Expression of hepatitis B antigens with a simian virus 40 vector

Association of core promoter/precore mutations and viral load in e antigen-negative chronic hepatitis B patients

Apart from core promoter A1762T/G1764A and precore G1896A mutations, other hepatitis B virus (HBV) mutants are detected in hepatitis B e antigen (HBeAg)-negative chronic hepatitis B (CHB). The aim of this study was to determine the effects of those mutants on clinical manifestation and viral loads of genotypes B and C HBV. Seventy-nine HBeAg-negative CHB patients with hepatitis flare were enrolled in this study and their HBV precore/core region were sequenced. Serial biochemical profiles and viral loads were assessed and compared. Fifty-three patients (67%) were infected by genotype B HBV and 26 (33%) were infected by genotype C HBV. The clinical manifestation and HBV viral loads were comparable between the two groups. However, genotype B was significantly associated with precore G1896A mutation (92.5%), and more mutations within nucleotide 1809-1817 were detected in patients infected by genotype B as compared with those infected by genotype C (18.9%vs 3.8%). Most of the cases had mutations at the -2, -3 or -5 position from the precore AUG initiation codon. Triple core promoter mutations T1753C/A1762T/G1764A [corrected] appeared to be linked to genotype C rather than genotype B HBV (19.2%vs 1.9%; P = 0.013). In multivariate analysis, the presence of either triple core promoter 1753/1762/1764 mutation or nucleotide 1809-1817 mutation was the only factor associated with lower HBV viral load (<70 Meq/mL) (odds ratio = 9.01; 95% CI 1.11-71.43; P = 0.04). In conclusion, minor HBV variants with mutations in the core promoter and precore region were detectable in genotypes B and C. Such HBV variants are genotype specific and related to viraemia levels.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

In vitro experimental infection of primary human hepatocytes with hepatitis B virus

BACKGROUND/AIMS

Studies on the interaction of hepatitis B virus (HBV) with its host cell require a suitable tissue culture system. This study used primary adult hepatocytes from healthy human liver tissue to establish productive infection in vitro.

METHODS

Hepatocytes were inoculated overnight with HBV. Production of viral proteins was assessed by radioimmunoassay and by [35S]methionine labeling, and production of viral DNA was assessed by Southern blotting and endogenous polymerase assay.

RESULTS

Secretion of high levels of hepatitis B surface antigen (HBsAg) and low levels of hepatitis B virus e antigen (HBeAg) into the medium was detectable 6 days after infection and reached maximum values after 12 days. Metabolic labeling showed production of viral proteins to be a result of de novo synthesis. The appearance of single-stranded HBV DNA in the cytoplasm of infected cells, typically present in immature cores, indicated viral replication. HBV DNA containing particles possessing an active viral DNA polymerase could be immunoprecipitated from the medium 12 days after infection. An antiserum specific for the preS1 region of the viral envelope was capable to block infection. Presence of dimethyl sulfoxide in the medium greatly improved the yield of viral proteins.

CONCLUSIONS

Primary adult human liver cells are competent for infection with HBV.

Hepatitis B virus core antigen (HBc Ag) accumulation in an HBV nonproducer clone of HepG2-transfected cells is associated with cytopathic effect

Two clones of the hepatoblastoma HepG2 cell line transfected with complete hepatitis B virus deoxyribonucleic acid (HBV DNA) were studied. The kinetics and cytopathic effect of HBV Ag production in these two clones (one of which was an HBV producer) were compared to those of the parent HepG2 cell line. The presence of hepatitis B surface antigen (HBs Ag) and hepatitis B e antigen (HBe Ag) was determined by commercial enzyme-linked immunosorbent assay (ELISA). A hepatitis B core antigen (HBc Ag)-specific ELISA assay was developed, using monoclonal anti-HBc to detect HBc Ag. Amounts of HBs, HBe, and HBC Ags were partially quantified in both intracellular and extracellular compartments. The HBV producer clone excreted high levels of HBc, HBe, and HBs Ags from the beginning of the growth phase, and no cytopathic effect was observed. The HBV nonproducer clone produced high levels of HBs and HBe Ags, but there was no detectable HBc Ag in the supernatant; instead, HBc Ag accumulated in the intracellular compartment. In this clone, significant cell death was observed 4 days after cell confluency, corresponding with notable HBc Ag release into the supernatant. These results suggest a cytopathic effect associated with HBc Ag accumulation in the HBV nonproducer clone, but no cytopathic effect in the HBV producer clone. This suggests that virological factors as well as the host's immune response may be considered in explaining liver injury occurring in hepatitis B.

