Combination Treatment with the Vimentin-Targeting Antibody hzVSF and Tenofovir Suppresses Woodchuck Hepatitis Virus Infection in Woodchucks

Current treatment options for patients infected with hepatitis B virus (HBV) are suboptimal, because the approved drugs rarely induce cure due to the persistence of the viral DNA genome in the nucleus of infected hepatocytes, and are associated with either severe side effects (pegylated interferon-alpha) or require life-long administration (nucleos(t)ide analogs). We report here the evaluation of the safety and therapeutic efficacy of a novel, humanized antibody (hzVSF) in the woodchuck model of HBV infection. hzVSF has been shown to act as a viral entry inhibitor, most likely by suppressing vimentin-mediated endocytosis of virions. Targeting the increased vimentin expression on liver cells by hzVSF after infection with HBV or woodchuck hepatitis virus (WHV) was demonstrated initially. Thereafter, hzVSF safety was assessed in eight woodchucks naïve for WHV infection. Antiviral efficacy of hzVSF was evaluated subsequently in 24 chronic WHV carrier woodchucks by monotreatment with three ascending doses and in combination with tenofovir alafenamide fumarate (TAF). Consistent with the proposed blocking of WHV reinfection, intravenous hzVSF administration for 12 weeks resulted in a modest but transient reduction of viral replication and associated liver inflammation. In combination with oral TAF dosing, the antiviral effect of hzVSF was enhanced and sustained in half of the woodchucks with an antibody response to viral proteins. Thus, hzVSF safely but modestly alters chronic WHV infection in woodchucks; however, as a combination partner to TAF, its antiviral efficacy is markedly increased. The results of this preclinical study support future evaluation of this novel anti-HBV drug in patients.

Innate and adaptive immunity associated with resolution of acute woodchuck hepatitis virus infection in adult woodchucks

Viral and/or host factors that are directly responsible for the acute versus chronic outcome of hepatitis B virus (HBV) infection have not been identified yet. Information on immune response during the early stages of HBV infection in humans is mainly derived from blood samples of patients with acute hepatitis B (AHB), which are usually obtained after the onset of clinical symptoms. Features of intrahepatic immune response in these patients are less studied due to the difficulty of obtaining multiple liver biopsies. Woodchuck hepatitis virus (WHV) infection in woodchucks is a model for HBV infection in humans. In the present study, five adult woodchucks were experimentally infected with WHV and then followed for 18 weeks. Blood and liver tissues were frequently collected for assaying markers of WHV replication and innate and adaptive immune responses. Liver tissues were further analyzed for pathological changes and stained for important immune cell subsets and cytokines. The increase and subsequent decline of viral replication markers in serum and liver, the elicitation of antibodies against viral proteins, and the induction of virus-specific T-cell responses indicated eventual resolution of acute WHV infection in all animals. Intrahepatic innate immune makers stayed unchanged immediately after the infection, but increased markedly during resolution, as determined by changes in transcript levels. The presence of interferon-gamma and expression of natural killer (NK) cell markers suggested that a non-cytolytic response mechanism is involved in the initial viral control in liver. This was followed by the expression of T-cell markers and cytolytic effector molecules, indicating the induction of a cytolytic response mechanism. Parallel increases in regulatory T-cell markers suggested that this cell subset participates in the overall immune cell infiltration in liver and/or has a role in regulating AHB induced by the cytolytic response mechanism. Since the transcript levels of immune cell markers in blood, when detectable, were lower than in liver, and the kinetics, except for NK-cells and interferon-gamma, did not correlate well with their intrahepatic expression, this further indicated enrichment of immune cells within liver. Conclusion: The coordinated interplay of innate and adaptive immunity mediates viral clearance in the woodchuck animal model of HBV infection. The initial presence of NK-cell associated interferon-gamma response points to an important role of this cytokine in HBV resolution.

