Duck hepatitis B virus can tolerate insertion, deletion, and partial frameshift mutation in the distal pre-S region

Genetic Heterogeneity and Mutated PreS Analysis of Duck Hepatitis B Virus Recently Isolated from Ducks and Geese in China

In this study, we detected 12 duck and 11 goose flocks that were positive for duck hepatitis B virus (DHBV) using polymerase chain reaction and isolated 23 strains between 2020 and 2022 in China. The complete genomes of goose strains E200801 and E210501 shared the highest identity (99.9%), whereas those of strains Y220217 and E210526 shared the lowest identity (91.39%). The phylogenetic tree constructed based on the genome sequences of these strains and reference strains was classified into three major clusters: the Chinese branch DHBV-I, the Chinese branch DHBV-II, and the Western branch DHBV-III. Furthermore, the duck-origin strain Y200122 was clustered into a separate branch and was predicted to be a recombinant strain derived from DHBV-M32990 (belonging to the Chinese branch DHBV-I) and Y220201 (belonging to the Chinese branch DHBV-II). Additionally, preS protein analysis of the 23 DHBV strains revealed extensive mutation sites, almost half of which were of duck origin. All goose-origin DHBV contained the mutation site G133E, which is related to increased viral pathogenicity. These data are expected to promote further research on the epidemiology and evolution of DHBV. Continuing DHBV surveillance in poultry will enhance the understanding of the evolution of HBV.

Spacer Domain in Hepatitis B Virus Polymerase: Plugging a Hole or Performing a Role?

Hepatitis B virus (HBV) polymerase is divided into terminal protein, spacer, reverse transcriptase, and RNase domains. Spacer has previously been considered dispensable, merely acting as a tether between other domains or providing plasticity to accommodate deletions and mutations. We explore evidence for the role of spacer sequence, structure, and function in HBV evolution and lineage, consider its associations with escape from drugs, vaccines, and immune responses, and review its potential impacts on disease outcomes.

Unusual Features of Sodium Taurocholate Cotransporting Polypeptide as a Hepatitis B Virus Receptor

Cell culture (cc)-derived hepatitis B virus (HBV) can infect differentiated HepaRG cells, but efficient infection requires addition of polyethylene glycol (PEG) during inoculation. Identification of sodium taurocholate cotransporting polypeptide (NTCP) as an HBV receptor enabled ccHBV infection of NTCP reconstituted HepG2 cells, although very little hepatitis B surface antigen (HBsAg) is produced. We found infection by patient serum-derived HBV (sHBV), which required purification of viral particles through ultracentrifugation or PEG precipitation, was PEG independent and much more efficient in HepaRG cells than in HepG2/NTCP cells. In contrast to hepatitis B e antigen (HBeAg), HBsAg was not a reliable marker of productive sHBV infection at early time points. A low HBsAg/HBeAg ratio by ccHBV-infected HepG2/NTCP cells was attributable to dimethyl sulfoxide (DMSO) in culture medium, NTCP overexpression, and HBV genotype D. HepG2/NTCP cells released more viral antigens than HepG2 cells after HBV genome delivery by adeno-associated virus, and stable expression of NTCP in a ccHBV producing cell line increased viral mRNAs, proteins, replicative DNA, and covalently closed circular DNA. NTCP protein expression in HepG2/NTCP cells, despite being driven by the cytomegalovirus promoter, was markedly increased by DMSO treatment. This at least partly explains ability of DMSO to promote ccHBV infection in such cell lines. In conclusion, NTCP appeared inefficient to mediate infection by serum-derived HBV. It could promote HBV RNA transcription while inhibiting HBsAg secretion. Efficient PEG-independent sHBV infection of HepaRG cells permits comparative studies of diverse clinical HBV isolates and will help identify additional factors on virion surface promoting attachment to hepatocytes.

