Abstract 2979: A balance of genomic instability, tumor-immune contexture and TGF-β signaling contributing to exclusion of T cells governs response to PD-L1 checkpoint blockade

Abstract Checkpoint inhibitor blockade can result in robust and durable anti-tumor responses in various cancers, including metastatic urothelial cancer (mUC). However, these responses only occur in a subset of patients. Identifying determinants of response and resistance to cancer immunotherapy is critical for extending therapeutic benefit to more patients. Atezolizumab (anti-PD-L1) was approved in the US for the treatment of mUC based on the single-arm Phase II study IMvigor210 (NCT02108652; n= 429 patients). Here, we examined the biology underlying primary immune escape and responsiveness to anti-PD-L1 in tumor samples of patients from IMvigor210. Methods: PD-L1 expression on tumor-infiltrating immune cells was assessed with SP142 IHC. Exploratory analyses in evaluable pre-treatment tissues included: (i) CD8 IHC analysis to define immune deserts, excluded and inflamed subtypes (ii) whole-transcriptome RNA sequencing to identify pathways associated with response and to perform tumor molecular subtyping, (iii) targeted mutational profiling (FoundationOne) to estimate tumor mutation burden, and (iv) whole-exome sequencing to predict putative neoantigens. EMT6-grafted BALB/c mice treated with anti-TGF-β and/or anti-PD-L1 antibodies were evaluated for tumor growth inhibition. Results: Response was associated with CD8+ T-effector gene expression and, to an even greater extent, high neoantigen or tumor mutation burden (TMB). Gene expression pathways significantly associated with high TMB were those involved in cell cycle, DNA replication, DNA damage response (DDR). Tumors with mutations in DDR gene sets also had significantly high TMB and response rates. Lack of response was associated with a signature of transforming growth factor β (TGF-β) signaling in fibroblasts, particularly in patients with CD8+ T cells that were excluded from the tumor parenchyma and instead found in the fibroblast- and collagen-rich peritumoral stroma. Using a mouse model that recapitulates this immune excluded phenotype, we found that therapeutic administration of a TGF-β blocking antibody together with anti-PD-L1 reduced TGF-β signaling in stromal cells, facilitated T cell penetration into the centre of the tumor, and provoked vigorous anti-tumor immunity and tumor regression. Conclusion: Pre-existing T-cell immunity and TMB are associated with response to atezolizumab in mUC, whereas TGF-β signaling in the stroma is a negative indicator of response, especially in immune-excluded tumors, a common phenotype of mUC. Integration of these three independent biological features provides a best basis for understanding clinical outcomes in this setting. Furthermore, the data suggests that TGF-β shapes the tumor microenvironment to restrain anti-tumor activity by restricting T cell infiltration. Citation Format: Sanjeev Mariathasan, Shannon J. Turley, Dorothee Nickles, Alessandra Castiglioni, Kobe Yuen, Yulei Wang, Edward E. Kadel, Hartmut Koeppen, Jillian L. Astarita, Rafael Cubas, Suchit Jhunjhunwala, Yagai Yang, Yasin Şenbabaoğlu, Ira Mellman, Daniel S. Chen, Priti Hegde, Richard Bourgon, Thomas Powles. A balance of genomic instability, tumor-immune contexture and TGF-β signaling contributing to exclusion of T cells governs response to PD-L1 checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2979.