Antiviral Efficacy and Host Immune Response Induction during Sequential Treatment with SB 9200 Followed by Entecavir in Woodchucks

SB 9200, an orally bioavailable dinucleotide, activates the viral sensor proteins, retinoic acid-inducible gene 1 (RIG-I) and nucleotide-binding oligomerization domain-containing protein 2 (NOD2) causing the induction of the interferon (IFN) signaling cascade for antiviral defense. The present study evaluated the overall antiviral response in woodchucks upon induction of immune response, first with SB 9200 followed by Entecavir (ETV) versus reduction of viral burden with ETV followed by SB 9200 immunomodulation. Woodchucks chronically infected with woodchuck hepatitis virus (WHV) were treated orally with SB 9200 (30 mg/kg/day) and ETV (0.5 mg/kg/day). Group 1 received ETV for 4 weeks followed by SB 9200 for 12 weeks. Group 2 received SB 9200 for 12 weeks followed by ETV for 4 weeks. At the end of treatment in Group 2, average reductions of 6.4 log₁₀ in serum WHV DNA and 3.3 log₁₀ in WHV surface antigen were observed whereas in Group 1, average reductions of 4.2 log₁₀ and 1.1 log₁₀ in viremia and antigenemia were noted. Both groups demonstrated marked reductions in hepatic WHV nucleic acid levels which were more pronounced in Group 2. Following treatment cessation and the 8-week follow-up, recrudescence of viral replication was observed in Group 1 while viral relapse in Group 2 was significantly delayed. The antiviral effects observed in both groups were associated with temporally different induction of IFN-α, IFN-β, and IFN-stimulated genes in blood and liver. These results suggest that the induction of host immune responses by pretreatment with SB 9200 followed by ETV resulted in antiviral efficacy that was superior to that obtained using the strategy of viral reduction with ETV followed by immunomodulation.

Quantitative evaluation of 2-deoxy-2[F-18]fluoro-D-glucose-positron emission tomography imaging on the woodchuck model of hepatocellular carcinoma with histological correlation

PURPOSE

The Eastern woodchuck (Marmota monax) is considered as a naturally occurring animal model of hepatocellular carcinoma (HCC). The performance of 2-deoxy-2-[F-18]fluoro-D-glucose (FDG) for imaging HCC on the woodchuck using Positron emission tomography (PET) was investigated in this study.

PROCEDURES

Dynamic FDG-PET scans were performed on five woodchucks with HCC and one healthy woodchuck before removal and processing of the liver tissues for histology. The parameters of a two-tissue compartment model with dual input were estimated using weighted least squares (WLS).

RESULTS

Ten HCCs were confirmed histologically. Six HCCs had a tumor-to-liver standardized uptake value (SUV) ratio < or =1.15, a k (4) / k (3) ratio similar to that in hepatic tissues and were well-differentiated. Four HCCs had a tumor-to-liver SUV ratio >1.15, a lower k (4) / k (3) ratio than the hepatic tissues and were moderately differentiated.

CONCLUSIONS

Increased FDG uptake was observed in HCCs that were the least differentiated and correlated with a lower k (4) / k (3) ratio.

The woodchuck as an animal model for pathogenesis and therapy of chronic hepatitis B virus infection

This review describes the woodchuck and the woodchuck hepatitis virus (WHV) as an animal model for pathogenesis and therapy of chronic hepatitis B virus (HBV) infection and disease in humans. The establishment of woodchuck breeding colonies, and use of laboratory-reared woodchucks infected with defined WHV inocula, have enhanced our understanding of the virology and immunology of HBV infection and disease pathogenesis, including major sequelae like chronic hepatitis and hepatocellular carcinoma. The role of persistent WHV infection and of viral load on the natural history of infection and disease progression has been firmly established along the way. More recently, the model has shed new light on the role of host immune responses in these natural processes, and on how the immune system of the chronic carrier can be manipulated therapeutically to reduce or delay serious disease sequelae through induction of the recovery phenotype. The woodchuck is an outbred species and is not well defined immunologically due to a limitation of available host markers. However, the recent development of several key host response assays for woodchucks provides experimental opportunities for further mechanistic studies of outcome predictors in neonatal- and adult-acquired infections. Understanding the virological and immunological mechanisms responsible for resolution of self-limited infection, and for the onset and maintenance of chronic infection, will greatly facilitate the development of successful strategies for the therapeutic eradication of established chronic HBV infection. Likewise, the results of drug efficacy and toxicity studies in the chronic carrier woodchucks are predictive for responses of patients chronically infected with HBV. Therefore, chronic WHV carrier woodchucks provide a well-characterized mammalian model for preclinical evaluation of the safety and efficacy of drug candidates, experimental therapeutic vaccines, and immunomodulators for the treatment and prevention of HBV disease sequelae.