IMPORTANCE

Currently in vitro infection with hepatitis B virus (HBV) depends on cell culture-derived HBV inoculated in the presence of polyethylene glycol. We found patient serum-derived HBV could efficiently infect differentiated HepaRG cells independent of polyethylene glycol, which represents a more physiological infection system. Serum-derived HBV has poor infectivity in HepG2 cells reconstituted with sodium taurocholate cotransporting polypeptide (NTCP), the currently accepted HBV receptor. Moreover, HepG2/NTCP cells secreted very little hepatitis B surface antigen after infection with cell culture-derived HBV, which was attributed to NTCP overexpression, genotype D virus, and dimethyl sulfoxide added to culture medium. NTCP could promote HBV RNA transcription, protein expression, and DNA replication in HepG2 cells stably transfected with HBV DNA, while dimethyl sulfoxide could increase NTCP protein level despite transcriptional control by a cytomegalovirus promoter. Therefore, this study revealed several unusual features of NTCP as an HBV receptor and established conditions for efficient serum virus infection in vitro.

Duck hepatitis B virus polymerase gene mutants associated with resistance to lamivudine have a decreased replication capacity in vitro and in vivo

BACKGROUND/AIMS

Hepatitis B virus mutants of the polymerase gene are frequently selected during lamivudine therapy for chronic hepatitis B. To study the biology of these mutants, we analyzed their replication capacity in the duck hepatitis B virus (DHBV) infection.

METHODS

The B and C domain polymerase mutants corresponding to the clinical isolates were engineered by site directed mutagenesis in the DHBV genome in different expression vectors.

RESULTS

The study of the enzymatic activity of the mutated viral polymerase polypeptides analyzed in a cell free system demonstrated a lower priming activity and a decreased capacity of elongation of viral minus strand DNA that was consistent with the lower replication capacity of these mutants in transfected leghorn male hepatoma cells compared to wild type genome. These mutants had a lower replication capacity in primary hepatocytes and in in vivo transfected ducklings. Although resistant to lamivudine, these mutants remained sensitive to PMEA.

CONCLUSION

YMDD mutants of the DHBV reverse transcriptase have a decreased replication capacity both in vitro and in vivo, and are not cross-resistant to PMEA. These results may be important to design new antiviral strategies to combat the replication of the lamivudine resistant viral strains.

Active human hepatitis B viral polymerase expressed in rabbit reticulocyte lysate system

Human HBV polymerase has been expressed in reticulocyte lysate system. The expressed protein shows the DNA-dependent DNA polymerase activity. In vitro transcription and translation produces a major protein product with an apparent molecular weight of approximately 100 kD. The HBV DNA polymerase has been characterized biochemically in the condition that the contaminating cellular DNA polymerases were fairly suppressed by aphidicolin and NEM. The polymerization reaction is optimal at pH 7.5 and 37 degrees C and the polymerase requires either MnCl2 or MgCl2, with a preference for MnCl2. The protein represented an optimal activity in the presence of either 75 mM NaCl or 100 mM KCl, with a higher activity at 75 mM NaCl than 100 mM KCl. Study of the polymerizing activity of the deleted versions of the polymerase protein suggests that the terminal protein is essential for full polymerase function and the spacer region may decrease the stability of the P protein.

In vivo selection of duck hepatitis B virus pre-S variants which escape from neutralization

To better understand the role of specific residues within the duck hepatitis B virus (DHBV) pre-S protein in neutralization and infectivity, we have selected and identified pre-S variants which escape neutralization. A highly neutralizing monoclonal antibody (Mab 900) which recognizes an epitope 83IPQPQWTP90 localized previously on the DHBV pre-S protein, within a region suspected to mediate the virus interaction with hepatocytes, was used as immune pressure. After only two in vivo neutralization rounds with Mab 900, five different pre-S mutant genomes were identified, which harbored point mutations affecting only proline residues located at position 90 within this epitope (83IPQPQWTP90) and/or at a distance at position 5. We have shown that a single (P5L) or double proline (P5L + P90H) substitution affect neither virus replication capacity nor in vivo infectivity. However, the P5 mutation reduces mutant recognition by Mab 900 twofold, while the substitution of both prolines 5 and 90 almost completely abolishes mutant P5L + P90H reactivity with this Mab and leads to a decrease of neutralization. Therefore we describe here an experimental system which allows rapid in vivo selection and identification of DHBV pre-S variants and provide evidence that residues within and at a distance from the neutralization epitope are important in DHBV neutralization but do not affect its replication capacity and infectivity.