Expression mechanism of the hepatitis B virus (HBV) C gene and biosynthesis of HBe antigen

The C gene of the hepatitis B virus, which contains two in-phase initiation codons delimiting the pre-C sequence and the C region, directs the synthesis of the major protein of the capsid (HBcAg) and of a precore protein which upon processing results in the secretion of the HBeAg. We used an adenovirus-based vector to study in the human 293 cell line the C gene products, the intermediates of the precore protein processing and the kind of protease involved in this processing. The synthesis of the 21-kDa HBcAg polypeptide was dependent on the deletion of the pre-C sequence suggesting that a pre-C mRNA is not used for the synthesis of the major capsid protein. With the construct containing the complete C gene, two proteins of 25 and 22 kDa were detected intracellularly, corresponding to the unprocessed and partially processed precore protein, respectively. In addition, a 15-kDa protein (HBeAg) was secreted in the culture medium. Using pepstatin, an inhibitor specific for aspartyl proteinases, reduction of HBeAg secretion and accumulation of the 22-kDa processing intermediate were observed, suggesting the involvement of an aspartyl proteinase in the conversion of the 22-kDa protein into HBeAg.

Intra- and extracellular distribution and immunochemical characterization of hepatitis B virus nucleocapsid proteins produced by a human hepatoma cell line transfected with cloned viral DNA

Hepatitis B virus (HBV)-related antigens produced by the human hepatoma cell line (HB611 cell), which had been transfected with a cloned HBV DNA and established as a stable producer of HBV (T. Tsurimoto, A. Fujiyama, and K. Matsubara, 1987, Proc. Natl. Acad. Sci. USA 84, 444-448), were investigated immunochemically and morphologically. All HBV-related antigens, HBV surface (HBsAg), e (HBeAg), and core (HBcAg), were semiquantitatively examined by the respective reversed passive hemagglutination assay (RPHA). RPHAs for HBcAg and for HBeAg were characterized as reacting only to the core particles and to the free form of nucleocapsid proteins, respectively. The amounts of HBsAg and nucleocapsid protein in culture medium were roughly related to the number of viable cells. The amount of core particles was, instead, proportional to the number of dead cells. Relative amounts of HBsAg, core particles, and nucleocapsid proteins in culture medium, cell surface, and cell lysate were determined and it was found that HBsAg and nucleocapsid proteins were effectively secreted into culture medium but core particles were not. Molecular species of nucleocapsid proteins were identified to be p17 and p18 (HBeAg polypeptides) in the culture medium and HBeAg polypeptides and p21.5 (HBcAg polypeptide) in the cytosol fraction. The p21.5 was preferentially found in the nuclear fraction.

DNA-binding activity of hepatitis B e antigen polypeptide lacking the protaminelike sequence of nucleocapsid protein of human hepatitis B virus

The characteristics of binding of hepatitis B core antigen (HBcAg) and hepatitis B e antigen (HBeAg) polypeptides to hepatitis B virus (HBV) DNA were analyzed. HBcAg polypeptide from recombinant HBV core particles and HBeAg polypeptide from partially purified serum HBeAg were prepared and verified to have molecular weights of 21,500 (P21.5) and of 17,000 (P17) and 18,000 (P18), respectively, by immunoblot analysis. By reaction of these proteins on a nitrocellulose membrane with cloned 32P-HBV DNA, it was revealed that the HBeAg polypeptide, which lacks the C-terminal 34 amino acids of P21.5, as well as the HBcAg polypeptide, bound to the DNA. The secondary structures of nucleocapsid proteins of HBV, woodchuck hepatitis virus, and ground squirrel hepatitis virus were predicted by the Garnier algorithm. Amino acid sequences which, in addition to those of the C-terminal regions, may contribute to binding were proposed to be the 21-amino-acid residues located at amino acids 100 to 120 of the nucleocapsid proteins of these hepadnaviruses.

