Impact of hepatitis B virus basic core promoter mutations on T cell response to an immunodominant HBx-derived epitope

The hepatitis B X (HBx) protein is a crucial component in HBV infection in vivo and has been implicated in HCC. In this study, we aimed to detect and characterize peripheral HBx-specific T cells in chronically infected patients at the inactive carrier state of the disease. HBx-specific IFN-gamma-secreting T cells were found in 36 of 52 patients (69%), and 78% (28/36) of responding patients had T cells targeting epitopes in the carboxy-terminal part of HBx. IL-10 secretion after the stimulation of T cells with HBx-derived peptides was weak or undetectable. IFN-gamma-secreting T cells recognized a previously unknown immunodominant CD4+ T cell epitope, HBx 126-140 (EIRLKVFVLGGCRHK), in 86% (24 of 28) of patients. This peptide bound several HLA-DR molecules (HLA-DRB1*0101, HLA-DRB1*0401, HLA-DRB1*1301, and HLA-DRB5*0101). Its coding sequence overlaps a domain of the HBV genome encompassing the basic core promoter (BCP) region. Taking into account the selection of viral core promoter mutants during HBV infection, we found that HBV variants with BCP mutations were present in patient sera. We further demonstrated that these viral mutant sequences activated T cells specific for the immunodominant epitope only weakly, if at all. This is the first study linking BCP mutations and HBx-specific T cell responses. Conclusion: Wild-type and variant peptides may represent potential tools for monitoring the HBV-specific T cell responses involved in sequence evolution during disease progression. Finally, the degenerate HI-A-DR binding of this promiscuous, immunodominant peptide would make it a valuable component of vaccines for protecting large and ethnically diverse patient populations.