Previously unsuspected cis-acting sequences for DNA replication revealed by characterization of a chimeric heron/duck hepatitis B virus

Heron hepatitis B virus (HHBV) is an avian hepadnavirus that is closely related to duck hepatitis B virus (DHBV). To learn more about the mechanism of hepadnavirus replication, we characterized a clone of HHBV that contains a substitution of DHBV sequence from nucleotide coordinates 403 to 1364. This clone, named HDE1, expresses a chimeric pregenomic RNA, a chimeric polymerase (P) protein, and a core (C) protein with a one-amino-acid substitution at its carboxy terminus. We have shown that HDE1 is defective for minus-strand DNA synthesis, resulting in an overall reduction of viral DNA. HDE1 was also defective for plus-strand DNA synthesis, resulting in aberrant ratios of replication intermediates. Genetic complementation assays indicated that HDE1 replication proteins, C and P, are functional for replication and wild-type HHBV proteins do not rescue either defect. These findings indicate that the HDE1 substitution mutation acts primarily in cis. By restoring nucleotides 403 to 902 to the HHBV sequence, we showed that cis-acting sequences for plus-strand DNA synthesis are located in the 5' half of the HDE1 chimeric region. These data indicate the presence of one or more formerly unrecognized cis-acting sequences for DNA synthesis within the chimeric region (nucleotides 403 to 1364). These cis-acting sequences in the middle of the genome might interact directly or indirectly with known cis elements that are located near the ends of the genome. Our findings suggest that a specific higher-order template structure is involved in the mechanism of hepadnavirus DNA replication.

Characterization of a 120-Kilodalton pre-S-binding protein as a candidate duck hepatitis B virus receptor

Infection by human and animal hepadnaviruses displays remarkable host and tissue tropism. The infection cycle probably initiates with binding of the pre-S domain of viral envelope protein to surface receptors present on the hepatocyte. Three types of neutralizing monoclonal antibodies against duck hepatitis B virus (DHBV) have their binding sites clustered within residues 83 to 107 of the pre-S protein, suggesting that this region may constitute a major receptor binding site. A 170- or 180-kDa duck protein (p170 or gp180) which binds DHBV particles through this part of the pre-S sequence has been identified recently. Although the p170 binding protein is host (duck) specific, its distribution is not restricted to DHBV-infectible tissues. Using the pre-S protein fused to glutathione S-transferase and immobilized on Sepharose beads, we have now identified an additional binding protein with a size of 120 kDa (p120). p120 expression is restricted to the liver, kidney, and pancreas, the three major organs of DHBV replication. While optimal p170 binding requires an intact pre-S protein, binding to p120 occurs much more efficiently with a few N- or C-terminally truncated forms. The p120 binding site was mapped to residues 98 to 102 of the pre-S region, which overlaps with a cluster of known virus-neutralizing epitopes. Site-directed mutagenesis revealed residues 100 to 102 (Phe-Arg-Arg) as the critical p120 contact site; nonconservative substitution in any of the three positions abolished p120 binding. Double mutations at positions 100 to 102 markedly reduced DHBV infectivity in cell culture. Short pre-S peptides covering the clustered neutralizing epitopes (also p170 and p120 binding sites) reduced DHBV infectivity in primary duck hepatocyte cultures. Thus, p120 represents a candidate component of the DHBV receptor complex.

Interaction between duck hepatitis B virus and a 170-kilodalton cellular protein is mediated through a neutralizing epitope of the pre-S region and occurs during viral infection

