The duck hepatitis B virus pre-C region encodes a signal sequence which is essential for synthesis and secretion of processed core proteins but not for virus formation

Characterization of Intracellular Precore-Derived Proteins and Their Functions in Hepatitis B Virus-Infected Human Hepatocytes

Hepatitis B virus (HBV) precore protein is not essential for viral replication but is thought to facilitate chronic infection. In addition to the secreted precore products, including the hepatitis B e antigen (HBeAg) and PreC protein, intracellular precore-derived proteins in HBV-infected human hepatocytes remain poorly characterized, and their roles, if any, remain largely unknown. Here, we detected multiple precore derivatives, including the nonprocessed precursor p25 and the processing intermediate p22, in HBV-infected human hepatocytes as well as human hepatoma cells overexpressing the HBV precore protein. Both p25 and p22 showed phosphorylated and unphosphorylated forms, which were located in different intracellular compartments. Interestingly, precore expression was associated with decreases in intracellular HBV core protein (HBc) and secreted DNA-containing virions but was also associated with an increase in secreted empty virions. The decrease in HBc by precore could be attributed to cytosolic p22, which caused HBc degradation, at least in part by the proteasome, and consequently decreased HBV pregenomic RNA packaging and DNA synthesis. In addition, cytosolic p22 formed chimeric capsids with HBc in the cell, which were further secreted in virions. In contrast, the PreC antigen, like HBeAg, was secreted via the endoplasmic reticulum (ER)-Golgi secretory pathway and was thus unable to form capsids in the cell or be secreted in virions. Furthermore, p25, as well as p22, were secreted in virions from HBV-infected human hepatocytes and were detected in the sera of HBV-infected chimpanzees. In summary, we have detected multiple intracellular precore-derived proteins in HBV-infected human hepatocytes and revealed novel precore functions in the viral life cycle. IMPORTANCE Chronic hepatitis B remains a worldwide public health issue. The hepatitis B virus (HBV) precore protein is not essential for HBV replication but may facilitate viral persistence. In this study, we have detected multiple precore protein species in HBV-infected human hepatocytes and studied their functions in the HBV life cycle. We found that the HBV precore proteins decreased intracellular HBV core protein and reduced secretion of complete virions but enhanced secretion of empty virions. Interestingly, the cytosolic precore protein species formed chimeric capsids with the core protein and were secreted in virions. Our results shed new light on the functions of intracellular precore protein species in the HBV life cycle and have implications for the roles of precore proteins in HBV persistence and pathogenesis.

Constrained evolution of overlapping genes in viral host adaptation: Acquisition of glycosylation motifs in hepadnaviral precore/core genes

Hepadnaviruses use extensively overlapping genes to expand their coding capacity, especially the precore/core genes encode the precore and core proteins with mostly identical sequences but distinct functions. The precore protein of the woodchuck hepatitis virus (WHV) is N-glycosylated, in contrast to the precore of the human hepatitis B virus (HBV) that lacks N-glycosylation. To explore the roles of the N-linked glycosylation sites in precore and core functions, we substituted T77 and T92 in the WHV precore/core N-glycosylation motifs (⁷⁵NIT⁷⁷ and ⁹⁰NDT⁹²) with the corresponding HBV residues (E77 and N92) to eliminate the sequons. Conversely, these N-glycosylation sequons were introduced into the HBV precore/core gene by E77T and N92T substitutions. We found that N-glycosylation increased the levels of secreted precore gene products from both HBV and WHV. However, the HBV core (HBc) protein carrying the E77T substitution was defective in supporting virion secretion, and during infection, the HBc E77T and N92T substitutions impaired the formation of the covalently closed circular DNA (cccDNA), the critical viral DNA molecule responsible for establishing and maintaining infection. In cross-species complementation assays, both HBc and WHV core (WHc) proteins supported all steps of intracellular replication of the heterologous virus while WHc, with or without the N-glycosylation sequons, failed to interact with HBV envelope proteins for virion secretion. Interestingly, WHc supported more efficiently intracellular cccDNA amplification than HBc in the context of either HBV or WHV. These findings reveal novel determinants of precore secretion and core functions and illustrate strong constraints during viral host adaptation resulting from their compact genome and extensive use of overlapping genes.

Hepadnaviruses infect a wide range of hosts. The human hepatitis B virus (HBV) and woodchuck hepatitis virus (WHV) are two closely related hepadnaviruses. In contrast to the WHV precore protein, which is N-glycosylated, the HBV precore protein lacks N-glycosylation. As precore and core proteins expressed from the overlapping precore/core genes share most of their sequences but have distinct functions, we investigated the roles of the N-linked glycosylation sequons in HBV and WHV precore/core genes. Our results revealed an important role of the N-linked glycosylation sequons in enhancing precore secretion levels and regulating core protein functions in virion secretion and infection. Furthermore, cross-species complementation assays using HBV and WHV core proteins and HBV or WHV genomes defective in core protein expression indicated that both HBV and WHV core proteins could support intracellular viral replication but not virion secretion of the heterologous virus. These results provide novel insights into the evolution of overlapping genes during host adaptation of hepadnaviruses.

Role of core protein mutations in the development of occult HBV infection

BACKGROUND & AIMS

Occult HBV infection (OBI) is associated with transfusion-transmitted HBV infection and hepatocellular carcinoma. Studies on OBI genesis have concentrated on mutations in the S region and the regulatory elements. Herein, we aimed to determine the role of mutations in the core region on OBIs.

METHODS

An OBI strain (SZA) carrying 9 amino acid (aa) substitutions in the core protein/capsid (Cp) was selected by sequence alignment and Western blot analysis from 26 genotype B OBI samples to extensively explore the impact of Cp mutations on viral antigen production in vitro and in vivo.

RESULTS

A large panel of 30 Cp replicons were generated by a replication-competent pHBV1.3 carrying SZA or wild-type (WT) Cp in a 1.3-fold over-length of HBV genome, in which the various Cp mutants were individually introduced by repairing site mutations of SZA-Cp or creating site mutations of WT-Cp by site-directed mutagenesis. The expression of HBcAg, HBeAg, and HBsAg and viral RNA was quantified from individual SZA and WT Cp mutant replicons in transfected Huh7 cells or infected mice, respectively. An analysis of the effect of Cp mutants on intracellular or extracellular viral protein production indicated that the W62R mutation in Cp had a critical impact on the reduction of HBcAg and HBeAg production during HBV replication, whereas P50H and/or S74G mutations played a limited role in influencing viral protein production invivo.

CONCLUSIONS

W62R and its combination mutations in HBV Cp might massively affect HBcAg and HBeAg production during viral replication, which, in turn, might contribute to the occurrence of OBI.

LAY SUMMARY

Occult hepatitis B virus infections (OBIs) have been found to be associated with amino acid mutations in the S region of the HBV, but the role of mutations in the core protein (Cp) remains unclear. In this study, an OBI strain (SZA) carrying 9 amino acid substitutions in Cp has been examined comprehensively in vitro and in vivo. The W62R mutation in Cp majorly reduces HBcAg and HBeAg production during HBV replication, potentially contributing to the occurrence of OBI.

