Three recently described animal virus models for human hepatitis B virus

Molecular Mechanisms and Animal Models of HBV-Related Hepatocellular Carcinoma: With Emphasis on Metastatic Tumor Antigen 1

Hepatocellular carcinoma (HCC) is an important cause of cancer death worldwide, and hepatitis B virus (HBV) infection is a major etiology, particularly in the Asia-Pacific region. Lack of sensitive biomarkers for early diagnosis of HCC and lack of effective therapeutics for patients with advanced HCC are the main reasons for high HCC mortality; these clinical needs are linked to the molecular heterogeneity of hepatocarcinogenesis. Animal models are the basis of preclinical and translational research in HBV-related HCC (HBV-HCC). Recent advances in methodology have allowed the development of several animal models to address various aspects of chronic liver disease, including HCC, which HBV causes in humans. Currently, multiple HBV-HCC animal models, including conventional, hydrodynamics-transfection-based, viral vector-mediated transgenic, and xenograft mice models, as well as the hepadnavirus-infected tree shrew and woodchuck models, are available. This review provides an overview of molecular mechanisms and animal models of HBV-HCC. Additionally, the metastatic tumor antigen 1 (MTA1), a cancer-promoting molecule, was introduced as an example to address the importance of a suitable animal model for studying HBV-related hepatocarcinogenesis.

Application of the woodchuck animal model for the treatment of hepatitis B virus-induced liver cancer

This review describes woodchucks chronically infected with the woodchuck hepatitis virus (WHV) as an animal model for hepatocarcinogenesis and treatment of primary liver cancer or hepatocellular carcinoma (HCC) induced by the hepatitis B virus (HBV). Since laboratory animal models susceptible to HBV infection are limited, woodchucks experimentally infected with WHV, a hepatitis virus closely related to HBV, are increasingly used to enhance our understanding of virus-host interactions, immune response, and liver disease progression. A correlation of severe liver pathogenesis with high-level viral replication and deficient antiviral immunity has been established, which are present during chronic infection after WHV inoculation of neonatal woodchucks for modeling vertical HBV transmission in humans. HCC in chronic carrier woodchucks develops 17 to 36 mo after neonatal WHV infection and involves liver tumors that are comparable in size, morphology, and molecular gene signature to those of HBV-infected patients. Accordingly, woodchucks with WHV-induced liver tumors have been used for the improvement of imaging and ablation techniques of human HCC. In addition, drug efficacy studies in woodchucks with chronic WHV infection have revealed that prolonged treatment with nucleos(t)ide analogs, alone or in combination with other compounds, minimizes the risk of liver disease progression to HCC. More recently, woodchucks have been utilized in the delineation of mechanisms involved in innate and adaptive immune responses against WHV during acute, self-limited and chronic infections. Therapeutic interventions based on modulating the deficient host antiviral immunity have been explored in woodchucks for inducing functional cure in HBV-infected patients and for reducing or even delaying associated liver disease sequelae, including the onset of HCC. Therefore, woodchucks with chronic WHV infection constitute a well-characterized, fully immunocompetent animal model for HBV-induced liver cancer and for preclinical evaluation of the safety and efficacy of new modalities, which are based on chemo, gene, and immune therapy, for the prevention and treatment of HCC in patients for which current treatment options are dismal.

Duck Hepatitis B Virus cccDNA Amplification Efficiency in Natural Infection Is Regulated by Virus Secretion Efficiency

Previous mutation based studies showed that ablating synthesis of viral envelope proteins led to elevated hepadnaviral covalently closed circular DNA (cccDNA) amplification, but it remains unknown how cccDNA amplification is regulated in natural hepadnaviral infection because of a lack of research system. In this study we report a simple procedure to prepare two identical duck hepatitis B virus inocula, but they possess 10-100-fold difference in cccDNA amplification in infected cell culture. We demonstrate that the infected cells with higher cccDNA amplification significantly reduce the virus secretion efficiency that results in higher accumulation of relaxed circular DNA (rcDNA) and DHBsAg in the cells. The infected cells with lower cccDNA amplification significantly increase the virus secretion efficiency that leads to lower intracellular rcDNA and DHBsAg accumulation. In contrast with the findings generated in the mutation based experimental system, the regulation of cccDNA amplification in natural hepadnaviral infection bypasses direct regulation of the cellular envelope proteins concentration, instead it modulates virus secretion efficiency that ultimately impacts the intracellular rcDNA concentration, an important factor determining the destination of the synthesized rcDNA in infected cells.

HBV induced HCC: major risk factors from genetic to molecular level

Hepatocellular carcinoma (HCC) is a deadly and emerging disease leading to death in Asian countries. High hepatitis B virus (HBV) load and chronic hepatitis B (CHB) infection increase the risk of developing HCC. HBV is a DNA virus that can integrate DNA into host genome thereby increase the yield of transactivator protein HBxAg that may deregulate many pathways involving in metabolism of cells. Several monogenic and polygenic risk factors are also involved in HCC development. This review summarizes the mechanism involved in HCC development and discusses some promising therapies to make HCC curative.

Transcriptome analysis of duck liver and identification of differentially expressed transcripts in response to duck hepatitis A virus genotype C infection

BACKGROUND

Duck is an economically important poultry and animal model for human viral hepatitis B. However, the molecular mechanisms underlying host-virus interaction remain unclear because of limited information on the duck genome. This study aims to characterize the duck normal liver transcriptome and to identify the differentially expressed transcripts at 24 h after duck hepatitis A virus genotype C (DHAV-C) infection using Illumina-Solexa sequencing.

RESULTS

After removal of low-quality sequences and assembly, a total of 52,757 unigenes was obtained from the normal liver group. Further blast analysis showed that 18,918 unigenes successfully matched the known genes in the database. GO analysis revealed that 25,116 unigenes took part in 61 categories of biological processes, cellular components, and molecular functions. Among the 25 clusters of orthologous group categories (COG), the cluster for "General function prediction only" represented the largest group, followed by "Transcription" and "Replication, recombination, and repair." KEGG analysis showed that 17,628 unigenes were involved in 301 pathways. Through comparison of normal and infected transcriptome data, we identified 20 significantly differentially expressed unigenes, which were further confirmed by real-time polymerase chain reaction. Of the 20 unigenes, nine matched the known genes in the database, including three up-regulated genes (virus replicase polyprotein, LRRC3B, and PCK1) and six down-regulated genes (CRP, AICL-like 2, L1CAM, CYB26A1, CHAC1, and ADAM32). The remaining 11 novel unigenes that did not match any known genes in the database may provide a basis for the discovery of new transcripts associated with infection.

