results in improved safety for certain in vivo applications.

Attacking hepatitis B virus cccDNA--The holy grail to hepatitis B cure

HBV deposits a covalently closed circular DNA form, called cccDNA, in the nucleus of infected cells. As the central transcription template, the cccDNA minichromosome is a key intermediate in the HBV life cycle. Its location in the nucleus makes cccDNA a difficult target for antivirals and immune response, and therefore it is responsible for chronicity of HBV infection. While little is known about the mechanisms involved in cccDNA formation, current research is accumulating data on the mechanisms regulating transcription from cccDNA, and the first potential targeting approaches have been reported. This review will summarize our knowledge about cccDNA biology and the latest advances in cccDNA targeting strategies in order to finally achieve an HBV cure.

AIC649 Induces a Bi-Phasic Treatment Response in the Woodchuck Model of Chronic Hepatitis B

AIC649 has been shown to directly address the antigen presenting cell arm of the host immune defense leading to a regulated cytokine release and activation of T cell responses. In the present study we analyzed the antiviral efficacy of AIC649 as well as its potential to induce functional cure in animal models for chronic hepatitis B. Hepatitis B virus transgenic mice and chronically woodchuck hepatitis virus (WHV) infected woodchucks were treated with AIC649, respectively. In the mouse system AIC649 decreased the hepatitis B virus titer as effective as the “gold standard”, Tenofovir. Interestingly, AIC649-treated chronically WHV infected woodchucks displayed a bi-phasic pattern of response: The marker for functional cure—hepatitis surface antigen—first increased but subsequently decreased even after cessation of treatment to significantly reduced levels. We hypothesize that the observed bi-phasic response pattern to AIC649 treatment reflects a physiologically “concerted”, reconstituted immune response against WHV and therefore may indicate a potential for inducing functional cure in HBV-infected patients.

Novel reporter system to monitor early stages of the hepatitis B virus life cycle

A recombinant hepatitis B virus (HBV) expressing NanoLuc (NL) (HBV/NL) was produced by cotransfecting a plasmid containing a 1.2‐fold HBV genome carrying the NL gene with a plasmid bearing a packaging‐defective 1.2‐fold HBV genome into a human hepatoma cell line, HepG2. We found that NL activity in HBV/NL‐infected primary hepatocytes or sodium taurocholate cotransporting polypeptide‐transduced human hepatocyte‐derived cell lines increased linearly for several days after infection and was concordant with HBV RNA levels in the cells. Treatment of the virus‐infected cells with HBV inhibitors reduced NL activity in a dose‐dependent manner. Detection of HBV/NL infection, monitored by NL activity, was highly sensitive and less expensive than detection using the conventional method to evaluate HBV infection. In addition, because we also studied host factors, this system is applicable not only for studying the HBV life cycle, but also for exploring agent(s) that regulate HBV proliferation.

Applications of human hepatitis B virus preS domain in bio- and nanotechnology

Human hepatitis B virus (HBV) is a member of the family Hepadnaviridae, and causes acute and chronic infections of the liver. The hepatitis B surface antigen (HBsAg) contains the large (L), middle (M), and small (S) surface proteins. The L protein consists of the S protein, preS1, and preS2. In HBsAg, the preS domain (preS1 + preS2) plays a key role in the infection of hepatocytic cells by HBV and has several immunogenic epitopes. Based on these characteristics of preS, several preS-based diagnostic and therapeutic materials and systems have been developed. PreS1-specific monoclonal antibodies (e.g., MA18/7 and KR127) can be used to inhibit HBV infection. A myristoylated preS1 peptide (amino acids 2-48) also inhibits the attachment of HBV to HepaRG cells, primary human hepatocytes, and primary tupaia hepatocytes. Antibodies and antigens related to the components of HBsAg, preS (preS1 + preS2), or preS1 can be available as diagnostic markers of acute and chronic HBV infections. Hepatocyte-targeting delivery systems for therapeutic molecules (drugs, genes, or proteins) are very important for increasing the clinical efficacy of these molecules and in reducing their adverse effects on other organs. The selective delivery of diagnostic molecules to target hepatocytic cells can also improve the efficiency of diagnosis. In addition to the full-length HBV vector, preS (preS1 + preS2), preS1, and preS1-derived fragments can be useful in hepatocyte-specific targeting. In this review, we discuss the literature concerning the applications of the HBV preS domain in bio- and nanotechnology.