Bicistronic woodchuck hepatitis virus core and gamma interferon DNA vaccine can protect from hepatitis but does not elicit sterilizing antiviral immunity

The immunity elicited against nucleocapsid of hepatitis B virus (HBV) and closely related woodchuck hepatitis virus (WHV) has been shown to be important in resolution of hepatitis and protection from infection. Further, activity of gamma interferon (IFN-gamma), which may directly inhibit hepadnavirus replication, promotes antiviral defense and favors T helper cell type 1 (Th1) response, which is seemingly a prerequisite of HBV clearance. In this study, to enhance induction of protective immunity against hepadnavirus, healthy woodchucks were immunized with a bicistronic DNA vaccine carrying WHV core (WHc) and woodchuck IFN-gamma (wIFN-gamma) gene sequences. Three groups, each group containing three animals, were injected once or twice with 0.5 mg, 0.9 mg, or 1.5 mg per dose of this vaccine. In addition, four animals received two injections of 0.6 mg or 1 mg WHc DNA alone. All animals were challenged with WHV. The results showed that four of nine animals injected with the bicistronic vaccine and one of four immunized with WHc DNA became protected from serologically evident infection and hepatitis. This protection was not linked to induction of WHc antigen-specific antibodies or T-cell proliferative response and was not associated with enhanced transcription of Th1 cytokines or 2',5'-oligoadenylate synthetase. Strikingly, all animals protected from hepatitis became reactive for WHV DNA and carried low levels of replicating virus in hepatic and lymphoid tissues after challenge with WHV. This study shows that the bicistronic DNA vaccine encoding both hepadnavirus core antigen and IFN-gamma was more effective in preventing hepatitis than that encoding virus core alone, but neither of them could mount sterile immunity against the virus or prevent establishment of occult infection.

Altered proteolysis and global gene expression in hepatitis B virus X transgenic mouse liver

Hepatitis B virus X (HBX) is essential for the productive infection of hepatitis B virus (HBV) in vivo and has a pleiotropic effect on host cells. We have previously demonstrated that the proteasome complex is a cellular target of HBX, that HBX alters the proteolytic activity of proteasome in vitro, and that inhibition of proteasome leads to enhanced viral replication, suggesting that HBX and proteasome interaction plays a crucial role in the life cycle and pathogenesis of HBV. In the present study, we tested the effect of HBX on the proteasome activities in vivo in a transgenic mouse model in which HBX expression is developmentally regulated by the mouse major urinary promoter in the liver. In addition, microarray analysis was performed to examine the effect of HBX expression on the global gene expression profile of the liver. The results showed that the peptidase activities of the proteasome were reduced in the HBX transgenic mouse liver, whereas the activity of another cellular protease was elevated, suggesting a compensatory mechanism in protein degradation. In the microarray analysis, diverse genes were altered in the HBX mouse livers and the number of genes with significant changes increased progressively with age. Functional clustering showed that a number of genes involved in transcription and cell growth were significantly affected in the HBX mice, possibly accounting for the observed pleiotropic effect of HBX. In particular, insulin-like growth factor-binding protein 1 was down-regulated in the HBX mouse liver. The down-regulation was similarly observed during acute woodchuck hepatitis virus infection. Other changes including up-regulation of proteolysis-related genes may also contribute to the profound alterations of liver functions in HBV infection.

The win locus involved in activation of the distal N-myc2 gene upon WHV integration in woodchuck liver tumors harbors S/MAR elements

Woodchuck hepatitis virus (WHV) and the woodchuck (Marmota monax) are models for hepatocellular carcinoma (HCC) induced by hepatitis B virus (HBV). In woodchuck liver tumors, the N-myc2 proto-oncogene is frequently activated by WHV integration either close to the gene or in the b3n and win downstream loci, located 10 and 150 kb from N-myc2, respectively. A scaffold/matrix attachment region (S/MAR) regulative element was shown to be in b3n, possibly mediating activation of the upstream N-myc2 gene upon WHV integration. To investigate if S/MAR elements are in win too, a 17-kb DNA fragment corresponding to the major region of WHV insertion in this locus was cloned and sequenced. Overlapping subcloned fragments spanning candidate S/MARs predicted by sequence analysis were tested by standard in vitro binding assays. Results showed the presence of two S/MAR elements in win. The distribution of previously described WHV insertions relative to the S/MARs reinforces the hypothesis that S/MARs nearby distal WHV insertions might be involved in long-range activation of N-myc2.

