In vitro infection of woodchuck hepatocytes with woodchuck hepatitis virus and ground squirrel hepatitis virus

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.

Advances and Challenges in Studying Hepatitis B Virus In Vitro

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

[(Methyl)1-(11)c]-acetate metabolism in hepatocellular carcinoma

PURPOSE

Studies have established the value of [(methyl)1-(11)C]-acetate ([(11)C]Act) combined with 2-deoxy-2[(18)F]fluoro-D-glucose (FDG) for detecting hepatocellular carcinoma (HCC) using positron emission tomography (PET). In this study, the metabolic fate of [(11)C]Act in HCC was characterized.

METHODS

Experiments with acetic acid [1-(14)C] sodium salt ([(14)C]Act) were carried out on WCH-17 cells and freshly derived rat hepatocytes. PET scans with [(11)C]Act were also carried out on woodchucks with HCC before injection of [(14)C]Act. The radioactivity levels in different metabolites were quantified with thin-layer chromatography.

RESULTS

In WCH-17 cells, the predominant metabolite was phosphatidylcholine (PC). Regions of HCCs with the highest [(11)C]Act uptake had higher radioactivity accumulation in lipid-soluble compounds than surrounding hepatic tissues. In those regions, PC and triacylglycerol (TG) accumulated more radioactivity than in surrounding hepatic tissues.

CONCLUSIONS

High [(11)C]Act uptake in HCC is associated with increased de novo lipogenesis. PC and TG are the main metabolites into which the radioactive label from [(11)C]Act is incorporated in HCC.

Hepatitis B virus X gene is implicated in liver carcinogenesis

Hepatitis B virus (HBV) is a small hepatotropic and highly species-specific enveloped DNA virus. The carcinogenicity of this virus has become focused on the X gene and its coded X protein. The X protein itself is unable to bind to DNA directly, but works as a potent transcriptional activator through multiple cis-acting elements and mediates several signal transduction cascades. Two regions of the X protein, aa.61-69 and aa.105-140, are found essential for the viral replication and expression as well. These functions interacting with transcription factors and signaling cascades are acting cooperatively to cause cell proliferation. Furthermore, the association of X protein with mitochondria causes loss of the mitochondrial membrane potential and subsequently causes cell death, the function of which is attributed to the aa.68-104 region of X protein. As a result, the X protein has two independent proliferative and cell death-promoting activities. Liver cancer has been shown to result from a series of mutations in specific oncogenes and tumor suppressor genes. In a recent study, X protein stimulates ROS generation in the mitochondria due to collapse of the membrane potential and increases the mutation frequency, that evokes malignant transformation. Inflammation as a result of HBV infection is concerned to cause DNA damage. In the past 10years, the possibility that several viral proteins directly engaged in the DNA damage has increased to some extent. From an evolutionary viewpoint, it is noteworthy that several arrangement proteins have been found in viruses. Thus, there is some clue that a small amount of X protein acts as an arrangement protein for HBV replication dependent upon cellular DNA damage due to generated ROS as an amplified signal.

Primary human hepatocytes are susceptible to infection by hepatitis delta virus assembled with envelope proteins of woodchuck hepatitis virus

Hepatitis B virus (HBV) and hepatitis delta virus (HDV) share the HBV envelope proteins. When woodchucks chronically infected with woodchuck hepatitis virus (WHV) are superinfected with HDV, they produce HDV with a WHV envelope, wHDV. Several lines of evidence are provided that wHDV infects not only cultured primary woodchuck hepatocytes (PWH) but also primary human hepatocytes (PHH). Surprisingly, HBV-enveloped HDV (hHDV) and wHDV infected PHH with comparable efficiencies; however, hHDV did not infect PWH. The basis for these host range specificities was investigated using as inhibitors peptides bearing species-specific pre-S (where S is the small envelope protein) sequences. It was found that pre-S1 contributed to the ability of wHDV to infect both PHH and PWH. In addition, the inability of hHDV to infect PWH was not overcome using a chimeric form of hHDV containing WHV S protein, again supporting the essential role of pre-S1 in infection of target cells. One interpretation of these data is that host range specificity of HDV is determined entirely by pre-S1 and that the WHV and HBV pre-S1 proteins recognize different receptors on PHH.

Lipid-mediated introduction of hepatitis B virus capsids into nonsusceptible cells allows highly efficient replication and facilitates the study of early infection events

The hepatitis B virus (HBV) is an enveloped DNA virus which is highly infectious in vivo. In vitro, only primary hepatocytes of humans and Tupaia belangeri or the novel HepaRG cell line are susceptible to HBV, but infection is inefficient and study of early infection events in single cells is unsatisfactory. Since hepatoma cells replicate the virus efficiently after transfection, this limited infection efficiency must be related to the initial entry phase. Here, we describe the lipid-based delivery of HBV capsids into nonsusceptible cells, circumventing the natural entry pathway. Successful infection was monitored by observing the emergence of the nuclear viral covalently closed circular DNA and the production of progeny virus and subviral particles. Lipid-mediated transfer initiated productive infection that was at least 100-fold more effective than infection of permissive cell cultures. High-dose capsid transfer showed that the uptake was not receptor limited and allowed the intracellular transport of capsids and genomes to be examined microscopically. The addition of inhibitors confirmed an entry pathway by fusion of the lipid with the plasma membrane. By indirect immune fluorescence and native fluorescence in situ hybridization, we followed the pathway of capsids and viral genomes in individual cells. We observed an active microtubule-dependent capsid transfer to the nucleus and a subsequent release of the viral genomes exclusively into the karyoplasm. Lipid-mediated transfer of viral capsids thus appears to allow efficient introduction of genetic information into target cells, facilitating studies of early infection events which are otherwise impeded by the small number of viruses entering the cell.

