Abstract 2993: STING signaling in breast tumor microenvironment modulates immune checkpoint blockade efficacy in the neu-N mouse model of breast cancer
Authors: Foote Jeremy B. , Kok Marlene , Leatherman James M. , Armstrong Todd D. , Marcinkowski Bridget , Kanne David B. , Jaffee Elizabeth M. , Dubensky Thomas W. , Emens Leisha A. 
Published: July 2017

Abstract
Background: The presence of tumor infiltrating T cells (TILs) is associated with improved clinical outcomes in multiple tumor types and is also necessary for response to immune checkpoint blockade. While T cell responses to some tumors occur spontaneously, the majority of cancers are not naturally recognized by the immune system. The lack of response is attributed primarily to insufficient T cell infiltration into the tumor microenvironment (TME). Activating the STING pathway represents one strategy for increasing infiltration of T cells into the TME. This induces interferon-β (IFN-β) production, leading to dendritic cell (DC) activation and priming of tumor antigen specific CD8+ T cells that mediate tumor regression. The intratumoral injection (IT) of ML-RR-S2-CDA (ADU-S100), a synthetic cyclic dinucleotide STING agonist, has antitumor efficacy in several aggressive mouse tumor models, including B16 melanoma, CT26 colon carcinoma, Panc02 pancreatic carcinoma, and 4T1 triple negative breast cancer. However, the impact of antigen-specific tolerance on tumor regression associated with STING-activation remains poorly understood. Therefore, we evaluated the efficacy of IT ADU-S100 in both non-tolerant parental FVB/N and the immune tolerant neu/N transgenic mice bearing established HER-2+ breast tumors.
Methods: First we first evaluated the impact of IT ADU-S100 on tumor regression, survival, innate sensing, and priming of HER-2 specific CD8+ T cells in both tumor-bearing non-tolerant FVB/N control and tolerant neu/N mice. Then we determined whether modulating the most highly expressed immune checkpoints on tumor infiltrating CD8+ T cells enhanced intratumoral STING activation with ADU-S100 in the tolerant neu/N mouse model of HER-2+ breast cancer.
Results: ADU S-100 induced HER-2-specific CD8+ T cell priming and durable tumor clearance in 100% of non-tolerant, FVB/N mice. In contrast, ADU S-100 failed to sufficiently prime HER-2-specific CD8+ T cells in tolerant neu/N mice, delaying tumor growth and clearing tumors in only 10% of mice. No differences in IFN-β production, DC priming, or HER-2-specific CD8+ T cell trafficking were detected between FVB/N and neu/N mice. However, activation and expansion of HER-2-specific CD8+ T cells was defective in neu/N mice. Immune cell infiltrates of untreated tumor-bearing neu/N mice expressed high levels of PD1 and OX40 on CD8+ T cells, and high levels of PD-L1 on both myeloid and tumor cells. Modulating PD-L1 and OX40 signaling combined with IT ADU S-100 enhanced HER-2-specific CD8+ T cell activity, clearing tumors from 40% of neu/N mice.
Conclusions: Intratumoral STING activation synergizes with PD1 pathway-blockade and OX40 receptor stimulation to overcome immune tolerance and prime tumor antigen-specific CD8+ T cell responses that mediate effective tumor regression.
Citation Format: Jeremy B. Foote, Marlene Kok, James M. Leatherman, Todd D. Armstrong, Bridget Marcinkowski, David B. Kanne, Elizabeth M. Jaffee, Thomas W. Dubensky, Leisha A. Emens. STING signaling in breast tumor microenvironment modulates immune checkpoint blockade efficacy in the neu-N mouse model of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2993. doi:10.1158/1538-7445.AM2017-2993