Helper-dependent adenoviral vector-mediated delivery of woodchuck-specific genes for alpha interferon (IFN-alpha) and IFN-gamma: IFN-alpha but not IFN-gamma reduces woodchuck hepatitis virus replication in chronic infection in vivo

Alpha interferon (IFN-alpha) and IFN-gamma are able to suppress hepadnavirus replication. The intrahepatic expression of high levels of IFN may enhance the antiviral activity. We investigated the effects of woodchuck-specific IFN-alpha (wIFN-alpha) and IFN-gamma(wIFN-gamma) on woodchuck hepatitis virus (WHV) replication in vivo by helper-dependent adenoviral (HD-Ad) vector-mediated gene transfer. The expression of biologically active IFNs was demonstrated in vitro after transduction of woodchuck cells with HD-Ad vectors encoding wIFN-alpha (HD-AdwIFN-alpha) or wIFN-gamma (HD-AdwIFN-gamma). The transduction efficacy of the HD-Ad vector in woodchuck liver in vivo was tested with a vector expressing green fluorescence protein (GFP). Immunohistochemical staining of liver samples on day 5 after injection showed expression of GFP in a high percentage of liver cells surrounding the central vein. The transduction of livers of WHV carriers in vivo with HD-AdwIFN-alpha or HD-AdwIFN-gamma induced levels of biologically active IFN, which could be measured in the sera of these animals. Expression of wIFN-alpha in the liver reduced intrahepatic WHV replication and WHV DNA in sera of about 1 log step in two of two woodchucks. Transduction with HD-AdwIFN-gamma, however, reduced WHV replicative intermediates only slightly in two of three animals, which was not accompanied with significant changes in the WHV DNA in sera. We demonstrated for the first time the successful HD-Ad vector-mediated transfer of genes for IFN-alpha and IFN-gamma in vivo and timely limited reduction of WHV replication by wIFN-alpha, but not by wIFN-gamma.

Interferon-gamma-associated responses to woodchuck hepatitis virus infection in neonatal woodchucks and virus-infected hepatocytes

Acute hepatitis and recovery from woodchuck hepatitis virus (WHV) infection involves increased intrahepatic expression of interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) mRNAs. In the present study, recovery correlated with increased intrahepatic expression of mRNAs for major histocompatibility complex class 1 (MHC1), beta(2)-microglobulin, 2'5'-oligoadenylate synthetase (2'5'-OAS), and indoleamine dioxygenase (IDO). By comparison, acute WHV infection progressing to chronicity was associated with diminished expression of these IFN-gamma-associated mRNAs in liver. Transfection of WHV-infected primary hepatocytes (WPH) from WHV carriers with an IFN-gamma-expressing plasmid (pIFN-gamma) resulted in dose-dependent accumulations of MHC1, TNF-alpha, 2'5'-OAS, and IDO mRNAs within 96 h. Markers of T cells and immune-mediated cytotoxicity that accumulate in recovering liver were not apparent in WPH based on the relative lack of CD3, CD4, Fas ligand, perforin, and granzyme B mRNAs. Expression of pIFN-gamma, and TNF-alpha-expressing plasmid (pTNF-alpha), did not affect total WHV RNA, or fully double-stranded WHV DNA in WPH, but each reduced some of the replicative intermediate (RI) species of WHV DNA synthesis. WPH treated with recombinant IFN-alpha protein had a higher fold induction of 2'5'-OAS mRNA associated with partial reductions in WHV RNAs and the major RI species. Thus, IFN-gamma expression in carrier WPH induced several host responses often observed in liver of recovering woodchucks, and impaired a stage of WHV DNA synthesis by a non-cytolytic mechanism mediated by TNF-alpha. Local enhancement of IFN-gamma-associated responses in chronic WHV-infected hepatocytes may promote therapeutic antiviral effects, but additional effector mechanisms evident during recovery appear necessary for more complete clearance of WHV infection.

