Abstract A08: Resolving STING-mediated tumor immune microenvironment shift at single-cell resolution
Authors: Tan Yee Sun , Heath Blake R. , Hill Brett D. , Luo Xiaobo , Rizvi Syed M. , Moghbeli Toktam , Hu Hongxiang , Siismets Erica , Wen Haitao , Xie Yuying , Sun Duxin , Young Simon , Zou Weiping , Wen Fei , Lei Yu L. 
Published: September 2020

Abstract
Background: The detection of double-stranded cytosolic DNA by cyclic GMP-AMP synthase (cGAS) leads to activation of stimulator of interferon genes (STING) and to the production of type-I interferon (IFN-I) as part of host defense mechanism. Recently, 2’3’ cyclic guanosine monophosphate-adenosine monophosphate (cGAMP), an endogenous activator of STING, has been shown to be a promising antitumor agent through the activation of innate and adaptive immunity. However, the mechanism by which cGAMP results in a tumor-specific T-cell expansion in the tumor microenvironment (TME) remains unclear. In addition, how highly anionic therapeutic STING agonists penetrate the hydrophobic phospholipid bilayer membrane to induce immune activation remains unknown.
Materials and Methods: We propose that serum albumin is an endogenous carrier of cGAMP into the cell cytoplasm. The efficacy of albumin-cGAMP on IFN-I signaling was evaluated using real-time PCR in human monocytic cell line THP-1, as well as primary mouse bone marrow-derived dendritic cells (BMDCs) and bone marrow-derived macrophages (BMDMs). To examine the effects of albumin-cGAMP on macrophage populations, flow cytometry was performed on BMDMs. Efficacy of albumin-cGAMP was examined in vivo using syngeneic mouse model of oral cancer, followed by a 40-marker panel for mass cytometry (CyTOF) to characterize the immune landscape at single-cell resolution.
Results and Discussion: We observed that the albumin-cGAMP mixture potently enhances the IFN-I signatures in THP-1 cells, BMDCs and BMDMs. Analysis of BMDMs reveals that cGAMP alone has a modest effect on macrophages and that albumin significantly increases cGAMP-induced M1/M2 polarization. Data analysis of CyTOF using SPADE and CITRUS reveals that the activation of STING results in significant reduction in subsets that compromise antitumor responses, such as regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSCs). An unbiased viSNE analysis reduces the high-dimension profiling data onto two dimensions and shows that CD4, CD8, B220, CD206, MHC-II, Dx5, F4/80, and Ly-6G are the main differentiating markers upon STING induction.
Conclusion: Taken together, this work presents albumin as a potential physiologic carrier of STING agonist cGAMP, and we characterize STING-mediated immune landscape change in the TME at single-cell resolution.
Citation Format: Yee Sun Tan, Blake R. Heath, Brett D. Hill, Xiaobo Luo, Syed M. Rizvi, Toktam Moghbeli, Hongxiang Hu, Erica Siismets, Haitao Wen, Yuying Xie, Duxin Sun, Simon Young, Weiping Zou, Fei Wen, Yu L. Lei. Resolving STING-mediated tumor immune microenvironment shift at single-cell resolution [abstract]. In: Proceedings of the AACR-AHNS Head and Neck Cancer Conference: Optimizing Survival and Quality of Life through Basic, Clinical, and Translational Research; 2019 Apr 29-30; Austin, TX. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(12_Suppl_2):Abstract nr A08.