Translocator protein 18 kDa (TSPO) acts as an immune resistance gene involved in the T-cell mediated immune control of glioblastoma

Glioblastoma is the most malignant and common type of primary brain tumor with a median survival of less than 21 months. It is particularly resistant to current immunotherapies. Thus, yet unknown mechanisms of immune resistance may be active in glioblastoma. Translocator protein 18 kDa (TSPO) expression is upregulated in glioblastoma and correlates with malignancy and poor prognosis but also with increased immune infiltration. An immunomodulatory role of TSPO in glioblastoma has not been reported. Here, we studied the role of TSPO in the regulation of immune resistance of human glioblastoma cells. We used public bulk and single-cell gene expression data to identify cell types, genes and signalling pathways correlating with TSPO expression. To validate inflammatory triggers and the role of TSPO experimentally, we co-cultured tumor-infiltrating T cells and cytotoxic T cell lines with autologous and antigen-loaded allogeneic, primary brain tumor initiating cells (BTICs). TSPO proficient and deficient BTIC lines and clones were generated through genetic manipulation. Inflammatory factors triggering TSPO expression were determined by ELISA and validated experimentally using blocking antibodies and recombinant proteins. The impact of TSPO on glioblastoma cell resistance against cytotoxic T cells was studied by cytotoxicity assays and by activity analysis of components of the apoptotic cascade. Individual death-inducing pathways suppressed by TSPO were determined using recombinant proteins and TSPO-regulated genes associated with apoptosis resistance were identified by gene expression analysis in TSPO proficient and deficient BTICs and subsequent functional analyses. TSPO expression in glioblastoma cells correlated with cytotoxic T cell infiltration, expression of TNFR and IFNGR, the activity of their downstream pathways, expression of TRAIL receptors and with PD-L1. Coculture of BTICs with tumor-reactive cytotoxic T cells or with T cell-derived factors induced TSPO up-regulation through TNFa and IFN?. Silencing of TSPO sensitized BTICs against T cell-mediated cytotoxicity. TSPO protected BTICs selectively against TRAIL-induced apoptosis by regulating both intrinsic and extrinsic apoptosis pathways. TSPO regulated the expression of multiple genes associated with apoptosis-resistance of tumors. Among them, we studied the peptidase inhibitors PI3 and SLPI, which sensitized BTICs towards TRAIL-induced apoptosis. Our data revealed that TSPO expression in glioblastoma cells is induced through T cell derived cytokines IFN? and TNFa. TSPO expression protects glioblastoma cells against cytotoxic T cell attack through the death inducing ligand TRAIL. Our data provided an indication that therapeutic targeting TSPO may be a suitable approach to sensitize glioblastoma to immune cell-mediated cytotoxicity by circumventing tumor intrinsic TRAIL-resistance. Overall design: BTIC13 and BTIC129 cell lines with shRNA mediated TSPO knockdown (Three biological replicates were analyzed for each of 4 conditions (two primary cell line from different patients ; two shRNA per cell line))