Abstract 2880: Novel alphavirus-based vaccine targets dendritic cells and efficiently breaks immunological tolerance to “self” tumor-associated antigen (PSA) in an HLA-DR mouse model of prostate cancer
Authors: Ryabov Vladimir , Pushko Peter , Tretyakova Irina , Saito Rikka , Alexander Richard B. , Klyushnenkova Elena N. 
Published: May 2015

Abstract
The development of immunotherapy for prostate cancer based on the induction of autoimmunity to prostate-specific differentiation antigens is an attractive concept. In this study, we constructed novel virus-like particles vector (VLPV) platform derived from live attenuated human TC-83 IND vaccine against Venezuelan equine encephalitis virus. We tested the ability of VLPV-PSA particles to target mouse CD11c+ dendritic cells (DCs) using in vitro infection assay. Incubation of mouse splenic DCs with VLPV-PSA resulted in robust accumulation of PSA in the cytoplasm of CD11c+ cells as detected by immunofluorescent staining and confocal microscopy. We also tested the immunogenicity and anti-tumor effect of the VLPV-PSA vaccine in a stringent mouse model of prostate cancer in which tolerance to human PSA expressed as a ‘self’ antigen in the prostate was established in a context of the HLA-DR2b chimeric transgene (DR2bxPSA F1 mice). Preventive vaccination with VLPV-PSA induced strong PSA-specific CD8+ T-cell response as detected by IFNγ ELISPOT assay and intracellular cytokine staining. Potent expansion of PSA-specific CD8+ effector T-cells was found in peripheral blood and spleens of vaccinated mice using PSA peptide-loaded MHC class I dextramers. VLPV-PSA vaccination also induced PSA-specific antibody production in immunized mice with predominance of Th1-associated IgG2a sub-isotype. Injection of TRAMP tumor cells expressing PSA into vaccinated mice resulted in elevated infiltration of tumor site with CD8+ T-cells and rapid elimination of PSA-expressing tumor cells during first several weeks after tumor challenge. Tumor survival analysis revealed significant delay of tumor growth in VLPV-PSA vaccinated mice, especially at the early time-points (p=0.044, Gehan-Breslow test). Interestingly, the combination treatment with VLPV-PSA and the blockade of the CTLA-4-mediated checkpoint did not enhance further CD8 T cell response to PSA, and did not show synergistic anti-tumor effect compared to the VLPV-PSA treatment alone. Our data demonstrate that VLPV-PSA are able to infect mouse DCs and induce production of heterologous antigen (PSA) resulting in efficient break of immunological tolerance to PSA and induction of anti-tumor response in a stringent mouse model of prostate cancer.
Citation Format: Vladimir Ryabov, Peter Pushko, Irina Tretyakova, Rikka Saito, Richard B. Alexander, Elena N. Klyushnenkova. Novel alphavirus-based vaccine targets dendritic cells and efficiently breaks immunological tolerance to “self” tumor-associated antigen (PSA) in an HLA-DR mouse model of prostate cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2880. doi:10.1158/1538-7445.AM2014-2880