Search Article Keyword:  

PubMed Submission Abstract PDF   Click Count: 2000 Download Count: 1024 

ISSN 1007-9327 CN 14-1219/R  World J Gastroenterol  2007 January 7;13(1):104-124

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

Stephan Menne, Paul J Cote

Stephan Menne, Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York, United States

Paul J Cote, Department of Microbiology and Immunology, Georgetown University Medical Center, Washington, DC,
United States

Supported by contract N01-AI-05399 to the College of Veterinary Medicine, Cornell University from the National Institute of Allergy and Infectious Diseases. PC and SM also have been supported by contract N01-AI-95390 to the Georgetown University Medical Center, Georgetown University from the National Institute of Allergy and Infectious Diseases

Correspondence to: Stephan Menne, PhD, Department of Clinical Sciences, College of Veterinary Medicine, Veterinary Medical Center, Cornell University, Ithaca, New York 14853, United States.

Telephone: +1-607-2533280  Fax: +1-607-2533289

Received: 2006-07-28         Accepted: 2006-10-04



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.


© 2007 The WJG Press. All rights reserved.


Key words: Woodchuck; Woodchuck hepatitis virus; Hepatitis B virus; Neonatal-acquired infection; Adult-acquired infection; Resolution; Chronicity; Humoral immune response; Cellular immune response; Antiviral therapy; Immunotherapy; Combination therapy; Hepatocellular carcinoma


Menne S, Cote PJ. The woodchuck as an animal model for pathogenesis and therapy of chronic hepatitis B virus infection. World J Gastroenterol 2007; 13(1): 104-124



Infection of adult humans with the hepatitis B virus (HBV) results characteristically in self-limited hepatic disease with recovery based on serological and clinical parameters. Progression to chronic HBV infection occurs infrequently in infected adults, but HBV infections often persist in unvaccinated infants born to HBV-carrier mothers. Chronic HBV infection can lead to chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma (HCC) later in life. Estimates indicate that more than 2 billion people worldwide have serological evidence of previous or current HBV infection, with at least 350 million chronic carriers, and an overall mortality rate from HBV-induced liver disease of 1.2 million deaths per year[1]. Although highly effective vaccines are licensed and have been in use since the early 1980’s to prevent HBV infection in neonates and adults, the large reservoir of chronic HBV carriers currently remaining could benefit immensely from the timely development of effective antiviral and/or immunotherapies that cure the infection or reduce the risk of disease progression.

Evidence from HBV-infected humans, and from animal models of HBV (i.e., HBV-transgenic mice, chimpanzees, pekin ducks, and woodchucks), indicate that the success or failure of humoral and cellular immune responses to the virus determine the initial outcome of acute HBV infection (i.e., as self-limited versus chronic), and that defective responses appear to play a role in the progression of chronic HBV infection (i.e., to chronic hepatitis, cirrhosis, and possibly HCC)[2-10]. Self-limited infections by HBV involving successful immune responses represent by far the more favorable outcome. Chronic HBV infections, where immune responses have failed or are sub-optimal for virus clearance, represent a daunting challenge to successful therapy against a background of continuing disease progression. Current treatment strategies for chronic HBV infection are suboptimal when compared to the curative process observed in self-limited HBV infection. Understanding the prevention and pathogenesis of HBV infection has advanced greatly through clinical studies in humans, and through experimental studies in the chimpanzee model of HBV infection; however, neither of these models is well-suited for the routine testing of therapeutic strategies for treatment of chronic HBV infection.

Woodchuck hepatitis virus (WHV) is a naturally occurring hepadnavirus of the Eastern woodchuck (Marmota monax) (Figure 1). WHV was described initially in 1977 at the Penrose Zoo in Philadelphia in a colony of woodchucks where high rates of chronic hepatitis and HCC had been observed[11]. Several strains of WHV have been identified since then, which are all very closely related genetically[12-16], but which may induce differing proportions of chronic infections in neonatal woodchucks[17]. WHV, and another HBV-like virus, the duck hepatitis B virus (DHBV)[18-20] have been used most extensively in the modeling of HBV infection and antiviral therapy (for previous reviews see[21-24]).

Research using the woodchuck began in 1978 and it was developed further into a laboratory model by 1980 when a woodchuck breeding colony was established at Cornell University. Early progress in model development at the Georgetown and Cornell Universities involved: (1) the production and validation of reagents and assays for WHV and for disease markers, (2) the characterization of infectious WHV inocula that induced predictably high rates of chronic infection when inoculated in neonatal woodchucks, and (3) basic studies of the natural history of virologic responses and tumor development associated with experimental infection of neonatal and adult woodchucks. Since 1988, the neonatal chronic WHV infection model has been applied primarily in the testing of antiviral nucleoside analogues for chronic HBV infection (for previous reviews see[10,25-30]).

Early studies in woodchucks also involved the testing of conventional vaccines for the prevention of acute, self-limited WHV infection in neonatal and adult woodchucks[28], and also for prevention of the chronic outcome and HCC in the vaccinated neonates challenged with higher doses of inoculum to enable breakthrough infections[31,32]. Immunomodulation of acute and chronic WHV infections using immunosuppressive drugs, such as cyclosporine A[33,34], was performed to gain an initial understanding of the role of the woodchuck immune response in the outcome and maintenance of WHV infection. The focus of investigations using the woodchuck has ranged widely since 1980, with flexible emphasis on both model development and model application in many areas of HBV research. These included viral and disease pathogenesis, and the prevention and treatment of HBV infection and disease sequelae (including HCC) using vaccines, antiviral drug candidates, and immunomodulators alone and in combination. The purpose of this review is to highlight the woodchuck as an animal model for pathogenesis and therapy of chronic HBV infection.



Experimental infection of woodchucks with WHV is a well-accepted model for many aspects of the pathogenesis of human HBV infection[7,10,26-29,35-38]. Recent studies of the host response of woodchucks to WHV infection and therapy have revealed numerous parallels to the immunopathogenesis of HBV infection. Certain immune markers in woodchucks cannot be analyzed currently to the same extent as those in mice and in humans. However, the patterns and profiles of those immune responses measured thus far in the woodchuck model are highly consistent with the underlying immunologic mechanisms defined in humans.

Experimental infection of neonatal or adult wood-chucks with WHV7P1[17], a well characterized inoculum of WHV, produces predictable proportions of acute, self-limited (i.e., resolved) infections versus chronic infections. This mimics the effects of age on outcome of HBV infection in humans[3,4]. In adult woodchucks, WHV7P1 infections result mainly in resolution, with less than 5% of woodchucks progressing to chronicity[17] (Figure 2).
However, transient suppression of cellular immune responses with cyclosporine A (CsA) during the incubation and acute phase of adult WHV infections results in 92% of these infections progressing to the chronic outcome; with CsA given only during the incubation period and very early acute stage (0 to 4 wk post infection), the result is up to 50% chronic outcomes in adult WHV infections[33,34]. This shows the importance and timing of early immune responses in the resolution of acute WHV infection. Experimental immunosuppression, however, does not necessarily mimic natural processes associated with the progression to chronic infection.

Most chronic HBV infections occur as a result of neonatally-acquired infection[39-41]. Experimental infection of neonatal woodchucks with WHV7P1 usually results in a 60%-75% frequency of chronic carriers and a 25%-40% frequency of naturally recovered infections[17] (Figure 2). Viral and host response kinetics are relatively uniform when neonatal woodchucks are inoculated with WHV7P1 in the spring of the year, thus enabling statistical modeling of serologic and hepatic responses using samples collected in successive years. Such features also enable co-temporal comparisons of early acute phase immune responses before the self-limited and chronic outcomes become evident serologically, which can help to differentiate and identify the underlying mechanisms involved in the onset versus maintenance of chronic WHV infection[42-46]
(Figure 3).

Chronic WHV infection involves life-long active viral replication and inevitable disease progression to chronic hepatitis and HCC[35,47-50]. In chronically infected woodchucks, there is no naturally occurring e-antigen to anti-e seroconversion and associated step-down of viral replication (i.e., as is commonly seen in chronic HBV infection; e.g.,[51-54]). In general, the high viral replication and high surface antigen and e-antigen loads present in the chronic WHV carrier appear to play a role in the maintenance of immunologic tolerance, and are associated with disease progression to HCC[43-45,55-57].

Self-limited WHV infection involves a relatively complete shut down of viral replication and a nearly complete clearance of virus from the system with full recovery. It has been suggested that trace amounts of residual WHV genomes often detected in long-term recovered woodchucks in liver, serum, and in peripheral blood mononuclear cells (PBMC), could actually represent an alternate form of persistent viral infection[58-65]. Residual HBV DNA has been documented also for humans recovered from self-limited HBV infection (e.g.,[66-71]). In woodchucks, even recovery from acute WHV infection incurs a discernable lifetime risk of HCC (5%-20%) when compared to control seronegative woodchucks (i.e., uninfected with WHV); however, this risk is significantly lower compared to the lifetime risk of HCC in chronic WHV carrier woodchucks, which is essentially 100%[58,72] (Figure 2). Such results provide direct experimental evidence for the carcinogenicity of WHV and, by analogy, for HBV where chronic infection also is associated with HCC.

In a recent study, we examined the reactivation of WHV replication and the generation of infectious WHV in long-term resolved adult woodchucks during experimental immunosuppression with CsA (Menne et al, unpublished data). Administration of CsA to serologically recovered woodchucks with evidence of residual WHV DNA in liver and PBMC, and with durable recall cellular immune responses of PBMC to WHV antigens, resulted in a transient reactivation of WHV replication during CsA treatment. This supports the idea that replication-competent WHV (and by analogy, HBV) can persist for many years after recovery from acute viral hepatitis, possibly as part of a continuing process. That is, the virus may be controlled by virus-specific immune responses that are primed continuously by trace amounts of virus and viral antigens. In any case, the presence of long-term recall cellular immune responses with mutual persistence of residual WHV covalently closed circular DNA (WHV cccDNA) is significant to the durability of recovery responses over the long term for the stable control of replication and shut down of the infectious process. The apparent lack (or need) of such immune responses with the apparent loss of WHV cccDNA is significant to the extent of viral immune clearance possible in recovery. One implication from the above studies is how much a relatively successful antiviral and/or immunotherapy for chronic HBV or WHV infection will improve the prospects for disease outcome beyond that observed in natural recovery from infection.



WHV is classified as a member of the genus Orthohepadnavirus, family Hepadnaviridae[73]. The genetic organization of WHV is similar to that of HBV and other mammalian hepadnaviruses, and their biological properties and replicative strategies are essentially the same[74]. Filaments and spherical particles are found in the serum of WHV-infected woodchucks which are composed of the envelope protein of the virus. Complete virions are 42 to 45 nm in diameter and are composed of an exterior envelope protein (WHV surface antigen; WHsAg), an inner nucleocapsid or core protein (WHcAg), and, within the nucleocapsid, the DNA genome[75,76]. The replicative cycle of WHV seems to be identical to that of HBV[75-78]. The role of cccDNA as the template for viral transcription, the mechanism of replenishment of the cccDNA pool, and the control of this pathway by surface antigen, have been investigated mainly using DHBV. For some studies, full-length clones of the WHV genome, cut and ligated to form a supercoiled cccDNA, have been used for in vivo molecular studies since direct injection into the hepatic parenchyma of woodchucks results in productive WHV infection[79]. Only a brief overview is provided below for background purposes.

During infection, HBV enters the hepatocyte, but the mechanism is poorly understood. No hepatocyte receptor has yet been defined for HBV, although studies suggest that the virus-cell recognition may be mediated all or in part by specific sequences located in the pre-S1 region of the large envelope protein. However, with numerous other potential envelope recognition sites for the cell suggested from in vivo neutralization studies with monoclonal antibodies, and the fact that antibodies elicited by vaccines to only the small envelope protein provide protective immunity, we are a long way from understanding the mechanisms of antibody-mediated neutralization of HBV attachment, entry, and uncoating during infection. It is known that the circular, partially double-stranded DNA genome makes its way to the nucleus where the partial DNA strand (i.e., positive strand) is completed via the endogenously linked virion reverse transcriptase-DNA polymerase, and the now fully circularized double strand is then ligated into a cccDNA. The cccDNA serves as the key template for viral mRNA transcription via the cellular RNA polymerase . One of the viral mRNAs (slightly larger than the genome length transcript) becomes encapsidated into maturing core particles along with the virion polymerase, where it is then reverse transcribed into the viral negative strand DNA via the RNA-dependent DNA polymerase activity of the encapsidated enzyme. The viral polymerase then uses its DNA-dependent DNA polymerase activity to partially complete the positive strand DNA to about 50%-75%, and this non-covalently closed circularized DNA is found in mature virions of HBV and WHV. Envelope acquisition occurs at the endoplasmic reticulum (ER) and mature virions are secreted from hepatocytes. Hepadnaviruses are not directly cytotoxic to infected cells.

Amplification and replenishment of cccDNA in the nucleus of the infected hepatocyte occurs when a portion of the maturing core particles complete positive strand DNA synthesis and are cycled back to the nucleus (i.e., instead of through the ER) where the new double strand DNA is processed into cccDNA. In HBV, most immunostaining of core is found in the nucleus, whereas in WHV, the core staining is primarily cytoplasmic, and not detected in the nucleus. This suggests a process of newly synthesized cytoplasmic core particles carrying out reverse transcription, partial or complete positive strand synthesis, and occasional re-entry into the nucleus for amplification of cccDNA (alternatively, cytoplasmic core staining may reflect incoming virus, but this seems far less likely). For HBV, cytoplasmic cores may go undetectable by immunostaining, and the denser staining of core particles within the nucleus may reflect maturation of HBV core particles there, with exit to the ER for envelope acquisition via a different cellular pathway. In established carrier woodchucks, WHV virions often circulate in 10- to 100-fold greater concentrations than do HBV virions in human chronic carriers. This may relate to the differential immuno-localization of core particles in the two models. 

