PP2Ac Deficiency Enhances Tumor Immunogenicity by Activating STING-Type I Interferon Signaling in Glioblastoma [RNA]

Glioblastoma (GBM) is an incurable cancer despite aggressive treatment paradigms. Current immunotherapies, such as CTLA-4 and/or PD-1 blockade, have yet to demonstrate benefits for GBM. A key barrier is the immunologically “cold” nature of GBM defined by insufficient tumor antigen presentation and lack of T cells infiltration. We previously demonstrated that pharmacologic inhibition of Protein Phosphatase-2A (PP2A), using a small molecule inhibitor, synergized with PD1 blockade in multiple preclinical tumor models including GBM. However, PP2A is ubiquitously expressed in many cell types and regulates many cellular pathways. The cell type or molecular mechanism responsible for PP2A-modulated tumor immunity is poorly understood. Here, we demonstrate that genetic ablation of PP2A in glioma cells sufficiently promotes tumor immunogenicity by enhancing 1) dsDNA production and thereby cGAS-Type I interferon (IFN) signaling, 2) MHC-I expression and 3) tumor mutational burden. PP2A deficient glioma enhances DC activation and cross presentation to promote clonal expansion of tumor antigen specific CD8 cells. PP2A deficiency also markedly sensitize tumors to immune checkpoint blockade and radiotherapy. Single cell analysis of murine glioma demonstrates that PP2A deficient tumors have 1) increased infiltration of CD8+ T cell, NK cell and cDC1 cells, 2) reduced infiltration of immunosuppressive tumor associated macrophages, 3) promotion of IFN signaling in both myeloid and tumor cells, and 4) reduced glioma-associated gene signature that predicts poor prognosis in glioma patients. This is the first study to establish a role for PP2A in regulating dsDNA-cGAS-STING signaling and collectively, our results suggest that PP2A is a promising novel target to enhance anti-tumor immunity for glioma. We implanted subcutanous or intracranial SB28 murine glioma in WT and LysMcrePP2Acfl/fl mice. 18 days after implantation, tumor tissue was harvested, and single cell suspensions were obtained and sent for sequencing.