Cultivation of HepG2.2.15 on Cytodex-3: higher yield of hepatitis B virus and less subviral particles compared to conventional culture methods

BACKGROUND/AIMS

Several novel systems are available to study human hepatitis B virus (HBV) replication in cell culture demanding for efficient cell culture based systems for HBV production. The aim was to enhance HBV production of the HBV stably producing cell line HepG2.2.15 by cultivation on spherical micro substrate.

METHODS

HepG2.2.15 was cultivated on microcarrier substrate Cytodex-3. HBV specific transcripts, viral protein and genome secretion, cell proliferation and MAP kinase signaling were analyzed. Infectivity of HBV particles was analyzed using primary tupaia hepatocytes.

RESULTS

Compared to stationary flask cultures, HepG2.2.15 on Cytodex-3 secreted 18-fold more HBV genomes, more HBeAg per culture volume and less HBV surface antigen per extracellular viral genome equivalent. This was reflected by a significantly higher infectivity of supernatant derived from carrier grown HepG.2.2.15 cells tested by infection of primary tupaia hepatocytes. The amount of phosphorylated ERK-2 was significantly elevated in cells cultivated on microcarrier.

CONCLUSIONS

The cultivation of HepG2.2.15 on Cytodex-3 increased production of infectious HBV particles and decreased secretion of subviral particles compared to the stationary cell cultivation. Microcarrier cultivation activates MAP kinase signaling that is crucial for HBV replication.

Pre-s1 antigen-dependent infection of Tupaia hepatocyte cultures with human hepatitis B virus

The susceptibility of the tree shrew Tupaia belangeri to human hepatitis B virus (HBV) has been demonstrated both in vivo and in vitro. In this study, we show that purified HBV infects primary T. belangeri hepatocyte cultures in a very specific manner, as detected by HBV covalently closed circular DNA, mRNA, HBV e antigen, and HBsAg production. A monoclonal antibody (MAb), MA18/7, directed against the pre-S1 domain of the large HBs protein, which has been shown to neutralize infectivity of HBV for primary human hepatocytes, also blocked infection of primary Tupaia hepatocytes. MAbs against the pre-S2 domain of HBs inhibited infection only partially, whereas an S MAb and polyvalent anti-HBs antibodies neutralized infection completely. Thus, both pre-S1 and S antigens are necessary for infection in the tupaia. Using subviral particles, >70% of primary Tupaia hepatocytes are capable of specific binding of pre-S1-rich HBsAg, showing localization in distinct membrane areas. The data show that the early steps of HBV infection in Tupaia hepatocyte cultures are comparable to those in the human system.

Comparison of anti-hepatitis B virus activities of lamivudine and clevudine by a quantitative assay

In this study, we used a quantitative assay to measure the concentration-dependent effects of antivirals on extracellular hepatitis B virus (HBV) DNA as well as on different cytoplasmic and nuclear forms of HBV DNA that participate in HBV replication. HBV recombinant baculovirus, which efficiently delivers the HBV genome to HepG2 cells, was used for this study because (i) antivirals can be administered prior to initiation of HBV infection or after HBV infection and (ii) sufficiently high HBV replication levels are achieved that HBV covalently closed circular (CCC) DNA can be easily detected and individual HBV DNA species can be quantitatively analyzed separately from total HBV DNA. The results showed that the levels of HBV replicative intermediate and extracellular DNA decreased in a concentration-dependent fashion following antiviral treatment. The 50% effective concentration (EC(50)) and EC(90) values and the Hill slopes differed for the different HBV DNA species analyzed. The data clearly indicated that (i) nuclear HBV DNAs are more resistant to antiviral therapy than cytoplasmic or extracellular HBV DNAs and (ii) nuclear HBV CCC DNA is more resistant than the nuclear relaxed circular form. This report presents the first in vitro comparison of the effects of two antivirals administered prior to initiation of HBV infection and the first thorough in vitro quantitative study of concentration-dependent antiviral effects on HBV CCC DNA.

