Therapeutic vaccination with ribonucleic acids encoding tumor/self antigens and a cytokine adjuvant in a transgenic murine model of prostatic carcinoma

Most human tumor antigens are non-mutated differentiation proteins, supporting the use of shared tumor/self antigens for active cancer immunotherapy; moreover, numerous studies have shown that vaccination with such antigens elicits paradoxical, tumor-reactive, self-tolerant cytotoxic lymphocyte (CTL) responses. Reproductive-origin neoplasia may represent an ideal target for this approach, as tolerance to hormonally-regulated antigens is low. In this study, we hypothesized that normal prostate contains useful prostatic carcinoma (CaP) rejection antigens, and therefore, immunization with a murine prostate antigen mixture would delay tumor progression in the Transgenic Adenocarcinoma of the Mouse Prostate (TRAMP) model. In pilot experiments, we demonstrated that vaccination with liposome-encapsulated ovalbumin mRNA efficiently generated tumor-protective CTL responses in C57BL/6 mice. Co-immunization with granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA was associated with an increase in splenic CD11c+/I-A b+ dendritic cells and potentiated anti-tumor responses. We evaluated this novel antigen/adjuvant combination in TRAMP mice; however, repeated immunization with a GM-CSF/prostate library mRNA mixture did not slow CaP growth, as measured by tumor weight and histologic grade. Because this failure presumably resulted from vaccine dose limitations, we investigated, in a second study, whether CaP progression could be inhibited by immunizing with either of two defined “self” antigens restrictively-expressed (SV-40 large T antigen [TAg]) or overexpressed (p53) in TRAMP tumors. Vaccination with GM-CSF-adjuvanted, wild-type or chimeric p53 mRNA constructs had no effect on neoplastic growth. In contrast, vaccine-elicited, TAg-specific CTL infiltrated tumors and significantly reduced CaP weight and histologic grade in TRAMP mice adoptively-transferred with naïve splenocytes prior to vaccination. The lack of response in TAg-vaccinated, non-transferred TRAMP mice suggested that de-novo-developed cognate T cells are specifically tolerized; in contrast, donor tumor-reactive CTL are constitutively activated and maintained at low levels for long periods (>3 months), without vaccine priming. Our results demonstrate that tumor-specific CTL induced by mRNA vaccination could control growth of a spontaneous neoplasm. Although tolerance precluded the effective use of tumor/self antigens in these experiments, adoptive transfer readily restored a long-lived, tumor/self-reactive T cell repertoire in tumor-bearing TRAMP hosts. Therefore, active immunization of clinical patients with tumor/self antigens may require similar reconstitution of the autoreactive T cell population prior to initiating therapy.