CONCLUSION

This study provided a gene expression pattern for normal duck liver and for the previously unrecognized changes in gene transcription that are altered during DHAV-C infection. Our data revealed useful information for future studies on the duck genome and provided new insights into the molecular mechanism of host-DHAV-C interaction.

Therapeutic vaccination in chronic hepatitis B: preclinical studies in the woodchuck

Recommended treatment of chronic hepatitis B with interferon-α and/or nucleos(t)ide analogues does not lead to a satisfactory result. Induction of HBV-specific T cells by therapeutic vaccination or immunotherapies may be an innovative strategy to overcome virus persistence. Vaccination with commercially available HBV vaccines in patients did not result in effective control of HBV infection, suggesting that new formulations of therapeutic vaccines are needed. The woodchuck (Marmota monax) is a useful preclinical model for developing the new therapeutic approaches in chronic hepadnaviral infections. Several innovative approaches combining antiviral treatments with nucleos(t)ide analogues, DNA vaccines, and protein vaccines were tested in the woodchuck model. In this paper we summarize the available data concerning therapeutic immunization and gene therapy using recombinant viral vectors approaches in woodchucks, which show encouraging results. In addition, we present potential innovations in immunomodulatory strategies to be evaluated in this animal model.

Liver cell transformation in chronic HBV infection

Epidemiological studies have provided overwhelming evidence for a causal role of chronic HBV infection in the development of hepatocellular carcinoma (HCC), but the molecular mechanisms underlying virally-induced tumorigenesis remain largely debated. In the absence of a dominant oncogene encoded by the HBV genome, indirect roles have been proposed, including insertional activation of cellular oncogenes by HBV DNA integration, induction of genetic instability by viral integration or by the regulatory protein HBx, and long term effects of viral proteins in enhancing immune-mediated liver disease. In this chapter, we discuss different models of HBV-mediated liver cell transformation based on animal systems of hepadnavirus infection as well as functional studies in hepatocyte and hepatoma cell lines. These studies might help identifying the cellular effectors connecting HBV infection and liver cell transformation.

Recombinant hepatitis B surface antigen and anionic phospholipids share a binding region in the fifth domain of beta2-glycoprotein I (apolipoprotein H)

Human beta2-glycoprotein I (beta 2GPI) binds to recombinant hepatitis B surface antigen (rHBsAg), but the location of the binding domain on beta 2GPI is unknown. It has been suggested that the lipid rather than the protein moiety of rHBsAg binds to beta 2GPI. Since beta 2GPI binds to anionic phospholipids (PL) through its lipid-binding region in the fifth domain of beta 2GPI, we predicted that this lipid-binding region may also be involved in binding rHBsAg. In this study, we examined rHBsAg binding to two naturally occurring mutants of beta 2GPI, Cys306Gly and Trp316Ser, or evolutionarily conserved hydrophobic amino acid sequence, Leu313-Ala314-Phe315 in the fifth domain of beta 2GPI. The two naturally occurring mutations and two mutagenized amino acids, Leu313Gly or Phe315Ser, disrupted the binding of recombinant beta 2GPI (rbeta 2GPI) to both rHBsAg and cardiolipin (CL), an anionic PL. These results suggest that rHBsAg and CL share the same region in the fifth domain of beta2GPI. Credence to this conclusion was further provided by competitive ELISA, where CL-bound rbeta 2GPI was incubated with increasing amounts of rHBsAg. As expected, pre-incubation of rbeta 2GPI with CL precluded binding to rHBsAg, indicating that CL and rHBsAg bind to the same region on beta 2GPI. Our data provide evidence that the lipid (PL) rather than the protein moiety of rHBsAg binds to beta 2GPI and that this binding region is located in the fifth domain of beta 2GPI, which also binds to anionic PL.

Age-related differences in amplification of covalently closed circular DNA at early times after duck hepatitis B virus infection of ducks

Inoculation of 3-day-old (3D) or 3-week-old (3W) ducklings with duck hepatitis B virus results in chronic or transient infection, respectively. We previously showed that rapid production of neutralizing antibody following inoculation of 3W ducklings prevents virus from spreading in the liver and leads to a transient infection (Y.-Y. Zhang and J. Summers, J. Virol. 78:1195-1201, 2004). In this study we further investigated early events of viral infection in both 3D and 3W ducks. We present evidence that a lower level of virus replication in the hepatocytes of 3W birds is an additional factor that probably favors transient infection. We suggest that lower virus replication is due to a less rapid covalently closed circular DNA amplification, leading to lower viremias and a slower spread of infection in the liver, and that the slower spread of infection in 3W ducks makes the infection more sensitive to interruption by the host immune responses.

Hepatocellular carcinoma in the woodchuck model of hepatitis B virus infection

The Eastern woodchuck ( Marmota monax ) harbors a DNA virus (Woodchuck hepatitis virus [WHV]) that is similar in structure and replicative life cycle to the human hepatitis B virus (HBV). Like HBV, WHV infects the liver and can cause acute and chronic hepatitis. Furthermore, chronic WHV infection in woodchucks usually leads to development of hepatocellular carcinoma (HCC) within the first 2-4 years of life. The woodchuck model has been important in the preclinical evaluation of safety and efficacy of the antiviral drugs now in use for treatment of HBV infection and continues to serve as an important, predictive model for innovative forms of therapy of hepatitis B using antiviral nucleosides and immune response modifiers alone or in combination. Almost all woodchucks that become chronic WHV carriers after experimental neonatal inoculation develop HCC with a median HCC-free survival of 24 months and a median life expectancy of 30-32 months. The woodchuck model of viral-induced HCC has been used effectively for the development of new imaging agents for enhancement of detection of hepatic neoplasms by ultrasound and magnetic resonance imaging. The chemoprevention of HCC using long-term antiviral nucleoside therapy has been shown in the woodchuck, and "proof of principal" has been established for some of the innovative, molecular methods for treatment of HCC. The model is available for fundamental investigations of the viral and molecular mechanisms responsible for hepatocarcinogenesis and should have substantial value for future development of innovative methods for chemoprevention and gene therapy of human HCC.