Drug discovery and development of antiviral agents for the treatment of chronic hepatitis B virus infection

A safe and effective vaccine for hepatitis B virus (HBV) has been available for nearly twenty years and currently campaigns to provide universal vaccination in developing countries are underway. Nevertheless, chronic HBV infection remains a leading cause of chronic hepatitis worldwide and there is a strong need for safe and effective antiviral therapies. Attempts to identify and develop antiviral agents to treat chronic HBV infection remains focused on nucleoside analogs such as 3TC (lamivudine), adefovir dipivoxil, (bis-POMPMEA), and others. However, advances in our understanding of the molecular biology of HBV and the development of new assays for HBV polymerase activity, such as the reconstitution of active HBV polymerase in vitro, should facilitate large screening efforts for non-nucleoside reverse transcriptase inhibitors. Recent advances have furthered our understanding of clinical resistance to lamivudine, have provided new approaches to treatment, and have offered new perspectives on the major challenges to the identification and development of antiviral agents for chronic HBV infection. Here, in an update to our previous review article that appeared in this series [59a], we focus on recent advances that have occurred in the areas of virus structure and replication, in vitro viral polymerase assays, cell culture systems, and animal models.

Efficient infection of primary tupaia hepatocytes with purified human and woolly monkey hepatitis B virus

The Asian tree shrew, Tupaia belangeri, has been proposed as a novel animal model for studying hepatitis B virus (HBV) infection. Here, we describe a protocol for efficient and reproducible infection of primary tupaia hepatocytes with HBV. We report that human serum interferes with HBV binding to the hepatocytes, thus limiting the maximum multiplicity of infection. Purification of HBV virions by gradient sedimentation greatly enhances virus binding and infectivity. Covalently closed circular DNA was clearly detectable by Southern blot analysis and newly synthesized single-stranded HBV DNA was visible 2 weeks postinoculation. Primary tupaia hepatocytes are also susceptible to infection with the recently discovered woolly monkey hepatitis B virus (WMHBV) but not to woodchuck hepatitis virus infection. Compared to HBV, WMHBV replicated at a higher rate with single-stranded DNA detectable within the first week postinoculation. Primary tupaia hepatocytes should represent a useful system for studying early steps of HBV and WMHBV infection.

In vitro and in vivo infectivity and pathogenicity of the lymphoid cell-derived woodchuck hepatitis virus

Woodchuck hepatitis virus (WHV) and human hepatitis B virus are closely related, highly hepatotropic mammalian DNA viruses that also replicate in the lymphatic system. The infectivity and pathogenicity of hepadnaviruses propagating in lymphoid cells are under debate. In this study, hepato- and lymphotropism of WHV produced by naturally infected lymphoid cells was examined in specifically established woodchuck hepatocyte and lymphoid cell cultures and coculture systems, and virus pathogenicity was tested in susceptible animals. Applying PCR-based assays discriminating between the total pool of WHV genomes and covalently closed circular DNA (cccDNA), combined with enzymatic elimination of extracellular viral sequences potentially associated with the cell surface, our study documents that virus replicating in woodchuck lymphoid cells is infectious to homologous hepatocytes and lymphoid cells in vitro. The productive replication of WHV from lymphoid cells in cultured hepatocytes was evidenced by the appearance of virus-specific DNA, cccDNA, and antigens, transmissibility of the virus through multiple passages in hepatocyte cultures, and the ability of the passaged virus to infect virus-naive animals. The data also revealed that WHV from lymphoid cells can initiate classical acute viral hepatitis in susceptible animals, albeit small quantities (approximately 10(3) virions) caused immunovirologically undetectable (occult) WHV infection that engaged the lymphatic system but not the liver. Our results provide direct in vitro and in vivo evidence that lymphoid cells in the infected host support propagation of infectious hepadnavirus that has the potential to induce hepatitis. They also emphasize a principal role of the lymphatic system in the maintenance and dissemination of hepadnavirus infection, particularly when infection is induced by low virus doses.

Kinetics of hepadnavirus loss from the liver during inhibition of viral DNA synthesis

Hepadnaviruses replicate by reverse transcription, which takes place in the cytoplasm of the infected hepatocyte. Viral RNAs, including the pregenome, are transcribed from a covalently closed circular (ccc) viral DNA that is found in the nucleus. Inhibitors of the viral reverse transcriptase can block new DNA synthesis but have no direct effect on the up to 50 or more copies of cccDNA that maintain the infected state. Thus, during antiviral therapy, the rates of loss of cccDNA, infected hepatocytes (1 or more molecules of cccDNA), and replicating DNAs may be quite different. In the present study, we asked how these losses compared when woodchucks chronically infected with woodchuck hepatitis virus were treated with L-FMAU [1-(2-fluoro-5-methyl-beta-L-arabinofuranosyl) uracil], an inhibitor of viral DNA synthesis. Viremia was suppressed for at least 8 months, after which drug-resistant virus began replicating to high titers. In addition, replicating viral DNAs were virtually absent from the liver after 6 weeks of treatment. In contrast, cccDNA declined more slowly, consistent with a half-life of approximately 33 to 50 days. The loss of cccDNA was comparable to that expected from the estimated death rate of hepatocytes in these woodchucks, suggesting that death of infected cells was one of the major routes for elimination of cccDNA. However, the decline in the actual number of infected hepatocytes lagged behind the decline in cccDNA, so that the average cccDNA copy number in infected cells dropped during the early phase of therapy. This observation was consistent with the possibility that some fraction of cccDNA was distributed to daughter cells in those infected hepatocytes that passed through mitosis.