In vitro expression of precore proteins of hepatitis B virus subgenotype A1 is affected by HBcAg, and can affect HBsAg secretion

HBeAg, a non-particulate protein of hepatitis B virus (HBV), is translated from the precore/core region as a precursor, which is post-translationally modified. Subgenotype A1 of HBV, which is a risk factor for hepatocellular carcinoma (HCC), has unique molecular characteristics in the basic core promoter/precore regions. Carriers of A1 exhibit early HBeAg loss. We sought to further characterize the precore proteins of A1 in vitro. HuH-7 cells were transfected with subgenomic constructs expressing individual precore proteins. Western blot analysis using DAKO anti-core antibody showed the expected sizes and a 1 kDa larger band for P22, P20 and P17. Using confocal microscopy, a cytoplasmic accumulation of HBeAg and precursors was observed with P25-expressing plasmid, whereas P22 localized both in the cytoplasm and nucleus. P20 and P17, which lack the carboxy end of P22 showed strong nuclear accumulation, implicating a nuclear localization signal in the N-terminal 10 amino acids. G1862T, unique to subgenotype A1, is frequently found in HBV from HCC patients. P25 with G1862T showed delayed and reduced HBeAg expression/secretion. Knock-out of core in the replication competent clones led to precore protein accumulation in the cytoplasm/perinuclear region, and decreased HBeAg secretion. Knock-out of precore proteins increased HBsAg secretion but intracellular HBsAg expression was unaffected. Over-expression of precore proteins in trans led to decreased HBsAg expression and secretion. Intracellular trafficking of HBV A1 precore proteins was followed. This was unaffected by the CMV promoter and different cell types. In the viral context, precore protein expression was affected by absence of core, and affected HBsAg expression, suggesting an interrelationship between precore proteins, HBcAg and HBsAg. This modulatory role of HBeAg and its precursors may be important in viral persistence and ultimate development of HCC.

Hepatitis B envelope antigen increases Tregs by converting CD4+CD25- T cells into CD4+CD25+Foxp3+ Tregs

Hepatitis B virus (HBV) can establish a lifelong chronic infection in humans, leading to liver cirrhosis, liver failure and hepatocellular carcinoma. Patients with chronic hepatitis B (CHB) exhibit a weak virus-specific immune response. Regulatory T cells (Tregs) play a key role in regulating the immune response in patients with CHB. Patients with hepatitis B envelope antigen (HBeAg)-positive CHB harbored a higher percentage of Tregs in their peripheral blood than those with HBeAg-negative CHB. However, whether and how HBeAg manipulates the host immune system to increase the population of Tregs remains to be elucidated. The present manuscript describes a preliminary immunological study of HBeAg in a mouse model. Multiple potential CD4⁺ T cell epitopes in HBeAg were identified using Immune Epitope Database consensus binding prediction. It was demonstrated that HBeAg treatment increased the numbers of Tregs in mouse spleens in vitro and in vivo. Furthermore, it was indicated that the HBeAg-mediated increase in Tregs occurred through the conversion of CD4⁺CD25^- T cells into CD4⁺CD25⁺Foxp3⁺ Tregs. Additionally, in vitro study illustrated that HBeAg stimulated murine spleen cells to produce increased transforming growth factor-β, which is required to enable HBeAg to convert T cells into Tregs. The results of the present study may provide further evidence of the effect of HBeAg on Tregs and aid in the development of novel HBeAg-based immunotherapy for CHB.

Tracing the evolutionary history of hepadnaviruses in terms of e antigen and middle envelope protein expression or processing

Hepatitis B virus (HBV) is the prototype of hepadnaviruses, which can be subgrouped into orthohepadnaviruses infecting mammals, avihehepadnaviruses of birds, metahepadnaviruses of fish, and herpetohepadnaviruses of amphibians and reptiles. The middle (M) envelope protein and e antigen are new additions in the evolution of hepadnaviruses. They are alternative translation products of the transcripts for small (S) envelope and core proteins, respectively. For HBV, e antigen is converted from precore/core protein by removal of N-terminal signal peptide followed by furin-mediated cleavage of the basic C-terminus. This study compared old and newly discovered hepadnaviruses for their envelope protein and e antigen expression or processing. The S protein of bat hepatitis B virus (BHBV) and two metahepadnaviruses is probably myristoylated, in addition to two avihepadnaviruses. While most orthohepadnaviruses express a functional M protein with N-linked glycosylation near the amino-terminus, most metahepadnaviruses and herpetohepadnaviruses probably do not. These viruses and one orthohepadnavirus, the shrew hepatitis B virus, lack an open precore region required for e antigen expression. Potential furin cleavage sites (RXXR sequence) can be found in e antigen precursors of orthohepadnaviruses and avihepadnaviruses. Despite much larger precore/core proteins of avihepadnaviruses and their limited sequence homology with those of orthohepadnaviruses, their proximal RXXR motif can be aligned with a distal RXXR motif for orthohepadnaviruses. Thus, furin or another basic endopeptidase is probably the shared enzyme for hepadnaviral e antigen maturation. A precore-derived cysteine residue is involved in forming intramolecular disulfide bond of HBV e antigen to prevent particle formation, and such a cysteine residue is conserved for both orthohepadnaviruses and avihepadnaviruses. All orthohepadnaviruses have an X gene, while all avihepadnaviruses can express the e antigen. M protein expression appears to be the most recent event in the evolution of hepadnaviruses.

Is the function of the HBeAg really unknown?

The immune response to the hepatitis B virus (HBV) vaccine in newborns of hepatitis B e antigen (HBeAg)-positive or HBeAg-negative mothers is the subject of Huang et al. The authors report no correlation between the HBeAg status of the mothers/cord blood and the newborns immune response to the vaccine, but, unfortunately, draw unfounded conclusions regarding the tolerogenic potential of in utero exposure to HBeAg. In this reply, I address the possible influence of in utero exposure to the HBeAg, and briefly review other characteristics of the HBeAg, that may promote HBV chronicity. I argue that the function of HBeAg should no longer be considered "unknown" and that immunotolerance/immunomodulation represent the dominant functions of the HBeAg in viral-host interactions.

Immunomodulatory Function of HBeAg Related to Short-Sighted Evolution, Transmissibility, and Clinical Manifestation of Hepatitis B Virus

Hepatitis B virus (HBV) infection, a global public health problem can be asymptomatic, acute or chronic and can lead to serious consequences of infection, including cirrhosis, and hepatocellular carcinoma. HBV, a partially double stranded DNA virus, belongs to the family Hepadnaviridae, and replicates via reverse transcription of an RNA intermediate. This reverse transcription is catalyzed by a virus-encoded polymerase that lacks proof reading ability, which leads to sequence heterogeneity. HBV is classified into nine genotypes and at least 35 subgenotypes, which may be characterized by distinct geographical distributions. This HBV diversification and distinct geographical distribution has been proposed to be the result of the co-expansion of HBV with modern humans, after their out-of-Africa migration. HBeAg is a non-particulate protein of HBV that has immunomodulatory properties as a tolerogen that allows the virus to establish HBV infection in vivo. During the natural course of infection, there is seroconversion from a HBeAg-positive phase to a HBeAg-negative, anti-HBe-positive phase. During this seroconversion, there is loss of tolerance to infection and immune escape-HBeAg-negative mutants can be selected in response to the host immune response. The different genotypes and, in some cases, subgenotypes develop different mutations that can affect HBeAg expression at the transcriptional, translational and post-translational levels. The ability to develop mutations, affecting HBeAg expression, can influence the length of the HBeAg-positive phase, which is important in determining both the mode of transmission and the clinical course of HBV infection. Thus, the different genotypes/subgenotypes have evolved in such a way that they exhibit different modes of transmission and clinical manifestation of infection. Loss of HBeAg may be a sign of short-sighted evolution because there is loss of tolerogenic ability of HBeAg and HBeAg-negative virions are less transmissible. Depending on their ability to lead to HBeAg seroconversion, the genotype/subgenotypes exhibit varying degrees of short-sighted evolution. The “arms race” between HBV and the immune response to HBeAg is multifaceted and its elucidation intricate, with transmissibility and persistence being important for the survival of the virus. We attempt to shed some light on this complex interplay between host and virus.

