STEM-13. GLIOMA STEM CELLS MIMIC REGULATORY T CELL FUNCTION TO SUPPRESS THE IMMUNE RESPONSE AND PROMOTE TUMOR PROPAGATION

Abstract The advent of immunotherapy has transformed our approach to cancer treatment; however, it is virtually ineffective in glioblastoma (GBM), the most common & malignant brain cancer in adults, due in part to the inherently immunosuppressive tumor microenvironment (TME). Recent research has identified GBM cancer stem cells (GSCs) as key modulators of the TME, mainly through activation and recruitment of pro-tumor immune cells (e.g., TAMs, regulatory T-cells (Tregs), MDSCs). We identified a novel immunosuppressive phenotype whereby GSCs mimic Treg function via TGF-b type II receptor (TGFBR2) signaling to suppress the immune response with the potential to promote tumor propagation and resist immunotherapy. Preliminary experiments revealed that TGFBR2 drives GBM cells to a CD44-high, mesenchymal-like GSC state. Transgenic TGFBR2 was found to induce a Treg-like transcriptional profile defined by expression of canonical Treg effector genes (e.g., CD274, ENTPD1, NT5E, LGALS1, TGFB1, PDCD1LG2) in human GBM neurospheres. Single-cell RNA-Seq and bioinformatics analyses confirmed the presence of this TGFBR2-mediated transcriptional signature in both patient-derived GSCs and patient tumors. Notably, in silico deconvolution of bulk GBM sequencing predicts that high expression of a TGFBR2-induced transcriptome associates with an immunosuppressive TME characterized by high infiltration of pro-tumor immune cells (e.g., TAMs, neutrophils). To target this Treg-like, immunosuppressive GSC phenotype, we utilized ITD-1, a small molecule inhibitor of TGFBR2. In vitro ITD-1 treatment reduced GSC proliferation capacity and downregulated expression of CD44 and Treg effectors. In T-cell co-cultures, pre-treating GSCs with ITD-1 increased T-cell viability and T-cell-mediated tumor cell killing. Administration of ITD-1 in tumor-bearing, immunocompetent mice reduced tumor volume and increased anti-tumor immune cell infiltration relative to vehicle controls. In all, ITD-1-mediated TGFBR2 inhibition attenuates a unique, Treg-like immunosuppressive and potentially tumor-protective phenotype in GSCs. Future studies will combine ITD-1 and checkpoint inhibitor therapy in an effort to increase the efficacy of immunotherapy in GBM.