Improved efficacy of DNA vaccination against prostate carcinoma by boosting with recombinant protein vaccine and by introduction of a novel adjuvant epitope

DNA vaccine represents an attractive approach for cancer treatment by inducing active immune-deprivation of gastrin-releasing peptide (GRP) from tumor cells, the growth of which is dependent on the stimulation of GRP. In this study, we developed a DNA vaccine using a plasmid vector to deliver the immunogen of six copies of the B cell epitope GRP_18–27 (GRP6). In order to increase the potency of this DNA vaccine, multiple strategies have been applied including DNA-prime protein-boost immunization and introduction of a foreign T-helper epitope into DNA vaccine. Mice vaccinated DNA vaccine boosting with HSP65-GRP6 protein induced high titer and relatively high avidity of anti-GRP antibodies as well as inhibition effect on the growth of murine prostate carcinoma, superior to the treatment using DNA alone or BCG priming HSP65-GRP6 protein boosting. Furthermore, the introduction of a novel foreign T-helper epitope into the GRP DNA vaccine showed a markedly stronger humoral immune response against GRP and tumor rejection even than the DNA-prime protein-boost strategy. No further stronger immunogenicity of this foreign T-helper epitope modified DNA vaccine was observed even using the strategy of modified DNA vaccine-priming and HSP65-GRP6 boosting method. The data presented demonstrate that improvement of potency of anti-GRP DNA vaccine with the above two feasible approaches should offer useful methods in the development of new DNA vaccine against growth factors for cancer immunotherapy.