STEM-08. GLIOBLASTOMA STEM CELLS EVADE NK CELL KILLING THROUGH DOWNREGULATION OF NKG2D LIGANDS

Abstract Glioblastoma (GBM) is the most aggressive primary brain tumor, characterized by inevitable recurrence and poor patient outcomes. GBM stem cells (GSCs) are key drivers of tumor progression, therapeutic resistance, and immune evasion. To thrive within the tumor microenvironment, GSCs must sustain their stem-like properties while escaping immune surveillance, including cytotoxic natural killer (NK) cell activity. While prior studies have largely focused on stemness-associated pathways, the mechanisms by which GSCs evade NK cell-mediated killing remain largely unexplored. Here, we assessed the susceptibility of GSCs and their non-stem GBM cell (NSGC) counterparts to NK cell cytotoxicity. Patient-derived GSC lines were differentiated into NSGCs via serum treatment. In co-culture assays with NK-92 cells, a human NK cell line, NSGCs were efficiently killed, whereas GSCs remained resistant. To investigate the molecular basis of this differential sensitivity, we profiled the expression of NKG2D ligands, which are activating molecules on tumor cells recognized by the NKG2D receptor on NK cells. GSCs exhibited markedly reduced expression of NKG2D ligands such as MICA, MICB, and ULBP1-3 compared to their differentiated NSGC counterparts. Functional relevance was confirmed by blocking NKG2D signaling with a neutralizing antibody, which partially rescued NSGCs from NK-92-mediated killing. Transcriptomic analysis of three matched GSC/NSGC pairs (MGG4, MGG6, MGG8) from the public dataset (GSE54791) validated the downregulation of NKG2D ligands in GSCs. Additionally, RNA-seq data from 44 GSC models and 9 normal brain cell lines (GSE119834) revealed that NKG2D ligand expression in GSCs was comparable to that in normal brain cells, underscoring a distinct NK cell evasion strategy. Collectively, these findings demonstrate that GSCs are intrinsically more resistant to NK cell surveillance, at least in part due to downregulation of NKG2D ligands. Therapeutic strategies that restore NKG2D ligand expression may sensitize GSCs to NK cell-mediated clearance and enhance the efficacy of NK-based immunotherapies for GBM.