Development of DNA vaccines to induce immunity against intracellular tumour antigens

We previously reported that the non-toxic Fragment C (FrC) region of tetanus toxin is a potent immunoenhancer. A DNA vaccine encoding a tumour protein linked to FrC induced protective immunity in vaccinated mice, in contrast to a similar vaccine lacking the FrC sequence. For intracellular tumour antigens, induction of CD8+ T cells is particular desirable. To activate CD8+ cell responses two changes were made to our vaccine design; first the N-terminal domain of FrC was removed to delete potentially competitive epitopes. Second, epitope presentation was enhanced by re-positioning minimal tumour peptides to the C-terminus of the remaining FrC sequence. This novel vaccine (p.DOM-peptide) proved an effective means to prime peptide-specific CTLs. To determine whether this design was applicable to a tumour antigen and to delineate the reasons for its success, we used a minimal H-2Ld-binding peptide, AH1, derived from the endogenous retroviral gene product, gp70, and expressed by the murine colon carcinoma, CT26. Vaccination with this second-generation design (p.DOM-AH1) activated functional AH1-specific CTLs capable of specifically lysing a panel of cells lines expressing the AH1 epitope. Importantly this vaccine also afforded protective immunity against CT26 tumour challenge in mice. Comparison with other AH10-containing DNA vaccines confirmed that both epitope competition and efficient tumour-peptide processing contribute to the efficacy of the p.DOM-AH1 vaccine. Finally, we have focused on the tumour-associated transcription factor, WT1, to investigate whether DNA vaccination can induce immunity against this clinically relevant auto-antigen. Encouragingly, administration of a p.DOM-peptide vaccine encoding a minimal WT1-derived HLA-A2-binding epitope induced strong responses in humanised transgenic mice following injection by the DNA/EP prime/boost vaccination schedule.