Expression of the precore region of an avian hepatitis B virus is not required for viral replication

The core-antigen-coding region of all hepadnaviruses is preceded by a short, in-phase open reading frame termed precore whose expression can give rise to core-antigen-related polypeptides. To explore the functional significance of precore expression in vivo, we introduced a frameshift mutation into this region of the duck hepatitis B virus (DHBV) genome and examined the phenotype of this mutant DNA by intrahepatic inoculation into newborn ducklings. Animals receiving mutant DNA developed DHBV infection, as judged by the presence in hepatocytes of characteristic viral replicative intermediates; molecular cloning and DNA sequencing confirmed that the original mutation was present in the progeny genomes. Infection could be efficiently transmitted to susceptible ducklings by percutaneous inoculation with serum from mutant-infected animals, indicating that infectious progeny virus was generated. These findings indicate that expression of the precore region of DHBV is not essential for genomic replication, core particle morphogenesis, or intrahepatic viral spread.

In vivo phosphorylation and protein analysis of hepatitis B virus core antigen

The C open reading frame of the hepatitis B virus contains two in-frame ATG codons that are separated by the precore region and encodes two major polypeptides that are antigenically distinct and that are probably synthesized from individual mRNAs. The precore region directs the secretion of the e antigen, whereas the core antigen can be expressed in the absence of these sequences. In this report a transient expression system was used to study the hepatitis B virus core antigen. By using a chimeric complex of adenovirus major late promoter-simian virus 40 enhancer sequences, we were able to achieve high levels of core antigen expression in transfected cells, permitting characterization of this protein and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The core polypeptide is a 20.9-kilodalton protein, and we show in this study that it is phosphorylated in vivo. Cell fractionation studies, the results of which are supported by indirect immunofluorescence, localized the phosphocore in the cytosol and the nucleus and indicated that it is associated with the membrane of transfected cells. Results of Triton X-114 solubilization studies indicated that the phosphocore is peripherally associated with cytoplasmic membranes. Expression of the membrane-associated phosphocore occurred in the absence of the precore sequences. The phosphocore also assembled into particles in the absence of other viral gene products or intact DNA.

Murine cells carrying integrated tandem genomes of hepatitis B virus DNA transcribe RNAs from endogenous promoters on both viral strands and express middle and major viral envelope proteins

Clone 4.10 cells were isolated as a methotrexate-resistant clone arising after cotransfection of mouse 3T3 cells with plasmid DNA containing a head-to-tail dimer of the hepatitis B virus (HBV) genome and DNA coding for methotrexate-resistant dihydrofolate reductase. The majority of methotrexate-resistant clones derived by this procedure have been found to contain multiple copies of the HBV genome, but the intact HBV dimer was rarely preserved. In contrast, 4.10 cells contained at least 40 copies of intact HBV dimer per cell. These cells produced large amounts of 22-nm hepatitis B surface antigen particles that included viral envelope proteins reactive with the pre-S2 region-specific antibody, indicating transcription and translation of the pre-S2 and S regions of the integrated viral genomes. The cells also synthesized viral e antigen, which was released into the culture medium. Characterization of polyadenylated viral RNAs transcribed from the long (minus) strand of the integrated HBV DNA demonstrated the presence of shorter-than-genome-length RNAs containing only X region sequences, shorter-than-genome-length RNAs containing both X and S region sequences, and longer-than-genome-length RNAs containing core, X, and S region sequences. Start sites for transcripts were mapped 5' to and within the pre-S region and 5' to and within the precore region at approximately the same sites as those utilized for HBV transcription during viral replication in infected livers. Polyadenylated RNA transcripts complementary to the short (plus) strand of HBV that initiated and terminated within the intact and integrated head-to-tail tandem viral genomes were also detected.

Production of hepatitis B virus in vitro by transient expression of cloned HBV DNA in a hepatoma cell line

Transfection of human hepatoma cell lines with cloned HBV DNA resulted in the secretion of large amounts of hepatitis B surface antigen (HBsAg) and core-related antigens (HBc/HBeAg) if well-differentiated cell lines were employed. Synthesis of both viral antigens was the highest in cell line HuH-7 and continued for approximately 25 days. Particles resembling hepatitis B virions (Dane particles) by morphology, density and by the presence of the preS1 surface antigen were released from the transfected HuH-7 cells into the culture medium. These particles produced in vitro were also indistinguishable from the naturally occurring hepatitis B virions in containing the virus-associated DNA polymerase and mature HBV genomes. Restriction analysis of these DNA molecules was compatible with the nucleotide sequence of the transfecting HBV DNA sequence. Viral surface antigens and core proteins present in the culture medium were fractionated and characterized by immunoprecipitation and SDS--PAGE after labeling with [35S]methionine. Antisera specific for X-gene products identified in cell extracts two hitherto unknown HBV gene products. This system thus provides a new approach to open questions regarding HBV-related gene function and HBV replication.

