Abstract A111: Distinct immunological landscapes of progressing and regressing solid tumors post anti-PD-1 immunotherapy

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Abstract Background: Advances in cancer immunotherapy in the form of checkpoint blockade such as anti-PD-1, have led to promising results and positive T-cell responses in a variety of human tumors. However, why some patients respond resulting in a decrease in tumor burden whilst others do not is not fully understood. Using a murine solid tumor model, which mirrors survival rates seen in humans undergoing anti-PD1 treatment, we have previously uncovered and identified a key tumor antigen involved in a protective and cytotoxic anti-tumor T-cell response in Treg depleted mice. Monitoring specific anti-tumor CD8 T cell responses over the course of a tumor challenge, we were able to dissect factors of the tumor microenvironment and phenotypic profiles of CD8 T-cells that may shape the fate of a tumor response at key stages of tumor growth during immunotherapy. Methods: Mice were challenged with CT26 tumor, subjected to three doses of anti-PD-1 antibody and survival and tumor growth monitored. These data were used to determine key time points to examine CD8 T cell responses in tumor infiltrating lymphocytes. Tumor antigen-specific CD8 T-cells were assessed by flow cytometry using MHC class I tetramer staining together with clonal exhaustion, effector function, memory and Vβ family markers. We also analyzed the same antigen-specific T-cells at a single-cell level to better understand the TCR clonotypes. The tumor microenvironment was also assessed for PD-L1 expression to dissect their role in inducing clonal T cell exhaustion. Results: Post anti-PD1 immunotherapy, we observed tumor regression and survival in ∼33% of mice. In mice where progression occurred, tumor growth was at a significantly slower rate compared to untreated mice. Examination of tumor-specific responses revealed the presence of low affinity, functional (IFN-γ+) CD8 T-cells was found to correlate with tumor regression and subsequent tumor free survival. Furthermore, surviving mice were also able to reject further tumor challenges in the absence of immunotherapy, suggesting the generation of protective anti-tumor memory responses. Conversely in progressing tumors, we observe the presence of high affinity tumor-specific T-cells both by staining intensity and competition assays. We highlight that these T-cells were absent in tumors that were regressing. This finding indicates that the presence of TCR clones with a strong TCR/peptide MHC interaction may correlate with a negative outcome. Further examination of these T-cell populations revealed differences in the Vβ families present within the two fates thus indicating clonotypic variances in T-cells exist despite responding to the same tumor antigen. Conclusion: Why immunotherapeutic strategies work in some cases whilst not in others is not yet fully understood. In our murine tumor model we were able to determine differences within the immunological landscape of the tumor microenvironment that exist in progressing and regressing tumors post anti-PD1 immunotherapy. Our findings indicate that the quality of CD8 T-cell responses can differ between the two fates and that TCR clones with different affinities/avidities for peptide-MHC, despite recognizing the same antigen, may determine the ability to respond to and reject tumors. Therefore the presence of T-cell clones with the lower affinities for antigen may prove more beneficial in tumor rejection. Being more important than the quantity of CD8 tumor responses. These findings provide valuable insights into protective anti-tumor immunity and understanding of what is required for successful tumor rejection. Citation Format: Gessa Sugiyarto, Osman Dadas, Tim Elliott, Edward James. Distinct immunological landscapes of progressing and regressing solid tumors post anti-PD-1 immunotherapy [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A111.