SRP308387 PRJNA705184 GSE167746 Targeting Treg cells with GITR activation alleviates resistance to immunotherapy in murine glioblastomas Immune checkpoint blockers (ICBs) have failed in all phase III glioblastoma (GBM) trials. Here, we show that regulatory T (Treg) cells play a key role in GBM resistance to ICBs in experimental gliomas. Targeting glucocorticoid-induced TNFR-related receptor (GITR) in Treg cells using an agonistic antibody (aGITR) promotes CD4 Treg cell differentiation into CD4 effector T cells, alleviates Treg cell-mediated suppression of anti-tumor immune response, and induces potent anti-tumor effector cells in GBM. The reprogrammed GBM-infiltrating Treg cells express genes associated with a Th1 response signature, produce IFN?, and acquire cytotoxic activity against GBM tumor cells while losing their suppressive function. aGITR and aPD1 antibodies increase survival benefit in three experimental GBM models, with a fraction of cohorts exhibiting complete tumor eradication and immune memory upon tumor re-challenge. Moreover, aGITR and aPD1 synergize with the standard of care treatment for newly-diagnosed GBM, enhancing the cure rates in these GBM models. Overall design: Treg cells were sorted from IgG (control; spleen n=7; tumor n=5) or aGITR+aPD1 (treated; spleen n=9; tumor=7) treated GBM-bearing mice. Each individual spleen or tumor sample was harvested from different mice, respectively. GSE167746 pubmed 33976133