Abstract 2501: Vaccination using epitopes with higher MHC class I affinity elicits CD8+ T cells with increased PD-1 expression

We have been interested in DNA vaccines as treatments for human prostate cancer, and have demonstrated that this approach can elicit antigen-specific CD8+ T-cells in humans that can persist over years. However, the persistence of antigen-expressing tumors demonstrates that mechanisms of tumor escape are at play. Consequently, in preclinical studies we have focused on mechanisms of resistance and efforts to improve the immunological activity of DNA vaccines. As one model antigen, we have used the synovial sarcoma, X breakpoint 2 (SSX2) protein, a cancer-testis antigen expressed in metastatic prostate cancer. We have previously demonstrated that SSX2 encodes two HLA-A2-restricted epitopes, and we have utilized HHD-II mice, in which HLA-A2 and HLA-DR1 (but not murine MHC class I and II) are expressed, as a means to study CD8+ T cells generated by vaccination. We have shown in this system that immunization with a DNA plasmid encoding SSX2 elicits robust CD8+ cells, and anti-tumor responses to syngeneic tumors expressing SSX2. We have recently identified that a DNA vaccine altered to encode peptide epitopes with increased HLA-A2 binding elicited a greater frequency of CD8+ T cells specific for SSX2 when compared to the native vaccine, however these T cells were less effective against tumors due to higher expression of PD-1. In the current study, we investigated the mechanisms underlying this regulation of T-cells following vaccination. Altered peptide ligands (APL) specific for one HLA-A2-restricted epitope were modified at anchor residues to generate peptides of varying affinities for HLA-A2. Groups of HHD-II mice were then immunized with one of five SSX2 APLs with varying HLA-A2 affinity. After one week the resulting antigen-specific CD8+ T cells were assessed for regulatory ligand expression and cytokine expression as an indicator of function. We found that a single immunization with high affinity APLs resulted in an increased percentage of SSX2 tetramer positive T-cells that displayed persistent PD-1 expression compared with lower-affinity APL. Upon ex vivo re-encounter of the native SSX2 peptide, T-cells from mice immunized with higher affinity APLs showed an increased multifunctional Th1 response as evidenced by increased IFNγ, IL2, and TNFα expression when compared to mice immunized with native or low affinity SSX2 peptides, but also higher PD-1 expression. Ongoing studies are further exploring the kinetics and persistence of PD-1 expression following antigen presentation. Notwithstanding, our results have implications for the design of anti-tumor vaccines that seek to incorporate epitopes with altered HLA-A2-binding ability as these may affect the persistence of PD-1 expression on CD8+ T cells following immunization. Citation Format: Christopher D. Zahm, Douglas McNeel. Vaccination using epitopes with higher MHC class I affinity elicits CD8+ T cells with increased PD-1 expression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2501. doi:10.1158/1538-7445.AM2015-2501