Limited proteolysis induces woodchuck hepatitis virus infectivity for human HepG2 cells

Previous work from our laboratory has shown that digestion of hepatitis B virus (HBV) with V8 protease rendered the virus infectious for human hepatoblastoma cell line (HepG2). It was hypothesized that the cleavage exposes a 16 amino acid region that includes a consensus 'fusion' motif necessary to mediate infectivity. Since woodchuck hepatitis virus (WHV) and HBV possess significant homology in this region of their envelope proteins, including the V8 protease cleavage site, the possibility that WHV infectivity for HepG2 cells could be induced by V8 digestion was explored. WHV isolated from the serum of chronically infected woodchucks, digested with V8 protease, was shown to loose its preS domain. V8 digested WHV eluted from gel filtration columns with a size similar to that of undigested virus, suggesting that digestion with V8 protease did not cause significant changes in virion size. The amount of progeny virus secreted into the culture medium following infection of HepG2 cells with V8 digested WHV reached 2.5 pg/ml, after 8 days. Moreover, WHV DNA replicative intermediates could be detected in the cells following infection with protease digested, but not undigested, viruses. These data suggest that protease modification of WHV, a non-human virus, induced infectivity for human tissue culture cells. These results are consistent with the hypothesis that exposure of an amino acid region of the envelope polypeptide that contains a consensus fusion motif is important in Hepadnavirus entry.

Increased hepatocyte turnover and inhibition of woodchuck hepatitis B virus replication by adefovir in vitro do not lead to reduction of the closed circular DNA

The aim of this study was to evaluate the inhibitory effect of the nucleotide analogue adefovir on woodchuck hepatitis B virus (WHV) replication and, in particular, to determine whether the pool of covalently closed circular DNA (cccDNA) could be reduced by adefovir treatment in primary cultures of woodchuck hepatocytes isolated from a chronic carrier. Strong reduction of WHV-DNA synthesis (90%) and secretion (up to 98%) was observed with all 3 doses of adefovir used (1, 10, and 100 micromol/L), whereas in the absence of the drug, high amounts of viral particles were continuously secreted in the culture medium until the end of the study (27 days). Secretion of envelope proteins and viral RNA levels remained constant both in the adefovir-treated and -untreated cultures for the entire course of the study. Intracellular core protein levels declined by approximately 50% in all the cultures, independent of adefovir treatment. There was no indication of cccDNA loss in the adefovir-treated hepatocyte cultures even when cell turnover was induced for 14 days by the addition of epidermal growth factor (EGF) to the culture medium. Our data show that adefovir has a very strong inhibitory effect on WHV-DNA synthesis in chronically infected primary hepatocyte cultures and indicate that cccDNA is a very stable molecule that appears to be efficiently transmitted to the dividing hepatocytes.

Hepatitis B virus biology

Hepadnaviruses (hepatitis B viruses) cause transient and chronic infections of the liver. Transient infections run a course of several months, and chronic infections are often lifelong. Chronic infections can lead to liver failure with cirrhosis and hepatocellular carcinoma. The replication strategy of these viruses has been described in great detail, but virus-host interactions leading to acute and chronic disease are still poorly understood. Studies on how the virus evades the immune response to cause prolonged transient infections with high-titer viremia and lifelong infections with an ongoing inflammation of the liver are still at an early stage, and the role of the virus in liver cancer is still elusive. The state of knowledge in this very active field is therefore reviewed with an emphasis on past accomplishments as well as goals for the future.

Therapy of chronic hepatitis B virus infection: inhibition of the viral polymerase and other antiviral strategies

Chronic hepatitis B infection remains a major public health problem worldwide. The hepatitis B virus belongs to the family of hepadnaviruses that replicate their DNA genome via a reverse transcription pathway. The chronicity of infection in infected hepatocytes is maintained by the persistence of the viral covalently closed circular DNA. The main strategies to combat chronic HBV infection rely on the stimulation of the specific antiviral immune response and on the inhibition of viral replication. While the prolonged administration of reverse transcriptase inhibitors is most often associated with a control of viral replication rather than eradication, it may select for resistant mutants. The search for new viral targets is therefore mandatory to design combination strategies to prevent the emergence of resistant mutants and eventually clear viral infection.

Emergence of drug-resistant populations of woodchuck hepatitis virus in woodchucks treated with the antiviral nucleoside lamivudine

Lamivudine [(-)-beta-L-2',3'-dideoxy-3'-thiacytidine] reduces woodchuck hepatitis virus (WHV) titers in the sera of chronically infected woodchucks by inhibiting viral DNA synthesis. However, after 6 to 12 months, WHV titers begin to increase toward pretreatment levels. Three WHV variants with mutations in the active site of the DNA polymerase gene are present at this time (W. S. Mason et al., Virology 245:18-32, 1998). We have asked if these mutant viruses were responsible for the lamivudine resistance and if their emergence caused an immediate rise in virus titers. Cell cultures studies implied that the mutants were resistant to lamivudine. Emergence of mutant WHV was not always associated, however, with an immediate rise in virus titers in the serum. One of the three types of mutant viruses became prominent in serum up to 7 months before titers in serum actually began to increase, at a time when wild-type virus was still predominant in the liver. The two other mutants did not show this behavior but were detected in serum and liver later, just at the time that virus titers began to rise. A factor linking all three mutants was that a similar duration of drug administration preceded the rise in titers, irrespective of which mutant ultimately prevailed. A simple explanation for these results is that the increase in virus titers following emergence of drug-resistant mutants can occur only as the preexisting wild-type virus is cleared from the hepatocyte population, allowing spread of the mutants. Thus, prolonged suppression of virus titers in the serum may sometimes be a measure of the stability of hepatocyte infection rather than of a successful therapeutic outcome.