Identification of cell surface viral binding proteins is important for understanding viral attachment and internalization. We have fused the pre-S domain of the duck hepatitis B virus (DHBV) large envelope protein to glutathione S-transferase and demonstrated a 170-kDa binding protein (p170) in [35S]methionine-labeled duck hepatocyte lysates. This glycoprotein was found abundantly in all extrahepatic tissues infectible with DHBV and in some noninfectible tissues, though it is not secreted into the blood. The interaction of pre-S fusion protein with p170 was competitively inhibited by wild-type DHBV in a dose-dependent manner. In addition, infection of hepatocytes with DHBV blocked the binding of pre-S fusion protein to p170, which suggests a biological role for p170 during natural infection. The p170 binding site was mapped to a conserved sequence of 16 amino acid residues (positions 87 to 102) by using 24 pre-S deletion mutants; this binding domain coincides with a major virus-neutralizing antibody epitope. Furthermore, site-directed mutagenesis revealed that an arginine residue at position 97 is critical for p170 binding. p170 was purified by a combination of ion-exchange and affinity chromatographies, and four peptide sequences were obtained. Two peptides showed significant similarities to human and animal carboxypeptides H, M, and N. Taken together, these results raise the possibility that the p170 binding protein is important during the replication cycle of DHBV.

Detection of an RNase H activity associated with hepadnaviruses

Replication of the hepadnavirus DNA genome is accomplished via reverse transcription of an intermediate, pregenomic RNA molecule. This process is likely to be carried out by a virally encoded, multifunctional polymerase which possesses DNA- and RNA-dependent DNA polymerase and RNase H activities. However, the nature of the product(s) of the polymerase gene predicted to mediate these functions is unclear. Biochemical studies of the polymerase protein(s) have been limited by its apparent low abundance in virus particles and, until recently, the inability to express active polymerase protein(s) heterologously. We have used activity gel assays to detect DNA- and RNA-dependent DNA polymerase activities associated with highly purified duck hepatitis B virus (DHBV) core particles (S. M. Oberhaus and J. E. Newbold, J. Virol. 67:6558-6566, 1993). Now we report that the same approach identifies a 35-kDa RNase H activity in association with highly purified DHBV core particles and crude preparations of virions from DHBV-infected ducks and woodchuck hepatitis virus-infected woodchucks. This is the first report of the detection of an hepadnavirus-associated RNase H activity. Its apparent size is smaller than any of the DNA polymerase activities that we detected previously and significantly smaller than the full-length protein predicted from the polymerase open reading frame (p85 for DHBV). These data suggest that the viral polymerase and RNase H activities are separable and that these enzymes may coordinate their activities in vivo by forming a complex.

Release of the hepatitis B virus-associated DNA polymerase from the viral particle by the proteolytic cleavage

Previous efforts for biochemical study of the human hepatitis B virus (HBV) DNA polymerase have been limited by its tight association with viral nucleocapsids. We report here that the soluble DNA polymerase from HBV particles was obtained by low pH treatment of the viral particles followed by incubation with small amounts of subtilisin. By these treatments, the approximately 100-kDa band in the activity gel assay was gradually converted to approximately 70 kDa, which subsequently showed reverse transcriptase activity on several exogenous templates. The single approximately 70-kDa active band, which did not show any DNA polymerase activity in endogenous reaction, was eluted through DEAE-Sepharose chromatography. These results suggest that the approximately 100-kDa protein, most likely the product of HBV Pol open reading frame, is tightly associated with viral nucleocapsids, and the approximately 70-kDa protein, the proteolytic cleavage product of approximately 100-kDa enzyme, is solubilized from viral particles as an active enzyme on exogenous templates.

Coordinate regulation of replication and virus assembly by the large envelope protein of an avian hepadnavirus

We have used linker scanning and site-directed mutagenesis in an attempt to distinguish among the known functions of the duck hepatitis B virus large envelope protein, p36. We found that linker-encoded amino acid substitutions in at least one region of the pre-S envelope protein p36 produced defects in both the production of enveloped virus and the regulation of covalently closed circular DNA (cccDNA) synthesis. Most linker substitutions, typically in the 5' two-thirds of the pre-S region of the p36 gene did not affect either cccDNA regulation or enveloped virus production but did destroy the infection competence of the enveloped particles produced. Single amino acid substitutions of residues 128 and 131 demonstrated a similar correlation between defects in the ability of p36 to support enveloped virus production and to control cccDNA levels. We concluded from these studies that virus production and cccDNA regulation probably require a common activity of p36.

