Abstract B67: Synthetic DNA multi-neoantigen vaccine drives predominately MHC class I CD8+ T cell-mediated effector immunity impacting tumor challenge

T-cell recognition of cancer neoantigens is important for effective immune checkpoint blockade therapy, and there is increasing interest in developing personalized tumor neoantigen vaccines. Previous studies utilizing RNA and synthetic long peptide neoantigen vaccines in preclinical and early-phase clinical studies have shown immune responses predominantly driven by MHC class II CD4+ T cells. Although it has been established that CD4+ T cells are able to recognize tumor neoantigens, the majority of naturally occurring tumor antigen-specific killer T cells identified in patients have been of CD8+ origin, indicating that additional immunization strategies aimed to stimulate neoantigen-specific CD8+ T cells may be useful. Here, we report the first preclinical study utilizing a synthetic DNA vaccine platform to target tumor neoantigens in mice. For this study, we identified neoepitopes by sequencing syngeneic mouse tumors, and designed optimized DNA plasmids encoding long strings of neoepitopes separated by highly efficient cleavage sites preserving epitope integrity. We tested the immunogenicity of 84 independent neoepitopes encoded into optimized DNA plasmids delivered by adaptive electroporation in mice in vivo. We observed that this approach generated robust T-cell immunity against a similar proportion of epitopes compared to other vaccine platforms (20/84, 24%). Strikingly, however, the synthetic neoantigen DNA vaccine platform generated a much larger proportion of CD8+ T-cell responses compared to the prior studies. Synthetic neoantigen DNA vaccines generated 75% CD8+ only or CD4/CD8+ T-cell responses, and 25% CD4+ only T-cell responses, showing a dramatic and important CD8+ T-cell bias. Inclusion of only high-affinity MHC class I ( Citation Format: Elizabeth K. Duperret, Alfredo Perales-Puchalt, Regina Stoltz, G.H. Hiranjith, Nitin Mandloi, James Barlow, Amitabha Chaudhuri, Niranjan Y. Sardesai, David B. Weiner. Synthetic DNA multi-neoantigen vaccine drives predominately MHC class I CD8+ T cell-mediated effector immunity impacting tumor challenge [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B67.