Metabolic labeling of woodchuck hepatitis B virus X protein in naturally infected hepatocytes reveals a bimodal half-life and association with the nuclear framework

In order to identify potential sites of hepadnavirus X protein action, we have investigated the subcellular distribution and the stability of woodchuck hepatitis virus (WHV) X protein (WHx) in primary hepatocytes isolated from woodchucks with persistent WHV infection. In vivo cell labeling and cell fractionation studies showed that the majority of WHx is a soluble cytoplasmic protein while a minor part of newly synthesized WHx is associated with a nuclear framework fraction (20%) and with cytoskeletal components (5 to 10%). Pulse-chase experiments revealed that cytoplasmic WHx has a short half-life and decays with bimodal kinetics (approximately 20 min and 3 h). The rates of association and turnover of nucleus-associated WHx suggest that compartmentalization may be responsible for the bimodal turnover observed in the cytoplasm.

Aberrant expression of a cytokeratin in a subset of hepatocytes during chronic WHV infection

Chronic infection of woodchucks with woodchuck hepatitis virus (WHV) invariably leads, within 2-4 years, to the appearance of hepatocellular carcinoma (HCC). HCC is preceded by an extended period of chronic liver damage, probably resulting from the immune response to viral antigens. It may be that infection itself also induces changes in the hepatocyte population. To begin to identify some of the changes in the liver prior to the appearance of HCC, monoclonal antibodies (MAbs) were generated from mice immunized with hepatocytes from a woodchuck chronically infected with WHV or with a tumor lysate. Immunofluorescence microscopy was used to select MAbs that reacted with host markers whose patterns of expression would distinguish chronically infected from uninfected liver or from liver tumors. One of these MAbs (2F2) reacted strongly with a subset of hepatocytes in chronically infected liver; a similar staining pattern was not detected in uninfected or transiently infected liver. Evidence is presented that this strong staining reaction reflects the overexpression or accumulation of the hepatocyte-specific intermediate filament protein, cytokeratin K18, a protein previously implicated in cryptogenic cirrhosis of the liver in humans (Ku, N. O. , Wright, T. L., Terrault, N. A., Gish, R., and Omary, M. B. J. Clin. Invest. 99: 19-23, 1997). Double immunofluorescent staining with antibodies to K18 and M-envelope protein of WHV suggested that strong reactivity to K18 was limited to cells expressing high levels of one or both of the large viral-envelope proteins, M and L; however, high expression of these viral proteins was not always associated with a strong K18 staining reaction.

The identification and development of antiviral agents for the treatment of chronic hepatitis B virus infection

Hepatitis B virus (HBV) is the leading cause of chronic hepatitis throughout the world. Notwithstanding the availability of a safe and effective vaccine, the world prevalence of HBV has not declined significantly, thus resulting in the need for a selective antiviral agent. HBV is a small, partially double-stranded DNA virus which replicates through an RNA intermediate. Most efforts to develop anti-HBV agents have been targeted to the viral DNA polymerase which possesses reverse transcriptase activity. Currently, the most promising anti-HBV agents are nucleoside analogs which interfere with viral DNA replication. Although earlier nucleoside analogs such as vidarabine (ara-A) and fialuridine (FIAU) have displayed unacceptable toxicities, newer analogs such as lamivudine (3TC), bis-POM PMEA (GS-840), lobucavir, and BMS-200,475 have demonstrated clinical utility. In particular, the use of lamivudine has generated considerable interest in the development of other L-enantiomeric nucleoside analogs for use against HBV. Here, we provide an overview of HBV structure and replication strategy and discuss the use of cell culture systems, in vitro viral polymerase systems, and animal models to identify and evaluate anti-HBV agents. We also discuss the various classes of nucleoside analogs in terms of structure, mechanism of action, status in clinical development, ability to select for resistant HBV variants, and use in combination therapies. Finally, we present a discussion of novel antiviral approaches, including antisense and gene therapy, and address the various challenges to successful anti-HBV chemotherapeutic intervention.