A safe and efficient hepatocyte-selective carrier system based on myristoylated preS1/21-47 domain of hepatitis B virus

A safe and efficient liver targeted PEGylated liposome (PEG-Lip) based on N-terminal myristoylated preS1/21-47 (preS1/21-47(myr)) of hepatitis B virus was successfully developed. The study aimed to elucidate the cellular uptake mechanism of preS1/21-47(myr) modified PEG-Lip (preS1/21-47(myr)-PEG-Lip) in hepatogenic cells and the distribution behavior of preS1/21-47(myr)-PEG-Lip in Vr:CD1 (ICR) mice. The cellular uptake results showed that preS1/21-47(myr)-PEG-Lip was effectively taken up by hepatogenic cells (including primary hepatocytes and liver tumor cells) through a receptor-mediated endocytosis pathway compared with non-hepatogenic cells. After systemic administration to H22 hepatoma-bearing mice, preS1/21-47(myr)-PEG-Lip showed significant liver-specific delivery and an increase in the distribution of preS1/21-47(myr)-PEG-Lip in hepatic tumor. Furthermore, the antitumor effect of preS1/21-47(myr)-PEG-Lip loaded with paclitaxel (PTX) was remarkably stronger than that of PTX injection and PTX loaded liposomes (including common liposomes and PEG-Lip). In safety evaluation, no acute systemic toxicity and immunotoxicity were observed after intravenous injection of preS1/21-47(myr)-PEG-Lip. No liver toxicity was observed despite the dramatic increase of preS1/21-47(myr)-PEG-Lip in liver. Taken together, preS1/21-47(myr)-PEG-Lip represents a promising carrier system for targeted liver disease therapy and imaging.

Virology. Response to Comment on "Specific and nonhepatotoxic degradation of nuclear hepatitis B virus cccDNA"

Chisari et al. challenge our central conclusion that the hepatitis B virus (HBV) persistent form, the covalently closed circular DNA (cccDNA), is degraded in a noncytotoxic and specific fashion in the nucleus of infected hepatocytes. Specificity of the assays used, exclusion of cell division or death, and activity of APOBEC3 deaminases in the nucleus, however, were addressed in the paper.

Recent advancements in optofluidics-based single-cell analysis: optical on-chip cellular manipulation, treatment, and property detection

Cellular analysis plays important roles in various biological applications, such as cell biology, drug development, and disease diagnosis. Conventional cellular analysis usually measures the average response from a whole cell group. However, bulk measurements may cause misleading interpretations due to cell heterogeneity. Another problem is that current cellular analysis may not be able to differentiate various subsets of cell populations, each exhibiting a different behavior than the others. Single-cell analysis techniques are developed to analyze cellular properties, conditions, or functional responses in a large cell population at the individual cell level. Integrating optics with microfluidic platforms provides a well-controlled microenvironment to precisely control single cell conditions and perform non-invasive high-throughput analysis. This paper reviews recent developments in optofluidic technologies for various optics-based single-cell analyses, which involve single cell manipulation, treatment, and property detection. Finally, we provide our views on the future development of integrated optics with microfluidics for single-cell analysis and discuss potential challenges and opportunities of this emerging research field in biological applications.

Is hepatitis B-virucidal validation of biocides possible with the use of surrogates?

The hepatitis B virus (HBV) is considered to be a major public health problem worldwide, and a significant number of reports on nosocomial outbreaks of HBV infections have been reported. Prevention of indirect HBV transmission by contaminated objects is only possible through the use of infection-control principles, including the use of chemical biocides, which are proven to render the virus non-infectious. The virucidal activity of biocides against HBV cannot be predicted; therefore, validation of the virucidal action of disinfectants against HBV is essential. However, feasible HBV infectivity assays have not yet been established. Thus, surrogate models have been proposed for testing the efficacy of biocides against HBV. Most of these assays do not correlate with HBV infectivity. Currently, the most promising and feasible assay is the use of the taxonomically related duck hepatitis B virus (DHBV), which belongs to the same Hepadnaviridae virus family. This paper reviews the application of DHBV, which can be propagated in vitro in primary duck embryonic hepatocytes, for the testing of biocides and describes why this model can be used as reliable method to evaluate disinfectants for efficacy against HBV. The susceptibility levels of important biocides, which are often used as ingredients for commercially available disinfectants, are also described.

New approaches to rapidly control foot-and-mouth disease outbreaks

Economically, foot-and-mouth disease is the most important viral-induced livestock disease worldwide. The disease is highly contagious and foot-and-mouth disease virus replicates and spreads extremely rapidly. Recent outbreaks in previously foot-and-mouth disease-free countries and the potential use of foot-and-mouth disease virus by terrorist groups have demonstrated the vulnerability of countries and the need to develop control strategies that can rapidly inhibit or limit spread of the disease. The current vaccine, an inactivated whole-virus preparation, has a number of limitations for use in outbreaks in disease-free countries. This review discusses the potential of the antiviral agent, Type I interferon, to produce rapid protection and proposes a combination strategy of an antiviral agent and a foot-and-mouth disease vaccine to induce both immediate and long-lasting protective responses.