The virological aspects of hepatitis B

Human hepatitis B virus (HBV) is a hepatotropic virus that is responsible for a significant burden of disease, causing liver disease and hepatocellular carcinoma. It is a small DNA virus with a replication strategy that is similar to that of a retrovirus. HBV is prone to mutagenesis and under the influence of diverse selection pressures, has evolved into a pool of quasispecies, genotypes and mutants, which confers a significant survival advantage. The genome is small, circular, and compact but has a complex replication strategy. The viral life cycle involves the formation of a covalently closed circular DNA (cccDNA), which is organized into a minichromosome that is the template for the synthesis of viral mRNA. HBV DNA (double-stranded linear form) can also integrate into the host genome, ensuring lifelong persistence of the virus. To date, despite great advances in therapeutics, once HBV is chronically established, it is incurable. This is by virtue of many aspects of its virological structure and viral life cycle. In this review, we aim to discuss important aspects of the virology of HBV with a focus on clinical implications.

Origins and Evolution of Hepatitis B Virus and Hepatitis D Virus

Members of the family Hepadnaviridae fall into two subgroups: mammalian and avian. The detection of endogenous avian hepadnavirus DNA integrated into the genomes of zebra finches has revealed a deep evolutionary origin of hepadnaviruses that was not previously recognized, dating back at least 40 million and possibly >80 million years ago. The nonprimate mammalian members of the Hepadnaviridae include the woodchuck hepatitis virus (WHV), the ground squirrel hepatitis virus, and arctic squirrel hepatitis virus, as well as a number of members of the recently described bat hepatitis virus. The identification of hepatitis B viruses (HBVs) in higher primates, such as chimpanzee, gorilla, orangutan, and gibbons that cluster with the human HBV, as well as a number of recombinant forms between humans and primates, further implies a more complex origin of this virus. We discuss the current theories of the origin and evolution of HBV and propose a model that includes cross-species transmissions and subsequent recombination events on a genetic backbone of genotype C HBV infection. The hepatitis delta virus (HDV) is a defective RNA virus requiring the presence of the HBV for the completion of its life cycle. The origins of this virus remain unknown, although some recent studies have suggested an ancient African radiation. The age of the association between HDV and HBV is also unknown.

Hepatitis B virus e antigen (HBeAg) may have a negative effect on dendritic cell generation

Hepatitis B virus (HBV) continues to be a serious worldwide health problem despite the use of protective HBV vaccines and therapeutic regimens against chronic HBV infection. Chronic HBV patients cannot induce sufficient immune responses against the virus. HBV and its antigens are believed to suppress immune responses during chronic infection. Hence, studying the role of HBV in immune suppression is very important for the development of alternative therapeutic strategies for HBV infections. In the present study, we investigated the effect of Hepatitis B virus e antigen (HBeAg) on the generation of bone marrow derived dendritic cells (BMDCs) and the stimulation of plasmacytoid DCs (pDCs). In the presence of HBeAg, the ratio of BMDCs was decreased, but the ratio of CD11b(+)Ly6G(+) immature myeloid cells was increased. The expression of 47 proteins was also changed during HBeAg treatment; however, CpG-induced MHC-II expression on pDCs was not affected. Our results indicate that HBeAg may have a negative effect on the generation of DCs from bone morrow precursors.

The hepatitis B virus e antigen suppresses the respiratory burst and mobility of human monocytes and neutrophils

The Hepatitis B virus (HBV) e antigen (HBeAg) is a secretory, non-structural protein, and associated with persistent infection of HBV. Previous studies indicate that HBeAg is able to regulate T cell-mediated responses, however, the interaction between HBeAg and the innate immune system is poorly understood. In this study, we demonstrated that recombinant HBeAg (rHBe) bound to human peripheral blood monocytes, neutrophils, and B lymphocytes but not to T lymphocytes. We focused on investigating the effects of HBeAg on monocytes and neutrophils and found that rHBe decreased the respiratory burst in both types of cells. Furthermore, we observed that cell migration in monocytes and neutrophils was suppressed by rHBe in a transwell assay. The attenuation of rHBe was not caused by a general cytotoxic effect because rHBe treatment stimulated low levels of cytokine and chemokine production by monocytes and it promoted neutrophil survival. Since the recruitment of monocytes and neutrophils to the infected site is crucial for the initiation of inflammation, HBeAg may modulate innate immune responses by diminishing the respiratory burst and migration of monocytes and neutrophils, which might interfere with the subsequent innate and adaptive immune responses against HBV, leading to the establishment of chronic infection.

Regulation of B7-H1 expression on peripheral monocytes and IFN-γ secretion in T lymphocytes by HBeAg

This study is to observe the expression of B7-H1, PD-1 and TLR2 on peripheral blood monocytes (PBMCs) regulated by HBeAg in chronic hepatitis B (CHB), and to illustrate the relation between HBeAg and persistent infection of HBV. In both CHB patients and healthy controls, the expression of B7-H7 was significantly increased on CD14(+) monocytes incubated with HBeAg, while that of TLR2 was significantly reduced; the expression of specific IFN-γ was significantly decreased in CD3(+)CD4(+) T lymphocytes incubated with HBeAg, while IL-6 and IL-10 in conditioned media were significantly increased. HBeAg is able to significantly up-regulate B7-H1, down-regulate TLR2 on monocytes, reduce IFN-γ produced by T lymphocytes and increase Th2-type cytokines secretion. These findings suggest that HBeAg suppresses the specific cellular immunity to clear the virus, and eventually lead to immune tolerance to HBV infection. Therefore, HBeAg plays an important role in immune suppression in chronic HBV patients.

Hepatitis B virus genetic variants: biological properties and clinical implications

Hepatitis B virus (HBV) causes a chronic infection in 350 million people worldwide and greatly increases the risk of liver cirrhosis and hepatocellular carcinoma. The majority of chronic HBV carriers live in Asia. HBV can be divided into eight genotypes with unique geographic distributions. Mutations accumulate during chronic infection or in response to external pressure. Because HBV is an RNA-DNA virus the emergence of drug resistance and vaccine escape mutants has become an important clinical and public health concern. Here, we provide an overview of the molecular biology of the HBV life cycle and an evaluation of the changing role of hepatitis B e antigen (HBeAg) at different stages of infection. The impact of viral genotypes and mutations/deletions in the precore, core promoter, preS, and S gene on the establishment of chronic infection, development of fulminant hepatitis and liver cancer is discussed. Because HBV is prone to mutations, the biological properties of drug-resistant and vaccine escape mutants are also explored.

Antigenic switching of hepatitis B virus by alternative dimerization of the capsid protein

Chronic hepatitis B virus (HBV) infection afflicts millions worldwide with cirrhosis and liver cancer. HBV e-antigen (HBeAg), a clinical marker for disease severity, is a nonparticulate variant of the protein (core antigen, HBcAg) that forms the building-blocks of capsids. HBeAg is not required for virion production, but is implicated in establishing immune tolerance and chronic infection. Here, we report the crystal structure of HBeAg, which clarifies how the short N-terminal propeptide of HBeAg induces a radically altered mode of dimerization relative to HBcAg (∼140° rotation), locked into place through formation of intramolecular disulfide bridges. This structural switch precludes capsid assembly and engenders a distinct antigenic repertoire, explaining why the two antigens are cross-reactive at the T cell level (through sequence identity) but not at the B cell level (through conformation). The structure offers insight into how HBeAg may establish immune tolerance for HBcAg while evading its robust immunogenicity.