Lack of effect of antiviral therapy in nondividing hepatocyte cultures on the closed circular DNA of woodchuck hepatitis virus

The template for synthesis of hepadnaviral RNAs is a covalently closed circular (ccc) DNA located in the nucleus of the infected hepatocyte. Hepatocytes are normally long-lived and nondividing, and antiviral therapies in chronically infected individuals face the problem of eliminating not only the replicative forms of viral DNA found in the cytoplasm but also the cccDNA from the nucleus. Because cccDNA does not replicate semiconservatively, it is not an obvious target for antiviral therapy. However, elimination of cccDNA might be facilitated if its half-life were short in comparison to the generation time of hepatocytes and if new cccDNA formation were effectively blocked. We have therefore measured cccDNA levels in woodchuck hepatocyte cultures following in vitro infection with woodchuck hepatitis virus and treatment with inhibitors of viral DNA synthesis. The viral reverse transcriptase inhibitors lamivudine (3TC) [(-)-beta-L-2',3'-dideoxy-3'-thiacytidine), FTC (5-fluoro-2',3'-dideoxy-3'-thiacytidine) and ddC (2',3'-dideoxycytidine) were added to the cultures beginning at 4 days postinfection. Treatment for up to 36 days with 3TC reduced the amount of cccDNA in the cultures not more than twofold compared to that of an untreated control. Treatment with ddC for 36 days and with FTC for 12 days resulted in effects similar to that of treatment with 3TC. Moreover, the declines in cccDNA appeared to reflect the loss of hepatocytes from the cultures rather than of cccDNA from hepatocytes. These results emphasize the important role of the longevity of the infected hepatocytes in the persistence of an infection.

Major differences between WHV and HBV in the regulation of transcription

Studies were carried out to further characterize enhancer and promoter elements on the woodchuck hepatitis virus (WHV) genome. We were able to confirm the existence of WHV promoters analogous to the major promoters of the related human hepatitis B virus (HBV) and of an enhancer analogous to the recently described WHV E2 element (Ueda, K., Wei, Y., and Ganem, D., Virology 217, 413, 1996). However, we were unable to identity an enhancer analogous to the E1 element of (HBV), despite the fact that these two viruses share a high degree of sequence homology and genetic organization. Some factor binding sites in the E1 region appeared to be conserved between the two viruses and may be required for the activity of the overlapping X gene promoter of WHV. Others did not appear to be essential for WHV X gene promoter activity, and their functional activity, if any, was not revealed. Our failure to detect a functional enhancer element in the region of WHV homologous to the HBV E1 enhancer may indicate that (i) fundamental differences exist in transcriptional regulation of the small circular genomes of WHV and HBV; (ii) WHV contains an E1 element which is functional in the context of the intact viral genome, but which is unable to function in the context of the various expression constructs used in our experiments; or (iii) correct regulation of WHV transcription via an E1 element is dependent upon transcription factors which are not expressed in the liver-specific cell lines used in our experiments.

Long-term expression of the hepatitis B virus core-e- and X-proteins does not cause pathologic changes in transgenic mice

BACKGROUND/AIMS

Chronic infections with the human hepatitis B virus can result in liver cirrhosis and primary hepatocellular carcinoma. The reasons for these long-term effects are unclear. The aim of this study was to generate transgenic mice expressing the HBV X- and c/e-gene under authentic and foreign promoter control and to test whether the respective gene products can cause pathologic effects during the lifespan of a mouse. Moreover, the temporal and the tissue-specific regulation of the crucial HBV c/e-gene promoter was analyzed.

METHODS

Eight transgenic mouse lines were generated. Four contained the c/e- and X-gene and two contained only the X-gene under authentic promoter control. Two lines expressed only the X-gene under control of the rat insulin promoter/enhancer. Gene expression was tested by protein and mRNA analyses. During an observation period of 2 years, mice were sacrificed and organs subjected to histologic examination. Mice expressing the X-gene in pancreatic beta cells were tested for the development of diabetes.

RESULTS

In the liver, slight histopathologic alterations but no neoplastic changes could be observed in mice expressing the X-gene. Activity of the c/e-gene promoter/enhancer was age dependent and was not restricted to hepatocytes.

CONCLUSION

No evidence was obtained that long-term expression of the HBV c/e- and X-gene products can cause neoplasia during the lifespan of a mouse.

Long-term productive episomal hepatitis B virus replication in primary cultures of adult human hepatocytes infected in vitro

We have previously increased the efficiency of hepatitis B virus (HBV) infection of human hepatocytes in vitro by using polyethylene glycol. After further documenting by neutralization experiments, this in vitro infection, we used this model to define new culture conditions that would maintain stable episomal replication for several weeks. We found that in the presence of 10% porcine serum and 2% dimethyl sulphoxide (DMSO), high-density cultures survived more than 3 months, while addition of hydrocortisone instead of DMSO resulted in survival of less than 1 month. HBV episomal replication was maintained without any evidence of viral integration into the host genome. The maintenance of HBV replication was demonstrated by: first, stability of the covalently-closed-circular DNA in the nucleus and relaxed circular and single-stranded replicative intermediates in the cytoplasm; second, detection of two major transcripts of 3.5 and 2.1-2.4 kb corresponding to the pregenomic and surface genes respectively; and third, continuous secretion of mature viral particles in the supernatant of infected cells. We showed that under these culture conditions, hepatocytes were blocked in the G1 phase of the cell cycle and did not spontaneously proliferate. Upon hepatocyte growth factor (HGF) stimulation, however, the ability of hepatocytes to divide was demonstrated and was compared in infected and non-infected cells. No change in proliferative capacity and no variation in c-myc and c-jun levels could be found. Hepatocyte survival was not modified in infected cells, confirming that HBV is not cytopathic for normal human hepatocytes. These new culture conditions represent substantial progress in the study of HBV-host cell interactions.