Detection of DNA polymerase activities associated with purified duck hepatitis B virus core particles by using an activity gel assay

Replication of hepadnaviruses involves reverse transcription of an intermediate RNA molecule. It is generally accepted that this replication scheme is carried out by a virally encoded, multifunctional polymerase which has DNA-dependent DNA polymerase, reverse transcriptase, and RNase H activities. Biochemical studies of the polymerase protein(s) have been limited by the inability to purify useful quantities of functional enzyme from virus particles and, until recently, to express enzymatically active polymerase proteins in heterologous systems. An activity gel assay which detects in situ catalytic activities of DNA polymerases after electrophoresis in partially denaturing polyacrylamide gels was used by M.R. Bavand and O. Laub (J. Virol. 62:626-628, 1988) to show the presence of DNA- and RNA-dependent DNA polymerase activities associated with hepatitis B virus particles produced in vitro. This assay has provided the only means by which hepadnavirus polymerase proteins have been detected in association with enzymatic activities. Since conventional methods have not allowed purification of useful quantities of enzymatically active polymerase protein(s), we have devised a protocol for purifying large quantities of duck hepatitis B virus (DHBV) core particles to near homogeneity. These immature virus particles contain DNA- and RNA-dependent DNA polymerase activities, as shown in the endogenous DNA polymerase assay. We have used the activity gel assay to detect multiple DNA- and RNA-dependent DNA polymerase proteins associated with these purified DHBV core particles. These enzymatically active proteins appear larger than, approximately the same size as, and smaller than an unmodified DHBV polymerase protein predicted from the polymerase open reading frame. This is the first report of the detection of active hepadnavirus core-associated DNA polymerase proteins derived from a natural host.

Duck hepatitis B virus (DHBV) as a model for understanding hepadnavirus neutralization

The role of the immune response to the human hepatitis B virus (HBV) envelope proteins in neutralization of viral infectivity has been well documented. The similarity between HBV, prototype member of the hepadnavirus family, and the closely related duck hepatitis B virus (DHBV) has allowed, use of the latter as a convenient model for the study of molecular mechanisms of HBV replication and neutralization. In this brief review, we will examine the HBV and DHBV envelope proteins and their role as targets for virus neutralization.

Viral spliced RNA are produced, encapsidated and reverse transcribed during in vivo woodchuck hepatitis virus infection

By the use of reverse transcription followed by polymerase chain reaction (RT-PCR), we have identified one shorter than full-length, pregenomic viral RNA species in liver samples of woodchucks chronically infected with the woodchuck hepatitis virus (WHV). The spliced WHV RNA of about 2.4 kb in length was cloned and partially sequenced. The splicing donor and acceptor sites of this novel RNA are located, respectively, 130 nucleotides downstream of the ATG initiation codon of the core gene and 21 nucleotides upstream of the initiation codon of the pre-S2 surface gene. The splicing event generates a new core-polymerase fusion protein and removes the terminal protein domain and the spacer region of the polymerase gene. A nucleotide probe specific for the splice junction was used following RT-PCR, to further confirm the existence of this spliced RNA in the liver of seven WHV-infected woodchucks. Deleted viral DNA molecules corresponding to the 2.4 kb spliced RNA were also detected in the liver and, to a lesser extent, in the serum of infected woodchucks, suggesting that this spliced RNA can be encapsidated and reverse-transcribed during the course of natural WHV infection.

Hepatitis B virus genomes that cannot synthesize pre-S2 proteins occur frequently and as dominant virus populations in chronic carriers in Italy

The heterogeneity of hepatitis B virus (HBV) pre-S sequences coding for envelope proteins was tested by DNA amplification and direct sequencing of viral genomes from sera of 22 unselected chronic carriers resident in Southern Italy. The sequences of the dominant viral genome populations from 15 carriers were very similar to known published "wildtype" HBV genomes and showed no deletions. In contrast, in the HBV populations of six patients, deletions in the pre-S region, mainly clustered at the amino terminal end of the pre-S2 region, were found. Four of these mutant genome populations and those from another patient cannot express pre-S2 proteins due to deletions or a mutation of the translation initiation codon. Emergence of the pre-S mutant viruses either during the natural course of infection or after interferon treatment was found in follow-up sera of one and two patients, respectively. These data indicate a high prevalence of pre-S mutant viruses which cannot express pre-S2 and normal-size pre-S1 proteins. This has important implications for the usefulness of diagnostic pre-S protein assays and possibly for interferon treatment and the efficacy of new vaccines containing pre-S proteins.

