DNA fusion gene vaccines induce cytotoxic T-cell attack on naturally processed peptides of human prostate-specific membrane antigen

For long-term attack on tumor cells in patients with prostate cancer, induction of cytolytic T cells is desirable. Several lineage-specific target proteins are known and algorithms have identified candidate MHC class I-binding peptides, particularly for HLA-A*0201. We have designed tolerance-breaking DNA fusion vaccines incorporating a domain of tetanus toxin fused to candidate tumor-derived peptide sequences. Using three separate peptide sequences from prostate-specific membrane antigen (PSMA) (peptides PSMA27, PSMA663, and PSMA711), this vaccine design induced high levels of CD8 1 T cells against each peptide in a HLA-A0201 preclinical model. In contrast, the full-length PSMA sequence containing all three epitopes was poorly immunogenic. Induced T cells were cytotoxic against peptide-loaded tumor cells, but only those against PSMA27 or PSMA663 peptides, and not PSMA711, were able to kill tumor cells expressing endogenous PSMA. Cytotoxicity was also evident in vivo. The preclinical model provides a powerful tool for generating CD8 1 T cells able to predict whether target cells can process and present peptides, essential for planning peptide vaccine-based clinical trials.