Abstract 4968: Tumor innate immunity primed by specific interferon stimulated endogenous retroviruses

Abstract

Abstract Introduction: Tumor cell heterogeneity is a key determinant of cancer progression and drug resistance, which is often mediated by mesenchymal cell subpopulations. While these subclones can secrete growth factors, chemokines and cytokines, the immune signaling networks that fuel this pro-tumorigenic state remain incompletely defined. Elucidating what underlies this state would provide insights into tumor biology and inform clinical strategies to improve anti-cancer therapies. Methods: Because of their well-defined nature, we used the phenotypically distinct H69M and H69AR Small Cell Lung Cancer (SCLC) mesenchymal subclones to uncover a novel mechanism of dysregulated innate immune signaling as compared with parental neuroendocrine H69 cells. Analysis of gene signatures across TCGA and CCLE databases, functional studies in additional cell lines, and ex vivo testing of patient-derived organotypic tumor spheroids (PDOTS) were conducted to determine the broader relevance across human cancers. Results: We discovered a novel epigenetically regulated subclass of endogenous retroviruses (ERVs) that engages innate immune signaling in mesenchymal cancer subpopulations. Stimulated 3 Prime Antisense Retroviral Coding Sequences (SPARCS) are oriented inversely in 3'UTRs of certain interferon-inducible genes and silenced by EZH2. De-repression of these loci resulted in dsRNA generation following IFNγ exposure due to bi-directional transcription from the STAT1-activated gene promoter and the 5' LTR of the antisense ERV. We found that dsRNA sensing preferentially by MAVS fuels activation of TBK1, IRF3, and STAT1 signaling, sustaining a positive feedback loop. SPARCS induction across specific human tumors and cell lines is tightly associated with downregulation of chromatin modifying enzymes, including EZH2, a mesenchymal AXL positive cell state, and B2M and MHC class 1 antigen expression. SPARCS high tumors were marked by immune infiltration, but also exhibited multiple features of tumor immune suppression. IFNγ treatment of PDOTS with de-repressed SPARCS markedly enhanced CXCL10 production and sensitized them to PD-1 blockade. Conclusions: Together, these data unveil a novel subclass of ERVs whose de-repression triggers pathologic innate immune signaling in cancer, with potentially important implications for cancer immunotherapy. Citation Format: David A. Barbie. Tumor innate immunity primed by specific interferon stimulated endogenous retroviruses [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 4968.