Low doses of hepadnavirus induce infection of the lymphatic system that does not engage the liver

Woodchuck hepatitis virus (WHV), which is closely related to human hepatitis B virus and is considered to be principally hepatotropic, invades the host's lymphatic system and persists in lymphoid cells independently of whether the infection is symptomatic and serologically evident or concealed. In this study, we show, with the woodchuck model of hepatitis B, that hepadnavirus can establish an infection that engages the lymphatic system, but not the liver, and persists in the absence of virus serological markers, including antiviral antibodies. This primary occult infection is caused by wild-type virus invading the host at a quantity usually not greater than 10(3) virions. It is characterized by trace virus replication progressing in lymphatic organs and peripheral lymphoid cells that, with time, may also spread to the liver. The infection is transmissible to virus-naive hosts as an asymptomatic, indefinitely long, occult carriage of small amounts of biologically competent virus. In contrast to residual silent WHV persistence, which normally endures after the resolution of viral hepatitis and involves the liver, primary occult infection restricted to the lymphatic system does not protect against reinfection with a large, liver-pathogenic WHV dose; however, the occult infection is associated with a swift recovery from hepatitis caused by the superinfection. Our study documents that the lymphatic system is the primary target of WHV infection when small quantities of virions invade a susceptible host.

In vivo transmission and dynamics of deleted genomes after experimental infection of woodchuck hepatitis B virus in adult animals

The presence of Deleted Genomes has been shown in a number of viral models including Hepadnaviridae. The analysis of woodchuck hepatitis B virus (WHV) population after experimental infection of woodchuck 197 (W197) with WHV7-PI inoculum revealed the presence of two Deleted Genomes: DG600 lacking a 1330 bp region (Core/Polymerase/PreS1) and DG900 showing a deletion of 869 nts (Pol/PreS/S). These mutants were also present in WHV7-PI. The successive WHV experimental infections in adult animals were performed using W197-w7 inoculum containing DG600 and DG900. Infections were divided into three groups presenting different patterns of viral replication, different presence of markers, occurrence of variants and persistence of infection. The first group displayed 2-3 weeks viremic phase and WHV-DNA titres of 10-30 ng/ml; the second a longer viremic phase (8-9 weeks) and higher WHV-DNA titres (up to 78 ng/ml). In contrast, the third group exhibited lifetime presence of WHV-DNA and WHVeAg in serum and viral replication in liver. The Deleted Genomes were transmitted in the newly infected animals with the same genomic organization. DG600 was persistently found only in chronically infected woodchuck, whereas a different pattern of presence was described for DG900. The characterization of these classes of deleted mutants in woodchuck-WHV model raises new questions on the link between DGs and persistent infections.

Helicobacter marmotae sp. nov. isolated from livers of woodchucks and intestines of cats

Woodchucks (Marmota monax) have a high incidence of hepatocellular carcinoma (HCC) associated with chronic infection with woodchuck hepatitis virus (WHV) and serve as a model of hepatitis B virus-associated HCC in humans. Helicobacter hepaticus, an enterohepatic helicobacter in mice, is known to cause hepatocellular adenomas and carcinomas in susceptible mouse strains. In long-term chemical bioassays conducted with B6C3F(1) mice, H. hepaticus has been regarded as a confounding factor because of its tumor-promoting activity. In order to determine if woodchucks harbor a Helicobacter sp. that might play a role in potentiating hepatic inflammation or neoplasia, a study was undertaken to determine whether woodchucks' livers were infected with a Helicobacter sp. Frozen liver samples from 20 (17 WHV-infected and 3 noninfected) woodchucks, 10 with WHV-associated hepatic tumors and 10 without tumors, were cultured by microaerobic techniques and analyzed by using genus- and species-specific helicobacter PCR primers. A 1,200-bp Helicobacter sp.-specific sequence was amplified from 14 liver samples. Southern hybridization confirmed the specific identity of the PCR products. Nine of the 10 livers with tumors had positive Helicobacter sp. identified by PCR, whereas 5 of the 10 livers without tumors were positive. By use of 16S rRNA species-specific primers for H. marmotae, two additional liver samples from the nontumor group had positive PCR amplicons confirmed by Southern hybridization. A urease-, catalase-, and oxidase-positive bacterium was isolated from one liver sample from a liver tumor-positive woodchuck. By 16S rRNA analysis and biochemical and phenotypic characteristics, the organism was classified as a novel Helicobacter sp. Subsequently, four additional bacterial strains isolated from feces of cats and characterized by biochemical, phenotypic, and 16S rRNA analysis were determined to be identical to the woodchuck isolate. We propose the name Helicobacter marmotae sp. nov. for these organisms. Further studies are required to ascertain if this novel Helicobacter sp. plays a tumor promotion role in hepadnavirus-associated tumors in woodchucks or causes enterohepatic disease in cats.