Novel recombinant hepatitis B virus vectors efficiently deliver protein and RNA encoding genes into primary hepatocytes

Hepatitis B virus (HBV) has extremely restricted host and hepatocyte tropism. HBV-based vectors could form the basis of novel therapies for chronic hepatitis B and other liver diseases and would also be invaluable for the study of HBV infection. Previous attempts at developing HBV-based vectors encountered low yields of recombinant viruses and/or lack of sufficient infectivity/cargo gene expression in primary hepatocytes, which hampered follow-up applications. In this work, we constructed a novel vector based on a naturally occurring, highly replicative HBV mutant with a 207-bp deletion in the preS1/polymerase spacer region. By applying a novel insertion strategy that preserves the continuity of the polymerase open reading frame (ORF), recombinant HBV (rHBV) carrying protein or small interfering RNA (siRNA) genes were obtained that replicated and were packaged efficiently in cultured hepatocytes. We demonstrated that rHBV expressing a fluorescent reporter (DsRed) is highly infective in primary tree shrew hepatocytes, and rHBV expressing HBV-targeting siRNA successfully inhibited antigen expression from coinfected wild-type HBV. This novel HBV vector will be a powerful tool for hepatocyte-targeting gene delivery, as well as the study of HBV infection.

Replication-competent infectious hepatitis B virus vectors carrying substantially sized transgenes by redesigned viral polymerase translation

Viral vectors are engineered virus variants able to deliver nonviral genetic information into cells, usually by the same routes as the parental viruses. For several virus families, replication-competent vectors carrying reporter genes have become invaluable tools for easy and quantitative monitoring of replication and infection, and thus also for identifying antivirals and virus susceptible cells. For hepatitis B virus (HBV), a small enveloped DNA virus causing B-type hepatitis, such vectors are not available because insertions into its tiny 3.2 kb genome almost inevitably affect essential replication elements. HBV replicates by reverse transcription of the pregenomic (pg) RNA which is also required as bicistronic mRNA for the capsid (core) protein and the reverse transcriptase (Pol); their open reading frames (ORFs) overlap by some 150 basepairs. Translation of the downstream Pol ORF does not involve a conventional internal ribosome entry site (IRES). We reasoned that duplicating the overlap region and providing artificial IRES control for translation of both Pol and an in-between inserted transgene might yield a functional tricistronic pgRNA, without interfering with envelope protein expression. As IRESs we used a 22 nucleotide element termed Rbm3 IRES to minimize genome size increase. Model plasmids confirmed its activity even in tricistronic arrangements. Analogous plasmids for complete HBV genomes carrying 399 bp and 720 bp transgenes for blasticidin resistance (BsdR) and humanized Renilla green fluorescent protein (hrGFP) produced core and envelope proteins like wild-type HBV; while the hrGFP vector replicated poorly, the BsdR vector generated around 40% as much replicative DNA as wild-type HBV. Both vectors, however, formed enveloped virions which were infectious for HBV-susceptible HepaRG cells. Because numerous reporter and effector genes with sizes of around 500 bp or less are available, the new HBV vectors should become highly useful tools to better understand, and combat, this important pathogen.

Truncated active human matrix metalloproteinase-8 delivered by a chimeric adenovirus-hepatitis B virus vector ameliorates rat liver cirrhosis

BACKGROUND

Liver cirrhosis is a potentially life-threatening disease caused by progressive displacement of functional hepatocytes by fibrous tissue. The underlying fibrosis is often driven by chronic infection with hepatitis B virus (HBV). Matrix metalloproteinases including MMP-8 are crucial for excess collagen degradation. In a rat model of liver cirrhosis, MMP-8 delivery by an adenovirus (Ad) vector achieved significant amelioration of fibrosis but application of Ad vectors in humans is subject to various issues, including a lack of intrinsic liver specificity.

METHODS

HBV is highly liver-specific and its principal suitability as liver-specific gene transfer vector is established. HBV vectors have a limited insertion capacity and are replication-defective. Conversely, in an HBV infected cell vector replication may be rescued in trans by the resident virus, allowing conditional vector amplification and spreading. Capitalizing on a resident pathogen to help in its elimination and/or in treating its pathogenic consequences would provide a novel strategy. However, resident HBV may also reduce susceptibility to HBV vector superinfection. Thus a size-compatible truncated MMP-8 (tMMP8) gene was cloned into an HBV vector which was then used to generate a chimeric Ad-HBV shuttle vector that is not subject to superinfection exclusion. Rats with thioacetamide-induced liver cirrhosis were injected with the chimera to evaluate therapeutic efficacy.

RESULTS

Our data demonstrate that infectious HBV vector particles can be obtained via trans-complementation by wild-type virus, and that the tMMP8 HBV vector can efficiently be shuttled by an Ad vector into cirrhotic rat livers. There it exerted a comparable beneficial effect on fibrosis and hepatocyte proliferation markers as a conventional full-length MMP-8Ad vector.

CONCLUSIONS

Though the rat cirrhosis model does not allow assessing in vivo HBV vector amplification these results advocate the further development of Ad-HBV vectors for liver-specific gene therapy, including and perhaps particularly for HBV-related disease.