A high level of mutation tolerance in the multifunctional sequence encoding the RNA encapsidation signal of an avian hepatitis B virus and slow evolution rate revealed by in vivo infection

In all hepadnaviruses, protein-primed reverse transcription of the pregenomic RNA (pgRNA) is initiated by binding of the viral polymerase, P protein, to the ε RNA element. Universally, ε consists of a lower stem and an upper stem, separated by a bulge, and an apical loop. Complex formation triggers pgRNA encapsidation and the ε-templated synthesis of a DNA oligonucleotide (priming) that serves to generate minus-strand DNA. In vitro systems for duck hepatitis B virus (DHBV) yielded important insights into the priming mechanism, yet their relevance in infection is largely unexplored. Moreover, additional functions encoded in the DHBV ε (Dε) sequence could affect in vivo fitness. We therefore assessed the in vivo performances of five recombinant DHBVs bearing multiple mutations in the upper Dε stem. Three variants with only modestly reduced in vitro replication competence established chronic infection in ducks. From one variant but not another, three adapted new variants emerged upon passaging, as demonstrated by increased relative fitness in coinfections with wild-type DHBV. All three showed enhanced priming and replication competence in vitro, and in one, DHBV e antigen (DHBeAg) production was restored. Pronounced impacts on other Dε functions were not detected; however, gradual, synergistic contributions to overall performance are suggested by the fact of none of the variants reaching the in vivo fitness of wild-type virus. These data shed more light on the P-Dε interaction, define important criteria for the design of future in vivo evolution experiments, and suggest that the upper Dε stem sequences provided an evolutionary playground for DHBV to optimize in vivo fitness.

Role of the propeptide in controlling conformation and assembly state of hepatitis B virus e-antigen

Hepatitis B virus "e-antigen" (HBeAg) is thought to be a soluble dimeric protein that is associated with chronic infection. It shares 149 residues with the viral capsid protein "core-antigen" (HBcAg), but has an additional 10-residue, hydrophobic, cysteine-containing amino-terminal propeptide whose presence correlates with physical, serological, and immunological differences between the two proteins. In HBcAg dimers, the subunits pair by forming a four-helix bundle stabilized by an intermolecular disulfide bond. The structure of HBeAg is probably similar but, instead, has two intramolecular disulfide bonds involving the propeptide. To compare the proteins directly and thereby clarify the role of the propeptide, we identified mutations and solution conditions that render both proteins as either soluble dimers or assembled capsids. Thermally induced unfolding monitored by circular dichroism, and electrophoresis of oxidized and reduced dimers, showed that the propeptide has a destabilizing effect and that the intramolecular disulfide bond forms preferentially and blocks the formation of the intermolecular disulfide bond that otherwise stabilizes the dimer. The HBeAg capsids are less regular than the HBcAg capsids; nevertheless, cryo-electron microscopy reconstructions confirm that they are constructed of dimers resembling those of HBcAg capsids. In them, a portion of the propeptide is visible near the dimer interface, suggesting that it intercalates there, consistent with the known formation of a disulfide bond between C(-7) in the propeptide and C61 in the dimer interface. However, this intercalation distorts the dimer into an assembly-reluctant conformation.

Bioinformatic analysis of the hepadnavirus e-antigen and its precursor identifies remarkable sequence conservation in all orthohepadnaviruses

The hepatitis B e-antigen (HBeAg) is a non-particulate secretory protein expressed by all viruses within the family Hepadnaviridae. It is not essential for viral assembly or replication but is important for establishment of persistent infection in vivo. Although the exact mechanism(s) by which the HBeAg manifests chronicity are unclear, the HBeAg elicits both humoral and cell-mediated immunity, down-regulates the innate immune response to infection, as well as functioning as a T cell tolerogen and regulating the immune response to the intracellular nucleocapsid. A bioinformatics approach was used to show that the HBeAg and precursory genetic codes share remarkable sequence conservation in all mammalian-infecting hepadnaviruses, irrespective of host, genotype, or geographic origin. Whilst much of this sequence conservation was within key immunomodulatory epitopes, highest conservation was observed at the unique HBeAg N-terminus, suggesting this sequence in particular may play an important role in HBeAg function.

Characterization of genotype-specific carboxyl-terminal cleavage sites of hepatitis B virus e antigen precursor and identification of furin as the candidate enzyme

Hepatitis B e antigen (HBeAg) is a secreted version of hepatitis B virus (HBV) core protein that promotes immune tolerance and persistent infection. It is derived from a translation product of the precore/core gene by two proteolytic cleavage events: removal of the amino-terminal signal peptide and removal of the carboxyl-terminal arginine-rich sequence. Four RXXR motifs are present at the carboxyl terminus of the HBeAg precursor, with the first two fused as (151)RRGRSPR(157). Genotype A possesses two extra amino acids at the first motif ((151)RRDRGRSPR(159)), which weakens the first motif and separates it from the second one. Western blot analysis of patient sera revealed a single HBeAg form for genotypes B to D but two additional forms of larger sizes for genotype A. Site-directed mutagenesis and transfection experiments with human hepatoma cell lines indicated that HBeAg of genotype B is derived from cleavage at the first ((151)RRGR(154)) motif. The major HBeAg form of genotype A corresponds to cleavage at the second ((156)RSPR(159)) motif, and the other two forms are cleavage products of the first ((151)RRDR(154)) and third ((166)RRRR(169)) motifs, respectively. Only the cleavage product of the third motif of genotype A was observed in furin-deficient LoVo cells, and an inhibitor of furin-like proprotein convertases blocked cleavage of the first and second motifs in human hepatoma cells. In conclusion, our study reveals genotypic differences in HBeAg processing and implicates furin as the major enzyme involved in the cleavage of the first and second RXXR motifs.

Inhibition of duck hepatitis B virus infection of liver cells by combined treatment with viral e antigen and carbohydrates

The e antigen (eAg) of duck hepatitis B virus (DHBV) is a glycosylated secretory protein with a currently unknown function. We concentrated this antigen from the supernatants of persistently infected primary duck liver cell cultures by ammonium sulphate precipitation, adsorption chromatography over concanavalin A Sepharose, preparative isoelectric focusing and molecular sieve chromatography. The combined treatment of duck liver cells with DHBV eAg (DHBe) concentrate and alpha-methyl-d-mannopyranoside strongly inhibited DHBV replication at de novo infection. When DHBe was added to non-infected primary duck liver cells, it was found to be associated with liver sinusoidal endothelial cells. This binding could be inhibited by the addition of alpha-methyl-d-mannopyranoside and other sugar molecules. The inhibitory effect of DHBe on infection could play a role in maintaining viral persistence.

Pre-P is a secreted glycoprotein encoded as an N-terminal extension of the duck hepatitis B virus polymerase gene

The duck hepatitis B virus (DHBV) pregenomic RNA is a bicistronic mRNA encoding the core and polymerase proteins. Thirteen AUGs (C2 to C14) and 10 stop codons (S1 to S10) are located between the C1 AUG for the core protein and the P1 AUG that initiates polymerase translation. We previously found that the translation of the DHBV polymerase is initiated by ribosomal shunting. Here, we assessed the biosynthetic events after shunting. Translation of the polymerase open reading frame was found to initiate at the C13, C14, and P1 AUGs. Initiation at the C13 AUG occurred through ribosomal shunting because translation from this codon was cap dependent but was insensitive to blocking ribosomal scanning internally in the message. C13 and C14 are in frame with P1, and translation from these upstream start codons led to the production of larger isoforms of P. We named these isoforms "pre-P" by analogy to the pre-C and pre-S regions of the core and surface antigen open reading frames. Pre-P was produced in DHBV16 and AusDHBV-infected duck liver and was predicted to exist in 80% of avian hepadnavirus strains. Pre-P was not encapsidated into DHBV core particles, and the viable strain DHBV3 cannot make pre-P, so it is not essential for viral replication. Surprisingly, we found that pre-P is an N-linked glycoprotein that is secreted into the medium of cultured cells. These data indicate that DHBV produces an additional protein that has not been previously reported. Identifying the role of pre-P may improve our understanding of the biology of DHBV infection.