Cell culture and animal models of viral hepatitis. Part I: hepatitis B

Despite the existence of a preventative vaccine, HBV represents a substantial threat to public health, suggesting the need for research to develop new treatments to combat the disease. The authors review the available in vitro and in vivo models, including recently developed transgenic and chimeric mouse models.

Rapid production of neutralizing antibody leads to transient hepadnavirus infection

Hepatitis B virus (HBV) frequently causes transient infections in adults but chronic infections in infants. The basis of these age-related outcomes is not known. Infection of ducks with duck hepatitis B virus (DHBV) displays a similar dependence of outcome on the age of the host at the time of infection. In this study we compared the infection of ducks at 3 days and 3 weeks of age. We found that the efficiency of infection of hepatocytes by virus in the inoculum was similar between the two age groups but that spread of the infection throughout the liver was severely inhibited in the 3-week-old-old ducks, while a rapid spread of the infection was observed in 3-day-old ducklings. Inhibition of virus spread was accompanied by the appearance in the serum of virus neutralizing activity, as assayed by blocking of infection of primary hepatocyte cultures. Neutralizing activity appeared as early as 1 or 2 days postinfection and increased during the next 2 weeks. Depletion of immunoglobulins from serum eliminated the neutralizing activity. The specific depletion of IgM indicated that IgM appeared as the dominant fraction of neutralizing antibody in the first 2 days postinfection, but declined from day 3 on while IgG antibody rose. We conclude that excess neutralizing antibody arising rapidly in birds inoculated at 3 weeks of age but not in newly hatched ducks prevented secondary cycles of infection, resulting in a limited infection in the liver and contributing to the eventual transient outcome of the infection.

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.

Preventing the spread of hepatitis B and C viruses: where are germicides relevant?

Hepatitis B virus (HBV) and hepatitis C virus (HCV) are the most prevalent bloodborne pathogens. Infections caused by these organisms can become chronic and may lead to liver cirrhosis and carcinoma. Limited chemotherapy is now available, but only HBV can be prevented through vaccination. Both viruses are enveloped and relatively sensitive to many physical and chemical agents; their ability to survive in the environment may not be as high as often believed. As a result, their spread occurs mainly through direct parenteral or percutaneous exposure to tainted body fluids and tissues. Careful screening of and avoiding contact with such materials remain the most effective means of protection. Nevertheless, the indirect spread of these viruses, although much less common, can occur when objects that are freshly contaminated with tainted blood enter the body or contact damaged skin. Germicidal chemicals are important in the prevention of HBV and HCV spread through shared injection devices, sharps used in personal services (such as tattooing and body piercing), and heat-sensitive medical/dental devices (such as flexible endoscopes) and in the cleanup of blood spills. Microbicides in vaginal gels may also interrupt their transmission. General-purpose environmental disinfection is unlikely to play a significant role in the prevention of the transmission of these viruses. Testing of low-level disinfectants and label claims for such products against HBV and HCV should be discouraged. Both viruses remain difficult to work with in the laboratory, but closely related animal viruses (such as the duck HBV) and the bovine viral diarrhea virus show considerable promise as surrogates for HBV and HCV, respectively. Although progress in the culturing of HBV and HCV is still underway, critical issues on virus survival and inactivation should be addressed with the use of these surrogates.

Purification of woodchuck hepatitis surface antigen using a monoclonal antibody raised against the antigen

Woodchuck hepatitis virus (WHV) is closely related to the human hepatitis B virus (HBV) and infection of woodchucks with WHV creates a useful model for studies of immunity, pathogenesis and therapy of HBV infection. To increase the usefulness of this model, monoclonal antibodies were raised to woodchuck hepatitis surface antigen (WHsAg) and one of these antibodies was used to purify the antigen by affinity chromatography from serum, a simpler and quicker method of purification than the current ultracentrifugation methods. The bands found by SDS-polyacrylamide gel electophoresis of WHsAg were the major 25 and 29 kilodalton (kDa) bands and a triplet of 45, 51 and 55 kDa which are thought to be the glycosylated and unglycosylated middle and large WHsAg. Both the antibody and the antigen are valuable reagents for the study of WHV infection.

Antiviral activity of beta-L-2',3'-dideoxy-2',3'-didehydro-5-fluorocytidine in woodchucks chronically infected with woodchuck hepatitis virus

The L-nucleoside analog beta-L-2',3'-dideoxy-2',3'-didehydro-5-fluorocytidine (beta-L-Fd4C) was first shown to exhibit potent activity against hepatitis B virus (HBV) in tissue culture and then to significantly inhibit viral spread during acute infection in the duck HBV model (F. Le Guerhier et al., Antimicrob. Agents Chemother. 44:111-122, 2000). We have therefore examined its antiviral activity in a mammalian model of chronic HBV infection, the woodchuck chronically infected with woodchuck hepatitis virus (WHV). Side-by-side comparison of beta-L-Fd4C and lamivudine administered intraperitoneally during short-term and long-term protocols demonstrated a more profound inhibition of viremia in beta-L-Fd4C-treated groups. Moreover, beta-L-Fd4C induced a marked inhibition of intrahepatic viral DNA synthesis compared with that induced by lamivudine. Nevertheless, covalently closed circular (CCC) DNA persistence explained the lack of clearance of infected hepatocytes expressing viral antigens and the relapse of WHV replication after drug withdrawal. Liver histology showed a decrease in the inflammatory activity of chronic hepatitis in woodchucks receiving beta-L-Fd4C. An electron microscopy study showed the absence of ultrastructural changes of hepatic mitochondria, biliary canaliculi, and bile ducts. However, a loss of weight was observed in all animals, whatever the treatment, as was a transient skin pigmentation in all woodchucks during beta-L-Fd4C treatment. There was no evidence that lamivudine or beta-L-Fd4C could prevent the development of hepatocellular carcinoma with the protocols used. These results indicate that beta-L-Fd4C exhibits a more potent antiviral effect than lamivudine in the WHV model but was not able to eradicate CCC DNA and infected cells from the liver at the dosage and with the protocol used.