Targeted delivery of interferon-α to hepatitis B virus-infected cells using T-cell receptor-like antibodies

During antiviral therapy, specific delivery of interferon-α (IFNα) to infected cells may increase its antiviral efficacy, trigger a localized immune reaction, and reduce the side effects caused by systemic administration. Two T-cell receptor-like antibodies (TCR-L) able to selectively bind hepatitis B virus (HBV)-infected hepatocytes of chronic hepatitis B patients and recognize core (HBc18-27) and surface (HBs183-91) HBV epitopes associated with different human leukocyte antigen (HLA)-A*02 alleles (A*02:01, A*02:02, A*02:07, A*02:11) were generated. Each antibody was genetically linked to two IFNα molecules to produce TCR-L/IFNα fusion proteins. We demonstrate that the fusion proteins triggered an IFNα response preferentially on the hepatocytes presenting the correct HBV-peptide HLA-complex and that the mechanism of the targeted IFNα response was dependent on the specific binding of the fusion proteins to the HLA/HBV peptide complexes through the TCR-like variable regions of the antibodies.

CONCLUSION

TCR-L antibodies can be used to target cytokines to HBV-infected hepatocytes in vitro. Fusion of IFNα to TCR-L decreased the intrinsic biological activity of IFNα but preserved the overall specificity of the protein for the cognate HBV peptide/HLA complexes. This induction of an effective IFNα response selectively in HBV-infected cells might have a therapeutic advantage in comparison to the currently used native or pegylated IFNα.

Hepatitis B virus X protein is essential to initiate and maintain virus replication after infection

BACKGROUND & AIMS

The molecular biology of hepatitis B virus (HBV) has been extensively studied but the exact role of the hepatitis B X protein (HBx) in the context of natural HBV infections remains unknown.

METHODS

Primary human hepatocytes and differentiated HepaRG cells allowing conditional trans complementation of HBx were infected with wild type (HBV(wt)) or HBx deficient (HBV(x-)) HBV particles and establishment of HBV replication was followed.

RESULTS

We observed that cells inoculated with HBx-deficient HBV particles (HBV(x-)) did not lead to productive HBV infection contrary to cells inoculated with wild type HBV particles (HBV(wt)). Although equal amounts of nuclear covalently closed circular HBV-DNA (cccDNA) demonstrated comparable uptake and nuclear import, active transcription was only observed from HBV(wt) genomes. Trans-complementation of HBx was able to rescue transcription from the HBV(x-) genome and led to antigen and virion secretion, even weeks after infection. Constant expression of HBx was necessary to maintain HBV antigen expression and replication. Finally, we demonstrated that HBx is not packaged into virions during assembly but is expressed after infection within the new host cell to allow epigenetic control of HBV transcription from cccDNA.

CONCLUSIONS

Our results demonstrate that HBx is required to initiate and maintain HBV replication and highlight HBx as the key regulator during the natural infection process.

A Tyr residue in the reverse transcriptase domain can mimic the protein-priming Tyr residue in the terminal protein domain of a hepadnavirus P protein

Hepadnaviruses are the only known viruses that replicate by protein-primed reverse transcription. Beyond the conserved reverse transcriptase (RT) and RNase H domains, their polymerases (P proteins) carry a unique terminal protein (TP) domain that provides a specific Tyr residue, Tyr96 in duck hepatitis B virus (DHBV), to which the first nucleotide of minus-strand DNA is autocatalytically attached and extended by three more nucleotides. In vitro reconstitution of this priming reaction with DHBV P protein and cellular chaperones had revealed strict requirements for the Dε RNA stem-loop as a template and for catalytic activity of the RT domain plus RNA-binding competence of the TP domain. Chaperone dependence can be obviated by using a truncated P protein (miniP). Here, we found that miniP with a tobacco etch virus (TEV) protease cleavage site between TP and RT (miniP(TEV)) displayed authentic priming activity when supplied with α-(32)P-labeled deoxynucleoside triphosphates; however, protease cleavage revealed, surprisingly, that the RT domain was also labeled. RT labeling had identical requirements as priming at Tyr96 and originated from dNMP transfer to a unique Tyr residue identified as Tyr561 in the presumed RT primer grip motif. Mutating Tyr561 did not affect Tyr96 priming in vitro and only modestly reduced replication competence of an intact DHBV genome; hence, deoxynucleotidylated Tyr561 is not an obligate intermediate in TP priming. However, as a first alternative substrate for the exquisitely complex protein-priming reaction, dNMP transfer to Tyr561 is a novel tool to further clarify the mechanism of hepadnaviral replication initiation and suggests that specific priming inhibitors can be found.