A mechanism to explain the selection of the hepatitis e antigen-negative mutant during chronic hepatitis B virus infection

Hepatitis B virus (HBV) expresses two structural forms of the nucleoprotein, the intracellular nucleocapsid (hepatitis core antigen [HBcAg]) and the secreted nonparticulate form (hepatitis e antigen [HBeAg]). The aim of this study was to evaluate the ability of HBcAg- and HBeAg-specific genetic immunogens to induce HBc/HBeAg-specific CD4(+)/CD8(+) T-cell immune responses and the potential to induce liver injury in HBV-transgenic (Tg) mice. Both the HBcAg- and HBeAg-specific plasmids primed comparable immune responses. Both CD4(+) and CD8(+) T cells were important for priming/effector functions of HBc/HBeAg-specific cytotoxic T-lymphocyte (CTL) responses. However, a unique two-step immunization protocol was necessary to elicit maximal CTL priming. Genetic vaccination did not prime CTLs in HBe- or HBc/HBeAg-dbl-Tg mice but elicited a weak CTL response in HBcAg-Tg mice. When HBc/HBeAg-specific CTLs were adoptively transferred into HBc-, HBe-, and HBc/HBeAg-dbl-Tg mice, the durations of the liver injury and inflammation were significantly greater in HBeAg-Tg recipient mice than in HBcAg-Tg mice. Importantly, liver injury in HBc/HBeAg-dbl-Tg mice was similar to the injury observed in HBeAg-Tg mice. Loss of HBeAg synthesis commonly occurs during chronic HBV infection; however, the mechanism of selection of HBeAg-negative variants is unknown. The finding that hepatocytes expressing wild-type HBV (containing both HBcAg and HBeAg) are more susceptible to CTL-mediated clearance than hepatocytes expressing only HBcAg suggest that the HBeAg-negative variant may have a selective advantage over wild-type HBV within the livers of patients with chronic infection during an immune response and may represent a CTL escape mutant.

The hepatitis B virus precore protein is retrotransported from endoplasmic reticulum (ER) to cytosol through the ER-associated degradation pathway

The hepatitis B virus precore protein is closely related to the nucleocapsid core protein but is processed distinctly in the cell and plays a different role in the viral cycle. Precore is addressed to the endoplasmic reticulum (ER) through a signal peptide, and the form present in the ER is the P22 protein. P22 is then cleaved in its C-terminal part to be secreted as HBe antigen. In addition, a cytosolic form of 22 kDa less characterized has been observed. Precore gene was shown to be implicated in viral persistence, but until now, the actual protein species involved has not been determined. Our work focuses on the cytosolic form of precore. Using human cells expressing precore and a convenient fractionation assay, we demonstrated that the cytosolic form is identical to the ER form and retrotransported in the cytoplasm through the ER-associated degradation pathway. This cellular machinery translocates misfolded proteins to the cytoplasm, where they are ubiquitinated on lysine residues and degraded by proteasome. We showed that precore escapes proteasome due to its low lysine content and accumulates in the cytosol. The role of this retrotransport was investigated. In the presence of precore, we found a specific redistribution of the Grp78/BiP chaperone protein to cytosol and demonstrated a specific interaction between precore and Grp78/BiP. Altogether, these data support the idea that the hepatitis B virus develops a strategy to take advantage of the ER-associated degradation pathway, allowing distinct subcellular localization and probably distinct roles for the viral precore protein.

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.

Impact of viral genotypes and naturally occurring mutations on biological properties of hepatitis B virus

Hepatitis B patients worldwide are infected with different viral genotypes. Within the same individual the dominant viral species evolves over the course of chronic infection to generate viral variants or mutants. The mutations, often selected by the host immune response or antiviral therapy, are sometimes restricted by viral genotypes. We are interested in characterizing mutations that affect the expression of hepatitis B e-antigen (HBeAg), a protein with a large effect on duration of infection and severity of liver diseases. HBeAg is encoded by the precore region in addition to the core gene. Core promoter mutations reduce HBeAg expression at the transcriptional level. We found that the hot spot mutations (A1762T/G1764A) only mildly reduced HBeAg expression and enhanced genome replication, while incorporation of additional core promoter mutations intensified both phenotypes. At the step of translation, a G1896A nonsense mutation in the precore region abolishes HBeAg expression. We first reportedthat the G1896A mutation rarely occurred in genotype A. Subsequent studies by others established the role of polymorphism at nucleotide 1858, rather than genotype, as the determinant for the G1896A mutation. Conversion of the precore/core protein to HBeAg requires proteolytic removal of both the amino and carboxy termini, and a (151)RRGR(154) motif has been implicated as the carboxy terminal cleavage site. In this regard, genotype A is unique in possessing a dipeptide insertion that expands the motif into (151)RRDRGR(156). We found that genotype A is cleaved primarily at R156, generating a mature HBeAg that is two amino acids longer than HBeAg from other genotypes. There are different avenues whereby HBeAg expression or its antigenicity can be modulated by viral genotype and naturally occurring mutations.

Avian hepatitis B viruses: Molecular and cellular biology, phylogenesis, and host tropism

The human hepatitis B virus (HBV) and the duck hepatitis B virus (DHBV) share several fundamental features. Both viruses have a partially double-stranded DNA genome that is replicated via a RNA intermediate and the coding open reading frames (ORFs) overlap extensively. In addition, the genomic and structural organization, as well as replication and biological characteristics, are very similar in both viruses. Most of the key features of hepadnaviral infection were first discovered in the DHBV model system and subsequently confirmed for HBV. There are, however, several differences between human HBV and DHBV. This review will focus on the molecular and cellular biology, evolution, and host adaptation of the avian hepatitis B viruses with particular emphasis on DHBV as a model system.

Overexpression of STAT-1 by adenoviral gene transfer does not inhibit hepatitis B virus replication

OBJECTIVES

Interferons are known to inhibit the replication of hepatitis B viruses (HBV) in several animal models in vitro and in vivo as well in humans. The STAT-1 protein plays a central role in the biological activity of both type I and type II interferons. The lack of functional STAT-1 renders cells and organisms susceptible to bacterial and viral infectious agents. We analysed whether the overexpression of STAT-1 protein enhances the biological interferon response and whether it elicits antiviral activity against HBV in vitro.

METHODS

To achieve an efficient STAT-1 overexpression in primary liver cells and hepatoma cells, we generated a recombinant, replication-deficient adenovirus expressing human STAT-1 (Adv-STAT-1). We analysed whether the overexpression of STAT-1 inhibits the replication of duck HBV and human HBV in vitro using Western blot analysis, the immunofluorescence of viral proteins and quantification of HBV-DNA copies, respectively.

RESULTS

In the duck model of HBV infection the overexpression of STAT-1 neither inhibited an established infection nor prevented the establishment of duck HBV replication when administered simultaneously with Adv-STAT-1. These observations were confirmed in an in-vitro model of human HBV infection using the human hepatoma cell line HepG2.2.15, which continuously replicates HBV.

CONCLUSION

These data demonstrate that the over-expression of STAT-1 alone is not sufficient to strengthen the biological response of interferon as an antiviral agent.

Relationship of genotypes of hepatitis B virus to mutations, disease progression and response to antiviral therapy

Phylogenetic analysis has led to the classification of hepatitis B virus into eight genotypes, designated A to H. The genotypes have differences in biological properties and show heterogeneity in their global distribution. These attributes of the genotypes may account not only for differences in the prevalence of hepatitis B virus mutants in various geographic regions, but also be responsible for differences in the clinical outcome and response to antiviral treatment in different population groups.