Animal models of hepadnavirus-associated hepatocellular carcinoma

Animal models of hepatitis B virus infection have been valuable for determining the mechanisms of hepadnavirus replication, for studies of pathogenesis, and for investigations of viral hepatocarcinogenesis. The woodchuck model also seems to be useful in the discovery and development of antiviral drugs to treat HBV infection and for testing new forms of immunotherapy. In particular, the woodchuck seems to be ideal for studying the effect of antiviral treatment and immunotherapy on the outcome of hepadnavirus infection and on survival. The median life expectancy of experimentally infected, chronic WHV carriers is approximately 29 months, and almost all develop HCC. New types of prophylaxis or therapy can be evaluated under controlled experimental conditions, in a relevant animal model, and within a reasonable time frame.

Isolation, characterization and biological significance of hepatitis B virus mutants from serum of a patient with immunologically negative HBV infection

BACKGROUND/AIM

A low-titered hepatitis B virus infection without immunological markers was identified in a prospective study in the serum of a kidney transplant recipient by PCR. The aim of this study was to analyze HBV genomes and their biological significance.

METHODS

The genome was amplified in two overlapping fragments A and B. Sequencing of 22 clones of the A- and 12 clones of the B-fragment revealed a heterogeneous virus population. A consensus and a mutant sequence were computed, representing the complete sequence of the virus population. The two sequences were compared with 41 published genomes of the different HBV geno- and serotypes.

RESULTS

Ninety-five point mutations and two deletions were identified. Two mutations were observed in all clones and 17 other mutations in three or more clones. The deletions were found in ten and seven of 22 clones. They were located in the C-gene and led to stop codons yielding truncated e- and/or core proteins. In vitro transfection of DNA constructs containing these deletions demonstrated a stop of HBV replication and of HBeAg expression. Cotransfection experiments demonstrated a dominant negative effect of the mutants containing the deletions. In addition, we describe new variants of naturally occurring HBsAg mutants that may cause HBV infection less detectable by standard HBsAg measurement assays. They were characterized by two point mutations which were observed in 9 of 12 and 13 of 22 clones of the S-gene. They significantly reduced the HBsAg expression in in vitro transfection experiments.

CONCLUSION

We found a patient with low-titered HBV infection, with mutations of the 'a' epitope of the Santigen as well as with mutations leading to truncated core proteins which may cause a dominant negative effect.

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.

Hepatitis B virus in hepatocarcinogenesis

Hepatitis B virus (HBV) is an important etiologic agent of chronic hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Although the mechanism whereby HBV causes HCC is not fully understood, it is likely that there are many relevant molecular pathways that contribute to the development of HBV-associated HCC. This review provides an overview of some of these proposed pathways and their relative importance. It also raises questions on basic and translational research that will signficantly contribute to the better understanding of underlying mechanisms, prevention, and treatment of this tumor type.

Perforin and Fas/Fas ligand-mediated cytotoxicity in acute and chronic woodchuck viral hepatitis

The Fas ligand (FasL)/Fas and the perforin-granzyme cytotoxic pathways presumably play a central role in the development of hepatocellular injury in viral hepatitis. To recognize the potential contribution of FasL and perforin-based cell killing in hepadnaviral infection, we adopted a cytotoxic assay using murine Fas+ P815 and human Fas- K562 cells as targets. Freshly isolated peripheral blood mononuclear cells (PBMC) from woodchucks with newly acquired woodchuck hepatitis virus (WHV) infection (n = 6), with chronic WHV hepatitis (n = 9), and from healthy animals (n = 11) were used as effector cells. We have found that woodchuck lymphoid cells kill cell targets via both the FasL/Fas and the perforin death pathways. The contribution of Fas-dependent cytolysis was ascertained in blocking experiments with anti-Fas antibody and by incubation of PBMC with cyclohexamide to prevent de novo synthesis of FasL. The involvement of the perforin pathway was confirmed by treatment of K562 cells with colchicine to inhibit the microtubule-dependent perforin release. Comparative analysis showed that peripheral lymphoid cells from acute WHV hepatitis, but not those from chronic WHV infection, are more cytotoxic and that this increase seems to be entirely due to activation of perforin-mediated killing. The data indicate that acute infection in woodchucks is associated with the augmented capacity of lymphoid cells to elicit perforin-dependent killing, but in chronic infection, independent of the severity of liver disease and duration of chronicity, these cells have the same or lower cytotoxic potential as PBMC from healthy controls. These findings suggest a role for non-specific cellular immunity, presumably natural killer (NK) cells, in the control of early WHV infection and in the progression of chronic hepatitis.

Detection of the human hepatitis B virus X-protein in transgenic mice after radioactive labelling at a newly introduced phosphorylation site

Besides the three essential genes encoding the envelope, core and polymerase proteins, all mammalian hepadnaviruses examined to date contain a fourth gene which is referred to as the x-gene. This gene is believed to encode a transcriptional transactivator which positively regulates viral gene expression. Attempts to detect X-protein in vivo or in tissue culture lead to varying results. Whereas some groups could detect a protein of the expected size, other groups did not. To establish optimal conditions for the isolation of the human hepatitis B virus X-protein, we introduced a recognition site for protein kinase A into the x-gene. Upon phosphorylation with radioactive ATP, this modified X-protein can be detected with very high specificity and sensitivity. Tissue culture experiments showed that X-protein expressed from a cytomegalovirus-driven plasmid is not soluble in non-ionic detergent but rather has to be extracted from the cell pellet by boiling with SDS at a slightly alkaline pH. This method was then used to examine the organs of several transgenic mouse lines which expressed the modified x-gene under control of the authentic promoter. The data show that expression of the x-gene and subsequent biosynthesis of the X-protein is not tissue-specific but rather can occur in most organs.