Expression of hepatitis B virus surface and core antigens: influences of pre-S and precore sequences

Amphotropic retroviral expression systems were used to synthesize hepatitis B virus surface antigen (HBsAg) and core antigen. The vectors permitted establishment of cell lines which expressed antigen from either the retroviral long terminal repeat or the mouse metallothionein-I promoter. HBsAgs were synthesized containing no pre-S sequences, pre-S(2) sequences alone, or pre-S(1) plus pre-S(2) sequences. Inclusion of pre-S(2) sequences did not affect the secretion or density of HBsAg particles but did reduce their mass by approximately 30%. Addition of pre-S(1) sequences almost completely abolished secretion of HBsAg and resulted in its localization in an aqueous-nonextractable pre- or early-Golgi cellular compartment. HBsAg was localized to the cytoplasm of the cell. This localization was unaffected by the presence of pre-S sequences in the antigen. Cell lines synthesizing hepatitis B antigens from core DNA fragments, containing or not containing precore sequences, secreted hepatitis B e antigen. However, the absence of precore DNA sequences resulted in additional synthesis of hepatitis core antigen, which was predominantly nuclear in localization.

Production of hepatitis B virus particles in Hep G2 cells transfected with cloned hepatitis B virus DNA

The hepatoblastoma cell line Hep G2 was transfected with a plasmid carrying the gene that confers resistance to G418 and four 5'-3' tandem copies of the hepatitis B virus (HBV) genome positioned such that two dimers of the genomic DNA are 3'-3' with respect to one another. Cells of one clone that grew in the presence of G418 produce high levels of hepatitis B e antigen and of hepatitis B surface antigen. HBV DNA is carried by these cells as chromosomally integrated sequences and episomally as relaxed circular, covalently closed, and incomplete copies of the HBV genome. Viral DNA was detected also in conditioned growth medium at the buoyant densities characteristic for infectious Dane and immature core particles. Finally, HBV-specific components morphologically identical to the 22-nm spherical and filamentous hepatitis B surface antigen particles as well as 42-nm Dane particles were visualized by immunoelectron microscopic analysis. Therefore, we have demonstrated that the Hep G2 cell line can support the assembly and secretion not only of several of the replicative intermediates of HBV DNA but also of Dane-like particles. This in vitro system can now be used to study the life cycle of HBV and the reaction of immunocompetent cells with cells carrying HBV.

Expression of hepatitis B viral core region in mammalian cells

We studied the expression of the core region of the hepatitis B virus genome in mammalian cells with recombinant plasmid vectors. Stably transformed rat fibroblast cell lines were established by transfection with vectors containing subgenomic and genome-length hepatitis B virus DNA, followed by G418 selection. The RNA transcripts directed by the core region were characterized by Northern blot hybridization and S1 nuclease mapping. Using the chloramphenicol acetyltransferase gene expression system, the promoter activity located upstream of the core open reading frame was confirmed. The synthesis of core and e polypeptides was studied with a commercial radioimmunoassay. These studies show that partial deletion of the precore sequences abolished secretion of the e antigen, but there was pronounced synthesis of the core antigen in transfected cells.

Novel RNA family structure of hepatitis B virus expressed in human cells, using a helper-free adenovirus vector