Transition of viral DNA to RNA during the life cycle of WHV has similarities to that of retroviruses[75,78], but integration of viral DNA into the host genome is not, however, essential for replication of hepadnaviruses, as is the case with retroviruses. Persistence of episomal cccDNA in infected hepatocytes is considered stable and this is problematic for its removal from the system, which appears to require elimination of the infected hepatocyte. It therefore represents the main target for attaining complete eradication of hepadnavirus from the system. When hepadnaviral DNA does integrate into host cell DNA, it is usually truncated and rearranged, and can target any number of sites in cellular DNA[80,81], some or all of which may be important in hepatocarcinogenesis. Morphological and molecular virological studies of the liver have shown that virtually 100% of hepatocytes become infected after experimental WHV infection[82]. Although replicative forms are cleared rapidly during recovery, WHV cccDNA persisted in a certain proportion of woodchucks long after evidence of WHV replication had ceased. That said, recovery is indeed durable and protective against disease progression in the vast majority of cases. On the other hand, persistence of the episomal cccDNA in chronic HBV (and WHV) infections remains a major conundrum in attempts to clear the virus via various therapeutic approaches (see below).

HBV generally is considered a hepatotropic virus, but hepadnavirus DNA can also be detected in extrahepatic tissues. For example, DHBV is often found replicating in the pancreas of ducks. HBV and WHV appear to infect the lymphatic system, although the exact significance of this observation is not well understood[58,60-63,65,83-86]. Some studies suggest that WHV replication and spread in the lymphatic compartment can proceed independently, even before infection of the liver[60,86]. Quiescent (non-replicating) WHV DNA molecules in PBMC from chronic WHV carriers can be activated to form replicative intermediates by stimulation of PBMC with lipopolysaccharide (LPS)[62]. The cell-free supernatants from LPS-stimulated WHV carrier PBMC (but not those from the unstimulated carrier PBMC) contain newly replicated infectious WHV that induce-acute hepatitis in WHV-susceptible adult woodchucks[84].

 WHV quiescence versus replication in the lymphatic compartment may vary depending on the state of the host lymphatic target cell (i.e., resting, dividing, circulating in blood, within lymphatic tissue, etc.). WHV DNA can be detected in bone marrow cells as early as one month post neonatal WHV infection, but the first signs of WHV replication in PBMC, lymph nodes, and spleens occur during the acute stage of hepatic infection[83]. During recovery lymphatic WHV replication subsides to a quiescent state (or approaches complete elimination). In chronic infections, WHV replication also becomes quiescent in circulating PBMC, but often continues in the spleen[83], and the quiescent WHV in PBMC often can be activated upon ex vivo stimulation using LPS, as indicated above[62,84]. More recent published studies indicate that long-term recovered woodchucks can also harbor infectious DNA in PBMC[58].

 From the above, woodchucks recovering from acute WHV infection and those progressing to chronicity seem to have similar PBMC infection profiles, and in both cases the PBMC respond robustly in proliferation assays to polyclonal mitogens such as ConA, PHA, and LPS[44,55,87-89]. Even with similar PBMC WHV DNA profiles, the PBMC proliferative responses to viral antigens are generally more robust in the recovery outcome compared to the chronic outcome[44,55,56,87-90]. Thus, immune response function in viral infection does not appear to be affected adversely by the ongoing lymphatic infection. In fact, lymphatic infection by WHV, either acutely or in chronic WHV carriers, does not result in any lymphadenopathy, lymphopenia, lymphoma, or generalized immunodeficiency enabling opportunistic infections. As with natural recovery, therapy of chronic WHV infection presumes to target all reservoirs and molecular forms of the virus in both the lymphatic system and liver.  



Resolution of experimental WHV infection in both neonatal and adult woodchucks involves a self-curative process with appropriate virus-specific immune responses in the periphery and liver (Figure 3). Natural recovery perhaps represents a benchmark for the possible induction of antiviral and/or immunotherapeutic effects in chronic WHV carriers. Specific activation of humoral and cellular immune responses is a prerequisite for viral clearance during acute HBV infection in adult patients, as reported in numerous studies[2-4,9,91]. However, the kinetic development of these responses during the early incubation and acute stages of adult HBV infection, and their influence on the course and outcome of infection, are less well characterized in humans, since patients usually do not present with clinical symptoms immediately after HBV transmission (except for a few rare cases involving known exposure times; e.g.,[92]).

Studies of self-limited WHV infection reveal numerous virus and host response patterns analogous to self-limited HBV infection[61,79,82,83,85,88,89,93-105]. In general, resolution of WHV infection in both the neonatal and adult settings is characterized by: (1) a transient peak of WHV DNA and antigen detection in serum and liver during the acute phase of infection, (2) timely and appropriate cell-mediated immunity (CMI) to viral antigens, (3) acute viral hepatitis with limited liver injury, (4) a transient peak and subsequent normalization of serum aminotransferases, and (5) seroconversion to virus-neutralizing antibodies, all leading to a substantial clearance of virus and viral antigens from the blood and liver. The humoral immune response to viral antigens (i.e., WHcAg and WHsAg) during resolution is associated with the development of robust titers of anti-core (anti-WHc), and of virus-neutralizing, protective, anti-surface antibodies (anti-WHs) with the onset and waning of the acute phase, all usually within several weeks after experimental infection[17,42].

In adult woodchucks, the CMI associated with recovery is characterized by activation of PBMC detected by in vitro stimulation of PBMC with WHsAg, WHcAg, and synthetic peptides of both antigens[87-90,93,94,106]. The successful PBMC response to WHcAg is associated co-temporally with viral clearance from serum, and this has been mapped extensively to several key epitopes of the WHcAg[87,93]. In fact, immunization with the dominant WHcAg epitope sequence between amino acids 97 to 110 significantly dampens acute WHV infections in adult woodchucks following experimental challenge with WHV, when compared to infections in unvaccinated control woodchucks[87]. Mapping of the PBMC responses to WHsAg and WHV x antigen (WHxAg) during the acute phase of resolution have been in progress.

The CMI to viral antigens during the acute phase in neonatal woodchucks experimentally infected with cWHV8P1 (from which neonates resolve more frequently) is similar to that in resolving adult woodchucks, which is independent of the WHV inoculum used[44]. Robust PBMC responses to WHcAg, WHsAg, and WHxAg, and to several non-overlapping core peptides, are associated temporally with the clearance of WHV DNA and WHsAg from serum. Detailed analysis of the WHcAg-specific PBMC responses revealed a broad recognition of several WHcAg epitopes representing apparently distinct regions of this antigen. Similar to adult WHV infections, neonatal woodchucks develop PBMC responses to important WHcAg peptides (residues 97 to 110, residues 100-113)[44].

In the liver, the CMI during resolution of adult WHV infection is characterized by moderate to marked hepatic inflammation and liver injury involving increased CD3- (cluster of differentiation 3) positive T lymphocyte accumulation, and apoptosis and regeneration of hepatocytes[96,97]. These events are accompanied by marked elevations of CD3, CD4, and CD8 mRNA expression and increased expression of the T-helper lymphocyte (Th)-type 1 cytokine mRNAs interferon gamma (IFN-g) and tumor necrosis factor alpha (TNF-a), and of the IFN-g -inducible oligoadenylate synthetase (2’-5’-OAS) mRNA[96,97]. In vitro testing of cell-mediated killing of hepatocytes revealed activation of both FasL- (ligand for the apoptosis-inducing factor Fas) and perforin-dependent pathways during resolution, and a comparative analysis demonstrated that acute hepatitis, but not established chronic WHV infection, is associated with elevated hepatocyte killing as a consequence of increased activation of the perforin-dependent pathway[95]. Further acute phase studies of adult self-limiting WHV infections are in progress to better define the kinetic interrelationships between mRNA expression in liver and PBMC mRNA expression ex vivo or following in vitro stimulation with antigens.

Neonatal WHV infections can be studied prospectively, in proportionate and adequate numbers of woodchucks, as the dichotomy in outcome proceeds dynamically in real time toward recovery versus chronicity. As with recovering WHV infections in adult woodchucks, resolution of neonatal WHV infection is associated with moderate hepatic inflammation and liver injury and accumulation of CD3-positive T-lymphocytes[42,43]. Hepatic inflammation is characterized further by significant accumulation of CD3, CD4, and CD8 mRNAs, with elevated expression of the Th-type 1 cytokine mRNAs IFN-g and TNF-a, and of the intracellular transcription factor STAT4 (signal transactivator of transcription) and T-bet (T box expressed in T lymphocytes) mRNAs, and also of Fas ligand and perforin mRNAs. When taken together, the results indicate that both non-cytolytic and cytolytic clearance of WHV-infected hepatocytes is occurring[43,45,46]. The results from the above studies suggest further that early virus-specific CMI in both the periphery and liver play a pivotal role in resolution of acute WHV infection in neonatal and adult woodchucks, and that the responses to WHcAg and to selected core peptides are instrumental in controlling viral infection.

Although immune responsiveness has been well-characterized in the periphery and liver of established HBV chronic carriers during chronic hepatitis, the actual acute phase responses associated with the early onset of chronic HBV infection are less well understood. Lack of immune responsiveness to HBV antigens in some HBV carriers may ensue with the establishment of the carrier state and have little to do with the early onset at a time when other individuals may recover normally. Moreover, chronic hepatitis is defective by definition, when compared with the acute hepatitis that results in recovery, because the chronic inflammation is incapable of clearing the hepatic infection and lends itself only to progressive liver disease. Adult patients presenting with acute hepatitis B are often well into the infection and only rarely progress to chronic infection. Studies of the early onset of chronic HBV infection in humans after neonatal transmission have obvious limitations. While studies in established chronic HBV carriers show defective immune responses associated with tolerance and chronic hepatitis and a failure to clear the infection[2-4,9,91], it is unclear whether such deficient responses are representative of the primary acute phase responses that predispose to the chronic outcome. Understanding how the chronic infection first becomes established kinetically near the time of the acute stage of infection could lead to the identification of important cause-effect relationships that will facilitate the rational development of therapies for successfully treating established chronic infections.

For testing the hypothesis that chronic WHV infection develops due to a diminished host response to acute infection, co-temporal comparisons were performed in the neonatal WHV infection model[7,17,42-46,107-109]. Using a bank of control and WHV-infected liver specimens that were obtained surgically at two acute phase time points of neonatal WHV infection (wk 8 and 14) and that were assigned to recovered or carrier woodchucks once outcome was known based on later serological profiles, the early onset of the chronic WHV carrier state (compared to co-temporal resolving infections) was characterized by: (1) higher acute phase viral loads in liver (at wk 8 and 14 post infection), (2) diminished acute hepatitis (at wk 14), (3) detectable but significantly diminished hepatic inflammation (at wk 14), and (4) reduced liver injury (at wk 14)[42,43,45,46]. This was associated further with: (1) absent or suboptimal intrahepatic accumulation of CD3, CD4, and CD8 mRNAs, and (2) reduced expression of Th-type 1 cytokine mRNAs, especially IFN-g and TNF-a, along with the key Th-type 1 transcription factor T-bet[43,45,46]. This represented an early primary deficiency in the Th-type 1 response in liver to acute WHV infection, and was not associated with any local antagonistic Th-type 2 immunoregulation[45].

Studies in the peripheral blood using serial measurements of PBMC responses in neonatal woodchucks experimentally infected with WHV7P1 or cWHV8P1 have shown thus far that all neonates with resolving infections had robust acute phase PBMC responses to WHcAg and to the key epitope of this antigen (core residues 97-110), with the majority of woodchucks also responding to WHsAg and WHxAg[44]. In contrast, prospective carriers responded less frequently or not at all, with only about one-third responding to WHcAg, and among these, only about half responded to the key core epitope and to other WHV antigens. Detailed mapping of the PBMC responses to WHcAg revealed that the epitopes recognized were localized to distinct regions of this antigen and were different from those recognized during resolving WHV infections[44]. In the prospective carriers with minimal acute phase PBMC responses to WHcAg, viremia and antigenemia developed later, and viral and antigen loads were lower compared to those seen in prospective carriers without any evidence of virus-specific PBMC responses[44]. In any case the levels of viremia and antigenemia in these prospective carriers were much higher than in neonates with resolving WHV infections[44]. Interestingly, the fact that virus-specific PBMC responses were undetectable in the majority of prospective carriers indicates an early genesis for the CMI defect commonly observed later in established chronic WHV infection[55,56,87-90,94]. Further studies to correlate the molecular immunologic responses of PBMC based on leukocyte surface marker and cytokine mRNA expression with outcome of neonatal WHV infection are in progress.

Established chronic HBV infection is associated with increased viral load and risk of severe liver disease sequelae[2-4,9,91]. Chronic HBV infections resulting from neonatal transmissions are characterized by T cell immunotolerance to viral antigens throughout most of life until end-stage disease, but may exhibit occasional exacerbations of liver disease before this time[51,110-114]. T cell proliferative responses to viral antigens in adult-acquired chronic HBV infections can be variable during disease progression, but are usually less responsive, except during periodic transient flare reactions and with seroconversion to anti-e antibodies (e.g.,[52-54,115,116]). 