Production of recombinant woodchuck IFNalpha and development of monoclonal antibodies

Interferon alpha (IFNalpha) is the first line treatment for chronic hepatitis B and C. In order to test new IFNalpha delivery systems and investigate the function of this cytokine in the woodchuck model, the best animal model of chronic hepatitis B, we produced and purified recombinant woodchuck IFNalpha and used it to produce monoclonal antibodies. wIFNalpha5 was cloned in a prokaryotic expression system, expressed as His-tagged protein and then purified. The rwIFNalpha5 protein was found to induce STAT-3 phosphorylation, to enhance 2',5'-oligoadenylate synthetase mRNA levels and to possess a potent antiviral activity. Two monoclonal antibodies were obtained through immunization of rats with rwIFNalpha5. Both recognized rwIFNalpha5 in western blot analysis and one was able to neutralize the antiviral activity of the rwIFNalpha5 and lymphoblastoid IFNalpha preparations. Finally, a capture rwIFNalpha5 ELISA was developed using both antibodies. In summary, the tools generated in this study will allow different forms of IFNalpha delivery as well as different combination therapies in woodchuck hepatitis virus infection to be tested, thus providing useful information for the design of new strategies to treat chronic hepatitis B in humans.

Assembly and export determine the intracellular distribution of hepatitis B virus core protein subunits

Little is known about the parameters and factors that determine the intracellular distribution of the hepatitis B virus core protein (HBc). In order to study HBc in living cells, HBc was tagged with green fluorescent protein (GFP). Being assembly-incompetent, the GFP-fusion protein was distributed equally throughout the cell. Mutational inactivation of known serine-phosphorylation sites within the C-terminal region led to predominantly intranuclear localization. Phosphorylation of these targets, presumably by an SR domain protein kinase, resulted in a predominantly cytoplasmic localization, which suggests active cytoplasmic export or retention. The phosphoserine itself, and not its negative charge, appears essential for the underlying mechanism. In addition, the arginine-rich, protamine-like domain surrounding these phosphorylation sites does not function as the dominant nuclear-localization signal, as had been assumed previously, because neither deleting nor altering these sequences led to a change in intracellular HBc subunit distribution. Restoring the capability of the fusion protein to form capsids by co-assembly with assembly-competent, sterically uncompromised HBc subunits provided a second assay that gave insight into the effects resulting from capsid formation. Assembly was found to be the dominant factor in the cytoplasmic retention of the GFP-HBc fusion protein. Furthermore, the stability of these empty capsids was influenced by the cell-cycle inhibitor nocodazole. Thus, the intracellular distribution of HBc is dominated by cytoplasmic assembly, which is supported by the active nuclear export of HBc subunits, and modulated during the cell cycle by the instability of capsids.

Heterologous replacement of the supposed host determining region of avihepadnaviruses: high in vivo infectivity despite low infectivity for hepatocytes

Hepadnaviruses, including hepatitis B virus (HBV), a highly relevant human pathogen, are small enveloped DNA viruses that replicate via reverse transcription. All hepadnaviruses display a narrow tissue and host tropism. For HBV, this restricts efficient experimental in vivo infection to chimpanzees. While the cellular factors mediating infection are largely unknown, the large viral envelope protein (L) plays a pivotal role for infectivity. Furthermore, certain segments of the PreS domain of L from duck HBV (DHBV) enhanced infectivity for cultured duck hepatocytes of pseudotyped heron HBV (HHBV), a virus unable to infect ducks in vivo. This implied a crucial role for the PreS sequence from amino acid 22 to 90 in the duck tropism of DHBV. Reasoning that reciprocal replacements would reduce infectivity for ducks, we generated spreading-competent chimeric DHBVs with L proteins in which segments 22–90 (Du-He4) or its subsegments 22–37 and 37–90 (Du-He2, Du-He3) are derived from HHBV. Infectivity for duck hepatocytes of Du-He4 and Du-He3, though not Du-He2, was indeed clearly reduced compared to wild-type DHBV. Surprisingly, however, in ducks even Du-He4 caused high-titered, persistent, horizontally and vertically transmissable infections, with kinetics of viral spread similar to those of DHBV when inoculated at doses of 10⁸ viral genome equivalents (vge) per animal. Low-dose infections down to 300 vge per duck did not reveal a significant reduction in specific infectivity of the chimera. Hence, sequence alterations in PreS that limited infectivity in vitro did not do so in vivo. These data reveal a much more complex correlation between PreS sequence and host specificity than might have been anticipated; more generally, they question the value of cultured hepatocytes for reliably predicting in vivo infectivity of avian and, by inference, mammalian hepadnaviruses, with potential implications for the risk assessment of vaccine and drug resistant HBV variants.

Hepatitis B virus (HBV) associated liver disease is a leading cause of death worldwide. Host range restrictions limit experimental HBV infections largely to chimpanzees or isolated human hepatocytes. A narrow host range is shared by the animal hepadnaviruses, e.g. from ducks (DHBV) and herons (HHBV); HHBV does not infect ducks though it can establish a low-level infection in cultured duck hepatocytes. Host tropism is thought to be mediated by the PreS domain of the large viral envelope protein, because certain duck virus PreS segments introduced into the envelope of spreading-incompetent HHBV pseudotypes enhanced infectivity for duck hepatocytes. Expecting that reciprocal exchanges in DHBV would negatively impact duck tropism, we generated chimeric DHBVs in which the PreS regions in question are derived from HHBV and which are autonomously spreading-competent; this allowed us to directly compare their infectivity for duck hepatocytes and ducks. Surprisingly, even the chimera with the largest portion of HHBV sequence was as infectious for ducks as authentic DHBV; in vitro infectivity, however, was substantially reduced. These unexpected differences question the value of cultured hepatocytes to reliably predict in vivo infectivity of avihepadnaviruses and, by inference, also that of vaccine escape and therapy resistant HBV variants.

