Efficacy of vaccination with plasmid DNA encoding for HER2/ neu or HER2/ neu-EGFP fusion protein against prostate cancer in rats

Abstract

Despite early diagnosis and improved therapy, 31,500 men will die from prostate cancer (PC) this year. The HER2/ neu oncoprotein is an important effector of cell growth found in the majority of high-grade prostatic tumors and is capable of rendering immunogenicity. The antigenicity of this oncoprotein might prove useful in the development of PC vaccines. Our goal is to prove the principle that a single DNA vaccine can provide reliable immunity against PC in the MatLyLu (MLL) translational tumor model. The parental rat MatLyLu PC cell line expresses low to moderate levels of the rat neu protein. To simulate in vivo human PC, MatLyLu cells were transfected with a truncated sequence of human HER2/ neu cDNA cloned into the pCI-neo vector. This HER2/ neu cDNA sequence encodes the first 433 amino acids of the extracellular domain (ECD). MatLyLu cells were also transfected with the same HER2/ neu cDNA sequence cloned into the N1-terminal sequence of EGFP reporter gene to produce a fusion protein. The partial ECD sequence of HER2/ neu includes five rat major histocompatibility (MHC)-II-restricted peptides with complete human-to-rat cross-species homology. The HER2/ neu protein overexpression was documented by Western Blot analysis, and the expression of fusion protein was monitored by confocal microscopy and fluorimetry. Vaccination with a single injection of HER2/ neu cDNA protected 50% of animals against HER2/ neu-MatLyLu tumors ( P<0.01). When the tumor cells were engineered to express HER2/ neu-EGFP fusion protein, the antitumor immunity was enhanced, as following vaccination with HER2/ neu-EGFP cDNA, 80% of these rats rejected HER2/ neu-EGFP-MatLyLu ( P<0.001). Both vaccines induced HER2/ neu-specific antibody titers. Rats vaccinated with EGFP-cDNA rejected 80% of EGFP-MatLyLu tumors and, interestingly, 40% of HER2/ neu-MatLyLu tumors. None of the cDNA vaccines induced immunity against parental MatLyLu cells. Our data clearly demonstrate that a single injection of HER2/ neu-EGFP cDNA is a very effective vaccine against PC tumors expressing the cognate tumor-associated antigen (TA). The antitumor immunity is significantly more pronounced if the tumors express xenogeneic HER2/ neu-EGFP fusion protein as opposed to only the syngeneic HER2/ neu oncoprotein. Our data suggests that the HER2/ neu-EGFP-MatLyLu tumor is a potential animal tumor model for investigating therapeutic vaccine strategies against PC in vivo and demonstrates the limitations of a cDNA vaccine only encoding for MHC-II-restricted HER2/ neu-ECD sequence peptides.