Immune-Induced Evolutionary Selection Focused on a Single Reading Frame in Overlapping Hepatitis B Virus Proteins

Viruses employ various means to evade immune detection. Reduction of CD8(+) T cell epitopes is one of the common strategies used for this purpose. Hepatitis B virus (HBV), a member of the Hepadnaviridae family, has four open reading frames, with about 50% overlap between the genes they encode. We computed the CD8(+) T cell epitope density within HBV proteins and the mutations within the epitopes. Our results suggest that HBV accumulates escape mutations that reduce the number of epitopes. These mutations are not equally distributed among genes and reading frames. While the highly expressed core and X proteins are selected to have low epitope density, polymerase, which is expressed at low levels, does not undergo the same selection. In overlapping regions, mutations in one protein-coding sequence also affect the other protein-coding sequence. We show that mutations lead to the removal of epitopes in X and surface proteins even at the expense of the addition of epitopes in polymerase. The total escape mutation rate for overlapping regions is lower than that for nonoverlapping regions. The lower epitope replacement rate for overlapping regions slows the evolutionary escape rate of these regions but leads to the accumulation of mutations more robust in the transfer between hosts, such as mutations preventing proteasomal cleavage into epitopes.