Studies in woodchucks indicate that viral antigen-specific CMI during established chronic WHV infection is also defective, similar to that observed in HBV
infection[45,55,56,87-89,94]. In the neonatal woodchuck, following an occasional, early and transient, but suboptimal acute hepatitis, WHV chronic carriage is characterized for some time with minimal chronic persistent hepatitis, and little or no liver injury based on serum enzyme markers up through at least 15 mo post infection[46]. This progresses subsequently to more active hepatitis and liver injury just before or at the time of HCC onset and tumor
growth[46,47,49,50,55,57]. PBMC remain essentially immuno-tolerant to WHV antigens throughout all of the chronic phase of neonatal WHV infection, including end stage disease[55,56,87,89,90,94]. The baseline expression of Th-type 1 cytokines in liver that is usually observed during the chronic phase can sometimes increase above normal with progressive chronic hepatitis (usually with increased TNF-a and less IFN-g)[45,97], but without affecting clearance of the infection. Less is known of the CMI in documented adult-acquired chronic WHV infections, but the apparent greater degree of chronic hepatitis in this setting may suggest some exception to the fully tolerant state as leading to and maintaining the chronic infection[96]. Indeed, some leukocyte surface and cytokine markers become elevated in liver during acute hepatitis in adult woodchucks that eventually become chronic carriers, although to a lesser extent than seen during the acute phase of woodchucks that resolve.



Antiviral drugs

Woodchucks with experimentally induced chronic WHV infection have been used successfully in the empiric screening and preclinical assessment of antiviral drugs being developed for treatment of chronic HBV infection (for previous reviews see[21,24,25,30,35,117]). Current strategies aim to suppress viral replication in liver and the concentration of viral DNA in serum during chronic HBV infection (i.e., reduce viral load) by treatment with nucleoside and nucleotide analogues. As indicated above, it has been difficult to target the viral cccDNA directly in this process, and so potent inhibition of viral replication is the main means to reduce viral load in blood and tissues, and perhaps diminish replenishment of cccDNA indirectly, until cells harboring this intermediate can turnover or be eliminated by immune responses. Accordingly, lifelong therapy with antiviral drugs is currently the accepted procedure, even though this often results in the selection of drug-resistant mutants (i.e., mutations of the polymerase gene), which has been observed and modeled in the woodchuck (e.g.,[118-122]).

Before testing in woodchucks, potential drug candidates are screened for antiviral activity against HBV in the 2.2.15 cell system, a HepG2 cell line that is engineered to produce HBV constitutively[123]. Drugs with significant antiviral activity in vitro also have been tested in a HBV-transgenic mouse model designed and validated for this purpose[124]. However, drug efficacy in an in vivo infection model is most usually assessed in the woodchuck model. Most nucleoside analogues with intermediate antiviral activity in vitro against HBV had comparable antiviral activity against WHV in woodchucks, but some exceptions exist. For example, fialuridine (D-FIAU) had modest activity in vitro, and potent antiviral activity in woodchucks; however, this was associated with a marked and delayed hepatotoxicity characterized by microvesicular steatosis and mitochondrial injury[125], similar to the unfortunate hepatotoxic effects this drug had in humans, where it was first tested[126].

In recent years, numerous nucleoside and nucleotide analogues designed to inhibit HBV replication were tested in the woodchuck model (e.g.,[125,127-140]. Some of these nucleoside analogues had demonstrated antiviral efficacy in chronic WHV carrier woodchucks, such as lamivudine (3TC, Epivir)[21,120,141-143], adefovir dipivoxil (ADV, Hepsera)[144,145], and entecavir (ETV, Baraclude)[139,146], and are now approved by the FDA for treatment of chronic HBV infection. Other nucleoside analogues, also having activity in woodchucks against WHV, are used for the treatment of human immunodeficiency virus (HIV), such as tenofovir disoproxil fumarate (TDF, Viread)[147] and emtricitabine (FTC, Coviracil)[148-150], and still others are in advanced clinical testing, such as telbivudine (LdT)[151-153], valtorcitabine (val-LdC)[151-153], and clevudine
(L-FMAU)[55,56,122,150,154-156]. FDA approval of these drugs for treatment of chronic HBV infection is expected in the near future (in fact, telbivudine was approved most recently). Table 1 summarizes the antiviral activities of these second and third-generation nucleosides in chronic WHV carrier woodchucks that were reported in selected studies. We note here in these experiments that viral recrudescence following cessation of drug is often a function of being unable to completely suppress viral replication sufficiently during a given treatment, or significantly enough over time in order to allow cells containing cccDNA to turnover or be eliminated by the immune response.

Lamivudine is a moderately potent antiviral drug in woodchucks and is without toxicity during daily, oral administration for up to 24 wk[21,141,157], and even longer[142]. The average reduction in serum WHV DNA after 4 or 12 wk of treatment with different doses (1, 5, or 15 mg/kg bodyweight) was approximately 2.5 and 1.5 logs, respectively. The average time to recrudescence of viral replication after drug withdrawal was within 1 to 2 wk. In woodchucks, lamivudine also has been shown to act synergistically both with alpha-interferon and with famciclovir[141,157]. An antiviral activity comparable to lamivudine has been reported for adefovir[144]. Daily oral administration of adefovir for 12 wk with doses of 5 and 15 mg/kg resulted in a reduction in serum viremia of 1.7 or 2.5 logs, respectively. Viral recrudescence after drug withdrawal occurred within 6 wk. No toxicity associated with administration of adefovir was observed. The antiviral activity of tenofovir in woodchucks[147] is comparable to those of lamivudine and adefovir. The reduction in serum WHV DNA observed after 4 wk of daily, oral treatment with tenofovir doses of 5 and 15 mg/kg was 1.5 or 1.2 logs, respectively. After drug withdrawal viral recrudescence occurred within 1 to 4 wk and treatment was without any evidence of drug-associated toxicity.

A higher antiviral activity on chronic WHV infection was reported for emtricitabine[149]. Daily oral treatment for 4 wk with doses of 10 or 30 mg/kg reduced serum viremia by 3.2 and 4.9 logs, respectively. Recrudescence of viral replication occurred within 1 to 2 wk after drug withdrawal. A dose of emtricitabine of 3 mg/kg in this study was less efficacious, but was comparable to those observed with 20 and 30 mg/kg, administered twice daily, intraperitoneally, for 4 wk[148]. There was no toxicity associated with this drug treatment. A more remarkable antiviral activity was obtained with valtorcitabine in chronic WHV carrier woodchucks after daily oral administration for 4 wk with a dose of 10 mg/kg[151-153]. In this case, serum WHV DNA became reduced by 4 to 6 logs with no evidence of drug-associated toxicity at the dose used. The time to recrudescence of viral replication after drug withdrawal was as little as 1 wk, but extended to 8 wk in many of the animals. Entecavir had an even higher antiviral activity in woodchucks[139]. Daily oral administration of entecavir for 12 wk at a dose of 0.02 mg/kg resulted in a reduction in serum viremia of 7 to 8 logs in 4 of 6 treated woodchucks (2 of the 6 treated woodchucks had only a modest antiviral effect). Recrudescence of viral replication after drug withdrawal occurred in as little as 2 wk, but was extended to 10 wk in several of the animals. Administration of entecavir at a dose of 0.1 mg/kg reduced serum viremia by 7 to 8 logs and viral recrudescence was observed within 6 to 10 wk. No toxicity associated with drug treatment was reported.

The most potent antiviral drugs that have been tested so far in woodchucks are telbivudine and clevudine. Daily oral administration of telbivudine for 4 wk at a dose of 10 mg/kg resulted in an 8 log reduction in serum viremia and viral recrudescence was observed within 4 to 8 wk after drug withdrawal[151,152]. Daily oral administration of clevudine for 4 wk at doses of 3 or 10 mg/kg reduced serum viremia by 9.2 and 8.2 logs, respectively[154]. With the lower dose of clevudine viral recrudescence after drug withdrawal was observed within 2 to 10 wk. The higher dose delayed viral recrudescence and serum WHV DNA concentrations reached pretreatment levels within 8 to 12 wk, and in one woodchuck, serum viremia was still suppressed at the end of the study. No toxicity was associated with the above short-term treatments using either telbivudine or clevudine.

The above studies in the woodchuck model demonstrate that a significant antiviral effect on chronic WHV infection could be achieved with all drugs. The relative antiviral efficacy against WHV, at the doses administered and for the duration of treatment used, was clevudine telbivudine entecavir > valtorcitabine emtricitabine tenofovir = adefovir = lamivudine. A prolonged suppression of WHV replication after drug withdrawal was achieved with clevudine, telbivudine, entecavir, and valtorcitabine, and the magnitude of these responses was often associated indirectly with transient or sustained reductions in WHV cccDNA potentially enabling some turnover of residually infected cells. The favorable safety and efficacy profile obtained thus far in the woodchuck model using relatively short-term treatments with clevudine, telbivudine, and valtorcitabine suggest that these drugs should be of value in the long-term control of chronic HBV infection in humans and support their continued clinical development.

The preclinical evaluation of antiviral drugs for treatment of lamivudine-resistant HBV infection has been modeled in woodchucks by the experimental induction of lamivudine-resistant WHV with nine or more months of lamivudine treatment, followed by continued therapy with lamivudine along with the new drug candidate of interest[122,145]. Prolonged treatment with lamivudine led to the establishment of drug-resistant WHV mutants, characterized mainly by mutations in the B domain of the WHV polymerase gene (i.e., HBV mutations occur in B and C domains). Supplemental daily oral treatment of these circulating B domain mutants with adefovir or clevudine (10 mg/kg per day, 12 wk and 7 wk, respectively) demonstrated that both drugs could suppress replication of these lamivudine-resistant WHV mutants. In a different study, lamivudine-resistant mutants of WHV were found to be cross-resistant to treatment with clevudine[122]. Studies are in progress using engineered lamivudine-resistant mutants of WHV that mimic the additional polymerase C domain mutants observed in human HBV patients treated with lamivudine. 

In addition to the testing of drugs in woodchucks for antiviral effects, applications also have been extended to the testing of entecavir, clevudine, or lamivudine for efficacy against disease progression[30,142,146,155]. Extended lamivudine treatment of woodchucks with chronic WHV infection delayed the development of HCC and significantly extended survival of woodchucks in one study[142]. In that study, twenty 8-mo-old chronic WHV carrier woodchucks were treated throughout the rest of their lifetime with lamivudine (5 mg and then 15 mg/kg,
orally, daily). Twenty placebo control WHV carrier woodchucks were included for comparison. Serum WHV DNA decreased by 4 to 5 logs in lamivudine-treated carrier woodchucks, with an antiviral effect that was sustained for more than one year with continued treatment. Importantly, there was a significant delay in time to onset of HCC and death due to HCC among lamivudine-treated woodchucks compared to placebo controls. In another study of lamivudine in WHV carrier woodchucks, no delay in hepatocarcinogenesis was observed with treatment, most likely because drug treatment began when woodchucks were at an older age, was of shorter duration, and less of an antiviral effect on serum WHV DNA was observed[120]. In both studies, lamivudine resistance developed that was associated with a high frequency of mutation in the WHV polymerase gene B domain[118,121].

In another study, long-term oral treatment with entecavir[146] in 8-mo-old woodchucks at 0.5 mg/kg per day for 8 wk, and then with a weekly dose of 0.5 mg/kg for 14 or 36 mo, produced sustained antiviral responses in half of the woodchucks treated for 14 mo, and in 80% of the woodchucks treated for 36 mo (i.e., reduced serum viremia of 5 to 8 logs). Here, the drug-treated woodchucks had marked reduction in viral load and did not develop HCC during the next 2 years follow-up. Compared in this case with historical controls, entecavir treatment significantly delayed the development of HCC and prolonged survival.

In another study, clevudine was administered orally to chronic WHV carrier woodchucks at 10 mg/kg per day for 32 wk[30,55,56,155] starting at 1 to 2 years of age. Half of the clevudine-treated woodchucks and half of the placebo recipients then received 4 doses of a conventional WHsAg vaccine (alum-adsorbed, formalin-inactivated WHsAg) during the next 16 wk. Combination treatment with clevudine and vaccine resulted in a sustained antiviral effect with reductions in serum viremia of more than 8 logs in many cases (Figure 4), and prevented the development of HCC altogether in up to 38% of treated woodchucks. In a subset of the woodchucks studied, where clevudine or placebo treatment was initiated at 1 year of age (and the data analyzed independent of combination with WHsAg vaccine), the development of HCC in clevudine-treated woodchucks was delayed significantly and long-term survival after 4 years likewise was increased significantly compared to woodchucks that did not receive clevudine. These studies show that chemotherapy with antiviral drugs can delay and reduce disease progression in chronic carrier woodchucks, and also show the correlation between reduced viral load and reduced disease progression, with noteworthy implications for HBV therapy in humans.

In addition to nucleoside or nucleotide analogues, various other compounds of organic and plant origin have been tested in woodchucks for their antiviral activity (e.g.,[141,158-161]), but these will not be discussed in detail in this review. Direct testing of anti-tumor agents against established HCC in woodchucks is also possible[162-164], but has not been fully developed to date.



The main goal of basic immunological studies described above in neonatal and adult WHV-infected woodchucks is to identify and differentiate factors that cause and maintain chronic infection from those that result from chronic carriage. By better defining cause-effect relationships, it should be possible to develop and test rational immunotherapies that can induce immune responses in the established chronic carrier that mimic those in recovery from WHV infection. In this way, it should be possible to enhance the immune elimination of cells harboring viral cccDNA (and/or control its level and expression), as occurs with successful immune responses leading to recovery.

