Natural immune escape mutations in the envelope proteins of hepatitis B virus (HBV) regulate the assembly and infectivity of human hepatitis delta virus (HDV).

The study examined how 35 natural immune escape mutations (IEMs) of hepatitis B virus (HBV) located in the major antigenic loop (MAL) of the envelope proteins regulate the assembly and infectivity of the natural sub-viral agent of HBV, hepatitis delta virus (HDV). All IEMs were permissive for the assembly and secretion of HDV virions. Only seven IEMs resulted in considerably decreased levels of secreted HDV virions in the context of one to three tested HBV genotypes, B, C, or D. The effects of IEMs on HDV infectivity were much more pronounced than on the assembly. Thirteen IEMs considerably inhibited the infectivity of HDV virions. All of them greatly reduced both the levels of HDV RNA genomes accumulated in the infected cells and the percentage of HDV-infected cells. Only one mutant R(169)P generated completely non-infectious HDV virions. Our analysis of the IEMs-mediated effects on the HDV life cycle therefore identified a number of critically important amino acid residues in the MAL, and aided the mechanistic understanding of the MAL functioning during the processes of HDV assembly and infectivity. The inhibition of either HDV assembly and/or infectivity is expected to suppress the viral spread and super-infection in vivo, and therefore to reduce the HDV reservoir in infected livers. Overall, the generated data showed that the anti-HBV immune response is capable of considerable down-regulation of the concomitant HDV infection by facilitating the selection for IEMs in the MAL. The study further advanced our understanding of the mechanisms of the complex HBV-HDV and HDV-host interactions.IMPORTANCEThe study demonstrated that anti-hepatitis B virus (HBV) immune response via selection for the immune escape mutations (IEMs) in HBV envelope proteins (surface antigen, HBsAg) can considerably reduce the assembly and/or infectivity of hepatitis delta virus (HDV) virions coated with HBsAg. The IEMs could down-regulate concomitant HDV infection mostly via affecting HDV infectivity, which was considerably inhibited by 13 out of 35 IEMs. The HDV spread/super-infection, which are expected to be suppressed by inhibited via HBV-specific IEMs the assembly and/or infectivity of HDV virions, should result in a reduced HDV reservoir in infected livers. The examination of the inhibition of HDV infectivity by IEMs apparently indicated that the major antigenic loop (MAL) of HBsAg could contain the residues primarily involved in viral attachment/entry and also the residues mostly regulating intracellular post-entry trafficking of HDV virions. The data advance our understanding of the MAL functioning during infection and of the indirect regulation of the HDV life cycle through the anti-HBV immune response.