Objective To analyze the mutation of PreS-S region in occult hepatitis B virus(OHBV) in HBV infected persons with positive HBsAb and investigate the biological mechanisms of the special infectious model.Methods A total of 38 HB-sAb positive OBI serum samples were amplified by Nested PCR and sequenced,HBV genotype and serotype were determined.The amino acid sequences of OHBV were compared to the corresponding sequence of wild-type strains of similar genotype obtained from the GenBank database.Results PreS-S segment of 11 samples were obtained and 8 samples were sequenced successfully.Among which,5 were genotype C and 3 were genotype B.Genotype B were all serotype adw,while genotype C were 1 adw and 4 adr.The mutation rates of PreS-S region,the immunoreactive area and the major hydrophilic region (MHR) were higher in OHBV than the wild-type strains (2.6％ vs 0.8％,x2 =40.23,3.2％ vs 0.3％,x2 =52.13,3.6％ vs 0.6％,x2 =13.25,all P＜0.01) and the substitutions of I126T,Q129R,M133T,F134I,D144E,G145K in α determinant were found in OBI samples.The mutation rate of amino acids in PreS-S region was higher in genotype C than genotype B (3.5％ vs 1.2％,x2--15.98,P＜0.01),meanwhile,the mutation rates in MHR,α determinant and immunoreactive region were higher in genotype C too,but no statistical significance was attained (4.7％ vs 1.7 ％,x2 =2.96,3.6 ％ vs 2.9％,x2 =0.25,4.1％ vs 2.3％,x2 =3.59,all P ＞0.05).Conclusion Mutations in PreS-S region,especially in immunoepitope,might change the virus'immunogenicity leading to escape from immune response and cause OBI with HBsAb positive.

Objective:To analyze the mutation of T-cell and B-cell epitopes derived from HBV PreS-S protein in occult hepatitis B virus (OHBV) and investigate the biological mechanisms of occult hepatitis B virus infection (OBI) and HBsAb positive OBI.Methods:The PreS-S region of OBI samples were amplified by nested PCR, the products were sequenced and HBV genotypes were determined. The mutations of T-cell and B-cell epitopes derived from HBV PreS-S protein were analyzed and compared among groups of HBV genotypes and the presence of HBsAb. The affinity of the high frequency of T-cell epitope substitutions were analyzed by SYF PEITHI, the changes of antigenic characteristics of high frequency of B-cell epitope substitutions were analyzed by Ab Designer, Expasy ProtParam tool, Epitope Prediction and Analysis Tools.Results:The PreS-S region of HBV was amplified in 21 samples, including 4 HBsAb+ OBI B, 6 HBsAb-OBI B, 6 HBsAb+ OBI C, 5 HBsAb-OBI C. The mutation rates in PreS-S region of OBI were significantly higher than wild type HBV strains(OBI Bvs. WT B: 2.64%: 0.66%, P<0.001; OBI Cvs. WT C: 3.67%: 1.19%, P<0.001). The mutation rates of the immunoreactive area were significantly higher than non-immunoreactive area in OBI (OBI B: 3.57%: 1.86%, P=0.005; OBI C: 4.78%: 2.65%, P<0.001). The mutation rates of the immunoreactive and non-immunoreactive area in OBI C were higher than OBI B, but there was no statistically significant difference (immunoreactive area: 4.78%: 3.57%, P=0.107; non-immunoreactive area: 2.65%: 1.86%, P=0.142). The mutation rates of T-cell and B-cell epitopes of HBsAb-OBI were higher than HBsAb+ OBI, although there was no significant difference (HBsAb-OBI Bvs. HBsAb+ OBI B: 4.17∶3.01, P=0.303; HBsAb-OBI Cvs. HBsAb+ OBI C: 5.65∶4.26, P=0.207). The affinity analysis of 4 high frequency T-cell epitope substitutions, including T47A/K, S174N, L175S, V177A, showed that the changes of affinity of most mutation sites were not obvious; the antigenicity analysis of 3 high frequency B-cell epitope substitutions, including G73S, K122R, I126M/T, did not show noticeable changes and the hydrophilicity, surface accessibility of some mutation sites were even better than wild strain. Conclusions:The mutation rates in PreS-S region of OBI were significantly higher than wild type HBV strains. The mutation rates of the immunoreactive area were higher than non-immunoreactive area in OBI. The variant activity of OBI C was higher than OBI B. The mutations of OBI might occur randomly and were not selected by antibody pressure. Single epitope and multi-epitopes combinational mutations might be a reason for OBI.