Immunization with Surface Antigen Vaccine Alone and after Treatment with 1-(2-Fluoro-5-Methyl-β-l-Arabinofuranosyl)-Uracil (l-FMAU) Breaks Humoral and Cell-Mediated Immune Tolerance in Chronic Woodchuck Hepatitis Virus Infection

More than 350 million people worldwide are chronically infected with hepatitis B virus (HBV) (19) and are at high risk of developing chronic hepatitis, hepatic cirrhosis, and hepatocellular carcinoma. Such individuals could benefit immensely from effective therapy, and a combination of antiviral drugs and immunomodulation is a strategy currently under consideration. Successful immunotherapeutic eradication of established HBV infection might be possible if there was better understanding of the underlying mechanisms of chronicity (1, 2, 11, 18, 22, 24, 30, 31, 39; reviewed in references 3 and 26). While approaches using antiviral drugs in combination with alpha interferon (IFN-α) or HBV surface antigen (HBsAg) vaccines have shown promise as therapies for established chronic HBV infection (9, 10, 12, 16, 20, 23, 35), such treatments have not been optimized based on a full understanding of the mechanisms of chronicity. Chronic HBV infection is associated with defects in immunity (reviewed in references 3 and 26) that hinder the development of successful immunotherapy. In contrast to self-limited HBV infection, the HBV carrier appears to be immunologically tolerant. The T helper (Th) cell responses to viral antigens are usually deficient, and antibodies to HBsAg (anti-HBs) are rarely detected. Whether additional B-cell tolerance is involved here is not clear. Antibodies to the HBV core antigen (HBcAg) (anti-HBc antibodies) are detected in carriers with hyporesponsive Th cells to core gene products (HBcAg and HBV e antigen [HBeAg]) because anti-HBc can be elicited via Th-independent mechanisms (32). Th cell responses to HBcAg and HBeAg can be favorable for the carrier when viral replication is diminished during seroconversion to antibodies to HBeAg (anti-HBe) (11, 18, 22, 24, 39). However, T-cell responses in the HBV carrier are generally dysfunctional overall and contribute more to disease progression than to viral clearance (11, 24, 39; reviewed in reference 3). The Eastern woodchuck (Marmota monax) chronically infected with the woodchuck hepatitis virus (WHV) has been used as an animal model to investigate the basic pathogenesis of chronic HBV infection and in the preclinical development of drugs for therapy of HBV (reviewed in references 6, 26, 36, and 38). This animal model mimics many of the immune response features observed in human HBV infections (4, 5, 7, 14, 15, 25, 27-29, 33; reviewed in references 6, 26, 36, and 38) and can predict human responses to antiviral agents as well (21). Experimental studies in chronic WHV carrier woodchucks have been performed using the new and potent antiviral drug 1-(2-fluoro-5-methyl-β-l-arabinofuranosyl)-uracil (l-FMAU) (34), which may represent a promising therapeutic approach for chronic HBV infection. l-FMAU significantly reduces the concentration in serum of both WHV DNA and WHV surface antigen (WHsAg) and also the levels of covalently closed circular viral DNA in the liver (34). A controlled study has now been performed in woodchucks involving treatment with a combination of l-FMAU followed by WHsAg vaccination. The purpose of this report is to show how it is possible using combination therapy to increasingly break immunologic tolerance in chronic WHV carrier woodchucks and to modulate the humoral and cellular immune response profiles toward that observed in self-limited WHV infections.