Protease-induced infectivity of hepatitis B virus for a human hepatoblastoma cell line

The human hepatoblastoma cell line HepG2 produces and secretes hepatitis B virus (HBV) after transfection of cloned HBV DNA. Intact virions do not infect these cells, although they attach to the surface of the HepG2 cell through binding sites in the pre-S1 domain. Entry of enveloped virions into the cell often requires proteolytic cleavage of a viral surface protein that is involved in fusion between the cell membrane and the viral envelope. Recently, we observed pre-S-independent, nonspecific binding between hepatitis B surface (HBs) particles and HepG2 cells after treatment of HBs antigen particles with V8 protease, which cleaves next to a putative fusion sequence. Chymotrypsin removed this fusion sequence and did not induce binding. In this study, we postulate that lack of a suitable fusion-activating protease was the reason why the HepG2 cells were not susceptible to HBV. To test this hypothesis, virions were partially purified from the plasma of HBV carriers and treated with either staphylococcal V8 or porcine chymotrypsin protease. Protease-digested virus lost reactivity with pre-S2-specific antibody but remained morphologically intact as determined by electron microscopy. After separation from the proteases, virions were incubated with HepG2 cells at pH 5.5. Cultures inoculated with either intact or chymotrypsin-digested virus did not contain detectable levels of intracellular HBV DNA at any time following infection. However, in cultures inoculated with V8-digested virions, HBV-specific products, including covalently closed circular DNA, viral RNA, and viral pre-S2 antigen, could be detected in a time-dependent manner following infection. Immunofluorescence analysis revealed that 10 to 30% of the infected HepG2 cells produced HBV antigen. Persistent secretion of virus by the infected HepG2 cells lasted at least 14 days and was maintained during several reseeding steps. The results show that V8-digested HBV can productively infect tissue cultures of HepG2 cells. It is suggested that proteolysis-dependent exposure of a fusion domain within the envelope protein of HBV is necessary during natural infection.

Woodchuck hepatitis virus infections: very rapid recovery after a prolonged viremia and infection of virtually every hepatocyte

Earlier studies have suggested that transient hepadnavirus infections in mammals are associated with virus replication in a large fraction of hepatocytes. Although the viremia that occurred during transient infections in some individuals would presumably lead to virus replication in all hepatocytes, these studies did not reveal if this was the case. The question of the extent of hepatocyte infection was therefore reinvestigated because of the implications of the results for the mechanisms of virus clearance. Woodchucks were inoculated with woodchuck hepatitis virus, and the course of hepatic infection was determined. These studies indicated that essentially 100% of the hepatocytes became infected in the majority of woodchucks. In 7 of 10 woodchucks, the viral infection was then rapidly cleared from the liver, generally in less than 4 weeks. In another three woodchucks, though productive infection was just as rapidly cleared, viral covalently closed circular DNA remained for weeks to months after other indicators of virus infection had disappeared from the liver. Bromodeoxyuridine labeling and anti-proliferating cell nuclear antigen staining to detect hepatocytes passing through S phase indicated an increase in hepatocyte proliferation during the recovery phase of infection. The rate of cell division appeared to be sufficient to replace no more than 2 to 3% of the hepatocytes per day, at the times at which the biopsies were performed. Histopathologic evaluation of the biopsy samples did not provide evidence for a massive amount of liver regeneration. Models to explain virus clearance, with or without massive immune system-mediated destruction of infected hepatocytes, are reviewed.

In vitro culture systems for hepatitis B and delta viruses

The development of tissue culture technology has led to invaluable information in many fields of modern virology. Until recently, the lack of an in vitro culture system for the hepatitis B virus (HBV) was a considerable impediment to the study of its life cycle at the cellular and molecular levels. However, it did not prevent its isolation and molecular cloning. Such has been the case also for the hepatitis delta virus (HDV), the genome of which was cloned and sequenced before its replication could be observed in cultured cells. In recent years, tissue culture systems for HBV and HDV have been developed progressively by the identification of permissive, established cell lines for production of virions and susceptible primary hepatocyte cultures for infection assays. I will briefly review here the recent experiments that have contributed to replicate HBV and HDV in cell culture systems.