Ad5-HBL is a type 5 adenovirus bearing the large BglII fragment (2.8 kilobases; 87% of the total genome) of hepatitis B virus (HBV), subtype adr. Eight HBV RNAs expressed in HeLa cells infected with Ad5-HBL were mapped by the nuclease S1 technique. Three major RNAs spanning 2.4, 2.0, and 0.7 kilobases of the HBV sequences cover the coding regions of "presurface" plus surface antigen, surface antigen alone, and "X" protein, respectively. The 5' segment of an RNA which could code for core antigen (HBcAg) was also detected. All major HBV RNAs initiate from mutually exclusive 5' ends, terminate at the unique 3' end within the HBcAg coding region (except readthrough species), and have no spliced deletion, forming a novel RNA family structure. No TATA box-like sequences were found near the 5' end of these RNAs, except in the case of the 2.4-kilobase RNA. About two thirds of total HBV RNA does not terminate at the mapped 3'-end position, suggesting the termination signal is functionally inefficient. Since the potential 5' end of HBcAg mRNA was mapped at the same position as the minus-strand nick of HBV DNA previously reported, we propose a model that requires inefficient poly(A) addition to produce an RNA which serves both as HBcAg mRNA and as the putative RNA template of minus-strand DNA synthesis in the HBV life cycle.

Expression of hepatitis B viral core region in mammalian cells

We studied the expression of the core region of the hepatitis B virus genome in mammalian cells with recombinant plasmid vectors. Stably transformed rat fibroblast cell lines were established by transfection with vectors containing subgenomic and genome-length hepatitis B virus DNA, followed by G418 selection. The RNA transcripts directed by the core region were characterized by Northern blot hybridization and S1 nuclease mapping. Using the chloramphenicol acetyltransferase gene expression system, the promoter activity located upstream of the core open reading frame was confirmed. The synthesis of core and e polypeptides was studied with a commercial radioimmunoassay. These studies show that partial deletion of the precore sequences abolished secretion of the e antigen, but there was pronounced synthesis of the core antigen in transfected cells.

Hepatitis B virus gene function: the precore region targets the core antigen to cellular membranes and causes the secretion of the e antigen

The core gene of the hepatitis B virus genome contains two conserved in-phase initiation codons separated by about 90 nucleotides. This region ("the precore region") encodes largely hydrophobic amino acids. We have expressed the coding sequence of the core gene with or without the precore region by using a simian virus 40-derived vector in heterologous mammalian cells. The results show that the precore region is not required for the expression either of core antigen (cAg) or of a related hepatitis B virus antigen, the e antigen (eAg). However, the precore region causes the cAg to become associated with cytoplasmic membranes, probably the endoplasmic reticulum. Further, the presence of the precore sequence results in the secretion of eAg. Our results suggest that the precore region plays a role in targeting core proteins to the membrane; this may be the direct cause of eAg secretion and also may aid in the interaction of the core and surface antigens in the formation of the viral particle.

Putative reverse transcriptase intermediates of human hepatitis B virus in primary liver carcinomas

Nucleocapsid-pol fusion proteins have been detected by serological screening hepatocellular carcinoma tissues that contain hepatitis B virus (HBV) DNA. The existence of these fusion proteins suggests that HBV may synthesize its reverse transcriptase in a fashion analogous to the way that retroviruses synthesize and process a precursor. The accumulation of HBV reverse transcriptase intermediates in tumorous tissues and not in other tissues may be related to the absence of viral core particles and possibly contributes to tumor development.

A novel expression selection approach allows precise mapping of the hepatitis B virus enhancer

We have used a novel approach called expression selection to precisely define the hepatitis B virus (HBV) enhancer. Expression selection is based on a shuttle vector containing an enhancerless SV40 T antigen gene, the SV40 origin of replication and a plasmid replicon. This vector is linearized, ligated with the sonicated DNA to be analyzed and transfected into eukaryotic cells, where only plasmids which have incorporated an enhancer can express T antigen and therefore replicate. Vectors amplified by replication are selectively rescued in E. coli and their inserts analyzed. When we performed this protocol with HBV DNA we rescued two overlapping fragments of 166 and 214 bp which in HBV DNA map about 500 bp upstream of the core antigen mRNA initiation site and 1150 bp downstream of the surface antigen mRNA initiation site. These results were confirmed by conventional deletion mapping. When compared to the SV40 enhancer in nonhepatic cell lines, the HBV enhancer is only 5 to 10% as active; nevertheless, it also acts in an orientation-independent manner and in a position downstream of a gene. The HBV enhancer is situated in the coding region of the potential reverse transcriptase, and thus is the first enhancer identified to map in a protein-coding region.