In vivo neutralization of duck hepatitis B virus by antibodies specific to the N-terminal portion of pre-S protein

The neutralization of duck hepatitis B virus (DHBV) infection using antibodies directed against the N-terminal portion of the large surface protein was examined in vitro and in vivo. We demonstrate here that a monoclonal antibody, directed against an epitope mapped between aa 77 and aa 100 on the DHBV pre-S, exerts a similar neutralizing activity (77%) both in vivo and in vitro. Furthermore, we have found that a polyclonal antiserum raised against the bacterially expressed 131 first amino acids of the DHBV pre-S region abolished the infectivity of DHBV in ducklings. Therefore, antibodies against a peptide representing most of the DHBV pre-S region (1-131) or a monoclonal antibody specific to an epitope within this region neutralizes in vivo DHBV infectivity.

Replication-defective missense mutations within the terminal protein and spacer/intron regions of the polymerase gene of human hepatitis B virus

We have analyzed 11 independent mutations located at various domains of the polymerase gene (pol) of human hepatitis B virus. Surprisingly, one of the two missense mutants within the spacer/intron region appears to be lethal. This result further defines the N-terminal limit of the reverse transcriptase domain. Alternatively, it suggests the potential existence of a novel domain with an unknown function. Two missense mutations within the terminal protein (TP) domain appear to be replication-defective as well, suggesting a functionally essential role of the TP domain in DNA replication.

Evidence that less-than-full-length pol gene products are functional in hepadnavirus DNA synthesis

Duck hepatitis B virus mutants containing frameshift or stop codon mutations in a portion of the viral pol gene separating the terminal protein and reverse transcriptase domains had a leaky phenotype and, depending on the location and type of mutation, synthesized up to 10% as much viral DNA as did the wild type. This region of the pol gene had previously been reported to be refractory to missense mutations; in fact, the leakiness of most of our mutants appeared attributable to translational suppression, which would also be expected to introduce amino acid changes. However, at least one mutant (pH1093 + 2), which was ca. 10% as active as the wild type, appeared to use a novel pathway to express the viral pol gene. Our analyses indicated that pH1093 + 2 synthesized the viral reverse transcriptase as a fusion protein with the amino-terminal portion of the pre-S envelope protein. Thus, in this case, the products of the terminal-protein and reverse transcriptase domains of the pol gene would function as separate protein species, though perhaps noncovalently joined in a dimeric structure during assembly of DNA replication complexes. Evidence was also obtained that was consistent with the idea that the wild-type pol gene may, at least in certain instances, be expressed as functional, subgenic polypeptides.

Effects of insertional and point mutations on the functions of the duck hepatitis B virus polymerase

The polymerase (P) gene of hepadnaviruses encodes a large polypeptide that appears to participate in several steps in the viral life cycle: packaging of viral RNA, providing the primer for synthesis of minus-strand DNA, synthesizing minus-strand DNA from an RNA template and plus-strand DNA from a DNA template, and degrading viral RNA in RNA-DNA hybrids. To assist in the assignment of these functions to domains of the duck hepatitis B virus polymerase protein, we have constructed a series of substitution mutations and a large insertion mutation, based in part on amino acid sequence comparisons with other proteins known to exhibit reverse transcriptase (RT) and RNase H activities. We found that changes in highly conserved sequences in putative RT and RNase H domains in the carboxy-terminal half of the protein dramatically reduced synthesis of both strands of viral DNA without major effects on RNA packaging into subviral cores. Thus we can uncouple RNA packaging and DNA synthesis but cannot separate RT and RNase H activities as has been done with human hepatitis B virus. The viability of a mutant with a large insertion (123 amino acids) upstream of the RT and RNase H domain indicates that a hinge region may separate parts of the polymerase protein implicated in priming and polymerization.