Replication of hepatitis B virus in differentiated adult rat hepatocytes transfected with cloned viral DNA

The possibility of obtaining expression of human hepatitis B virus (HBV) genes and production of virus particles in normal liver cells from heterologous species like normal adult rat hepatocytes, by transfecting the complete HBV genome, was investigated. Various techniques for hepatocyte transfection were assayed including the usual calcium-phosphate coprecipitation technique, the Pasco and Fagan modified calcium-phosphate procedure, and the lipofection technique. Transfection efficiency was determined by measuring the production of HBV surface antigen under various culture conditions. Transfection was the most efficient when assayed 1 or 2 days after hepatocyte plating at low density. Few variations in the efficiency were observed between the different transfection procedures. We show that under these culture conditions, replication of HBV can be achieved in differentiated adult rat hepatocytes. Synthesis of relaxed circular and single-stranded DNA forms and of viral transcripts including pregenome RNA occurred in the cells whereas viral antigens and mature and immature viral particles were released into the culture medium. The production of viral proteins was always higher in hepatocytes cocultivated with rat liver epithelial cells and maintained at a low density. In contrast, viral replication was not obtained by transfecting undifferentiated rat liver epithelial cells. These results demonstrate that replication of HBV can occur in hepatocytes from mammalian species non-closely related to primates and strongly support the idea that attachment of the virus and its penetration into the cells are critical steps in the host-specificity of the infection process and that hepatic-specific regulating factors could be essential for viral replication.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of hepatitis delta virus and suppression of helper hepatitis B virus in a human hepatoma cell line

The hepatitis delta virus (HDV) is a defective virus with a coat composing of the surface antigen of its helper virus, hepatitis B virus (HBV). Replication of HDV in the absence of HBV has been shown in cell cultures by transient transfection of the HDV plasmid. However, the formation and release of HDV virions have not been observed. In this report, a human hepatoma cell line HuH-7 was transiently cotransfected with HDV and HBV plasmids. The production of monomeric and multimeric antigenomic RNAs of HDV in the transfected cells indicated replication of the HDV genome. The major 3.5- and 2.1-kb RNAs of HBV were also expressed. Virions of both HDV and HBV were released from the cotransfected cells, as shown by the detection of monomeric genomic HDV RNA and partially double-stranded HBV DNA in the culture medium. Thus, this is the first report that describes the assembly and the release of HDV viral particles in an in vitro cell culture. The HDV virions released possessed physicochemical properties identical to those of the HDV virions found in infected human serum. Furthermore, expression of both the 3.5- and 2.1-kb RNAs of HBV was shown to be dramatically decreased by the presence of HDV, indicating suppression of the expression of HBV genes by HDV. The amount of HBV virions released was similarly suppressed by HDV. Cotransfection of HBV with an expression plasmid of the HDV delta antigen remarkably reduced the levels of the 3.5- and 2.1-kb HBV RNAs, indicating that suppression of the expression of HBV RNAs by HDV occurs via the action of the delta antigen. This HBV- and HDV-cotransfected human hepatoma cell line should provide an excellent system for the study of the function of the delta antigen and the interaction between HDV and its helper, HBV.

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.

Antiviral strategies in chronic hepatitis B virus infection: I. Establishment of an in vitro system using the duck hepatitis B virus model

Primary duck hepatocyte (PDH) cultures were established from ducklings congenitally infected with the duck hepatitis B virus (DHBV), plated onto feeder cell layers of irradiated human embryonic lung fibroblasts, and observed for 2 to 3 weeks. This system permitted the survival of the PDH in a differentiated form free of fibroblastic overgrowth for at least 3 weeks. The hepatocytes were shown to contain all the replicative DNA intermediates found during DHBV replication as well as the DHBV structural proteins PRE-S1, PRE-S2, and S of duck hepatitis B surface antigen (DHBsAg). The pool of supercoiled (SC) DHBV DNA increased dramatically from days 10 to 14 postplating. This PDH-feeder cell layer cell culture model provides a convenient system to study the effects of conventional inhibitors of DHBV replication and compounds targeted at the supercoiled form of DHBV DNA. This approach should allow the evaluation of a variety of strategies for treating chronic carriers of hepadnaviruses.

In hepatocytes infected with duck hepatitis B virus, the template for viral RNA synthesis is amplified by an intracellular pathway

During the productive phase of chronic hepadnaviral infections, virion DNA synthesis occurs in the cytoplasm of the infected hepatocyte, but viral RNA is synthesized in the nucleus, apparently from a covalently closed, circular (CCC) viral DNA. J. Tuttleman, C. Pourcel, and J. Summers (1986a, Cell 47, 451-460) have shown that the intracellular levels of CCC DNA can increase during initiation of infection of duck hepatocytes in vitro with duck hepatitis B virus and during long term culture of infected duck hepatocytes in vitro. This amplification of CCC DNA occurs through the reverse transcription pathway. To distinguish between an entirely intracellular process of amplification and amplification due to multiple infections by extracellular virus in the virus producing cultures, suramin was added to the infected cultures to block superinfection. We found that CCC DNA amplification occurred at least as efficiently in the presence of suramin as in its absence. First, there was a net increase in the total amount of CCC DNA in the cultures both in the presence and in the absence of suramin. Second, synthesis of CCC DNA in the presence and absence of suramin was observed by density labeling of this viral DNA by growth of the cultures in medium containing BUdR. Amplification was also demonstrable in the presence of neutralizing duck antibodies. These results support the hypothesis of Tuttleman et al. (1986a) that CCC DNA amplification in chronically infected cultures and, by inference, the mechanism of persistent infection involves primarily intracellular regulatory mechanisms.