Abstract 1648: Ionizing radiation switches the function of tumor-derived exosomes from messengers of tolerance to inducers of antitumor immunity

Tumor-derived exosomes (TEX) are constantly shed by cancer cells and have been shown to carry tumor-derived antigens to dendritic cells (DCs). However, presentation of tumor antigens derived from TEX in the absence of activation signals for DCs leads to immune tolerance. Our lab has demonstrated that ionizing radiotherapy (RT) can convert the irradiated tumor into an in situ vaccine, leading to anti-tumor immune responses. We hypothesized that TEX derived from irradiated cancer cells play a role in RT-induced anti-tumor immunity by delivering tumor antigens together with pro-inflammatory signals that activate DCs and prime tumor-specific effector T cells. To begin to address this hypothesis we first studied how RT modifies TEX molecular composition and their ability to induce DC activation in vitro. Mouse carcinoma cells TSA cultured in exosome-free media were treated in vitro with sham RT (control TEX), or 3 fractions of 8Gy (RT-TEX). TEX were isolated from supernatants 48 hr later using differential ultracentrifugation and purified with a sucrose gradient. Electron microscopy was used to confirm TEX size and morphology. Mass spectrometry (LFQ-MS) followed by MS/MS analyses was used to characterize protein composition. miRNA were analyzed by nanoString nCounter Mouse miRNA expression assay kit using a panel of 578 mouse miRNAs. Normalized results were analyzed with MultiExperiment Viewer. Mouse bone marrow-derived dendritic cells (BMDC) were cultured with TEX for 48 hours. Cells were analyzed by flow cytometry for expression of activation markers. Significant changes in microRNA and protein compositions were seen in RT-TEX compared to control TEX. Specifically, RT-TEX showed increase in proteins involved in the Antigen Processing and Presentation pathway. Additionally, 17 unique proteins were present only in RT-TEX and included proteins involved in T cell development, MHC class I peptide processing, and pro-inflammatory lipid signaling. Analysis of DCs cultured with RT-TEX, but not control TEX, revealed an increase in cell surface expression of CD80 (1428 MFI vs. 963 MFI) and CD86 (3487 MFI vs. 2578 MFI). Interestingly, culturing BMDC with RT-TEX also resulted in an increase in DCs expressing CD103 and CD8a (2% vs 1.2% of CD11c+ cells), suggesting that RT-TEX may influence differentiation of BMDC towards this subset, which is critical for cross-presentation of tumor antigens to T cells. Obtained data support the hypothesis that RT-TEX may switch tolerogenic DCs into activated DCs by providing activation signals together with tumor-derived antigens. Further in vivo experiments are ongoing to determine the ability of RT-TEX to stimulate anti-tumor immune responses. Citation Format: Julie M. Diamond, Jessica R. Chapman, Beatrix Ueberheide, Silvia C. Formenti, Sandra Demaria. Ionizing radiation switches the function of tumor-derived exosomes from messengers of tolerance to inducers of antitumor immunity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1648.