As indicated in the sections above, chronicity as an outcome of neonatal WHV infection appears to result from a failed or suboptimal primary immune response relatively early during the acute phase of infection in the periphery and in the liver. The onset of chronic infection (compared to resolution) is characterized by deficiencies in the CD8-positive cytolytic T lymphocyte (CTL) response, and reduced expression of Th-type 1 cytokines and intracellular transcription factors, minimal acute hepatitis, and humoral and cellular immunologic tolerance to viral antigens[42-46]. Negative immunoregulation of the intrahepatic Th-type 1 response by excessive intrahepatic Th-type 2 immune responses is not a defining factor in this outcome[45,46]. Chronicity then appears to develop due to reduced immune-mediated clearance of infected hepatocytes by both non-cytolytic and cytolytic processes[45,46]. The above studies indicate further that early induction of immune tolerance may be a factor in the onset of chronic neonatal WHV infection, and a similar mechanism may be involved in the onset of chronic HBV infection in unvaccinated infants born to HBV-carrier mothers. This may include the deletion of higher affinity virus-specific T cells by negative selection of precursor T cells in the thymus (central tolerance), or clonal anergy or exhaustion of virus-specific T cells that escaped early negative selection in the thymus, which are then rendered unresponsive due to higher viral and antigen loads (peripheral tolerance).

Several studies have used WHV-naïve woodchucks for testing experimental vaccines, including conventional and DNA vaccines, and adjuvants, for later therapeutic vaccination of chronic WHV carrier woodchucks. In these studies antibody responses against WHsAg or WHcAg were induced[31,55,87,165-172], and partial or full protection against viral infection and disease by challenge with WHV was observed[31,87,165,166,168-172]. A few studies also determined that cellular immune responses were induced in addition to the humoral responses[87,168-171].

Unlike when WHV-naive woodchucks are immunized, the detection of free anti-WHs in serum of WHV carriers vaccinated with WHsAg is more problematic due to an excess of WHsAg in the serum samples. However, positive signals for anti-WHs can often be detected by enzyme immunoassay under these conditions, even though it may be complexed in native serum. This is because of the exchange of bound anti-WHs between WHsAg in the sample solution, and the WHsAg adsorbed to the solid phase assay matrix. Unvaccinated WHV carriers rarely if ever show detectable anti-WHs of this nature, even though they may have some complexed anti-WHs in serum. Thus, the vaccination of WHV carriers with WHsAg most likely increases the levels of anti-WHs in complex, which enables its subsequent detection (at generally low levels) in the various enzyme immunoassay formats. Assays able to detect anti-WHs in complex with WHsAg are being developed to better study such responses.

One approach to immunotherapy is to modulate the deficient humoral and cellular immune responses of chronic HBV carriers by conventional vaccination (Table 2). In one study chronic WHV carrier woodchucks received up to 6 immunizations with a serum-derived WHsAg vaccine that was adsorbed to aluminum salt and contained monophosphoryl lipid A[173]. Following immunization, all of the carrier woodchucks developed an antibody response against WHsAg that was directed mainly against the WHV preS region, but there was little in the way of positive CMI to the antigen used in the vaccine. Despite the induction of anti-WHs antibodies, serum levels of WHV DNA and WHsAg in vaccinated carriers remained unchanged. This was consistent with another study in which four doses of an alum-adsorbed, formalin-inactivated WHsAg vaccine were administered[55,56,155]. In the latter study, CMI to WHsAg and WHsAg peptides was detected in the majority of vaccinated carriers, but, again, there was little effect on serum viral load (Figures 4 and 5). Therapeutic vaccination of chronic WHV carrier woodchucks with a serum-derived WHsAg in combination with an experimental adjuvant (i.e., a peptide carrying a Th epitope from sperm whale myoglobin) induced anti-WHs antibody responses and minor transient reductions in serum WHV DNA in a few of the vaccinated carriers[174]. Caution in the use of this therapeutic WHsAg vaccine was recommended, however, since some of the carriers died during the vaccinations. Such adverse effects could have been related to the experimental adjuvant and/or to liver disease present in the woodchucks at entry into the study. The results from these studies indicate that immunization of chronic WHV carrier woodchucks with WHsAg can partially induce (or boost) B cell responses to WHsAg. Additional modulation, however, seems necessary for inducing a response profile that resembles that observed during resolution of WHV infection.

Another approach to immunotherapy of chronic HBV infection involves direct reconstitution of the deficient Th-type 1 immune responses in the liver to mimic natural recovery from infection. Cytokines such as IFN-g and TNF-a have been reported to have direct, non-cytolytic antiviral effects in HBV transgenic mice[8,175-177]. However, increased expression of these cytokines can occur in established chronic WHV carriers with progressing chronic hepatitis and liver injury[45,96,97,178], but with little concurrent reduction in viral replication. This indicates that additional responses would be important to developing a more complete therapeutic effect resembling recovery. Recent studies have shown that woodchuck IFN-g (and TNF-a) does not significantly deplete WHV RNA or WHV DNA replicative intermediates in vitro in virus-infected primary hepatocytes from chronic carriers[179]. Other studies in primary hepatocyte cultures from established WHV carriers suggest that expression of IFN-g from a transfected plasmid (and also of TNF-a) can induce partial host response profiles with similarity to recovering liver, and also impair a later step in viral replication by a non-cytolytic mechanism that is probably mediated by TNF-a[180].

The effects of woodchuck IFN-a and IFN-g on WHV replication were determined in vivo in a recent study in chronic WHV carrier woodchucks using an adenoviral vector for the expression of these cytokines[181]. Following vector administration directly into the liver, a slight but transient reduction in intrahepatic WHV DNA replication and in serum WHV DNA of about 1 log was obtained with the IFN-a expressing vector. The intrahepatic expression of IFN-g, however, had no effect on WHV, thus leading to the conclusion that hepatocytes of chronic WHV carrier woodchucks may be functionally altered in their response to IFN-g or resistant to this cytokine. In another study, the administration of woodchuck IFN-a using an adeno-associated viral vector for intrahepatic delivery of this cytokine into chronic WHV carrier woodchucks had a significant antiviral effect in that serum WHV DNA was reduced by 2 logs on average (range 1.5 to 4 logs)[182]. The antiviral effect observed was transient in the majority of woodchucks, but two woodchucks appeared to have sustained suppression in serum WHV DNA concentration. The results from these studies indicate that in vivo therapeutic gene delivery to augment the deficient Th-type 1 cytokine responses in liver may restore some of the failed antiviral and immunologic functions in human chronic HBV infection. 

Another approach to immunotherapy of chronic HBV infection involves the restoration and stimulation of higher-affinity Th and CTL clones in the periphery (or locally in the liver). Rather than to supplement a specific cytokine deficiency, it may be possible to reconstitute a complete and successful cellular immune response to acute infection by transfer of autologous or histocompatible T cell clones. The efficacy of the latter approach has been demonstrated in recent clinical studies of lymphocompatible bone marrow or PBMC transplantation from HBV-recovered or anti-HBV immunized donors into chronic carrier recipients (e.g.,[183-187]). Studies of cell-based therapies in chronic WHV carrier woodchucks involving adoptive lymphocyte transfer from vaccinated or WHV-resolved donors are in progress using neonatal-infected carrier woodchucks made lymphocompatible with their sires by co-injection of parental bone marrow and/or lymphocytes at birth. Later, after the neonates become established WHV carriers, they are re-administered parental lymphocytes therapeutically that were primed in the parent by immunization or recovery from acute WHV infection (Menne et al, unpublished data). Recently, another approach involving adoptive immunotransfer via liver transplantation from vaccinated WHV-naïve woodchucks into chronic carrier woodchucks was tested[188]. Following vaccination of donor woodchucks with DNA plasmids encoding WHcAg, WHsAg, and WHsAg in combination with a plasmid expressing IFN-g livers were transplanted into recipient woodchucks, and the therapeutic effect determined. Two of 3 recipient carriers demonstrated a reduction in serum WHV DNA below the limit of detection by Southern hybridization analysis immediately following transplantation that lasted for up to 7 wk. WHV DNA in serum samples was detected when a more sensitive PCR assay was used. Nevertheless, the reductions in serum viremia were consistent with parallel reductions in intrahepatic levels of WHV RNA and DNA replicative intermediates.


Combination therapy

The high viral and antigen loads in serum during the chronic phase of infection are believed to maintain immunologic tolerance in established carrier woodchucks[55,56]. In some cases, treatment with antiviral drug alone may unmask host immune responses as seen during treatment of adult-acquired chronic HBV infection with lamivudine[189,190]; however, such responses appear sub-optimal for bringing about a complete recovery phenotype. To facilitate the emergence of the host immune response from a tolerant state maintained by high antigen load, combination therapy with a nucleoside analogue followed by modulation of the deficient immune responses represents a promising approach. Such an approach might even be able to improve upon natural recovery by creating optimal conditions for more rapid and complete eradication of viral cccDNA from the system. 

In one study, chronic WHV carrier woodchucks were treated with lamivudine at a relatively high daily dose of 200 mg/kg given orally for 23 wk[143]. At the time, WHV DNA and WHsAg serum levels had declined by 3 to 5 logs or 1 log, respectively, woodchucks were vaccinated with three doses of a serum-derived WHsAg in combination with a peptide carrying a Th epitope from sperm whale myoglobin. In contrast to a previous study[174], therapeutic vaccination did not induce detectable anti-WHs antibody responses in carriers; the levels of viremia and antigenemia remained nearly unchanged from that achieved by drug treatment, and they returned to pretreatment levels following drug withdrawal. One important finding of this study was that the combination of lamivudine and vaccine, but not treatment with drug alone, induced CMI to WHsAg and WHcAg, presumably by shifting the cytokine profile from Th-type 2 to that of Th-type 0/1.

In another study chronic WHV carrier woodchucks received lamivudine treatment for 6 mo, again at a relatively high dose (200 mg/kg per day, oral) in order to reduce serum WHV DNA by 1 to 3 logs, and were then superinfected with an adenoviral vector expressing b-galactosidase[191]. Compared to control woodchucks, combination treatment resulted in further reductions of serum WHV DNA (10-20-fold) in the majority of woodchucks. The vector itself induced local immune responses in liver, and a bystander antiviral effect was observed on intrahepatic WHV DNA, WHV cccDNA, and WHV RNA that correlated with the inflammatory responses involving increased intrahepatic expression of woodchuck leukocyte markers and cytokines. The suppression of WHV replication was transient, but prolonged compared to woodchucks receiving lamivudine monotherapy. Similar results were obtained following superinfection of chronic WHV carriers with adenoviral vectors expressing IFN-g, TNF-a, or b-galactosidase in combination with orally administered clevudine at 10 mg/kg per day[156]. Adenovirus superinfection led to declines in the intrahepatic WHV DNA levels, but a long-term benefit of combination treatment over clevudine alone was not observed. However, in contrast to monotherapy with lamivudine[191], recrudescence of WHV replication was deltayed until 14 wk after withdrawal of clevudine.

The antiviral effect of a combination of two nucleoside analogues in addition to an adenoviral vector expressing IFN-g also has been tested in chronic WHV carrier woodchucks[150]. Woodchucks received clevudine and emtricitabine simultaneously at daily oral doses of 10 mg/kg and 30 mg/kg, respectively, for 8 wk, with two intravenous injections of the vector at wk 4 and 8. Combination treatment with clevudine and emtricitabine resulted in an antiviral effect on WHV replication, with reductions in serum viremia by 4 logs, and associated declines in intrahepatic levels of WHV DNA replicative intermediates and WHV cccDNA. The antiviral effect was sustained in a few woodchucks following drug withdrawal. The additional administration of the adenoviral vector led to increased liver inflammation, but enhancements of the antiviral effect compared to combination treatment with clevudine and emtricitabine were not observed.

In our study in chronically WHV-infected woodchucks described above[55,56,155], combination therapy with clevudine (10 mg/kg per day, oral, 32 wk), followed by 4 doses of a conventional WHsAg vaccine (alum-adsorbed, formalin-inactivated WHsAg), enhanced the virus-specific CMI to WHsAg, and resulted in additional collateral responses to other viral antigens (Figures 4 and 5). Vaccination alone elicited low-level antibody responses to WHsAg in most woodchucks but did not affect serum WHV DNA or WHsAg levels compared to placebo-treated control woodchucks. Chronic WHV carrier woodchucks treated first with clevudine to reduce serum WHV DNA (> 6 to 8 log reduction) and WHsAg (> 50- to 500-fold reduction), and then vaccinated, developed a more robust anti-WHs antibody response. After vaccination, WHsAg-specific CMI was shown in both vaccinated groups, but was significantly enhanced in woodchucks treated initially with clevudine, and was broadened to include responses to WHcAg and to selected peptide epitopes of WHcAg and WHsAg.

Thus, the long-term drug treatment combined with therapeutic vaccination was shown to break humoral and cellular immune tolerance in treated WHV carrier woodchucks better than the component monotherapies, and to produce a more complete immune response profile resembling that in recovery from acute WHV infection, including an associated and marked reduction in the concentration of WHV cccDNA in liver. While the inclusion of vaccine after clevudine treatment did not result in a significant further antiviral effect beyond that of clevudine alone (i.e., clevudine is so potent that further antiviral effects would be difficult to measure), the combination therapy did have an additive benefit over the monotherapies in delaying the onset and occurrence of disease progression, including chronic hepatitis and HCC[30,155]. The results of this study suggest that the delay in the onset of chronic hepatitis and HCC is due to the uniformly high degree of suppression of viral load, especially the expression of viral antigens in serum and liver, any of which could act to maintain immune tolerance during chronic carriage. Longer term protection against the onset or development of HCC then appeared to be a function of the improved cellular and humoral immune responsiveness to viral antigens, which could no longer serve as endogenous tolerogens after reduction by drug.