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.

Hepatitis B viruses: reverse transcription a different way

Hepatitis B virus (HBV), the causative agent of B-type hepatitis in humans, is the type member of the Hepadnaviridae, hepatotropic DNA viruses that replicate via reverse transcription. Beyond long-established differences to retroviruses in gene expression and overall replication strategy newer work has uncovered additional distinctions in the mechanism of reverse transcription per se. These include protein-priming by the unique extra terminal protein domain of the reverse transcriptase (RT) utilizing an RNA hairpin for de novo initiation of first strand DNA synthesis, and the strict dependence of this process on cellular chaperones. Recent in vitro reconstitution systems enabled first biochemical insights into this multifactorial reaction, complemented by high resolution structural information on the RNA, though not yet the protein, level. Genetic approaches have revealed long-distance interactions in the nucleic acid templates as an important factor enabling the puzzling template switches required to produce the relaxed circular (RC) DNA found in infectious virions. Finally, the failure of even potent HBV RT inhibitors to eliminate nuclear covalently closed circular (ccc) DNA, the functional equivalent of integrated proviral DNA, has spurred a renewed interest in the mechanism of cccDNA generation. These new developments are in the focus of this review.

Immune selection during chronic hepadnavirus infection

PURPOSE

Late-stage outcomes of chronic hepatitis B virus (HBV) infection, including fibrosis, cirrhosis, and hepatocellular carcinoma (HCC) result from persistent liver injury mediated by HBV antigen specific cytotoxic T lymphocytes (CTLs). Two other outcomes that often accompany chronic infection, the emergence of mutant viruses, including HBe-antigen negative (HBeAg (-)) HBV, and a reduction over time in the fraction of hepatocytes productively infected with HBV, may also result from persistent immune attack by antiviral CTLs. To gain insights into how these latter changes take place, we employed computer simulations of the chronically infected liver.

METHODS

Computational programs were used to model the emergence of both virus-free hepatocytes and mutant strains of HBV.

RESULTS

The computer modeling predicted that if cell-to-cell spread of virus is an efficient process during chronic infections, an HBV mutant that replicated significantly more efficiently than the wild type would emerge as the prevalent virus in a few years, much more rapidly than observed, while a mutant that replicated with the same or lower efficiency would fail to emerge. Thus, either cell-to-cell spread is inefficient or mutants do not replicate appreciably more efficiently than wild type. In contrast, with immune selection and a higher rate of killing of hepatocytes infected with wild-type virus, emergence of mutant virus can be explained without the need for a higher replication rate. Immune selection could also explain the emergence of virus-free hepatocytes that are unable to support HBV infection, since they should have a lower turnover rate than infected hepatocytes.

Ribavirin and IFN-α combination therapy induces CD4+ T-cell proliferation and Th1 cytokine secretion in patients with chronic hepatitis B

AIM: To investigate the anti-viral mechanism of combination therapy of interferon (IFN)-α and ribavirin in patients with chronic hepatitis B.

METHODS: Twenty patients were assigned to receive either IFN-α plus ribavirin (group A, n = 14) or no treatment as a control (group B, n = 6). Patients were analyzed for T-cell proliferative responses specific for hepatitis B virus (HBV)-antigen and cytokine production by peripheral blood mononuclear cells (PBMCs).

RESULTS: Combination therapy induced HBV-antigen specific CD4+ T-cell proliferative responses in four patients (28.6%). Production of high levels of HBV-specific IFN-γ, tumor necrosis factor (TNF)-α, interleukin (IL)-12 by PBMCs was found in five patients (35.7%), who showed significantly lower HBV DNA levels in serum at 12 mo after treatment ended (P = 0.038) and at 24 mo of follow-up (P = 0.004) than those without high levels of cytokine production.

CONCLUSION: HBV-antigen specific CD4+ T cells may directly control HBV replication and secretion of anti-viral T helper 1 (Th1) cytokines by PBMCs during combination therapy of chronic hepatitis B with ribavirin and IFN-α.