Hepatitis B virus transgenic mouse model of chronic liver disease

A model for hepatitis B virus-associated chronic liver disease has been made using cloned hepatitis B virus DNA as a transgene in a severe combined immunodeficient host. These mice consistently support virus gene expression and replication. After adoptive transfer of unprimed, syngeneic splenocytes, these mice cleared virus from liver and serum, and developed chronic liver disease. This model will permit identification of the host and virus contributions to chronic liver disease in the absence of tolerance.

Hepatocellular carcinoma

Hepatocellular carcinoma (HCC) for most patients is a terminal complication of chronic inflammatory and fibrotic liver disease. With regrettably few exceptions, treatment is largely palliative, and long-term survival is rare. However, the major causes of HCC worldwide are known and preventable. Hepatitis B and C exist only in man; the viruses have no known non-human reservoirs. Transmission of the viruses can be interrupted by vaccination against hepatitis B virus infection and improvements in medical techniques for hepatitis C, for which no vaccine has yet been developed.

Treatment of duck hepatitis B virus by antisense poly-2'-O-(2,4-dinitrophenyl)-oligoribonucleotides

The poly-2'-O-(2,4-dinitrophenyl)-oligoribonucleotide (poly-DNP-RNA) with antisense sequence 5'ggguguauggaaaagccguc-3' was designed to target the sequence 2468-2487 in the polymerase gene of duck hepatitis B virus (DHBV). The stereochemically pure RNA was synthesized by using T7 RNA polymerase with synthetic DNA template and subsequently derivatized with 2,4-dinitrofluorobenzene in mild basic conditions to make the poly-DNP-RNA with an average DNP/base ratio of 0.7. In vitro studies showed that this antisense poly-DNP-RNA can hybridize with sense DNA and has high resistance to RNase A digestion. These poly-DNP-RNA were also found to be potent sequence-independent inhibitors of the reverse transcriptase activity of DHBV DNA-polymerase. For in vivo studies, DHBV-infected ducks were treated with antisense, sense, and random noncomplementary sequence poly-DNP-RNA, respectively. The data showed that the antisense poly-DNP-RNA completely inhibited the duck viremia in all nine ducks that had been treated with a dose of 1 mg/kg (i.v.) per day for 25 days. The viremia did not come back after 10 months recession. In the sense group, three of the four ducks showed no inhibition, and in the random group, both ducks maintained their viremia. After 45 days of treatment with the antisense poly-DNP-RNA, followed by 2 weeks of recession, PCR as well as QC-PCR assay and microscopic examination showed that viral DNA had disappeared in liver and that the histology of the damaged liver (filled with fat granules) had returned to normal.

Aflatoxin exposure produces serum alphafetoprotein elevations and marked oval cell proliferation in young male Pekin ducklings

Feeding of aflatoxin to ducks produces extensive oval cell proliferation in the liver associated with a prolonged elevation of serum alphafetoprotein (AFP). Short term feeding of 0.075-0.6 microgram/g of aflatoxin to young male Pekin ducks results in rapid and massive dose-related proliferation of "oval" cells, which extend from the portal zone across the hepatic lobule within three to five weeks. Longer term feeding of 0.15 microgram/g and 0.3 microgram/g results in prolonged elevations of serum AFP. Prolonged elevation of serum AFP serves as a marker of oval cell proliferation preceding hepatocellular carcinoma (HCC) development. These results confirm that ducks are sensitive to low amounts of aflatoxin and develop early lesions that have been shown in other studies to be associated with hepatocarcinogenesis. These findings in ducks support the likelihood that aflatoxin exposure contributes to the risk for development of HCC in humans.

Metabolic activation of aflatoxin B1 to aflatoxin B1-8,9-epoxide in woodchucks undergoing chronic active hepatitis

Chronic hepatitis B virus infection as well as consumption of food contaminated with the mycotoxin aflatoxin B1 are considered to be 2 major risk factors for the development of primary liver cancer in humans. Furthermore, epidemiological surveys indicate that hepatitis B virus and aflatoxin B1 might act synergistically to induce primary liver cancer. In the present study, we have tested the hypothesis that the metabolic activation of aflatoxin B1 to aflatoxin B1-8,9-epoxide, the ultimate mutagenic and carcinogenic mycotoxin metabolite, is enhanced in an experimental model of chronic hepatitis using woodchucks, chronically infected with the woodchuck hepatitis virus. Woodchuck liver microsomes were incubated with radiolabeled aflatoxin B1, the resulting aflatoxin B1-8,9-epoxide was trapped as a glutathione conjugate and its formation rate was determined by a reversed-phase HPLC analysis. In woodchuck hepatitis virus-positive woodchucks, activation of aflatoxin B1 to aflatoxin B1-8,9-epoxide was reduced when compared to woodchuck hepatitis virus-free animals, and the extent of the reduction was dependent on the severity of the hepatitis. Hence, at least in woodchucks, a chronic hepadnaviral infection does not lead to an enhanced activation of aflatoxin B1.

Woodchuck hepatitis virus-induced carcinoma as a relevant natural model for therapy of human hepatoma

BACKGROUND

Eastern American woodchuck (Marmota monax), naturally infected with woodchuck hepatitis virus, a virus similar to human hepatitis B virus, develops liver cancer with a high prevalence.

AIMS

The aim of this work was to assess Marmota monax as a model of human hepatocellular carcinoma, especially to assess new potential adjuvant therapies after surgical resection.

METHODS

Forty-four woodchuck hepatitis virus-infected animals were regularly screened by ultrasound examination from the age of 18 months and for a 30-month period. One or more liver tumors were diagnosed in 31 animals (70%). Five of them with multifocal tumor or poor general status were considered unsuitable for surgery. The other 26 were operated on. At laparotomy no tumor was found in three.

RESULTS

The 18 liver tumors studied were hepatocellular carcinomas, grossly and microscopically similar to human hepatocellular carcinoma. Peritumoral parenchyma studied in 13 specimens was always non-cirrhotic but adequate staining demonstrated patterns of fibrosis in four cases. Clear evidence of chronic active hepatitis, periportal hepatitis and steatosis were demonstrated in five, seven and one of the 13 specimens, respectively. Tumors were treated by tumorectomy in eight animals, by alcoholization in seven and by laser photocoagulation in one. A simple tumor biopsy was performed in the other seven. Ten animals died postoperatively. All the survivors in the tumorectomy group died from tumor recurrence within 10-18 months after surgery.