The woodchuck animal model of chronic HBV infection has been valuable in determining the mechanisms of hepadnavirus replication and for studies of viral pathogenesis including associated disease sequelae and host immune responses. Continued modeling of early acute phase immune responses leading to resolution versus chronicity in the neonatal woodchuck may help to identify useful predictive markers of outcome that will facilitate the early identification of the carrier state, and the rational development of antiviral and/or immunotherapies for established chronic HBV infection. Colony-born woodchucks infected as neonates with well-characterized inocula also enable the evaluation of efficacy and toxicity of new types of prophylaxis or therapy under controlled experimental conditions in a relevant animal model within a reasonable time frame. Continued testing of new therapeutic approaches empirically and rationally in the woodchuck model will ultimately improve the chances for successful therapeutic eradication of established chronic HBV infection and its disease sequelae.



The authors graciously acknowledge the support of Drs. Bud C Tennant (Cornell University) and John L Gerin (Georgetown University).



1      Vryheid RE, Kane MA, Muller N, Schatz GC, Bezabeh S. Infant and adolescent hepatitis B immunization up to 1999: a
  global overview. Vaccine 2000; 19: 1026-1037

2      Bertoletti A, Ferrari C. Kinetics of the immune response during HBV and HCV infection. Hepatology 2003; 38: 4-13

3      Chisari FV, Ferrari C. Hepatitis B virus immunopathology. Springer Semin Immunopathol 1995; 17: 261-281

4      Chisari FV. Rous-Whipple Award Lecture. Viruses, immunity, and cancer: lessons from hepatitis B. Am J Pathol 2000;
: 1117-1132

5      Jilbert AR, Kotlarski I. Immune responses to duck hepatitis B virus infection. Dev Comp Immunol 2000; 24: 285-302

6      Menne S, Tennant BC. Unraveling hepatitis B virus infection of mice and men (and woodchucks and ducks). Nat Med
  1999; 5: 1125-1126

7      Cote PJ, Toshkov I, Nakamura I, Menne S, Korba B, Tennant B, Gerin J. Chronicity as an outcome of experimental
  neonatal woodchuck hepatitis virus infection results from a deficient type 1 immune response to acute infection.
  Margolis HS, Alter MJ, Liang TJ, Dienstag JL, eds. Viral hepatitis and liver diseases: proceedings of the 10th international
  symposium on viral hepatitis and liver disease. Atlanta: International Medical Press Ltd, 2002: 280-285

8      Chisari FV. Hepatitis B virus transgenic mice: models of viral immunobiology and pathogenesis. Curr Top Microbiol
1996; 206: 149-173

9      Guidotti LG, Chisari FV. Noncytolytic control of viral infections by the innate and adaptive immune response. Annu Rev
2001; 19: 65-91

10    Menne S, Cote PJ. The woodchuck as an emerging animal model for immunopathogenesis and immunotherapy of
  human HBV infection In: Recent research developments in virology, vol. 5. Kerala, India: Transworld Research Network,
  2003: 117-141

11    Summers J, Smolec JM, Snyder R. A virus similar to human hepatitis B virus associated with hepatitis and hepatoma in
  woodchucks. Proc Natl Acad Sci USA 1978; 75: 4533-4537

12    Cummings IW, Browne JK, Salser WA, Tyler GV, Snyder RL, Smolec JM, Summers J. Isolation, characterization, and
  comparison of recombinant DNAs derived from genomes of human hepatitis B virus and woodchuck hepatitis virus. Proc
  Natl Acad Sci USA
1980; 77: 1842-1846

13    Galibert F, Chen TN, Mandart E. Nucleotide sequence of a cloned woodchuck hepatitis virus genome: comparison with
  the hepatitis B virus sequence. J Virol 1982; 41: 51-65

14    Kodama K, Ogasawara N, Yoshikawa H, Murakami S. Nucleotide sequence of a cloned woodchuck hepatitis virus
  genome: evolutional relationship between hepadnaviruses. J Virol 1985; 56: 978-986

15    Cohen JI, Miller RH, Rosenblum B, Denniston K, Gerin JL, Purcell RH. Sequence comparison of woodchuck hepatitis virus
  replicative forms shows conservation of the genome. Virology 1988; 162: 12-20

16    Girones R, Cote PJ, Hornbuckle WE, Tennant BC, Gerin JL, Purcell RH, Miller RH. Complete nucleotide sequence of a
  molecular clone of woodchuck hepatitis virus that is infectious in the natural host. Proc Natl Acad Sci USA 1989; 86:

17    Cote PJ, Korba BE, Miller RH, Jacob JR, Baldwin BH, Hornbuckle WE, Purcell RH, Tennant BC, Gerin JL. Effects of age and
  viral determinants on chronicity as an outcome of experimental woodchuck hepatitis virus infection. Hepatology 2000;
: 190-200

18    Mandart E, Kay A, Galibert F. Nucleotide sequence of a cloned duck hepatitis B virus genome: comparison with
  woodchuck and human hepatitis B virus sequences. J Virol 1984; 49: 782-792

19    Mason WS, Seal G, Summers J. Virus of Pekin ducks with structural and biological relatedness to human hepatitis B
  virus. J Virol 1980; 36: 829-836

20    Omata M, Uchiumi K, Ito Y, Yokosuka O, Mori J, Terao K, Wei-Fa Y, O’Connell AP, London WT, Okuda K. Duck hepatitis B
  virus and liver diseases. Gastroenterology 1983; 85: 260-267

21    Korba BE, Cote P, Hornbuckle W, Tennant BC, Gerin JL. Treatment of chronic woodchuck hepatitis virus infection in the
  Eastern woodchuck (Marmota monax) with nucleoside analogues is predictive of therapy for chronic hepatitis B virus
  infection in humans. Hepatology 2000; 31: 1165-1175

22    Main J, McCarron B, Thomas HC. Treatment of chronic viral hepatitis. Antivir Chem Chemother 1998; 9: 449-460

23    De Clercq E. Perspectives for the treatment of hepatitis B virus infections. Int J Antimicrob Agents 1999; 12: 81-95

24    Zoulim F. Evaluation of novel strategies to combat hepatitis B virus targetting wild-type and drug-resistant mutants in
  experimental models. Antivir Chem Chemother 2001; 12 Suppl 1: 131-142

25    Tennant BC, Gerin JL. The woodchuck model of hepatitis B virus infection. ILAR J 2001; 42: 89-102

26    Roggendorf M, Tolle TK. The woodchuck: an animal model for hepatitis B virus infection in man. Intervirology 1995;
: 100-112

27    Paronetto F, Tennant BC. Woodchuck hepatitis virus infection: a model of human hepatic diseases and hepatocellular
  carcinoma. Prog Liver Dis 1990; 9: 463-483

28    Gerin JL, Tennant BC, Ponzetto A, Purcell RH, Tyeryar FJ. The woodchuck animal model of hepatitis B-like virus infection
  and disease. Prog Clin Biol Res 1983; 143: 23-28

29    Cote PJ, Gerin JL. The woodchuck as a model of hepadnavirus infection, pathogenesis and therapy. Forum Trends Exp
  Clin Med
1996; 6: 131-159

30    Tennant BC, Toshkov IA, Peek SF, Jacob JR, Menne S, Hornbuckle WE, Schinazi RD, Korba BE, Cote PJ, Gerin JL.
  Hepatocellular carcinoma in the woodchuck model of hepatitis B virus infection. Gastroenterology 2004; 127: S283-S293

31    Gerin JL, Tennant BC, Popper H, Tyeryar FJ, Purcell RH. The woodchuck model of hepadnavirus infection and disease.
  In: Brown F, Chanock R, Lerner R, eds. Vaccines 86: new approaches to immunization. Cold Spring Harbor: Cold Spring
  Harbor Laboratory, 1986: 383-386

32    Cote PJ, Korba BE, Tennant BC, Gerin JL. Immunopatho-genesis and immunomodulation of woodchuck hepatitis virus
  infection In: Hollinger FB, Lemon SM, Margolis HS, eds. Viral hepatitis and liver disease. Baltimore: Williams & Wilkins,
1991: 483-486

33    Cote PJ, Korba BE, Baldwin B, Hornbuckle WE, Tennant BC, Gerin JL. Immunosuppression with cyclosporine during the
  incubation period of experimental woodchuck hepatitis virus infection increases the frequency of chronic infection in adult
  woodchucks. J Infect Dis 1992; 166: 628-631

34    Cote PJ, Korba BE, Steinberg H, Ramirez-Mejia C, Baldwin B, Hornbuckle WE, Tennant BC, Gerin JL. Cyclosporin A
  modulates the course of woodchuck hepatitis virus infection and induces chronicity. J Immunol 1991; 146: 3138-3144

35    Tennant BC. Animal models of hepadnavirus-associated hepatocellular carcinoma. Clin Liver Dis 2001; 5: 43-68

36    Michalak TI. Occult persistence and lymphotropism of hepadnaviral infection: insights from the woodchuck viral hepatitis
  model. Immunol Rev 2000; 174: 98-111

37    Robinson WS. Molecular events in the pathogenesis of hepadnavirus-associated hepatocellular carcinoma. Annu Rev
1994; 45: 297-323

38    Gerin JL, Cote PJ, Korba BE, Tennant BC. Hepadnavirus-induced liver cancer in woodchucks. Cancer Detect Prev 1989;
: 227-229

39    Chang MH. Chronic hepatitis virus infection in children. J Gastroenterol Hepatol 1998; 13: 541-548

40    Norkrans G. Epidemiology of hepatitis B virus (HBV) infections with particular regard to current routes of transmission
  and development of cirrhosis and malignancy. Scand J Infect Dis Suppl 1990; 69: 43-47

41    Ghendon Y. Perinatal transmission of hepatitis B virus in high-incidence countries. J Virol Methods 1987; 17: 69-79

42    Cote PJ, Toshkov I, Bellezza C, Ascenzi M, Roneker C, Ann Graham L, Baldwin BH, Gaye K, Nakamura I, Korba BE,
  Tennant BC, Gerin JL. Temporal pathogenesis of experimental neonatal woodchuck hepatitis virus infection: increased
  initial viral load and decreased severity of acute hepatitis during the development of chronic viral infection. Hepatology
  2000; 32: 807-817

43    Nakamura I, Nupp JT, Cowlen M, Hall WC, Tennant BC, Casey JL, Gerin JL, Cote PJ. Pathogenesis of experimental
  neonatal woodchuck hepatitis virus infection: chronicity as an outcome of infection is associated with a diminished acute
  hepatitis that is temporally deficient for the expression of interferon gamma and tumor necrosis factor-alpha messenger
  RNAs. Hepatology 2001; 33: 439-447

44    Menne S, Roneker CA, Roggendorf M, Gerin JL, Cote PJ, Tennant BC. Deficiencies in the acute-phase cell-mediated
  immune response to viral antigens are associated with development of chronic woodchuck hepatitis virus infection
  following neonatal inoculation. J Virol 2002; 76: 1769-1780

45    Wang Y, Menne S, Jacob JR, Tennant BC, Gerin JL, Cote PJ. Role of type 1 versus type 2 immune responses in liver
  during the onset of chronic woodchuck hepatitis virus infection. Hepatology 2003; 37: 771-780 

46    Wang Y, Menne S, Baldwin BH, Tennant BC, Gerin JL, Cote PJ. Kinetics of viremia and acute liver injury in relation to
  outcome of neonatal woodchuck hepatitis virus infection. J Med Virol 2004; 72: 406-415

47    Ponzetto A, Forzani B. Animal models of hepatocellular carcinoma: hepadnavirus-induced liver cancer in woodchucks.
  Ital J Gastroenterol
1991; 23: 491-493

48    Seeger C, Baldwin B, Hornbuckle WE, Yeager AE, Tennant BC, Cote P, Ferrell L, Ganem D, Varmus HE. Woodchuck
  hepatitis virus is a more efficient oncogenic agent than ground squirrel hepatitis virus in a common host. J Virol 1991;
: 1673-1679

49    Gerin JL. Experimental WHV infection of woodchucks: an animal model of hepadnavirus-induced liver cancer.
  Gastroenterol Jpn
1990; 25 Suppl 2: 38-42

50    Popper H, Roth L, Purcell RH, Tennant BC, Gerin JL. Hepatocarcinogenicity of the woodchuck hepatitis virus. Proc Natl
  Acad Sci USA
1987; 84: 866-870

51    Hsu HY, Chang MH, Hsieh KH, Lee CY, Lin HH, Hwang LH, Chen PJ, Chen DS. Cellular immune response to HBcAg in
  mother-to-infant transmission of hepatitis B virus. Hepatology 1992; 15: 770-776

52    Ferrari C, Penna A, Bertoletti A, Valli A, Antoni AD, Giuberti T, Cavalli A, Petit MA, Fiaccadori F. Cellular immune
  response to hepatitis B virus-encoded antigens in acute and chronic hepatitis B virus infection. J Immunol 1990; 145:

53    Tsai SL, Chen PJ, Lai MY, Yang PM, Sung JL, Huang JH, Hwang LH, Chang TH, Chen DS. Acute exacerbations of chronic
  type B hepatitis are accompanied by increased T cell responses to hepatitis B core and e antigens. Implications for
  hepatitis B e antigen seroconversion. J Clin Invest 1992; 89: 87-96

54    Marinos G, Torre F, Chokshi S, Hussain M, Clarke BE, Rowlands DJ, Eddleston AL, Naoumov NV, Williams R. Induction of
  T-helper cell response to hepatitis B core antigen in chronic hepatitis B: a major factor in activation of the host immune
  response to the hepatitis B virus. Hepatology 1995; 22: 1040-1049

