Truncated or chimeric endogenous protein antigens gain immunogenicity for B cells by stress protein-facilitated expression.

Truncated variants of the SV40 large T antigen (T-Ag) with an intact N terminus are as efficiently expressed in eukaryotic transfectants as wild-type (wt) T-Ag. Coprecipitation of N-terminal T-Ag fragments with the constitutively expressed, cytosolic stress protein hsp73 suggests that this chaperone stabilized expression of the truncated T-Ag fragments. In contrast to T-Ag, the 163-residue N-terminal preS domain of the hepatitis B surface antigen (HBsAg) is difficult to express. When the preS domain is C-terminally fused to a hsp73-binding cytoplasmic T-Ag (cT-Ag) fragment its stable expression as a chimeric cT-preS protein is obtained. DNA-based vaccination with plasmid DNA encoding either wt or hsp-associated mutant T-Ag elicited potent MHC class I-restricted, T-Ag-specific T cell responses. In contrast, DNA vaccination with hsp73-binding (mutant or chimeric) T-Ag variants, but not with wt T-Ag elicited T-Ag-specific antibody responses. Furthermore, vaccination with cT-preS-encoding plasmid DNA induced antibodies binding to the preS domain of the large HBsAg. Hence, hsp73-bound endogenous antigens efficiently stimulate antibody responses. These findings may be relevant for tumor immunology and autoimmunity.