[Functional analysis of hepatitis B virus immune escape mutants with insertion mutations in the surface antigen]

To evaluate the influence of insertion mutations occurring in the hydrophobic region, between amino acids 114 and 115, of the hepatitis B surface antigen (HBsAg) on viral antigenicity and replication.

Hepatitis B virus (HBV) DNA was obtained from patients with HBsAg-positive chronic hepatitis B (CHB) infection and subjected to sequence analysis and comparison to GenBank reference sequences for HBV genotype B (AB073826) and genotype C (AF286594). Insertion mutations detected in the HBsAg region were used to make recombinant expression plasmids via site-directed mutagenesis. After transfecting the recombinant HBsAg into Huh7 cells, the mutants' effects on viral antigenicity and replication were evaluated by chemiluminescence microparticle immunoassay (CMIA) and Southern blot hybridization, respectively. The viral antigenicity of each mutant was predicted by bioinformatic analysis, using the Jameson-Wolf method to predict the antigenic index, the Hopp-Woods method to predict hydrophilicity, the Emini method to predict the probability of a region lying of the protein's surface, and the Karplus-Schulz method to predict the flexibility of the protein backbone.

Two CHB patients harbored HBV with insertion mutations in HBsAg: one with two (NT) and one with three (NTT) inserted amino acids between 114 and 115. The NTT recombinant HBsAg mutant showed no impact on viral replication and reacted weakly with anti-HBs in CMIA (P = 0.02). The antigen indices for the insertion of NTT were 1.00, -0.16, and 0.18, and insertion of the three amino acids affected the index values of five proximal amino acid sites (with an average increase of 0.13). The hydrophilic indices for the insertion of NTT were 0.2, -0.4, and -0.4, with no significant effect on the proximal amino acids. The insertion of the three amino acids changed both the surface probability (range: -0.55 to 2.97; affecting eight proximal amino acids) and the flexibility (range: -0.01 to 1.1; affecting five proximal amino acids).

The NTT three amino acids insertion in the HBsAg open reading frame, between 114 and 115 of the wild-type sequence, detected in a CHB patient may play a role in HBV immune escape without influencing the viral replicative capacity.