55    Menne S, Roneker CA, Tennant BC, Korba BE, Gerin JL, Cote PJ. Immunogenic effects of woodchuck hepatitis virus
  surface antigen vaccine in combination with antiviral therapy: breaking of humoral and cellular immune tolerance in
  chronic woodchuck hepatitis virus infection. Intervirology 2002; 45: 237-250

56    Menne S, Roneker CA, Korba BE, Gerin JL, Tennant BC, Cote PJ. Immunization with surface antigen vaccine alone and
  after treatment with 1-(2-fluoro-5-methyl-beta-L-arabinofuranosyl)-uracil (L-FMAU) breaks humoral and cell-mediated
  immune tolerance in chronic woodchuck hepatitis virus infection. J Virol 2002; 76: 5305-5314

57    Korba BE, Cote PJ, Gerin JL, Menne S, Toshkov IA, Tennant BC. Therapy with Clevudine followed by vaccine delays the
  progression of WHV-induced hepatitis and hepatocellular carcinoma in chronically-infected woodchucks. Hepatology
  2001; 34: 583 Part 582 Suppl

58    Michalak TI, Pardoe IU, Coffin CS, Churchill ND, Freake DS, Smith P, Trelegan CL. Occult lifelong persistence of
  infectious hepadnavirus and residual liver inflammation in woodchucks convalescent from acute viral hepatitis.
1999; 29: 928-938

59    Mulrooney PM, Michalak TI. Quantitative detection of hepadnavirus-infected lymphoid cells by in situ PCR combined
  with flow cytometry: implications for the study of occult virus persistence. J Virol 2003; 77: 970-979

60    Lew YY, Michalak TI. In vitro and in vivo infectivity and pathogenicity of the lymphoid cell-derived woodchuck hepatitis
  virus. J Virol 2001; 75: 1770-1782

61    Korba BE, Cote PJ, Wells FV, Baldwin B, Popper H, Purcell RH, Tennant BC, Gerin JL. Natural history of woodchuck
  hepatitis virus infections during the course of experimental viral infection: molecular virologic features of the liver and
  lymphoid tissues. J Virol 1989; 63: 1360-1370

62    Korba BE, Cote PJ, Gerin JL. Mitogen-induced replication of woodchuck hepatitis virus in cultured peripheral blood
  lymphocytes. Science 1988; 241: 1213-1216

63    Korba BE, Wells F, Tennant BC, Cote PJ, Gerin JL. Lymphoid cells in the spleens of woodchuck hepatitis virus-infected
  woodchucks are a site of active viral replication. J Virol 1987; 61: 1318-1324

64    Korba BE, Wells F, Tennant BC, Yoakum GH, Purcell RH, Gerin JL. Hepadnavirus infection of peripheral blood
  lymphocytes in vivo: woodchuck and chimpanzee models of viral hepatitis. J Virol 1986; 58: 1-8 

65    Coffin CS, Michalak TI. Persistence of infectious hepadnavirus in the offspring of woodchuck mothers recovered from
  viral hepatitis. J Clin Invest 1999; 104: 203-212

66    Yuki N, Nagaoka T, Yamashiro M, Mochizuki K, Kaneko A, Yamamoto K, Omura M, Hikiji K, Kato M. Long-term histologic
  and virologic outcomes of acute self-limited hepatitis B. Hepatology 2003; 37: 1172-1179

67    Blum HE, Liang TJ, Galun E, Wands JR. Persistence of hepatitis B viral DNA after serological recovery from hepatitis B
  virus infection. Hepatology 1991; 14: 56-63

68    Michalak TI, Pasquinelli C, Guilhot S, Chisari FV. Hepatitis B virus persistence after recovery from acute viral hepatitis. J
  Clin Invest
1994; 93: 230-239

69    Penna A, Artini M, Cavalli A, Levrero M, Bertoletti A, Pilli M, Chisari FV, Rehermann B, Del Prete G, Fiaccadori F, Ferrari
  C. Long-lasting memory T cell responses following self-limited acute hepatitis B. J Clin Invest 1996; 98: 1185-1194

70    Rehermann B, Ferrari C, Pasquinelli C, Chisari FV. The hepatitis B virus persists for decades after patients’ recovery
  from acute viral hepatitis despite active maintenance of a cytotoxic T-lymphocyte response. Nat Med 1996; 2: 1104-1108

71    Yotsuyanagi H, Yasuda K, Iino S, Moriya K, Shintani Y, Fujie H, Tsutsumi T, Kimura S, Koike K. Persistent viremia after
  recovery from self-limited acute hepatitis B. Hepatology 1998; 27: 1377-1382

72    Korba BE, Wells FV, Baldwin B, Cote PJ, Tennant BC, Popper H, Gerin JL. Hepatocellular carcinoma in woodchuck
  hepatitis virus-infected woodchucks: presence of viral DNA in tumor tissue from chronic carriers and animals
  serologically recovered from acute infections. Hepatology 1989; 9: 461-470

73    Gust ID, Burrell CJ, Coulepis AG, Robinson WS, Zuckerman AJ. Taxonomic classification of human hepatitis B virus.
1986; 25: 14-29

74    Ganem D. Hepadnaviridae: the viruses and their replication. In: Fields BN, Knipe DM, Howley PM, eds. Fields virology,
  3rd ed. Philadelphia: Lippincott-Raven, 1996: 2703-2737

75    Ganem D, Varmus HE. The molecular biology of the hepatitis B viruses. Annu Rev Biochem 1987; 56: 651-693

76    Tiollais P, Pourcel C, Dejean A. The hepatitis B virus. Nature 1985; 317: 489-495

77    Seeger C, Ganem D, Varmus HE. Biochemical and genetic evidence for the hepatitis B virus replication strategy. Science
  1986; 232: 477-484

78    Summers J. The replication cycle of hepatitis B viruses. Cancer 1988; 61: 1957-1962

79    Chen HS, Miller RH, Hornbuckle WE, Tennant BC, Cote PJ, Gerin JL, Purcell RH. Titration of recombinant woodchuck
  hepatitis virus DNA in adult woodchucks. J Med Virol 1998; 54: 92-94

80    Mason WS, Jilbert AR, Summers J. Clonal expansion of hepatocytes during chronic woodchuck hepatitis virus infection.
  Proc Natl Acad Sci USA
2005; 102: 1139-1144

81    Summers J, Jilbert AR, Yang W, Aldrich CE, Saputelli J, Litwin S, Toll E, Mason WS. Hepatocyte turnover during
  resolution of a transient hepadnaviral infection. Proc Natl Acad Sci USA 2003; 100: 11652-11659 

82    Kajino K, Jilbert AR, Saputelli J, Aldrich CE, Cullen J, Mason WS. Woodchuck hepatitis virus infections: very rapid
  recovery after a prolonged viremia and infection of virtually every hepatocyte. J Virol 1994; 68: 5792-5803

83    Korba BE, Brown TL, Wells FV, Baldwin B, Cote PJ, Steinberg H, Tennant BC, Gerin JL. Natural history of experimental
  woodchuck hepatitis virus infection: molecular virologic features of the pancreas, kidney, ovary, and testis. J Virol 1990;
: 4499-4506

84    Korba BE, Cote PJ, Shapiro M, Purcell RH, Gerin JL. In vitro production of infectious woodchuck hepatitis virus by
  lipopolysaccharide-stimulated peripheral blood lymphocytes. J Infect Dis 1989; 160: 572-576 

85    Korba BE, Gowans EJ, Wells FV, Tennant BC, Clarke R, Gerin JL. Systemic distribution of woodchuck hepatitis virus in
  the tissues of experimentally infected woodchucks. Virology 1988; 165: 172-181

86    Michalak TI, Mulrooney PM, Coffin CS. Low doses of hepadnavirus induce infection of the lymphatic system that does
  not engage the liver. J Virol 2004; 78: 1730-1738

87    Menne S, Maschke J, Tolle TK, Lu M, Roggendorf M. Characterization of T-cell response to woodchuck hepatitis virus
  core protein and protection of woodchucks from infection by immunization with peptides containing a T-cell epitope. J
1997; 71: 65-74

88    Cote PJ, Gerin JL. In vitro activation of woodchuck lymphocytes measured by radiopurine incorporation and interleukin-2
  production: implications for modeling immunity and therapy in hepatitis B virus infection. Hepatology 1995; 22: 687-699

89    Menne S, Maschke J, Tolle T, Kreuzfelder E, Grosse-Wilde H, Roggendorf M. Determination of peripheral blood
  mononuclear cell responses to mitogens and woodchuck hepatitis virus core antigen in woodchucks by 5-bromo-2’-
  deoxyuridine or 2[3H]adenine incorporation. Arch Virol 1997; 142: 511-521

90    Gujar SA, Michalak TI. Flow cytometric quantification of T cell proliferation and division kinetics in woodchuck model of
  hepatitis B. Immunol Invest 2005; 34: 215-236

91    Ferrari C, Penna A, DegliAntoni A, Fiaccadori F. Cellular immune response to hepatitis B virus antigens. An overview. J
1988; 7: 21-33

92    Webster GJ, Reignat S, Maini MK, Whalley SA, Ogg GS, King A, Brown D, Amlot PL, Williams R, Vergani D, Dusheiko
  GM, Bertoletti A. Incubation phase of acute hepatitis B in man: dynamic of cellular immune mechanisms. Hepatology
  2000; 32: 1117-1124

93    Menne S, Maschke J, Lu M, Grosse-Wilde H, Roggendorf M. T-Cell response to woodchuck hepatitis virus (WHV)
  antigens during acute self-limited WHV infection and convalescence and after viral challenge. J Virol 1998; 72: 6083-

94    Menne S, Maschke J, Klaes R, Grosse-Wilde H, Roggendorf M. Cellular immune response of woodchucks to woodchuck
  hepatitis virus surface protein during acute WHV infection. In: Rizzetto M, Purcell R, Gerin J, Verme G, eds. Viral
  hepatitis and liver diseases: proceedings of the IX triennial international symposium on hepatitis viruses and liver
  disease. Torino: Edizoni Minerva Medica, 1997: 453-457

95    Hodgson PD, Grant MD, Michalak TI. Perforin and Fas/Fas ligand-mediated cytotoxicity in acute and chronic woodchuck
  viral hepatitis. Clin Exp Immunol 1999; 118: 63-70

96    Hodgson PD, Michalak TI. Augmented hepatic interferon gamma expression and T-cell influx characterize acute
  hepatitis progressing to recovery and residual lifelong virus persistence in experimental adult woodchuck hepatitis virus
  infection. Hepatology 2001; 34: 1049-1059

97    Guo JT, Zhou H, Liu C, Aldrich C, Saputelli J, Whitaker T, Barrasa MI, Mason WS, Seeger C. Apoptosis and regeneration
  of hepatocytes during recovery from transient hepadnavirus infections. J Virol 2000; 74: 1495-1505

98    Jacob JR, Ascenzi MA, Roneker CA, Toshkov IA, Cote PJ, Gerin JL, Tennant BC. Hepatic expression of the woodchuck
  hepatitis virus X-antigen during acute and chronic infection and detection of a woodchuck hepatitis virus X-antigen
  antibody response. Hepatology 1997; 26: 1607-1615

99    Chemin I, Vermot-Desroches C, Baginski I, Lamelin JP, Hantz O, Jacquet C, Rigal D, Trepo C. Monitoring of early events
  of experimental woodchuck hepatitis infection: studies of peripheral blood mononuclear cells by cytofluorometry and
  PCR. FEMS Immunol Med Microbiol 1993; 7: 241-249

100  Feitelson MA, Clayton MM. X antigen/antibody markers in hepadnavirus infections. Antibodies to the X gene product(s).
1990; 99: 500-507

101  Feitelson MA, Clayton MM, Blumberg BS. X antigen/antibody markers in hepadnavirus infections. Presence and
  significance of hepadnavirus X gene product(s) in serum. Gastroenterology 1990; 98: 1071-1078

102  Michalak TI, Lin B, Churchill ND, Dzwonkowski P, Desousa JR. Hepadna virus nucleocapsid and surface antigens and the
  antigen-specific antibodies associated with hepatocyte plasma membranes in experimental woodchuck acute hepatitis.
  Lab Invest
1990; 62: 680-689

103  Tyler GV, Summers JW, Synder RL. Woodchuck hepatitis virus in natural woodchuck populations. J Wildl Dis 1981; 17:

104  Lindberg J, Pichoud C, Hantz O, Vitvitski L, Grimaud JA, Gilbert JM, Joubert L, Frommel D, Trepo C. Woodchuck hepatitis
  virus infection: serologic and histopathologic course and outcome. Eur J Clin Microbiol 1985; 4: 59-61

105  Ponzetto A, Cote PJ, Ford EC, Purcell RH, Gerin JL. Core antigen and antibody in woodchucks after infection with
  woodchuck hepatitis virus. J Virol 1984; 52: 70-76

106  Menne S, Cote PJ. Measurement of cell-mediated immune response in woodchucks. Methods Mol Med 2004; 96: 27-36

107  Menne S, Wang Y, Butler SD, Gerin JL, Cote PJ, Tennant BC. Real-time polymerase chain reaction assays for leukocyte
  CD and cytokine mRNAs of the Eastern woodchuck (Marmota monax). Vet Immunol Immunopathol 2002; 87: 97-105

108  Nakamura I, Nupp JT, Rao BS, Buckler-White A, Engle RE, Casey JL, Gerin JL, Cote PJ. Cloning and characterization of
  partial cDNAs for woodchuck cytokines and CD3epsilon with applications for the detection of RNA expression in tissues by
  RT-PCR assay. J Med Virol 1997; 53: 85-95

