Characterization of occult hepatitis B virus from blood donors carrying genotype A2 or genotype D strains.

Background/aims: Nucleic acid testing (NAT) for hepatitis B virus (HBV) DNA in blood donations identified occult HBV infection (OBI) as a potential threat to blood safety.

Methods: A collaborative study was undertaken to explore the molecular basis of OBIs prevalent in Europe in relation to clinical and serological data.

Results: Ninety-one percent of 77 donor samples of European origin HBV DNA positive but HBV surface antigen (HBsAg) negative were confirmed. Viral load ranged between unquantifiable and 5640 IU/mL (median 25 IU/mL). Fifty-two strains were genotyped (14 HBV(A2) and 38 HBV(D)). Compared to HBsAg+ samples, genotype D was significantly more frequent than genotype A2 in OBIs from Poland or Italy (P<0.04). Amino acid substitutions were concentrated in the immunologically active parts of the Pre-S/S proteins (P<0.0001) affecting both cellular CD8 T-cell epitopes and B-cell neutralizing Major Hydrophilic Region epitopes. Substitutions were more frequent in OBIs than in HBsAg+ strains of both genotype D (P<0.001) and A2 (P<0.01), in OBIs of genotype D than A2 in the 'a' region (P<0.001) but not cellular epitopes, and in anti-HBs+ than anti-HBs- OBIs (P<0.001).

Conclusions: Results support the hypothesis that humoral and cellular immune pressure on the HBV envelope proteins are major mechanisms generating OBI.