Chaperones activate hepadnavirus reverse transcriptase by transiently exposing a C-proximal region in the terminal protein domain that contributes to epsilon RNA binding

All hepatitis B viruses replicate by protein-primed reverse transcription, employing a specialized reverse transcriptase, P protein, that carries a unique terminal protein (TP) domain. To initiate reverse transcription, P protein must bind to a stem-loop, epsilon, on the pregenomic RNA template. TP then provides a Y residue for covalent attachment of the first nucleotide of an epsilon-templated DNA oligonucleotide (priming reaction) that serves to initiate full-length minus-strand DNA synthesis. epsilon binding requires the chaperone-dependent conversion of inactive P protein into an activated, metastable form designated P*. However, how P* differs structurally from P protein is not known. Here we used an in vitro reconstitution system for active duck hepatitis B virus P combined with limited proteolysis, site-specific antibodies, and defined P mutants to structurally compare nonactivated versus chaperone-activated versus primed P protein. The data show that Hsp70 action, under conditions identical to those required for functional activation, transiently exposes the C proximal TP region which is, probably directly, involved in epsilon RNA binding. Notably, after priming and epsilon RNA removal, a very similar new conformation appears stable without further chaperone activity; hence, the activation of P protein is triggered by energy-consuming chaperone action but may be completed by template RNA binding.

A structural model for duck hepatitis B virus core protein derived by extensive mutagenesis

Duck hepatitis B virus (DHBV) shares many fundamental features with human HBV. However, the DHBV core protein (DHBc), forming the nucleocapsid shell, is much larger than that of HBV (HBc) and, in contrast to HBc, there is little direct information on its structure. Here we applied an efficient expression system for recombinant DHBc particles to the biochemical analysis of a large panel of mutant DHBc proteins. By combining these data with primary sequence alignments, secondary structure prediction, and three-dimensional modeling, we propose a model for the fold of DHBc. Its major features are a HBc-like two-domain structure with an assembly domain comprising the first about 185 amino acids and a C-terminal nucleic acid binding domain (CTD), connected by a morphogenic linker region that is longer than in HBc and extends into the CTD. The assembly domain shares with HBc a framework of four major alpha-helices but is decorated at its tip with an extra element that contains at least one helix and that is made up only in part by the previously predicted insertion sequence. All subelements are interconnected, such that structural changes at one site are transmitted to others, resulting in an unexpected variability of particle morphologies. Key features of the model are independently supported by the accompanying epitope mapping study. These data should be valuable for functional studies on the impact of core protein structure on virus replication, and some of the mutant proteins may be particularly suitable for higher-resolution structural investigations.

Monoclonal antibodies providing topological information on the duck hepatitis B virus core protein and avihepadnaviral nucleocapsid structure

The icosahedral capsid of duck hepatitis B virus (DHBV) is formed by a single core protein species (DHBc). DHBc is much larger than HBc from human HBV, and no high-resolution structure is available. In an accompanying study (M. Nassal, I. Leifer, I. Wingert, K. Dallmeier, S. Prinz, and J. Vorreiter, J. Virol. 81:13218-13229, 2007), we used extensive mutagenesis to derive a structural model for DHBc. For independent validation, we here mapped the epitopes of seven anti-DHBc monoclonal antibodies. Using numerous recombinant DHBc proteins and authentic nucleocapsids from different avihepadnaviruses as test antigens, plus a panel of complementary assays, particle-specific and exposed plus buried linear epitopes were revealed. These data fully support key features of the model.

Chaperone activation of the hepadnaviral reverse transcriptase for template RNA binding is established by the Hsp70 and stimulated by the Hsp90 system

Hepadnaviruses are DNA viruses that replicate by protein-primed reverse transcription, employing a specialized reverse transcriptase (RT), P protein. DNA synthesis from the pregenomic RNA is initiated by binding of P to the ε signal. Using ε as template and a Tyr-residue for initiation, the RT synthesizes a DNA oligo (priming) as primer for full-length DNA. Priming strictly requires prior RT activation by chaperones. Active P–ε complexes have been reconstituted in vitro, but whether in addition to the heat-shock protein 70 (Hsp70) system the Hsp90 system is essential has been controversial. Here we quantitatively compared Hsp70 versus Hsp70 plus Hsp90 RT activation, and corroborated that the Hsp70 system alone is sufficient; however, Hsp90 as well the Hsp70 nucleotide exchange factor Bag-1 markedly stimulated activation by increasing the steady-state concentration of the activated metastable RT form P*, though by different mechanisms. Hsp90 inhibition in intact cells by geldanamycin analogs blocked hepadnavirus replication, however not completely and only at severely cytotoxic inhibitor concentrations. While compatible with a basal level of Hsp90 independent in vivo replication, unambiguous statements are precluded by the simultaneous massive upregulation of Hsp70 and Hsp90.

High expression of hepatitis B virus based vector with reporter gene in hepatitis B virus infection system

AIM: To construct a hepatitis B virus (HBV)-based vector with a reporter gene and to establish an HBV infection system to evaluate the availability of the vector.

METHODS: The HBV-based vectors with green fluorescence protein (GFP) were packaged into the liver of immunodeficient mice through transfer and helper plasmid using hydrodynamic technology. Wild type HBV (wt HBV) was provided by plasmid MC2009. Primary human hepatocytes (PHH) were isolated and infected with recombinant HBV (rHBV) or wt HBV. GFP expression was monitored by confocal and flow cytometry. HBV DNA and HBV surface antigen (HBSAg) were analyzed by PCR and ELISA.