109  Cote PJ, Nakamura I, Bellezza C, Tennant BC, Gerin JL. Immunobiology of the woodchuck and woodchuck hepatitis virus
  infection. In: Rizzetto M, Purcell R, Gerin J, Verme G, eds. Viral hepatitis and liver diseases: proceedings of the IX
  triennial international symposium on hepatitis viruses and liver disease. Torino, Italy: Edizoni Minerva Medica, 1997:

110  Chisari FV, Ferrari C. Hepatitis B virus immunopathogenesis. Annu Rev Immunol 1995; 13: 29-60

111  Abbott WG, Geursen A, Fraser JD, Marbrook J, Skinner MA, Tan PL. The influence of a maternal chronic hepatitis B
  virus infection on the repertoire of transcribed T-cell receptor beta chain variable region genes in human cord blood.
1995; 22: 1034-1039

112  Itoh Y, Okanoue T, Sakamoto S, Nishioji K, Kashima K. The effects of prednisolone and interferons on serum
  macrophage colony stimulating factor concentrations in chronic hepatitis B. J Hepatol 1997; 26: 244-252

113  Zielinska W, Paszkiewicz J, Korczak A, Wlasiuk M, Zoltowska A, Szutowicz A, Cummins JM, Georgiades JA. Treatment of
  fourteen chronic active HBsAg+, HBeAg+ hepatitis patients with low dose natural human interferon alpha administered
  orally. Arch Immunol Ther Exp (Warsz) 1993; 41: 241-251

114  Hsu HY, Chang MH, Ni YH, Lee PI. Cytokine release of peripheral blood mononuclear cells in children with chronic
  hepatitis B virus infection. J Pediatr Gastroenterol Nutr 1999; 29: 540-545

115  Jung MC, Diepolder HM, Spengler U, Wierenga EA, Zachoval R, Hoffmann RM, Eichenlaub D, Frosner G, Will H, Pape GR.
  Activation of a heterogeneous hepatitis B (HB) core and e antigen-specific CD4+ T-cell population during seroconversion
  to anti-HBe and anti-HBs in hepatitis B virus infection. J Virol 1995; 69: 3358-3368

116  Lohr HF, Weber W, Schlaak J, Goergen B, Meyer zum Buschenfelde KH, Gerken G. Proliferative response of CD4+ T cells
 and hepatitis B virus clearance in chronic hepatitis with or without hepatitis B e-minus hepatitis B virus mutants.
1995; 22: 61-68

117  Zoulim F, Berthillon P, Guerhier FL, Seigneres B, Germon S, Pichoud C, Cheng YC, Trepo C. Animal models for the study
 of HBV infection and the evaluation of new anti-HBV strategies. J Gastroenterol Hepatol 2002; 17 Suppl: S460-S463

118  Tatti KM, Korba BE, Stang HL, Peek S, Gerin JL, Tennant BC, Schinazi RF. Mutations in the conserved woodchuck
 hepatitis virus polymerase FLLA and YMDD regions conferring resistance to lamivudine. Antiviral Res 2002; 55: 141-150

119  Yamamoto T, Litwin S, Zhou T, Zhu Y, Condreay L, Furman P, Mason WS. Mutations of the woodchuck hepatitis virus
 polymerase gene that confer resistance to lamivudine and 2’-fluoro-5-methyl-beta-L-arabinofuranosyluracil. J Virol 2002;
: 1213-1223

120  Mason WS, Cullen J, Moraleda G, Saputelli J, Aldrich CE, Miller DS, Tennant B, Frick L, Averett D, Condreay LD, Jilbert
 AR. Lamivudine therapy of WHV-infected woodchucks. Virology 1998; 245: 18-32

121  Zhou T, Saputelli J, Aldrich CE, Deslauriers M, Condreay LD, Mason WS. Emergence of drug-resistant populations of
 woodchuck hepatitis virus in woodchucks treated with the antiviral nucleoside lamivudine. Antimicrob Agents Chemother
 1999; 43: 1947-1954

122  Zhu Y, Yamamoto T, Cullen J, Saputelli J, Aldrich CE, Miller DS, Litwin S, Furman PA, Jilbert AR, Mason WS. Kinetics of
 hepadnavirus loss from the liver during inhibition of viral DNA synthesis. J Virol 2001; 75: 311-322

123  Korba BE, Gerin JL. Use of a standardized cell culture assay to assess activities of nucleoside analogs against hepatitis B
 virus replication. Antiviral Res 1992; 19: 55-70

124  Morrey JD, Korba BE, Sidwell RW. Transgenic mice as a chemotherapeutic model for hepatitis B virus infection. Antivir Ther 1998; 3: 59-68

125  Tennant BC, Baldwin BH, Graham LA, Ascenzi MA, Hornbuckle WE, Rowland PH, Tochkov IA, Yeager AE, Erb HN,
 Colacino JM, Lopez C, Engelhardt JA, Bowsher RR, Richardson FC, Lewis W, Cote PJ, Korba BE, Gerin JL. Antiviral activity
 and toxicity of fialuridine in the woodchuck model of hepatitis B virus infection. Hepatology 1998; 28: 179-191

126  Kleiner DE, Gaffey MJ, Sallie R, Tsokos M, Nichols L, McKenzie R, Straus SE, Hoofnagle JH. Histopathologic changes
 associated with fialuridine hepatotoxicity. Mod Pathol 1997; 10: 192-199

127  Lewis W, Griniuviene B, Tankersley KO, Levine ES, Montione R, Engelman L, de Courten-Myers G, Ascenzi MA,
 Hornbuckle WE, Gerin JL, Tennant BC. Depletion of mitochondrial DNA, destruction of mitochondria, and accumulation of
 lipid droplets result from fialuridine treatment in woodchucks (Marmota monax). Lab Invest 1997; 76: 77-87

128  Josephson L. Severe toxicity of fialuridine (FIAU). N Engl J Med 1996; 334: 1135-1136; author reply 1137-1138

129  Fourel I, Hantz O, Watanabe KA, Jacquet C, Chomel B, Fox JJ, Trepo C. Inhibitory effects of 2’-fluorinated arabinosyl-
 pyrimidine nucleosides on woodchuck hepatitis virus replication in chronically infected woodchucks. Antimicrob Agents
1990; 34: 473-475

130  Le Guerhier F, Pichoud C, Jamard C, Guerret S, Chevallier M, Peyrol S, Hantz O, King I, Trepo C, Cheng YC, Zoulim F.
 Antiviral activity of beta-L-2’,3’-dideoxy-2’,3’-didehydro-5-fluorocytidine in woodchucks chronically infected with
 woodchuck hepatitis virus. Antimicrob Agents Chemother 2001; 45: 1065-1077

131  Zahm FE, d’Urso N, Bonino F, Ponzetto A. Treatment of woodchuck hepatitis virus infection in vivo with 2’, -3’-
 dideoxycytidine (ddC) and 2’,-3’-dideoxycytidine monophosphate coupled to lactosaminated human serum albumin (L-HSA
 ddCMP). Liver 1996; 16: 88-93

132  Korba BA, Xie H, Wright KN, Hornbuckle WE, Gerin JL, Tennant BC, Hostetler KY. Liver-targeted antiviral nucleosides:
 enhanced antiviral activity of phosphatidyl-dideoxyguanosine versus dideoxyguanosine in woodchuck hepatitis virus
 infection in vivo. Hepatology 1996; 23: 958-963

133  Zahm FE, Bonino F, Giuseppetti R, Rapicetta M. Antiviral activity of ganciclovir, 9-(1,3-dihydroxy-2-propoxymethyl)
 guanine against woodchuck hepatitis virus: quantitative measurement of woodchuck hepatitis virus DNA using storage
 phosphor technology. Ital J Gastroenterol Hepatol 1998; 30: 510-516

134  Chu CK, Boudinot FD, Peek SF, Hong JH, Choi Y, Korba BE, Gerin JL, Cote PJ, Tennant BC, Cheng YC. Preclinical
 investigation of L-FMAU as an anti-hepatitis B virus agent. Antivir Ther 1998; 3: 113-121

135  Hostetler KY, Beadle JR, Hornbuckle WE, Bellezza CA, Tochkov IA, Cote PJ, Gerin JL, Korba BE, Tennant BC. Antiviral
 activities of oral 1-O-hexadecylpropanediol-3-phosphoacyclovir and acyclovir in woodchucks with chronic woodchuck
 hepatitis virus infection. Antimicrob Agents Chemother 2000; 44: 1964-1969 

136  Hurwitz SJ, Tennant BC, Korba BE, Gerin JL, Schinazi RF. Pharmacodynamics of (-)-beta-2‘,3‘-dideoxy-3‘-thiacytidine in
 chronically virus-infected woodchucks compared to its pharmacodynamics in humans. Antimicrob Agents Chemother
 1998; 42: 2804-2809

137  Enriquez PM, Jung C, Josephson L, Tennant BC. Conjugation of adenine arabinoside 5’-monophosphate to
 arabinogalactan: synthesis, characterization, and antiviral activity. Bioconjug Chem 1995; 6: 195-202

138  Ponzetto A, Fiume L, Forzani B, Song SY, Busi C, Mattioli A, Spinelli C, Marinelli M, Smedile A, Chiaberge E. Adenine
 arabinoside monophosphate and acyclovir monophosphate coupled to lactosaminated albumin reduce woodchuck
 hepatitis virus viremia at doses lower than do the unconju-gated drugs. Hepatology 1991; 14: 16-24

139  Genovesi EV, Lamb L, Medina I, Taylor D, Seifer M, Innaimo S, Colonno RJ, Standring DN, Clark JM. Efficacy of the
 carbocyclic 2’-deoxyguanosine nucleoside BMS-200475 in the woodchuck model of hepatitis B virus infection. Antimicrob
 Agents Chemother
1998; 42: 3209-3217

140  Choi JR, Cho DG, Roh KY, Hwang JT, Ahn S, Jang HS, Cho WY, Kim KW, Cho YG, Kim J, Kim YZ. A novel class of
 phosphonate nucleosides. 9-[(1-phosphonomethoxycyclopropyl)methyl]guanine as a potent and selective anti-HBV agent.
 J Med Chem
2004; 47: 2864-2869

141  Korba BE, Cote P, Hornbuckle W, Schinazi R, Gangemi JD, Tennant BC, Gerin JL. Enhanced antiviral benefit of
 combination therapy with lamivudine and alpha interferon against WHV replication in chronic carrier woodchucks. Antivir Ther 2000; 5: 95-104

142  Peek SF, Toshkov I, Erb H, Schinazi R, Korba B, Cote P, Gerin J, Tennant B. Lamivudine (2,3-dideoxy-3-thiacytidine,3TC)
 delays the onset of hepatocellular carcinoma (HCC) and increases survival in the woodchuck model of chronic hepatitis B
 virus (HBV) infection. Hepatology 2002; 36: 1855 Part 1852 Suppl

143  Hervas-Stubbs S, Lasarte JJ, Sarobe P, Vivas I, Condreay L, Cullen JM, Prieto J, Borras-Cuesta F. T-helper cell
 response to woodchuck hepatitis virus antigens after therapeutic vaccination of chronically-infected animals treated with
 lamivudine. J Hepatol 2001; 35: 105-111

144  Cullen JM, Li DH, Brown C, Eisenberg EJ, Cundy KC, Wolfe J, Toole J, Gibbs C. Antiviral efficacy and pharmacokinetics of
 oral adefovir dipivoxil in chronically woodchuck hepatitis virus-infected woodchucks. Antimicrob Agents Chemother 2001;
: 2740-2745

145  Jacob JR, Korba BE, Cote PJ, Toshkov I, Delaney WE 4th, Gerin JL, Tennant BC. Suppression of lamivudine-resistant B-
 domain mutants by adefovir dipivoxil in the woodchuck hepatitis virus model. Antiviral Res 2004; 63: 115-121

146  Colonno RJ, Genovesi EV, Medina I, Lamb L, Durham SK, Huang ML, Corey L, Littlejohn M, Locarnini S, Tennant BC,
 Rose B, Clark JM. Long-term entecavir treatment results in sustained antiviral efficacy and prolonged life span in the
 woodchuck model of chronic hepatitis infection. J Infect Dis 2001; 184: 1236-1245

147  Menne S, Cote PJ, Korba BE, Butler SD, George AL, Tochkov IA, Delaney WE 4th, Xiong S, Gerin JL, Tennant BC. Antiviral
 effect of oral administration of tenofovir disoproxil fumarate in woodchucks with chronic woodchuck hepatitis virus
 infection. Antimicrob Agents Chemother 2005; 49: 2720-2728

148  Cullen JM, Smith SL, Davis MG, Dunn SE, Botteron C, Cecchi A, Linsey D, Linzey D, Frick L, Paff MT, Goulding A, Biron K.
 In vivo antiviral activity and pharmacokinetics of (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine in
 woodchuck hepatitis virus-infected woodchucks. Antimicrob Agents Chemother 1997; 41: 2076-2082

149  Korba BE, Schinazi RF, Cote P, Tennant BC, Gerin JL. Effect of oral administration of emtricitabine on woodchuck
 hepatitis virus replication in chronically infected woodchucks. Antimicrob Agents Chemother 2000; 44: 1757-1760

150  Jacquard AC, Nassal M, Pichoud C, Ren S, Schultz U, Guerret S, Chevallier M, Werle B, Peyrol S, Jamard C, Rimsky LT,
 Trepo C, Zoulim F. Effect of a combination of clevudine and emtricitabine with adenovirus-mediated delivery of gamma
 interferon in the woodchuck model of hepatitis B virus infection. Antimicrob Agents Chemother 2004; 48: 2683-2692