Efficient duck hepatitis B virus production by an avian liver tumor cell line

Duck hepatitis B virus (DHBV) is produced in small amounts following transfection of human hepatoma or hepatoblastoma cell lines with cloned viral DNA. In a search for better hosts for DHBV replication, two avian liver cell lines were investigated. One of these cell lines, LMH, produced 5 to 10 times more DNA replicative intermediates and 10 to 20 times more infectious DHBV than did either of the two human cell lines, HuH-7 and Hep G2. Utilization of cell lines in genetic analyses of virus replication is often dependent upon obtaining efficient complementation between cotransfected viral genomes. We assayed transcomplementation of a viral polymerase (pol) gene mutant, which is rather inefficient in transfected human cells, and found that viral DNA synthesis was at least 20 times more efficient following cotransfection of LMH cells than in similarly transfected HuH-7 cells. Recombination, a potential interpretation problem in complementation assays, occurred at low levels in the cotransfected cultures but was substantially reduced or eliminated by creation of an LMH subline stably expressing the viral polymerase. This cell line, pol-7, supported the replication of DHBV pol mutants at ca. 10 to 15% of the level of virus replication obtained following transfection with wild-type viral DNA. By transcomplementation of a pol gene mutant in LMH cells, we were able to produce sufficient virus with the mutant genome to investigate the role of polymerase in covalently closed circular DNA amplification. Our results substantiate the hypothesis that covalently closed circular DNA is synthesized by the viral reverse transcriptase.

Active hepatitis B virus replication in the presence of anti-HBe is associated with viral variants containing an inactive pre-C region

Although rise of anti-HBe immunity in the course of hepatitis B virus (HBV) infection is generally followed by clearance of the infectious virions, ongoing chronic liver disease with circulating virions has been repeatedly observed in a significant number of anti-HBe patients, especially in Mediterranean countries. To investigate the possible role of HBV variants, we cloned HBV DNA from the serum of three such anti-HBe cases. Comparative restriction mapping of HBV clones suggested circulation of different HBV genomes in the three cases. DNA sequencing revealed an inactive pre-C region in all 11 HBV clones derived from the three cases, either as one or two point mutations in the 3' terminus generating an in-frame TAG stop codon, or a 1 nucleotide insertion in the 5' terminus resulting in frameshift mutation. Furthermore, for one clone the complete 3182 nucleotide sequence was determined and no significant mutation was found in the remainder of the genome. We conclude that chronic hepatitis cases positive for anti-HBe are associated with HBV variants containing an inactive pre-C region and hence cannot synthesize pre-C region-derived HBeAg. This finding may provide a molecular explanation for the continued viral replication despite presence of anti-HBe immunity.

Virus-neutralizing monoclonal antibody to a conserved epitope on the duck hepatitis B virus pre-S protein

In this study we used duck hepatitis B virus (DHBV)-infected Pekin ducks and heron hepatitis B virus (HHBV)-infected heron tissue to search for epitopes responsible for virus neutralization on pre-S proteins. Monoclonal antibodies were produced by immunizing mice with purified DHBV particles. Of 10 anti-DHBV specific hybridomas obtained, 1 was selected for this study. This monoclonal antibody recognized in both DHBV-infected livers and viremic sera a major (36-kilodalton) protein and several minor pre-S proteins in all seven virus strains used. In contrast, pre-S proteins of HHBV-infected tissue or viremic sera did not react. Thus, the monoclonal antibody recognizes a highly conserved DHBV pre-S epitope. For mapping of the epitope, polypeptides from different regions of the DHBV pre-S/S gene were expressed in Escherichia coli and used as the substrate for immunoblotting. The epitope was delimited to a sequence of approximately 23 amino acids within the pre-S region, which is highly conserved in four cloned DHBV isolates and coincides with the main antigenic domain as predicted by computer algorithms. In in vitro neutralization assays performed with primary duck hepatocyte cultures, the antibody reduced DHBV infectivity by approximately 75%. These data demonstrate a conserved epitope of the DHBV pre-S protein which is located on the surface of the viral envelope and is recognized by virus-neutralizing antibodies.