Transcription of woodchuck hepatitis virus in the chronically infected liver

The transcription of woodchuck hepatitis virus (WHV) genome was studied in the liver of chronically infected woodchucks by Northern blot, nuclease mapping and primer extension analysis. Two major transcripts, 2.1 and 3.7 kb in length, and several minor transcripts were found in samples which supported active WHV replication. The 2.1-kb RNA represents the major transcript of the S gene, encoding the viral surface antigen (WHsAg) as demonstrated by blot-hybridization experiments. Two transcription initiation sites were localized downstream of the second AUG of the pre-S region, 139 and 152 nucleotides upstream of the translation initiation codon of the S gene. The 3.7-kb transcript, present in an equal amount, is slightly larger than the WHV genome and could be involved in the expression of all viral proteins. The data derived from RNA mapping strongly suggest that this transcript is initiated approximately 70 nucleotides upstream of the C gene, encoding the viral core antigen (WHcAg), and represents the message for WHcAg. It might also serve in the viral replication cycle as a potential template for reverse transcription. All WHV-specific transcripts were found to be processed at a unique site, 20 nucleotides downstream of the polyadenylation signal situated within the core gene. A different set of WHV-specific mRNAs was observed in a woodchuck hepatocellular carcinoma when only integrated forms of WHV DNA could be detected. Two RNA species of 2.3 and 4.6 kb were characterized. The 3.7-kb RNA was absent, reinforcing the hypothesis that this transcript corresponds to the pre-genome.

Comparative sequence analysis of duck and human hepatitis B virus genomes

We have cloned and sequenced an infectious, functionally active genome of a duck hepatitis B virus (DHBV). It is 3,021 base pairs (bp) in length and shows little DNA sequence homology to the genome of human hepatitis B virus (HBV). However, the amino acid sequences of predicted viral gene products are similar between DHBV and HBV, and the genome organization present in DHBV reflects that of HBV. As in the mammalian virus the long minus strand of the DHBV genome encodes three long overlapping reading frames designated as P, S, and C. The fourth open reading frame, termed X, is absent in DHBV. A comparison with a sequence of a second DHBV isolate [Mandart et al, Journal of Virology 49:782-792, 1984] revealed a nucleotide sequence variation of 5.6% and confirmed the presented overall gene organization of DHBV.

Transcripts and the putative RNA pregenome of duck hepatitis B virus: implications for reverse transcription

Duck hepatitis B virus (DHBV) is a DNA virus that replicates by reverse transcription. We have examined transcripts of DHBV to elucidate mechanisms of gene expression and replication. Three major transcripts were characterized and related to the expression of the genes for the core antigen (DHBcAg), the surface antigen (DHBsAg), and the pre-S/DHBs protein, respectively. They are unspliced and overlap partially since they start at different promoter sites, but they terminate near a common polyadenylation signal. The messenger RNA related to DHBcAg expression is the only major transcript of more than genome length and therefore also represents the putative template for reverse transcription (pregenome). Its structure predicts a strategy of replication of hepatitis B viruses different from that of retroviruses.

Cloned duck hepatitis B virus DNA is infectious in Pekin ducks

Approximately 10% of German-bred Pekin ducks were found to be chronically infected with duck hepatitis B virus (DHBV). The genomes of three German DHBV isolates analyzed were closely related but showed substantial restriction site polymorphism compared with U.S. isolates. We tested the infectivity of three sequence variants of cloned DHBV DNA by injecting them into the liver of virus-free ducklings. Most of these animals injected with double-stranded closed-circular or plasmid-integrated dimer DHBV DNA developed viremia, demonstrating the infectivity of all three cloned DHBV DNA variants. The cloned viruses produced were indistinguishable from those from naturally infected animals, implying that our experimental approach can be used to perform a functional analysis of the DHBV genome.

Hepatitis B virus transcription in the infected liver

Hepatitis B virus (HBV) transcription was studied in the liver of an infected chimpanzee and compared with HBV transcription in heterologous systems. Besides the well characterized 2.3-kb surface antigen mRNA produced in most systems, a second major transcript was identified in the liver. This 3.8-kb transcript (+/- 300 bases) is slightly larger than the HBV genome and is probably involved both in core/e antigen synthesis and in HBV replication via reverse transcription. In addition, minor variants of the 2.3-kb surface antigen mRNA were characterized as probably being involved in the expression of HBsAg-related minor proteins. Finally, several potential transcription signals, identified on the HBV genome using heterologous expression systems, were found to be poorly active if at all in the infected liver, thereby stressing the importance of HBV transcription studies performed with liver material.