151  Standring DN, Bridges EG, Placidi L, Faraj A, Loi AG, Pierra C, Dukhan D, Gosselin G, Imbach JL, Hernandez B,
 Juodawlkis A, Tennant B, Korba B, Cote P, Cretton-Scott E, Schinazi RF, Myers M, Bryant ML, Sommadossi JP. Antiviral
 beta-L-nucleosides specific for hepatitis B virus infection. Antivir Chem Chemother 2001; 12 Suppl 1: 119-129

152  Bryant ML, Bridges EG, Placidi L, Faraj A, Loi AG, Pierra C, Dukhan D, Gosselin G, Imbach JL, Hernandez B, Juodawlkis
 A, Tennant B, Korba B, Cote P, Marion P, Cretton-Scott E, Schinazi RF, Sommadossi JP. Antiviral L-nucleosides specific
 for hepatitis B virus infection. Antimicrob Agents Chemother 2001; 45: 229-235 

153  Bryant ML, Bridges EG, Placidi L, Faraj A, Loi AG, Pierra C, Dukhan D, Gosselin G, Imbach JL, Hernandez B, Juodawlkis
 A, Tennant B, Korba B, Cote P, Cretton-Scott E, Schinazi RF, Sommadossi JP. Anti-HBV specific beta-L-2’-
 deoxynucleosides. Nucleosides Nucleotides Nucleic Acids 2001; 20: 597-607

154  Peek SF, Cote PJ, Jacob JR, Toshkov IA, Hornbuckle WE, Baldwin BH, Wells FV, Chu CK, Gerin JL, Tennant BC, Korba BE.
 Antiviral activity of clevudine [L-FMAU, (1-(2-fluoro-5-methyl-beta, L-arabinofuranosyl) uracil)] against woodchuck
 hepatitis virus replication and gene expression in chronically infected woodchucks (Marmota monax). Hepatology 2001;
: 254-266

155  Korba BE, Cote PJ, Menne S, Toshkov I, Baldwin BH, Wells FV, Tennant BC, Gerin JL. Clevudine therapy with vaccine
 inhibits progression of chronic hepatitis and delays onset of hepatocellular carcinoma in chronic woodchuck hepatitis virus
 infection. Antivir Ther 2004; 9: 937-952

156  Zhu Y, Cullen JM, Aldrich CE, Saputelli J, Miller D, Seeger C, Mason WS, Jilbert AR. Adenovirus-based gene therapy
 during clevudine treatment of woodchucks chronically infected with woodchuck hepatitis virus. Virology 2004; 327: 26-40

157  Korba BE, Cote P, Hornbuckle W, Schinazi R, Gerin JL, Tennant BC. Enhanced antiviral benefit of combination therapy
 with lamivudine and famciclovir against WHV replication in chronic WHV carrier woodchucks. Antiviral Res 2000; 45: 19-

158  Block TM, Lu X, Mehta AS, Blumberg BS, Tennant B, Ebling M, Korba B, Lansky DM, Jacob GS, Dwek RA. Treatment of
 chronic hepadnavirus infection in a woodchuck animal model with an inhibitor of protein folding and trafficking. Nat Med
 1998; 4: 610-614

159  Bartholomew RM, Carmichael EP, Findeis MA, Wu CH, Wu GY. Targeted delivery of antisense DNA in woodchuck
 hepatitis virus-infected woodchucks. J Viral Hepat 1995; 2: 273-278

160  Blumberg BS, Millman I, Venkateswaran PS, Thyagarajan SP. Hepatitis B virus and primary hepatocellular carcinoma:
 treatment of HBV carriers with Phyllanthus amarus. Vaccine 1990; 8 Suppl: S86-S92

161  Gerin JL, Korba BE, Cote PJ, Tennant BC. A preliminary report of a controlled study of thymosin alpha-1 in the
 woodchuck model of hepadnavirus infection. Adv Exp Med Biol 1992; 312: 121-123

162  Putzer BM, Stiewe T, Rodicker F, Schildgen O, Ruhm S, Dirsch O, Fiedler M, Damen U, Tennant B, Scherer C, Graham
 FL, Roggendorf M. Large nontransplanted hepatocellular carcinoma in woodchucks: treatment with adenovirus-mediated
 delivery of interleukin 12/B7.1 genes. J Natl Cancer Inst 2001; 93: 472-479 

163  Bilbao R, Gerolami R, Bralet MP, Qian C, Tran PL, Tennant B, Prieto J, Brechot C. Transduction efficacy, antitumoral
 effect, and toxicity of adenovirus-mediated herpes simplex virus thymidine kinase/ ganciclovir therapy of hepatocellular
 carcinoma: the woodchuck animal model. Cancer Gene Ther 2000; 7: 657-662 

164  Gouillat C, Manganas D, Zoulim F, Vitrey D, Saguier G, Guillaud M, Ain JF, Duque-Campos R, Jamard C, Praves M, Trepo
 C. Woodchuck hepatitis virus-induced carcinoma as a relevant natural model for therapy of human hepatoma. J Hepatol
 1997; 26: 1324-1330

165  Roos S, Fuchs K, Roggendorf M. Protection of woodchucks from infection with woodchuck hepatitis virus by
 immunization with recombinant core protein. J Gen Virol 1989; 70 (Pt 8): 2087-2095

166  Schodel F, Neckermann G, Peterson D, Fuchs K, Fuller S, Will H, Roggendorf M. Immunization with recombinant
 woodchuck hepatitis virus nucleocapsid antigen or hepatitis B virus nucleocapsid antigen protects woodchucks from
 woodchuck hepatitis virus infection. Vaccine 1993; 11: 624-628 

167  Cote PJ, Shapiro M, Engle RE, Popper H, Purcell RH, Gerin JL. Protection of chimpanzees from type B hepatitis by
 immunization with woodchuck hepatitis virus surface antigen. J Virol 1986; 60: 895-901

168  Garcia-Navarro R, Blanco-Urgoiti B, Berraondo P, Sanchez de la Rosa R, Vales A, Hervas-Stubbs S, Lasarte JJ, Borras
 F, Ruiz J, Prieto J. Protection against woodchuck hepatitis virus (WHV) infection by gene gun coimmunization with WHV
 core and interleukin-12. J Virol 2001; 75: 9068-9076

169  Siegel F, Lu M, Roggendorf M. Coadministration of gamma interferon with DNA vaccine expressing woodchuck hepatitis
 virus (WHV) core antigen enhances the specific immune response and protects against WHV infection. J Virol 2001; 75:

170  Lu M, Hilken G, Kruppenbacher J, Kemper T, Schirmbeck R, Reimann J, Roggendorf M. Immunization of woodchucks with
 plasmids expressing woodchuck hepatitis virus (WHV) core antigen and surface antigen suppresses WHV infection. J Virol
 1999; 73: 281-289

171  Lu M, Isogawa M, Xu Y, Hilken G. Immunization with the gene expressing woodchuck hepatitis virus nucleocapsid protein
 fused to cytotoxic-T-lymphocyte-associated antigen 4 leads to enhanced specific immune responses in mice and
 woodchucks. J Virol 2005; 79: 6368-6376

172  Argentini C, Giuseppetti R, D’Ugo E, La Sorsa V, Tritarelli E, Orobello S, Canitano A, Gluck R, Rapicetta M. A pre-S/S
 CHO-derived hepatitis B virus vaccine protects woodchucks from WHV productive infection. Vaccine 2005; 23: 3649-3656

173  Lu M, Klaes R, Menne S, Gerlich W, Stahl B, Dienes HP, Drebber U, Roggendorf M. Induction of antibodies to the PreS
 region of surface antigens of woodchuck hepatitis virus (WHV) in chronic carrier woodchucks by immunizations with WHV
 surface antigens. J Hepatol 2003; 39: 405-413

174  Hervas-Stubbs S, Lasarte JJ, Sarobe P, Prieto J, Cullen J, Roggendorf M, Borras-Cuesta F. Therapeutic vaccination of
 woodchucks against chronic woodchuck hepatitis virus infection. J Hepatol 1997; 27: 726-737 

175  Guidotti LG, Guilhot S, Chisari FV. Interleukin-2 and alpha/beta interferon down-regulate hepatitis B virus gene
 expression in vivo by tumor necrosis factor-dependent and -independent pathways. J Virol 1994; 68: 1265-1270

176  Guidotti LG, Borrow P, Hobbs MV, Matzke B, Gresser I, Oldstone MB, Chisari FV. Viral cross talk: intracellular inactivation
 of the hepatitis B virus during an unrelated viral infection of the liver. Proc Natl Acad Sci USA 1996; 93: 4589-4594

177  Guidotti LG, Ishikawa T, Hobbs MV, Matzke B, Schreiber R, Chisari FV. Intracellular inactivation of the hepatitis B virus
 by cytotoxic T lymphocytes. Immunity 1996; 4: 25-36

178  Schildgen O, Fiedler M, Dahmen U, Li J, Lohrengel B, Lu M, Roggendorf M. Fluctuation of the cytokine expression in the
 liver during the chronic woodchuck hepatitis virus (WHV) infection is not related to viral load. Immunol Lett 2006; 102:

179  Lu M, Lohrengel B, Hilken G, Kemper T, Roggendorf M. Woodchuck gamma interferon upregulates major
 histocompatibility complex class I transcription but is unable to deplete woodchuck hepatitis virus replication
 intermediates and RNAs in persistently infected woodchuck primary hepatocytes. J Virol 2002; 76: 58-67 

180  Wang Y, Jacob JR, Menne S, Bellezza CA, Tennant BC, Gerin JL, Cote PJ. Interferon-gamma-associated responses to
 woodchuck hepatitis virus infection in neonatal woodchucks and virus-infected hepatocytes. J Viral Hepat 2004; 11: 404-

181  Fiedler M, Rodicker F, Salucci V, Lu M, Aurisicchio L, Dahmen U, Jun L, Dirsch O, Putzer BM, Palombo F, Roggendorf M.
 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. J
2004; 78: 10111-10121

182  Berraondo P, Ochoa L, Crettaz J, Rotellar F, Vales A, Martinez-Anso E, Zaratiegui M, Ruiz J, Gonzalez-Aseguinolaza G,
 Prieto J. IFN-alpha gene therapy for woodchuck hepatitis with adeno-associated virus: differences in duration of gene
 expression and antiviral activity using intraportal or intramuscular routes. Mol Ther 2005; 12: 68-76

183  Ilan Y, Nagler A, Shouval D, Ackerstein A, Or R, Kapelushnik J, Adler R, Slavin S. Development of antibodies to hepatitis
 B virus surface antigen in bone marrow transplant recipient following treatment with peripheral blood lymphocytes from
 immunized donors. Clin Exp Immunol 1994; 97: 299-302

184  Ilan Y, Nagler A, Adler R, Tur-Kaspa R, Slavin S, Shouval D. Ablation of persistent hepatitis B by bone marrow
 transplant-ation from a hepatitis B-immune donor. Gastroenterology 1993; 104: 1818-1821

185  Lau GK, Suri D, Liang R, Rigopoulou EI, Thomas MG, Mullerova I, Nanji A, Yuen ST, Williams R, Naoumov NV. Resolution
 of chronic hepatitis B and anti-HBs seroconversion in humans by adoptive transfer of immunity to hepatitis B core antigen.
2002; 122: 614-624

186  Lau GK, Liang R, Lee CK, Yuen ST, Hou J, Lim WL, Williams R. Clearance of persistent hepatitis B virus infection in
 Chinese bone marrow transplant recipients whose donors were anti-hepatitis B core- and anti-hepatitis B surface
 antibody-positive. J Infect Dis 1998; 178: 1585-1591

187  Brugger SA, Oesterreicher C, Hofmann H, Kalhs P, Greinix HT, Muller C. Hepatitis B virus clearance by transplantation of
 bone marrow from hepatitis B immunised donor. Lancet 1997; 349: 996-997

188  Dahmen U, Dirsch O, Li J, Fiedle M, Lu M, Rispeter K, Picucci M, Broelsch CE, Roggendorf M. Adoptive transfer of
 immunity: a new strategy to interfere with severe hepatitis virus reinfection after woodchuck liver transplantation.
Transplantation 2004; 77: 965-972

189  Boni C, Bertoletti A, Penna A, Cavalli A, Pilli M, Urbani S, Scognamiglio P, Boehme R, Panebianco R, Fiaccadori F, Ferrari
 C. Lamivudine treatment can restore T cell responsiveness in chronic hepatitis B. J Clin Invest 1998; 102: 968-975

190  Boni C, Penna A, Ogg GS, Bertoletti A, Pilli M, Cavallo C, Cavalli A, Urbani S, Boehme R, Panebianco R, Fiaccadori F,
 Ferrari C. Lamivudine treatment can overcome cytotoxic T-cell hyporesponsiveness in chronic hepatitis B: new
 perspectives for immune therapy. Hepatology 2001; 33: 963-971

191  Zhou T, Guo JT, Nunes FA, Molnar-Kimber KL, Wilson JM, Aldrich CE, Saputelli J, Litwin S, Condreay LD, Seeger C,
       Mason WS.
Combination therapy with lamivudine and adenovirus causes transient suppression of chronic woodchuck
       hepatitis virus infections.
J Virol 2000; 74: 11754-11763


S- Editor  Liu Y    L- Editor  Alpini GD    E- Editor  Ma WH





Reviews Add

Related Articles:
p73beta inhibits transcriptional activities of enhancer I and X promoter in hepatitis B virus more efficiently than p73alpha
Anti-HBV hairpin ribozyme-mediated cleavage of target RNA in vitro
Inhibition of hepatitis B virus by oxymatrine in vivo
Humoral and cellular immunogenecity of DNA vaccine based on hepatitis B core gene in rhesus monkeys
A comparative study on serologic profiles of virus hepatitis B