Evolution of cytotoxic T-lymphocyte epitopes in hepatitis B virus.

In hepatitis B virus (HBV), while mutations that escape from cytotoxic T-lymphocyte (CTL) recognition have been described it has been difficult to determine how natural selection by host CTL has influenced long-term evolution of HBV. We used statistical analysis of published HBV genomic sequences to examine the role of natural selection in evolution of CTL epitopes. Based on a phylogenetic analysis, we identified 25 pairs of closely related genomes isolated from different HBV genotypes and examined pattern of nucleotide substitution in genomic regions encoding well-characterized CTL epitopes. On average, both epitope and non-epitope regions are subject to purifying selection acting at non-synonymous sites. However, certain CTL epitopes showed a pattern of nucleotide substitution suggesting repeated positive selection across the population. The results support the hypothesis that CTL-driven selection has been an important factor in long-term evolution of HBV.