Improved immunogenicity of HIV-1 epitopes in HBsAg chimeric DNA vaccine plasmids by structural mutations of HBsAg

To improve the immunogenicity of epitopes from the envelope protein of HIV- 1, we have developed gene gun-delivered subunit DNA vaccines by inserting t he sequences encoding the V3 region into the hepatitis B virus (HBV) envelo pe gene, often called the surface antigen (HBsAg). We have examined the pos sibility of modifying the immune response to V3 by introducing modification s into the carrier HBsAg in gene gun DNA immunization of mice. In some plas mid constructions, the V3 sequence was introduced into the preS2 region of the HBsAg, Although this region is not present in all protein subunits of t he HBsAg particles produced, abolishing the internal translational initiati on site for the S protein had no effect on the immune response to V3, Expre ssion of V3 at the N-terminal or C-terminal part of the HBsAg protein resul ted in equal anti-V3 antibody and cytotoxic T-lymphocyte (CTL) responses. H owever, elimination of secretion by single amino-acid mutations in the HBsA g decreased the anti-HBsAg antibody response but enhanced the anti-V3 antib ody response, In contrast, the CTL response to V3 was independent of the st ructural mutations but could be improved by a total deletion of the HBsAg s equence part. Thus, the immune response to heterologous epitopes can be alt ered by modifications in the carrier HBsAg protein, Modifications of the HB sAg carrier might interfere with the dominant immune response to the HBsAg epitopes, allowing better antibody induction to less immunogenic foreign ep itopes, However, for induction of CTL responses, the expression of minimal epitopes may be advantageous.