In anti-hepatitis B virus (HBV) therapy using peginterferon, seroconversion of HBV surface antigen (HBsAg) can be achieved in a small percentage of patients, which is considered a cure of the disease. Of 245 consecutive chronic hepatitis B patients receiving peginterferon therapy in our center, 8 (3.27%) achieved HBsAg seroclearance. Surprisingly, 2 of the 8 patients remained viremic by standard HBV-DNA assays. The coding regions of HBV pre-S/S gene derived from serial serum samples were analyzed. Site directed-mutagenesis experiment was performed to verify the phenotypic alterations in Huh-7 cells.

In patient-1, an sT125A mutant developed in the HBsAg-negative stage, constituting 11.2% of the viral population. HBV-DNA level was 2.73 × 10(4) IU/mL at the time of detection. This mutant was not detectable in the HBsAg-positive stages. Phenotypic study in Huh-7 cells showed significantly reduction of antigenicity. In patient-2, an sW74* truncation mutation was found in the HBsAg-negative stage, constituting 83.1% of the viral population. HBV-DNA level was 4.12 × 10(4) IU/mL at the time of detection. Phenotypic study in Huh-7 cells showed a complete loss of the antigenicity.

Patient-2 subsequently experienced an episode of hepatitis relapse 7 months after the end of treatment with negative HBsAg throughout the hepatitis flare. In conclusion, during peginterferon antiviral therapy, achievement of HBsAg seroconversion not necessarily indicated viral eradication. Emergence of the S gene mutants was an alternative possibility and relapse with "HBsAg-negative hepatitis" could occur.