Our laboratory’s work in adult high-grade glioma has discovered a therapeutically-targetable pathway through the inducible expression of indoleamine 2,3 dioxygenase 1 (IDO1). IDO1 is an immunosuppressive enzyme that metabolizes tryptophan. Our project’s purpose is to delineate the role of T-cell-driven IDO1 expression in human DIPG, and to develop the therapeutic potential for inhibiting human IDO1 with novel, specific, small inhibitory (si)RNA oligonucleotides. METHODS: Gene expression for human IDO1, T-cell specific, CD3e, and the proinflammatory cytokines, IFNγ and IFNb, were investigated among surgically-resected pediatric brain tumor specimens obtained from the Childhood Brain Tumor Tissue Consortium (CBTTC). IDO1 and CD3e were also explored in a novel, humanized DIPG mouse model (NSG-SGM3-BLT), by injecting SF8628 into the brain stem. Humanized mice were either untreated or depleted for human CD4+ and CD8+ T cells. In vitro analysis included unique patient-derived DIPG cell lines, stimulated with human IFNγ, and analyzed for IDO1 mRNA and protein levels, with and without the addition of IDO1 siRNA. RESULTS: IDO1 expression is normally low in cultured human DIPG, but rapidly induced by IFNγ. Analysis of surgically-resected human pediatric brain tumor specimens confirms the association between IDO1 levels and the co-expression of IFNγ and IFNb. Strikingly, human T cells directly increase human IDO1 expression in intracranial DIPG, while our siRNA specifically decreases IDO1 mRNA, protein and enzyme activity/levels. DISCUSSION: The poor prognosis of children with DIPG, combined with the lack of therapies, emphasizes the importance of understanding immunosuppressive IDO1 and its potential therapeutic value. These data also confirm that, immunotherapies aimed at enhancing T cell effector functions in DIPG patients, also need to consider resulting effects on intratumoral IDO1-mediated immunosuppression. Ongoing work by our laboratory aims to weaponize our novel IDO1-targeted siRNA for in vivo delivery, and for adjuvant treatment in DIPG patients receiving immunotherapy.