RESULTS: 3 × 10⁷ wt HBV copies/mL and 5 × 10⁶ rHBV copies/mL were collected from mice serum. In the wt HBV infected group, HBV progeny was 2 × 10⁷ copies/mL and HBSAg was 770 ng/mL. In the rHBV infected group, GFP fluorescence was detected on d 3 post-infection and over 85% of the parenchymal cells expressed green fluorescence on d 12 post-infection. Compared with wt HBV in the PHH infection system, no rHBV DNA or HBSAg were detected in PHH culture media.

CONCLUSION: An effective HBV based vector was developed, which proved to be a useful HBV infection system. This vector and infection system can be applied to develop a therapeutic vector and study the HBV life cycle and viral pathogenesis.

Development of Gene Vectors for Pinpoint Targeting to Human Hepatocytes by Cationically Modified Polymer Complexes

We developed a vector that might enable gene therapy of metabolic liver disease or hepatoma. Here we demonstrate the use of cationically modified biocompatible phospholipid polymer conjugated with hepatitis B surface (HBs) antigen for the specific transfer of genes into human hepatocytes. Poly(2-methacryloyloxyethyl phosphorylcholine (MPC)- co-N,N-dimethylaminoethyl methacrylate (DMAEMA)-co- p-nitrophenylcarbonyloxyethyl methacrylate(NPMA))(polyMDN) was prepared as a frame of vector. The specific expression of sFlt-1 or GFP by polyMDN conjugated with HBs containing plasmid (plasmid/polyMDN-HBs), polyMDN containing plasmid (plasmid/polyMDN), plasmid only and PBS were assessed in tumor cells (HepG2 or WiDr) in vitro and in vivo. The histological findings, organ weight changes, and degree of liver dysfunction were examined in the mice administered by several reagents. The sFlt-1 and GFP expression was observed only in the HepG2 cells transfected with sFlt-1 or GFP/polyMDN-HBs. None of the side effects mentioned above was observed. In conclusion, these results suggest that polyMDN-HBs is a human hepatocyte-specific gene delivery vector that might not have serious side effects.

Hepatitis B virus replication

Hepadnaviruses, including human hepatitis B virus (HBV), replicate through reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA). Despite this kinship to retroviruses, there are fundamental differences beyond the fact that hepadnavirions contain DNA instead of RNA. Most peculiar is the initiation of reverse transcription: it occurs by protein-priming, is strictly committed to using an RNA hairpin on the pgRNA, ε, as template, and depends on cellular chaperones; moreover, proper replication can apparently occur only in the specialized environment of intact nucleocapsids. This complexity has hampered an in-depth mechanistic understanding. The recent successful reconstitution in the test tube of active replication initiation complexes from purified components, for duck HBV (DHBV), now allows for the analysis of the biochemistry of hepadnaviral replication at the molecular level. Here we review the current state of knowledge at all steps of the hepadnaviral genome replication cycle, with emphasis on new insights that turned up by the use of such cell-free systems. At this time, they can, unfortunately, not be complemented by three-dimensional structural information on the involved components. However, at least for the ε RNA element such information is emerging, raising expectations that combining biophysics with biochemistry and genetics will soon provide a powerful integrated approach for solving the many outstanding questions. The ultimate, though most challenging goal, will be to visualize the hepadnaviral reverse transcriptase in the act of synthesizing DNA, which will also have strong implications for drug development.

Hepatitis B virus X protein and the estrogen receptor variant lacking exon 5 inhibit estrogen receptor signaling in hepatoma cells

Hepatitis B virus (HBV) X protein (HBx) is considered to play a role in the development of hepatocellular carcinoma (HCC) during HBV infection. HCC was shown to be more prevalent in men than in women. Estrogen, which exerts its biological function through estrogen receptor (ER), can inhibit HBV replication. ERDelta5, an ERalpha variant lacking exon 5, was found to be preferentially expressed in patients with HCC compared with patients with normal livers. Here, we report the biological role of ERDelta5 and a novel link between HBx and ERalpha signaling in hepatoma cells. ERDelta5 interacts with ERalpha in vitro and in vivo and functions as a dominant negative receptor. Both ERalpha and ERDelta5 associate with HBx. HBx decreases ERalpha-dependent transcriptional activity, and HBx and ERDelta5 have additive effect on suppression of ERalpha transactivation. The HBx deletion mutant that lacks the ERalpha-binding site abolishes the HBx repression of ERalpha. HBx, ERalpha and histone deacetylase 1 (HDAC1) form a ternary complex. Trichostatin A, a specific inhibitor of HDAC enzyme, can restore the transcriptional activity of ERalpha inhibited by HBx. Our data suggest that HBx and ERDelta5 may play a negative role in ERalpha signaling and that ERalpha agonists may be developed for HCC therapy.