Identification and characteristics of mutations promoting occult HBV infection by ultrasensitive HBsAg assay.

The significance of occult hepatitis B virus (HBV) infection (OBI) has been increasingly recognized while the underlying mechanisms remain incompletely understood. This study aimed to identify high-frequency OBI-related mutations in HBV surface antigen (HBsAg)-negative samples tested by the ultrasensitive Lumipulse G HBsAg-Quant assay. OBI samples were collected from 32 blood establishments across 14 provinces in China. Lumipulse G HBsAg-Quant assay was performed for the re-testing and reclassification of OBI. Mutations in genotypes B (GTB) and C (GTC) were analyzed to identify high-frequency single and combined mutations. Additionally, the efficacy of commercial reagents commonly employed in clinical diagnostics for detecting mutant HBsAg was evaluated. Western Blot was used for the confirmation of extracellular HBsAg as well as the detection of intracellular HBsAg. Hydrophilicity analysis and transmembrane distribution prediction of HBsAg were utilized for further validation. Single mutations at 17 sites and 9 combined mutations in GTB indicated a significantly elevated mutation frequency. In GTC, there were single mutations at 16 sites and 9 combined mutations. Several commercial reagents commonly demonstrated limited capacity toward mutant HBsAg with T123A/P, K141C, and P142R/I/K/L (GTB) and S114A/P (GTC). The findings indicated that mutations including T123A/C/K/S, S132G/Y, P142L/R/S/T, T143M, D144G, G145A, K160R+V168A, I4T+V168A, M103I+K122R, and M103I+Q181R (GTB), along with Q101H, M103I, R160K+C221Y (GTC), were associated with reduced levels of HBsAg both extracellularly and intracellularly. Additionally, K160R (GTB) and E2G (GTC) were associated with intracellular aggregation. This study elucidates the mutations associated with decreased extracellular HBsAg with ultrasensitive HBsAg assay, providing insight for further investigation into the mechanisms of OBI. IMPORTANCE: The sensitivity of HBsAg detection reagents directly impacts the identification of occult hepatitis B virus (HBV) infection (OBI). This study aims to identify high-frequency OBI-related mutations in HBV surface antigen (HBsAg)-negative samples evaluated using a Fujirebio-Lumipulse ultrasensitive HBsAg assay and to investigate the implications of these mutations on the antigenicity of HBsAg, the detection capacities of various HBsAg assays, and the effects on intracellular and extracellular levels of HBsAg. Generally, our study offers a new perspective on OBI-related mutations by ultrasensitive HBsAg assay and lays the groundwork for further research on the OBI mechanism and the enhancement of HBsAg detection reagents.