CONCLUSIONS

It is concluded that woodchuck hepatitis virus-induced liver carcinoma is a natural model of human hepatocellular carcinoma with similar pathology and natural history, including early ultrasonic detection and tumor recurrence after resection. Tumor excision is feasible in this animal model, which now provides the basis for assessment of new potential adjuvant therapies for human hepatocellular carcinoma in an attempt to reduce the high recurrence rate after surgical resection in humans.

Woodchuck hepatitis virus X protein is present in chronically infected woodchuck liver and woodchuck hepatocellular carcinomas which are permissive for viral replication

The woodchuck hepatitis virus (WHV) X gene (WHx) is required for infectivity of WHV in woodchucks, and the gene encodes a broadly acting transcription factor. Several lines of evidence from cell culture and transgenic mice suggest that X proteins can promote hepatocarcinogenesis. To determine whether WHx-encoded proteins are present during persistent infection and hepatocellular carcinoma (HCC) in woodchucks, we surveyed livers and HCCs from a panel of WHV carrier woodchucks for the presence of WHx by utilizing an immunoprecipitation-Western blot (immunoblot) procedure. We detected a single 15.5-kDa WHx gene product in 100% of the persistently infected livers but not in livers from animals which had recovered from acute infection or in those of uninfected woodchucks. Analysis of HCCs revealed that all of the tumors which contained WHV replication intermediates were also positive for WHx. In contrast, WHx was undetectable in HCCs which did not contain replicative intermediates. Subcellular localization studies detected WHx in the cytoplasm but not in the nuclei of primary woodchuck hepatocytes. Comparative immunoprecipitation experiments revealed that there were 4 X 10(4) to 8 X 10(4) molecules of WHx per primary woodchuck hepatocyte. Four lines of WHx transgenic mice did not develop HCC spontaneously. However, when one line was treated with diethylnitrosamine, the occurrence of precancerous lesions was enhanced compared with that in diethylnitrosamine-treated nontransgenic controls. The apparent absence of WHx in some woodchuck HCCs indicates that WHx may not be required to maintain the tumor phenotype, whereas its presence in all persistently infected livers leaves open the possibility that it plays a role in hepatocarcinogenesis.

In vivo antiviral effects of mismatched double-stranded RNA on duck hepatitis B virus

The antiviral activity and ability of mismatched double-stranded RNA (m-dsRNA), r(I)n.r(C12-U)n, to induce interferon (IFN) were evaluated in ducks chronically infected with duck hepatitis B virus (DHBV). When m-dsRNA was administered intravenously at a single dose of 5 mg/kg, serum DHBV DNA concentrations decreased significantly for 3 days (P < 0.002). However, the DHBV DNA concentrations returned to the pretreatment levels 4 days after treatment. Inhibition of DHBV DNA replication in the liver was also observed 2 days after treatment. Serum IFN activity peaked 3 hours after administration of m-dsRNA, then rapidly declined. 2'-5' Oligo-adenylate synthetase (2'-5'AS) activity increased gradually after treatment and remained elevated for at least 48 hours. In ducks receiving m-dsRNA once daily for 7 consecutive days, serum DHBV DNA concentrations on the last day of treatment were decreased by 76 +/- 12% (P < 0.05) in ducks that received 0.2 mg of m-dsRNA per kg and by 65 +/- 12% (P < 0.05) in ducks that received 1 mg of m-dsRNA per kg. This decrease persisted for at least 2 weeks after the cessation of treatment in all ducks. These results suggest that m-dsRNA effectively inhibits DHBV replication in vivo, and that IFN induction and stimulation of 2'-5'AS activity contribute to the inhibition of DHBV replication by m-dsRNA.

Evaluation of anti-hepadnavirus activity of Phyllanthus amarus and Phyllanthus maderaspatensis in duck hepatitis B virus carrier Pekin ducks

Extracts of the two traditional Indian herbs, Phyllanthus amarus (P. amarus) and Phyllanthus maderaspatensis (P. maderaspatensis), described by others as useful in the treatment of chronic hepatitis B virus infection were studied for antiviral properties on duck hepatitis B virus infection. One hundred and fourteen ducks infected posthatch with the duck hepatitis B virus (DHBV) were divided into groups at three months of age and treated intraperitoneally with the aqueous, butanol, and alcoholic extracts of these two plants at doses of 25, 50, or 200 mg/kg body weight. Saline-treated animals served as controls. In the ducks negative for DHBV in serum after treatment, we observed replicative intermediates in the liver. There was no definite antiviral property observed in the treated ducks.

Immuno disc assay for screening duck hepatitis B surface antigen in serum, liver tissue and cultured hepatocytes

An immuno disc assay (IDA) for semi-quantitative analysis of the surface antigen (DHBsAg) of duck hepatitis B virus (DHBV) is described. Unpurified antigen preparations were adsorbed onto punched-out nitrocellulose membrane discs. Rabbit antiserum raised against serum-derived gradient-purified DHBsAg was used for detecting the antigen. Cross-reacting antibodies in the rabbit antiserum were removed using normal duck serum and normal duck hepatocytes. The sensitivity of the IDA was compared with that of the Western blot analysis and was observed to be of the same order, but differed slightly for DHBsAg in liver and sera. In contrast to Western blot analysis, antigen specificity for the IDA included the S-protein. Immunodetection was carried out in microtitre plates, but the procedure was accelerated by attaching the antigen-adsorbed discs to an adhesive plate sealer. The IDA was exemplified for measuring DHBsAg in duck serum, duck liver homogenates and viral protein synthesis in cultures of DHBV-infected hepatocytes.