The woodchuck model of hepatitis B virus infection

The woodchuck hepatitis virus (WHV) was the first of the mammalian and avian hepadnaviruses described after discovery of the virus of hepatitis B (HBV). Woodchucks chronically infected with WHV develop progressively severe hepatitis and hepatocellular carcinoma, which present as lesions that are remarkably similar to those associated with HBV infection in humans. The initial virological studies and studies of pathogenesis utilized woodchucks that had been trapped in the wild and had acquired WHV infection naturally. Research with wild woodchucks was complicated by lack of knowledge of their backgrounds (e.g., dietary history, exposure to parasites or environmental toxins, and source and duration of WHV infection). Breeding colonies of woodchucks have been established and maintained in laboratory animal facilities, and laboratory-reared woodchucks are superior for experimental studies of pathogenesis or hepatocarcinogenesis. It is possible to infect neonatal woodchucks born in the laboratory with standardized inocula and produce a high rate of chronic WHV carriers that are useful for controlled investigations. WHV has been shown experimentally to cause hepatocellular carcinoma, supporting conclusions based on epidemiological and molecular virological studies that HBV is an important etiological factor in human hepatocarcinogenesis. Chronic WHV carrier woodchucks have become a valuable animal model for the preclinical evaluation of antiviral therapy for HBV infection, providing useful pharmacokinetic and pharmacodynamic results in a relevant animal disease model. It also has been shown that the pattern of toxicity and hepatic injury observed in woodchucks treated with certain fluorinated pyrimidines is remarkably similar to that observed in humans that were treated with the same drugs, suggesting the woodchuck has significant potential for the preclincial assessment of antiviral drug toxicity.

Animal models of hepatitis delta virus infection and disease

Hepatitis delta virus (HDV) is a defective RNA virus with similarities to unusual subviral pathogens of higher plants. It requires hepatitis B virus (HBV) for its replication/transmission, and HBV-infected humans are the only established host. HDV causes both severe acute hepatitis and rapidly progressive chronic disease in some individuals. The HDV life cycle involves remarkable features, such as ribozyme- mediated autocatalytic processes, Pol II-directed RNA synthesis from a single-stranded circular RNA template, and RNA editing. Much of our understanding of the nature of this pathogen derives from experimental studies in the chimpanzee model of HBV infection. The hepadnavirus-infected eastern woodchuck also is capable of supporting HDV replication and offers opportunities for the development of control strategies that might be applicable to human type D hepatitis.

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.

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.

Correct binding of viral X protein to UVDDB-p127 cellular protein is critical for efficient infection by hepatitis B viruses

A fully effective treatment of chronic human hepatitis B virus (HBV) infection is still missing and HBV remains the first etiological agent of liver cancer. Although the viral regulatory X protein is essential for infection, its mode of action remains obscure, due the lack of an in vitro infection system. In the accompanying study, we showed the functional importance of interaction between X and the host protein UVDDB-p127, in the transactivation and apoptotic properties of the viral protein. Here, we addressed the biological role of X-UVDDB interaction in the infectious process using a genetic approach in the woodchuck virus closely related to HBV. We show that (i) mutations in X, which markedly affect UVDDB-binding, also abolished productive infection in woodchucks, (ii) in the few cases where mutant viruses led to infection, compensatory mutations had occurred in the X gene of the viral progeny, which restored correct UVDDB-binding. We conclude that efficient viral replication in vivo requires proper X-UVDDB interaction. The interaction may thus provide a novel therapeutic target for the treatment of hepatitis

Effects of age and viral determinants on chronicity as an outcome of experimental woodchuck hepatitis virus infection