Immune Tolerance Split between Hepatitis B Virus Precore and Core Proteins

The function of the hepatitis B virus (HBV) precore or HBeAg is largely unknown because it is not required for viral assembly, infection, or replication. However, the HBeAg does appear to play a role in viral persistence. It has been suggested that the HBeAg may promote HBV chronicity by functioning as an immunoregulatory protein. As a model of chronic HBeAg exposure and to examine the tolerogenic potential of the HBV precore and core (HBcAg) proteins, HBc/HBeAg-transgenic (Tg) mice crossed with T cell receptor (TCR)-Tg mice expressing receptors for the HBc/HBeAgs (i.e., TCR-antigen double-Tg pairs) were produced. This study revealed three phenotypes of HBe/HBcAg-specific T-cell tolerance: (i) profound T-cell tolerance most likely mediated by clonal deletion, (ii) T-cell clonal ignorance, and (iii) nondeletional T-cell tolerance mediated by clonal anergy and dependent on the structure, location, and concentration of the tolerogen. The secreted HBeAg is significantly more efficient than the intracellular HBcAg at eliciting T-cell tolerance. The split T-cell tolerance between the HBeAg and the HBcAg and the clonal heterogeneity of HBc/HBeAg-specific T-cell tolerance may have significant implications for natural HBV infection and especially for precore-negative chronic hepatitis.

Envelopment of the hepatitis B virus nucleocapsid

The hepatitis B virus (HBV) is an enveloped DNA virus with an icosahedral capsid replicating via reverse transcription. The crystal structure of the capsid is known. It has a diameter of 36 nm and is formed by one protein species (C protein). The viral envelope contains three different coterminal proteins (S, M, and L proteins) spanning the membrane several times. These proteins are not only released from infected cells as components of the viral envelope but in 10,000-fold excess as subviral lipoprotein particles with a diameter of 22 nm containing no capsid. Assembly of the capsid occurs in the cytosol and results in packaging of a 3.5 kb RNA molecule together with viral and cellular factors. This newly formed capsid cannot be enveloped. Rather, synthesis of the viral DNA genome in the lumen of the capsid by reverse transcription is required to induce a budding competent state. Envelopment then takes place at an intracellular membrane of the pre-Golgi compartment. The S and the L protein, but not the M protein, is required for this process. The L protein forms two different transmembrane topologies. The isoform exposing the N-terminal part at the cytosolic side of the membrane is essential for budding. In this domain, a 22 amino acid (aa) long linear stretch has been mapped genetically to play a vital role in the morphogenetic process. This domain probably mediates the contact to the capsid. A second matrix domain was mapped to the cytosolic loop of the S protein. A similar genetic approach identified two small areas on the capsid surface, which might interact with the envelope proteins during envelopment.

Hepatitis B Virus e Antigen Variants

More than 300 million people worldwide are chronically infected with hepatitis B virus (HBV). Considering the very short generation time for a virus, and the high error rate associated with the reverse transcription step of HBV replication, decades of HBV infection are probably equivalent to million years of human evolution. The most important selective force during the natural course of HBV infection appears to be the immune response. The development of anti-HBe antibody in hepatitis B patients usually correlates with reduction of HBV viremia. As a consequence, escape mutants of anti-HBe are selected. The core promoter mutants express less HBe antigen (HBeAg) through transcriptional down regulation, while precore mutants express truncated products. We recently identified additional mutations that modulate HBeAg translation initiation, proteolytic cleavage, and secondary structure maintenance through a disulfide bond. The core promoter mutants have been associated with the development of fulminant hepatitis during acute infection and liver cancer during chronic infection. Consistent with their enhanced pathogenicity, core promoter mutants were found to replicate at up to 10-fold higher levels in transfected human hepatoma cells than the wild-type virus. Moreover, some core promoter mutants are impaired in virion secretion due to missense mutations in the envelope gene. These virological properties may help explain enhanced pathogenicity of core promoter mutants in vivo.

A function of the hepatitis B virus precore protein is to regulate the immune response to the core antigen

A unique characteristic of the hepatitis B virus is the production of a secreted form (precore or HBeAg) of the structural nucleocapsid (core or HBcAg). By using T cell receptor (TCR) transgenic (Tg) and TCR x HBc/HBeAg double- and triple-Tg pairs, we demonstrate that HBeAg elicits T cell tolerance, whereas HBcAg is nontolerogenic in this system. In fact, TCR x HBc double-Tg mice spontaneously seroconvert to IgG anti-HBc positivity at an early age. However, the presence of HBeAg in the serum of TCR x HBc x HBe triple-Tg mice prevents anti-HBc seroconversion. HBeAg mediates its immunoregulatory effect by eliciting tolerance in HBc/HBeAg-specific T cells. The results suggest that hepadnaviruses have retained a secretory form of the nucleoprotein because it functions as a T cell tolerogen and regulates the immune response to the intracellular nucleocapsid. This HBeAg-mediated immune regulation may predispose to chronicity during perinatal infections and prevent severe liver injury during adult infections.

Exploring the biological basis of hepatitis B e antigen in hepatitis B virus infection

The function of the hepatitis B e antigen (HBeAg) is largely unknown because it is not required for viral assembly, replication, or infection. In this report we chronicle clinical and experimental studies in an attempt to understand the role of HBeAg in natural infection. These studies largely have focused on clinical-pathologic features of HBeAg-negative variants in acute and chronic HBV infection, mutational analysis in animal models of hepadnavirus infection, and the use of transgenic murine models. The clinical and experimental data suggest that serum HBeAg may serve an immunoregulatory role in natural infection. To the contrary, cytosolic HBeAg serves as a target for the inflammatory immune response. These dual roles of the HBeAg and its ability to activate or tolerize T cells show the complexity of the interactions between the HBeAg and the host during HBV infection.

Hepatitis B virus downregulates the human interferon-inducible MxA promoter through direct interaction of precore/core proteins

The human MxA protein is an interferon (IFN)-inducible GTPase with proven antiviral activity against diverse viruses. IFN responsiveness is impaired in chronic hepatitis B virus (HBV) infection. Accordingly, initial experiments showed that, in contrast to parental HepG2 cells, when HepG2-derived 2.2.15 liver cells carrying the HBV genome were treated with IFN, they could not synthesize the MxA protein. Furthermore, MxA expression was reduced in HepG2 cells transiently transfected with the HBV genome. To assess whether HBV-encoded precore/core (preC/C) proteins interact with the IFN-signalling pathway, HepG2, Chang and HeLa cells were transfected with preC/C expression plasmids; the levels of signal transducers remained unaffected. Next, full-length and deletion mutants fused to the CAT reporter gene were tested to investigate whether MxA inhibition occurs at the promoter level. In co-transfection experiments, IFN-induced CAT activity was inhibited by preC/C expression in a dose-dependent manner. Analysis of deletion mutants showed that the region affected by the preC/C proteins comprises IFN-stimulated response elements 2 and 3, upstream of the putative start codon of the MxA promoter. In addition, HBV preC/C proteins interacted directly with the MxA promoter, as shown by electrophoretic mobility shift assays. These results demonstrate a mechanism that HBV probably uses to downregulate an element of the IFN-induced host antiviral responses, which accounts for the impairment observed in HBV-infected patients.