Evaluation of Phyllanthus amarus and Phyllanthus maderaspatensis as agents for postexposure prophylaxis in neonatal duck hepatitis B virus infection

The therapeutic potential of plant extracts of Phyllanthus amarus and Phyllanthus maderas patensis for postexposure prophylaxis against infection by Hepadnaviruses was studied in ducklings infected by the duck hepatitis B virus (DHBV). Forty-four Pekin ducklings were inoculated intraperitoneally with DHBV at 24 hr post-hatch. They were treated by intraperitoneal injection of Phyllanthus amarus (aqueous extract) (100 mg/kg body weight) or Phyllanthus mad eraspatensis (alcoholic extract) (100 mg/kg body weight) for a period of 4 weeks. Infected ducklings treated with saline served as controls. Weekly serum samples obtained before, during, and after treatment were analysed for the presence of DHBV DNA in serum by dot blot hybridisation using alpha 32P-labelled probes. Liver tissue was collected after killing the ducks at various time intervals and was studied for replicative status of the viral DNA and liver histopathology; 17 of 21 ducks were viraemic on completion of treatment with Phyllanthus amarus. At 16 week posttreatment follow-up four of seven animals remained viraemic. Similar results were obtained with Phyllanthus maderaspatensis. There was no alteration in DHBV replication in the liver. No toxicity was observed with this treatment. These observations suggest that Phyllanthus amarus and Phyllanthus maderaspatensis are not useful as therapeutic agents for postexposure prophylaxis against DHBV infection.

Identification of a binding site in the hepatitis B virus RNA pregenome for the viral Pol gene product

The hepatitis B virus, although containing a DNA genome, replicates by reverse transcription of an RNA pregenome. The viral Pol gene encodes the reverse transcriptase which catalyzes viral DNA synthesis. To study the interaction of this protein with HBV RNA, the entire Pol gene product was expressed except its eight amino-terminal codons in Escherichia coli as fusion protein with beta-galactosidase. In the absence of competing nucleic acids full-length expression products were able to nonspecifically bind in vitro synthesized HBV RNAs of different polarity and length. However, if competed with an excess of unspecific RNA, only those HBV RNAs were bound which contained besides the direct repeats 1 and 2 nucleotide sequences downstream of direct repeat 1. The corresponding binding site was found to be located within the adjacent 134 nucleotides downstream of DR1. We conclude from our data that this region which is in part homologous to the U5 region of retroviral genomes may be important for the binding of the HBV Pol gene product to the viral pregenome.

Hepatocellular carcinoma following chronic delta virus hepatitis in a patient cured of leukemia

A 20-year-old patient with chronic delta virus hepatitis (CDVH), cured of acute lymphoblastic leukemia (ALL), developed hepatocellular carcinoma (HCC) 14 years after hepatitis B virus (HBV) infection. The association between chronic HBV infection and HCC is well known, but CDVH patients affected by HCC are rarely reported in literature. To our knowledge, the case we describe is the first HCC case reported in literature occurring in a young boy with CDVH. We could expect further similar cases, considering 1) the high prevalence of HDV infection in children affected by ALL in our series, 2) the previous ALL treatment, and 3) a possible natural predisposition to cancer.

Veno-occlusive disease in snow leopards (Panthera uncia) from zoological parks

Livers from 54 snow leopards, 4 days to 23 years old, that had died in 23 US zoos, were evaluated histopathologically to determine if the hepatic fibrosis, which has been noted to be prevalent in this species, was due to chronic active hepatitis from hepadnaviral infection, Ito cell proliferation, or hemosiderosis. Forty-two of 54 snow leopards had subintimal vascular fibrosis with partial or total occlusion of central and sublobular veins (veno-occlusive disease) of unknown origin. All 21 leopards older than 5 years were affected. Four leopards had chronic active hepatitis, and 12 leopards had cholangiohepatitis; but these lesions were not connected anatomically to central and sublobular venous fibrosis. Hepatocellular and Kupffer cell siderosis and Ito cell proliferation were prevalent and often coexisted with perisinusoidal, central, and sublobular venous fibrosis; but fibrosis was present in leopards without siderosis or Ito cell proliferation. The pattern and prevalence of veno-occlusive disease in these leopards was similar to that reported in captive cheetah (Acinonyx jubatus), suggesting that a common extrinsic factor may cause the majority of hepatic disease in these large felid animals in captivity.

Phenotypic patterns of preneoplastic and neoplastic hepatic lesions in woodchucks infected with woodchuck hepatitis virus

Chronic infection of woodchucks with woodchuck hepatitis virus (WHV) was associated with the development of hepatitis, foci of altered hepatocytes and hepatocellular adenomas and carcinomas. The cytomorphological and cytochemical analysis permitted the identification of three different types of focal lesions; namely, glycogen-storage foci, mixed-cell foci and intermediate-cell foci, each showing a characteristic pattern. The cells of the glycogen-storage foci had clear to acidophilic cytoplasm, and were overloaded with glycogen. They showed a marked elevation in the activity of glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase (MDH), increased activity of succinate dehydrogenase (SDH), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and glycerol-3-phosphate dehydrogenase (G3PDH), reduction in the activity of glycogen phosphorylase (PHO), glucose-6-phosphatase (G6Pase), adenosine triphosphatase (ATPase) and adenyl cyclase (ADC), and unchanged activity of glycogen synthase (SYN) and gamma-glutamyl transferase (GGT). The mixed-cell foci mainly consisted of basophilic cells poor in glycogen, but were intermingled with cells containing glycogen. These foci were characterized by a marked decrease in activity of PHO, SYN, G6Pase, G6PDH, ATPase and ADC, and increased activity of GGT, SDH, MDH and GAPDH. The intermediate-cell foci consisted of cells with both basophilic and glycogenotic cytoplasmic compartments, and showed a similar enzyme histochemical profile to the mixed-cell foci, with slight differences in the degree of elevation or reduction of some enzymes. The phenotypic similarities and the close spatial relationship between the foci of altered hepatocytes, and the hepatocellular adenomas and carcinomas in WHV-infected woodchucks, suggest that these lesions are preneoplastic. The focal morphological and metabolic aberrations emerging during hepatocarcinogenesis in WHV-infected woodchuck, are in principle similar to those identified in the course of chemical hepatocarcinogenesis in various species. The focal metabolic aberrations apparently represent a general biological response of the liver parenchyma to oncogenic agents and are closely linked to neoplastic transformation of the hepatocytes.