Acute hepadnavirus infections either resolve or progress to chronicity. Factors that influence chronicity as an outcome of hepatitis B virus (HBV) infection in humans can be studied experimentally in the woodchuck model. Accordingly, several woodchuck hepatitis virus (WHV) inocula were characterized. Representative inocula had high titers of infectious virus (approximately 10(7.7)-10(9.5) woodchuck 50% infectious doses per milliliter [WID(50%)/mL] by subcutaneous inoculation), with 1 WID(50%) ranging between 21 and 357 physical virion particles. WHV7P1 (standard high dose, 5 x 10(6) WID(50%)) produced a 72% chronicity rate (i.e., percent chronic of total infected) in neonatal woodchucks (1-3 days old). Comparable doses of WHV8P1 resulted in a lower chronicity rate in neonates (34% chronic) indicating that it represented a strain different from WHV7P1. Neonatal woodchucks were more susceptible to chronic infection by high doses of WHV7P1 (range, 65%-75% chronic) compared with 8-week-old weanlings (33% chronic) and adult woodchucks (0% chronic; i.e., all resolved). High doses of cloned wild-type viruses also induced high rates of chronicity in neonates (70%-80% chronic). Chronicity rates in neonates were decreased for low doses of WHV7P1 (500 WID(50%), 9% chronic) and for high doses of a precore WHeAg-minus mutant WHV8 clone (17% chronic). Thus, both age and viral determinants can influence chronicity as an outcome of experimental WHV infection. Standardized inocula will enable the study of mechanisms that initiate and maintain chronic hepadnavirus infection and also provide a means for developing WHV carriers for therapeutic studies.

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.

Hepatocarcinogenesis in woodchuck hepatitis virus/c-myc mice: sustained cell proliferation and biphasic activation of insulin-like growth factor II

Transgenic mice carrying the c-myc oncogene under control of woodchuck hepatitis virus (WHV) DNA sequences invariably develop hepatocellular carcinoma (HCC), despite a temporally limited expression of the transgene in the neonatal liver. To better characterize the different steps of the tumorigenic process, we analyzed the liver expression of the c-myc transgene and several growth-related genes by in situ hybridization and Northern blotting. In parallel studies, proliferated changes were investigated by detection of bromodeoxy-uridine-positive S-phase nuclei and apoptosis was evaluated by in situ nick end-labeling of DNA. During the neonatal period, high levels of c-myc messenger RNAs (mRNAs) were detected in all hepatocytes, and the expression of insulin-like growth factor II (IGF II) was frequently enhanced, correlating with increased cell proliferation. Despite elevated expression of the p53 gene, no change in liver cell apoptosis was observed. After weaning, c-myc transgene expression decreased to undetectable levels in all hepatocytes, whereas proliferation decreased but remained notably higher than in age-matched controls. The expression of c-fos, c-jun, and c-H-ras was highly variable during the preneoplastic period and in the tumors, with no consistent increase compared with controls. Resurgence of c-myc transgene expression was evidenced in all cells from hyperplastic lesions and carcinomas, accompanied with frequent focal reactivation of IGF II. Thus the strong proliferative stimulus induced by the combined effects of c-myc and IGF II in the neonatal liver might initiate a process characterized by persistent, dysregulated hepatocyte proliferation, in turn greatly increasing the risk of hepatocellular transformation.

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.

Woodchuck hepatitis virus X protein is required for viral infection in vivo

The X gene of the mammalian hepadnaviruses is believed to encode a protein of 17 kDa which has been shown to transactivate a wide range of viral and cellular promoters. The necessity for X gene expression during the viral life cycle in vivo has recently been suggested (H.-S. Chen, S. Kaneko, R. Girones, R. W. Anderson, W. E. Hornbuckle, B. C. Tennant, P. J. Cote, J. L. Gerin, R. H. Purcell, and R. H. Miller, J. Virol. 67:1218-1226, 1993). We have independently constructed two variants of woodchuck hepatitis virus (WHV) with mutations in the X coding region. Transient transfection of two different hepatoma cell lines showed that these WHV X gene mutants were competent for virus replication in vitro. To determine whether X expression was required for viral replication in vivo, we injected mutant and wild-type genomes into the livers of susceptible woodchucks. While the wild-type WHV genomes were infectious in all animals examined, the mutant genomes did not initiate a WHV infection in woodchucks. These results indicate that the X gene of the hepadnaviruses plays a major role in viral replication in vivo.