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

Abstract

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.

Figure 1

Eastern wood-chuck (Marmota mo-nax).

Figure 2

Schematic profiles for serum viremia in adult and neonatal models of experimental WHV infection. A: Adult woodchucks. Adult woodchucks born to WHV-negative dams are infected with 1 x 10⁷ woodchuck infectious doses 50% of a defined WHV inoculum by the intravenous route. The proportions of chronic and resolved outcomes of adult woodchucks usually are less than 5% and more than 95%, respectively. The lifetime risk for the development of HCC in established chronic and resolved WHV infections is 100% and 15%-20%, respectively; B: Neonatal woodchucks. Neonatal woodchucks born to WHV-negative dams are infected with 5 x 10⁶ woodchuck infectious doses 50% of a defined WHV inoculum by the subcutaneous route. The proportions of chronic and resolved outcomes range between 60%-75% and 25%-40%, respectively. The lifetime risk for the development of HCC in established chronic and resolved WHV infections is 100% and 15%-20%, respectively. HCC in uninfected, WHV-negative woodchucks is not observed. Approximate time intervals for the development of innate and acquired immunity are shown.

Figure 3

Schematic profiles for viremia, acute hepatitis, and disease progression in the neonatal model of experimental WHV infection. A: Self-limited, resolved WHV infection; B: Chronic WHV infection. Neonatal woodchucks born to WHV-negative dams are infected experimentally at 3 d of age with 5 x 10⁶ woodchuck infectious doses 50% of a defined WHV inoculum by the subcutaneous route. Approximate time intervals for self-limited acute hepatitis and progressive chronic hepatitis are shown. Use of the neonatal WHV infection model enables co-temporal comparisons of acute self-limiting and chronic outcome of WHV infections as they develop. Comparison of acute phase events at early time points before or near the times when self-limited, resolved and chronic outcomes begin to segregate based on serologic criteria allows the definition of immune mechanisms involved in progression toward recovery versus chronicity. Comparison at later time points enables the investigation of mechanisms that are important in the maintenance of the established WHV infection state and that lead to disease progression and tumor development. During this time the established chronic WHV carrier woodchucks are used mainly for the testing of antiviral nucleosides, immunotherapeutic strategies, and for the prevention of onset and development of HCC.

Figure 4

Combination treatment with clevudine and WHsAg vaccine suppresses serum viremia and antigenemia and induces a humoral response in chronic WHV carrier woodchucks. Changes in serum WHV DNA, serum WHsAg, and anti-WHs antibodies of chronic carriers in response to treatment with placebo (CLV-V-), vaccine (CLV-V+), drug (CLV+V-), and combination of drug and vaccine (CLV+V+) are shown. Horizontal bars denote the period of clevudine (CLV) administration for 32 wk. Arrowheads represent the 4 immunizations (V) using 50 μg doses of an alum-adsorbed, formalin-inactivated WHsAg vaccine at wk 32, 36, 40, and 48. WHVge, WHV genomic equivalents (virion or WHV DNA-containing particles).

Figure 5

Combination treatment with clevudine and WHsAg vaccine enhances and expands the pattern of cell-mediated immune responses to WHV antigens in chronic WHV carrier woodchucks. Changes in the PBMC responses to WHsAg, WHcAg, WHeAg, and WHxAg of chronic carriers in response to treatment with placebo (CLV-V-), vaccine (CLV-V+), drug (CLV+V-), and combination of drug and vaccine (CLV+V+) are shown. Horizontal bars denote the period of clevudine (CLV) administration for 32 wk. Arrowheads represent the 4 immunizations (V) using 50 μg doses of an alum-adsorbed, formalin-inactivated WHsAg vaccine at wk 32, 36, 40, and 48.