Occult hepatitis B infection in blood donors from South East Asia: molecular characterisation and potential mechanisms of occurrence.

OBJECTIVE: To investigate the molecular basis of occult hepatitis B virus (HBV) infection (OBI) in Asian blood donors. DESIGN: OBI donors from Hong Kong, Malaysia, Singapore, Taiwan and Thailand were tested for HBV serological markers, and strains were molecularly characterised. RESULTS: Among 138 confirmed OBI carriers (median age 47 years), HBV genotypes B and C were dominant (60% and 34%, respectively) in agreement with the genotype distribution in chronically infected donors in the region. Viral load ranged between unquantifiable and 3670 IU/ml (median 11 IU/ml). Eleven per cent of OBIs showed an unusual anti-HBs-only serological profile without evidence of past vaccination for most of these individuals. Occult HBV strains showed a higher genetic diversity than strains from matched hepatitis B surface antigen (HBsAg)+ donors, irrespective of genotype. No unique genetic signature or evidence of reduced replication competence was found. Mutations in the vicinity of the pre-S2/S splice donor site were common in OBI(B) (44%) and OBI(C) (36%) strains. S regions from four OBI cases were transfected in HuH7 cells. Results showed limited HBsAg secretion and suggested that mutations disrupting the splice donor site structure may affect pre-S2/S mRNA splicing. CONCLUSIONS: There is indirect evidence that incomplete immune control is involved in the occurrence of OBI in Asian blood donors infected with genotypes B and C as observed in Europe with genotype A2 but to a lower extent than with genotype D. A post-transcriptional mechanism may play a role in HBsAg expression in some OBIs irrespective of HBV genotype.

CD4 and CD8 T cell responses to tumour-associated Epstein-Barr virus antigens in nasopharyngeal carcinoma patients.

Nasopharyngeal carcinoma (NPC), an Epstein-Barr virus (EBV)-associated tumour common in Southern Chinese populations, is a potentially important target for T cell-based immunotherapy. The tumour cells are HLA class I- and II-positive and express a limited subset of EBV latent proteins, namely the nuclear antigen EBNA1 and the latent membrane proteins LMP2 and (in some cases) LMP1. To ask whether the tumour develops in the presence of a potentially protective host response or in its absence, we set out to determine the prevailing levels of CD4+ and CD8+ T cell memory to these proteins in NPC patients at tumour diagnosis. We first screened healthy Chinese donors against Chinese strain EBNA1, LMP1 and LMP2 sequences in Elispot assays of interferon-gamma release and identified the immunodominant CD4+ and CD8+ epitope peptides presented by common Chinese HLA alleles. Then, comparing 60 patients with >70 healthy controls on peptide epitope mini-panels, we found that T cell memory to CD4 epitopes in all three proteins was unimpaired in the blood of patients at diagnosis. In most cases NPC patients also showed detectable responses to CD8 epitopes relevant to their HLA type, the one consistent exception being the absence in patients of a B*4001-restricted response to LMP2. We infer that NPC arises in patients whose prevailing levels of T cell memory to tumour-associated EBV proteins is largely intact; the therapeutic goal must therefore be to re-direct the existing memory repertoire more effectively against antigen-expressing tumour cells.