Genome replication, virion secretion, and e antigen expression of naturally occurring hepatitis B virus core promoter mutants

The core promoter mutants of hepatitis B virus (HBV) emerge as the dominant viral population at the late HBeAg and the anti-HBe stages of HBV infection, with the A1762T/G1764A substitutions as the hotspot mutations. The double core promoter mutations were found by many investigators to moderately enhance viral genome replication and reduce hepatitis B e antigen (HBeAg) expression. A much higher replication capacity was reported for a naturally occurring core promoter mutant implicated in the outbreak of fulminant hepatitis, which was caused by the neighboring C1766T/T1768A mutations instead. To systemically study the biological properties of naturally occurring core promoter mutants, we amplified full-length HBV genomes by PCR from sera of HBeAg(+) individuals infected with genotype A. All 12 HBV genomes derived from highly viremic sera (5 x 10(9) to 5.7 x 10(9) copies of viral genome/ml) harbored wild-type core promoter sequence, whereas 37 of 43 clones from low-viremia samples (0.2 x 10(7) to 4.6 x 10(7) copies/ml) were core promoter mutants. Of the 11 wild-type genomes and 14 core promoter mutants analyzed by transfection experiments in human hepatoma cell lines, 6 core promoter mutants but none of the wild-type genomes replicated at high levels. All had 1762/1764 mutations and an additional substitution at position 1753 (T to C), at position 1766 (C to T), or both. Moreover, these HBV clones varied greatly in their ability to secrete enveloped viral particles irrespective of the presence of core promoter mutations. High-replication clones with 1762/1764/1766 or 1753/1762/1764/1766 mutations expressed very low levels of HBeAg, whereas high-replication clones with 1753/1762/1764 triple mutations expressed high levels of HBeAg. Experiments with site-directed mutants revealed that both 1762/1764/1766 and 1753/1762/1764/1766 mutations conferred significantly higher viral replication and lower HBeAg expression than 1762/1764 mutations alone, whereas the 1753/1762/1764 triple mutant displayed only mild reduction in HBeAg expression similar to the 1762/1764 mutant. Thus, core promoter mutations other than those at positions 1762 and 1764 can have major impact on viral DNA replication and HBeAg expression.

New hepatitis B virus of cranes that has an unexpected broad host range

All hepadnaviruses known so far have a very limited host range, restricted to their natural hosts and a few closely related species. This is thought to be due mainly to sequence divergence in the large envelope protein and species-specific differences in host components essential for virus propagation. Here we report an infection of cranes with a novel hepadnavirus, designated CHBV, that has an unexpectedly broad host range and is only distantly evolutionarily related to avihepadnaviruses of related hosts. Direct DNA sequencing of amplified CHBV DNA as well a sequencing of cloned viral genomes revealed that CHBV is most closely related to, although distinct from, Ross' goose hepatitis B virus (RGHBV) and slightly less closely related to duck hepatitis B virus (DHBV). Phylogenetically, cranes are very distant from geese and ducks and are most closely related to herons and storks. Naturally occurring hepadnaviruses in the last two species are highly divergent in sequence from RGHBV and DHBV and do not infect ducks or do so only marginally. In contrast, CHBV from crane sera and recombinant CHBV produced from LMH cells infected primary duck hepatocytes almost as efficiently as DHBV did. This is the first report of a rather broad host range of an avihepadnavirus. Our data imply either usage of similar or identical entry pathways and receptors by DHBV and CHBV, unusual host and virus adaptation mechanisms, or divergent evolution of the host genomes and cellular components required for virus propagation.

Proteolytic processing of the hepatitis B virus e antigen precursor. Cleavage at two furin consensus sequences

The Hepatitis B virus P22 protein is a nonstructural protein that is the precursor of the 17-kDa secreted e antigen (HBeAg). The mature HBeAg is obtained after the removal of the C-terminal region of P22, a process which involves a proprotein convertase. Our studies show first that the protease could cleave P22 at the C-terminal side of Arg(167) or Arg(154) and second, that the maturation process can be either done in one step or in two steps with the generation of a processing intermediate (P20). Our data also demonstrate that the removal of the P22 C terminus, which occurs mainly in the trans-Golgi network, can also be achieved after exocytosis. Keeping in mind this characteristic and the amino acid sequence of the cleavage sites, we concluded that furin is involved in the maturation of the HBeAg. In addition, we show that in our experimental system, the HBeAg is a 164-amino acid protein and not a 159-amino acid protein as previously reported.

Structural and functional heterogeneity of naturally occurring hepatitis B virus variants

Most organisms have developed sophisticated machineries to preserve their genomic integrity. On the contrary hepatitis B virus (HBV), like a lot of other viruses can undergo rapid and drastic sequence changes, especially if the virus has to cope with natural or therapy induced antiviral mechanisms in the host. Here, we try to summarize possible implications for the molecular pathogenesis of HBV based on the extensive research on the genetic variants of HBV.

Transfer of hepatitis B virus genome by adenovirus vectors into cultured cells and mice: crossing the species barrier

For the study of hepatitis B virus infection, no permissive cell line or small animal is available. Stably transfected cell lines and transgenic mice which contain hepadnavirus genomes produce virus, but--unlike in natural infection--from an integrated viral transcription template. To transfer hepadnavirus genomes across the species barrier, we developed adenovirus vectors in which 1.3-fold-overlength human and duck hepatitis B virus genomes were inserted. The adenovirus-mediated genome transfer efficiently initiated hepadnavirus replication from an extrachromosomal template in established cell lines, in primary hepatocytes from various species, and in the livers of mice. Following the transfer, hepatitis B virus proteins, genomic RNA, and all replicative DNA intermediates were detected. Detection of covalently closed circular DNA in hepatoma cell lines and in primary hepatocytes indicated that an intracellular replication cycle independent from the transferred linear viral genome was established. High-titer hepatitis B virions were released into the culture medium of hepatoma cells and the various primary hepatocytes. In addition, infectious virions were secreted into the sera of mice. In conclusion, adenovirus-mediated genome transfer initiated efficient hepatitis B virus replication in cultured liver cells and in the experimental animals from an extrachromosomal template. This will allow development of small-animal systems of hepatitis B virus infection and will facilitate study of pathogenicity of wild-type and mutant viruses as well as of virus-host interaction and new therapeutic approaches.

Sequence comparison of an Australian duck hepatitis B virus strain with other avian hepadnaviruses

The genome of an Australian strain of duck hepatitis B virus (AusDHBV) was cloned from a pool of congenitally DHBV-infected-duck serum, fully sequenced and found by phylogenetic analyses to belong to the 'Chinese' DHBV branch of the avian hepadnaviruses. Sequencing of the Pre-S/S gene of four additional AusDHBV clones demonstrated that the original clone (pBL4.8) was representative of the virus present in the pool, and a head-to-tail dimer of the clone was infectious when inoculated into newly hatched ducks. When the published sequences of 20 avian hepadnaviruses were compared, substitutions or deletions in the polymerase (POL) gene were most frequent in the 500 nt segment encoding the 'spacer' domain that overlaps with the Pre-S domain of the Pre-S/S gene in a different reading frame. In contrast, substitutions and deletions were rare within the adjacent segment that encodes the reverse transcriptase domain of the POL protein and the S domain of the envelope protein, presumably because they are more often deleterious.

Rapid detection of genotypes and mutations in the pre-core promoter and the pre-core region of hepatitis B virus genome: correlation with viral persistence and disease severity

BACKGROUND/AIMS

We aimed to clarify the clinical relevance of hepatitis B virus pre-core mutant detection in patients with chronic hepatitis B using a newly developed assay.

METHODS

Viral genotypes and pre-core mutations were studied in relation to viral persistence and liver disease severity using INNO-LiPA methodology. The study group included 151 patients with chronic hepatitis B, 85 positive for HBeAg (group I) and 66 positive for anti-HBe (group II).

RESULTS

The prevalence of viral genotypes in group I was: 64% A, 1% B, 15% C, 19% D, 0% E, 0% F and in group II: 39% A, 0% B, 2% C, 56% D, 2% E, 2% F (p<0.001). The prevalence of mutations at pre-core codon 28 (M2) was lower in group I (5%) than in group II (64%) (p<0.001). The prevalence of pre-core promoter mutations was also lower in group I (21%) than in group II (61%) (p<0.001). M2 mutations were more frequently detected in genotype D than in genotype A (p<0.001), while the other mutations were not influenced by viral genotype. Serum HBV DNA levels were significantly lower in group II versus group I (p<0.001), and in patients with any of the pre-core mutations versus wild-type sequence (p<0.01). Although cirrhosis was more frequent in group II (37%) versus group I (22%) and in patients with either one of the pre-core mutation (31%) versus wild-type sequence (25%), there was no statistical difference in liver severity assessed by ALT levels and Knodell score.

