DNA fusion gene vaccination mobilizes effective anti-leukemic cytotoxic T lymphocytes from a tolerized repertoire

The majority of known human tumor-associated antigens, derive from non-mutated self proteins. T cell tolerance, essential to prevent autoimmunnity must therefore be cautiously circumvented to generate cytotoxic T cell responses against these targets. Our strategy uses DNA fusion vaccines to activate high levels of peptide-specific CTL. Key foreign sequences from tetanus toxin activate tolerance-breaking CD4⁺ T cell help. Candidate MHC class I binding tumor peptide sequences are fused to the C terminus for optimal processing and presentation. To model performance against a leukemia-associated antigen in a tolerized setting, we constructed a fusion vaccine encoding an immunodominant CTL epitope derived from Friend murine leukemia virus gag protein (FMuLV^gag) and vaccinated tolerant FMuLV^gag-transgenic (gag-Tg) mice. Vaccination with the construct induced epitopespecific IFN-γ-producing CD8⁺ T cells in normal and gag-Tg mice. The frequency and avidity of activated cells were reduced in gag-Tg mice, and no autoimmune injury resulted. However, these CD8⁺ T cells did exhibit gag-specific cytotoxicity in vitro and in vivo. Also, epitope-specific CTL killed FBL-3 leukemia cells expressing endogenous FMuLV^gag antigen and protected against leukemia challenge in vivo. These results demonstrate a simple strategy to engage anti-microbial T cell help to activate epitope-specific polyclonal CD8⁺ T cell responses from a residual tolerized repertoire.