Experimental WHV infection of woodchucks: an animal model of hepadnavirus-induced liver cancer

Woodchuck hepatitis virus (WHV), a member of the Hepadnaviridae, is closely related to HBV in its virus structure, genetic organization and mechanism of replication. Natural infection of woodchucks is associated with chronic liver disease and primary hepatocellular carcinoma (HCC). A concerted effort to develop the woodchuck as an experimental animal model of hepadnavirus-induced disease was initiated in 1980. The experimental studies have established the following: (1) Chronic WHV carriage as an outcome of infection is a function of age of exposure, virus dose and, possibly, virus strain. As in humans, animals infected as newborns develop chronic antigenemia at high rates compared to young adults. (2) WHV causes primary hepatocellular carcinoma (HCC) in woodchucks. Hundred percent of experimentally-induced chronic WHsAg carriers developed HCC within three years; no HCC has occurred in concurrent uninfected control animals born and held in the same laboratory environment. The predictable course of experimental WHV infection leading to liver disease in woodchucks makes this an ideal model in which to study the natural history of hepadnavirus and to develop effective prophylactic and therapeutic strategies.

X antigen/antibody markers in hepadnavirus infections. Antibodies to the X gene product(s)

Antibodies to the X antigen of hepatitis B virus and woodchuck hepatitis virus were assayed in serial sera from infected individuals and compared with other markers of infection. Antibody to the X antigen was found in 11 of 17 (65%) patients and 17 of 40 (42%) woodchucks that were surface-antigen positive. In comparison, this antibody was found in 5 of 14 (36%) patients and in none of 4 woodchucks that were surface-antigen negative. In 5 of 6 patients showing seroconversion from hepatitis B e antigen to antibody, antibody to X appeared at or near the time of seroconversion. In patients persistently positive for e antigen, X antibody often appeared when viral DNA became undetectable in the serum. In 14 of 17 (82%) woodchucks positive for antibody to X antigen, it also appeared near or after the time that viral DNA in serum disappeared. X antibodies were detected with great frequency only in populations with high frequencies of other hepatitis B virus markers. The results are consistent with the conclusion that antibody to X antigen is a marker of hepadnavirus infections that seems to be associated with a decrease in viral replication. Antibodies to the X antigen, then, may be a host response to the replication complex of the virus.

Monoclonal antibodies raised to purified woodchuck hepatitis virus core antigen particles demonstrate X antigen reactivity

Woodchuck hepatitis core antigen (WHcAg) particles purified from the liver of chronically infected animals were used for monoclonal antibody production. Most of the putative clones demonstrated anti-WHc specificity. However, the supernatants from several putative clones bound X antigen sequences from woodchuck hepatitis virus (WHV) and hepatitis B virus (HBV). One monoclonal antibody, designated WC9-85 (an IgM), specifically bound hepatitis B X antigen (HBxAg) residues spanning positions 115-131 (peptide 100). WC9-85 also specifically detected liver-derived WHcAg and duck hepatitis B core antigen (DHBcAg) particles in the same CsCl density gradient fractions as did specific anticore and cross-reactive polyclonal anti-x. WC9-85 did not bind to HBcAg particles made by recombinant DNA techniques, in which only the C-gene sequences are expressed, but did bind to liver-derived HBcAg in identical assays. A second monoclonal anti-x, WC8-62, had similar characteristics. Identification of the immunoreactive species in liver-derived core particles by Western blotting showed that WC9-85 bound the major DHBcAg polypeptide having an apparent molecular weight of 35,000 Da. WC9-85 also bound WHcAg-associated bands at approximately 37,000 and 27,000 Da, but little or no binding at the apparent molecular weight of the major WHcAg polypeptide (about 21,000 Da) was observed. These results are consistent with the conclusions that X determinants are associated with core particles purified from naturally infected livers, that such determinants are associated with the major DHBcAg polypeptide and at least two minor WHcAg-associated polypeptides, and that X reactivity is distinct from core and/or e reactivity in hepadnavirus core particles.

Epitope-specific antibody response to the surface antigen of duck hepatitis B virus in infected ducks

In order to investigate the immune response to duck hepatitis B virus (DHBV) infection, newly hatched DHBV DNA negative ducklings were injected with infectious serum of sufficiently low DHBV-DNA titer to allow clearance of viremia. Of 20 injected ducklings, 13 (65%) became viremic. Of these, 6 (46%) cleared virus from the serum 3 to 22 weeks postinjection. The convalescent sera of these 6 animals were tested for an epitope-specific antibody response in a highly specific competitive inhibition assay using a panel of monoclonal antibodies against duck hepatitis B surface antigen (DHBsAg) that had been well-characterized. All 6 animals recovering from DHBV infection developed antibodies to epitopes on the preS and S proteins of DHBV. Antibody responses were highly variable with marked differences between animals in the extent and specificity of the antibody response. The humoral response to DHBsAg was prolonged in some animals but transient in others. No antibody to preS or S was detected in either preimmune sera or sera of control animals from an uninfected flock. Infected animals that did not clear viremia also remained antibody negative. The humoral responses to neutralizing preS epitopes III and V were weak but antibodies to two immunodominant epitopes on the preS region (II and B) were present in all 6 animals. The humoral response to the two epitopes in the S region was transient and of lower titer when compared to the two immunodominant preS epitopes. The two immunodominant preS epitopes may play an important role in clearance of DHBV infection in ducks.

Involvement of an AFP1-binding site in cell-specific transcription of the pre-S1 region of the human hepatitis B virus surface antigen gene

Human hepatitis B virus infection is characterized by a high degree of hepatotropism which may be due to the dependency of viral genes on specific host factors for their expression. To learn more about such a requirement and the molecular basis of the viral tissue tropism we analyzed the promoter function in the pre-S1 region of the surface antigen gene. DNase I footprinting and competition gel retardation assays showed that a sequence with an AT-rich core (AT motif) in the pre-S1 promoter region interacts with AFP1, a hepatoma nuclear factor that binds to the alpha-fetoprotein enhancer and promoter. Functional analysis of the pre-S1 AT motif by transient transfection assays showed that this element is important in cell-specific transcriptional initiation. These results suggest that AFP1 may be one of the factors determining the liver specificity of human hepatitis B virus.