CONCLUSION

Pre-core mutants, whose molecular pattern is strongly dependent on viral genotypes, are associated with viral persistence in anti-HBe positive patients with ongoing chronic hepatitis B. The availability of this rapid assay should allow a precise monitoring of viral pre-core mutants during the course of chronic hepatitis B.

H-2(d) mice born to and reared by HBeAg-transgenic mothers do not develop T cell tolerance toward the hepatitis B virus core gene products

The function of the secretory core gene product (HBeAg) of the human hepatitis B virus (HBV) is unknown. It has been proposed that this protein may be passed from the mother to her offspring at the perinatal stage where it might induce immune tolerance. In a previous study we have shown that the murine placenta presents an efficient barrier for the HBe protein and that H-2(b) mice born to HBeAg-positive transgenic mothers do not develop tolerance of specific cytotoxic T cells. In the present work we demonstrate that transgenic mice expressing high serum levels of HBeAg secrete only small amounts of this protein into their milk and excrete minute amounts of the viral gene product in their urine. Furthermore, it is shown that nontransgenic H-2(d) mice born to and reared by HBeAg-positive mothers exhibit a reactivity of HBc/eAg-specific CD4(+) Th cells and CD8(+) cytotoxic T cells comparable to that of normal isogenic control mice. In accordance with this observation the humoral immune responses directed against the HBeAg were comparable between these two groups of animals. This finding indicates that H-2(d) mice potentially exposed to small amounts of maternal HBeAg transferred by the transplacental, lactogenic, or renal route do not develop tolerance toward the HBV core gene products. These data challenge the hypothesis that a potential function of the HBeAg may be to operate as a tolerogen at the perinatal developmental stage.

Endotoxin stimulates liver macrophages to release mediators that inhibit an early step in hepadnavirus replication

Hepadnaviruses are known to be sensitive to various extracellular mediators. Therefore, bacterial endotoxin, which induces the secretion of proinflammatory mediators in the liver, was studied for its effect on hepadnavirus infection in vitro using the duck hepatitis B virus (DHBV) model. In initial experiments, endotoxin was shown to inhibit DHBV replication in primary duck hepatocyte cultures prepared by standard collagenase perfusion. As a primary endotoxin target, hepatic nonparenchymal cells (NPC) contaminating primary hepatocyte cultures, and among these probably macrophages (Kupffer cells), were identified to secrete polypeptide mediators into the cell culture medium. When added during DHBV infection, these mediators elicited the principal antiviral effect in a dose-dependent fashion. On the molecular level, they inhibited accumulation of viral proteins as well as amplification of the nuclear extrachromosomal DHBV DNA templates. In hepatocytes with an established DHBV infection, DHBV protein and progeny virus production was inhibited while the levels of established nuclear DHBV DNA templates and viral transcripts remained unaffected. Finally, in hepatocytes infected with a replication-deficient recombinant DHBV-green fluorescent protein (GFP) virus, the endotoxin-induced mediators markedly reduced GFP expression from chimeric DHBV-GFP transcripts, indicating that the major effect is at a level of translation of viral RNAs. Taken together, the data obtained demonstrate that antiviral mediators, and among these the cytokines alpha interferon (IFN-alpha) and IFN-gamma, are released from hepatic NPC, most probably liver macrophages, upon endotoxin stimulation; furthermore, these mediators act at a posttranscriptional step of hepadnavirus replication.

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

Hepatitis B viruses specifically target the liver, where they efficiently infect quiescent hepatocytes. Here we show that human and avian hepatitis B viruses can be converted into vectors for liver-directed gene transfer. These vectors allow hepatocyte-specific expression of a green fluorescent protein in vitro and in vivo. Moreover, when used to transduce a type I interferon gene, expression of interferon efficiently suppresses wild-type virus replication in the duck model of hepatitis B virus infection. These data suggest local cytokine production after hepatitis-B-virus-mediated gene transfer as a promising concept for the treatment of acquired liver diseases, including chronic hepatitis B.

Enrichment of a precore-minus mutant of duck hepatitis B virus in experimental mixed infections

A precore-deficient mutant of duck hepatitis B virus (DHBV) produced by site-directed mutagenesis was tested for its ability to compete with wild-type virus in a mixed infection of 3-day-old ducklings. The mutation was shown to produce a cis-acting defect, resulting in a replication rate that was about one-half that of wild-type virus. Accordingly, wild-type virus was rapidly selected during the spread of infection. During the chronic phase of the infection, however, two selection patterns were seen. In 4 of 10 ducks, the wild-type virus slowly replaced the precore mutant. In another four ducks, the precore mutant virus slowly replaced the wild-type virus. In the remaining two ducklings, ratios of wild-type and precore mutant virus fluctuated, with wild-type virus slowly predominating. The replacement of wild-type virus was not due to the emergence of a rapidly replicating variant of the precore mutant, since genomes cloned from the infected ducks retained their original replication defect. Replacement of wild-type virus, however, correlated with elevated anti-core antibody titers, which continued to increase with time. The selection of a precore-negative strain of DHBV may be analogous to the selection for precore mutants of HBV during chronic hepatitis in humans.

The C terminus of the hepatitis B virus e antigen precursor is required for a tunicamycin-sensitive step that promotes efficient secretion of the antigen

The Hepatitis B virus encodes the secreted e antigen (HBe) whose function in the viral life cycle is unknown. HBe derives from a 25-kDa precursor that is directed to the secretory pathway. After cleavage of the signal sequence, the resulting 22-kDa protein (P22) is processed in a post-endoplasmic reticulum compartment to mature HBe by removal of the 34-amino acid C-terminal domain. The efficiency of HBe secretion is specifically decreased in cells grown in the presence of tunicamycin, an inhibitor of N-glycosylation. Inasmuch as HBe precursor is not N-glycosylated, our data suggest that a cellular tunicamycin-sensitive protein increases the intracellular transport through the HBe secretory pathway. The study of the secretion of HBe derived from C-terminal-truncated precursors demonstrates that the tunicamycin-sensitive secretion absolutely requires a part of the C-terminal region that is removed to form mature HBe, indicating that the cellular tunicamycin-sensitive protein increases the efficiency of the intracellular transport of P22. We have also shown that the Escherichia coli beta-galactosidase can be secreted when fused to the HBe precursor signal sequence and that the P22 C-terminal domain renders the secretion of this reporter protein also tunicamycin-sensitive.

Glucagon treatment interferes with an early step of duck hepatitis B virus infection

The effect of glucagon on the establishment of hepadnavirus infection was studied in vitro with the duck hepatitis B virus (DHBV) model. The presence of the peptide hormone throughout infection or starting up to 8 h after virus uptake resulted in a dose-dependent reduction in the levels of intra- and extracellular viral gene products and of secreted virions. Treatment with forskolin or dibutyryl-cyclic AMP, two drugs that also stimulate the cyclic AMP (cAMP) signal transduction pathway, resulted in comparable inhibition, suggesting that the inhibitor effect is related to changes in the activity of protein kinase A. In persistently infected hepatocytes, only a slight, but continuous, decrease in viral replication was observed upon prolonged drug treatment. Time course analysis, including detection of DHBV covalently closed circular (ccc) DNA templates, revealed that glucagon acts late during the establishment of infection, at a time when the virus is already internalized, but before detectable ccc DNA accumulation in the nucleus. These data suggest that nuclear import (and reimport) of DHBV DNA genomes from cytosolic capsids is subject to cAMP-mediated regulation by cellular factors responding to changes in the state of the host cell.