A novel monoclonal antibody targeting a conserved inner region of the hepatitis B virus envelope enables broad detection of immune escape variants.

The detection of hepatitis B surface antigen (HBsAg) is fundamental for the diagnosis of chronic hepatitis B (CHB). However, current diagnostic assays that rely on antibodies targeting the conformational "a" determinant of HBsAg are frequently compromised by mutations in this region, leading to undetected immune escape variants. In this study, we generated a novel monoclonal antibody, designated S1705, by immunizing mice with CHO-derived HBsAg. This antibody was identified to recognize a conserved linear epitope located within the internal loop region of the HBsAg particle. Western blot analysis under denaturing conditions confirmed that S1705 robustly detects HBsAg from multiple genotypes (A-D) and common clinical mutants, including the prevalent G145R variant. Moreover, S1705 demonstrated effective utility in applications such as flow cytometry and immunofluorescence. Notably, it exhibited superior performance compared to commercial antibodies targeting conformational epitopes in detecting HBsAg escape variants. We conclude that S1705, by targeting a conserved linear epitope, enables broad and reliable detection of both wild-type and mutant HBsAg. Critically, our findings demonstrate that immunodominant antigenic regions exist beyond the conventional boundaries of the 'a' determinant. This antibody thus holds significant promise for enhancing the diagnostic coverage of HBV variants and supporting future antiviral research and clinical monitoring.