IMMU-13. COMBINED MODULATION OF THE TGF-Β PATHWAY AND GITR IMMUNE CHECKPOINT SIGNALING PROMOTES ANTI-TUMOR IMMUNITY IN SYNGENEIC GLIOMA MODELS

Abstract

Abstract The profound local immunosuppressive microenvironment is one hallmark of glioblastoma, which results in resistance to most immunotherapeutic strategies that have been explored so far. Reverting this condition in order to reinvigorate anti-glioma immunity might be a promising therapeutic approach. Transforming growth factor (TGF)-β signaling is deregulated in different cancer types and contributes to the malignant phenotype of glioma cells. Glioma-derived TGF-β is also a major immunosuppressive factor in the tumor microenvironment. Furthermore, intratumoral regulatory T (Treg) cells and activated T effector cells express high levels of the co-stimulatory immune checkpoint glucocorticoid-induced tumor necrosis factor receptor (GITR). Agonistic anti-GITR antibodies have been explored in preclinical tumor models and are under investigation in clinical trials for the treatment of solid tumors. We evaluated the effect of TGF-β and GITR targeting on anti-tumor immune responses in syngeneic mouse glioma models. In co-culture settings, GITR modulation with a GITR ligand (GITRL)-Fc fusion protein, given alone or in combination with a pharmacological TGF-β receptor inhibitor, led to increased T cell activation. Furthermore, the combined targeting of the two pathways resulted in significantly higher immune cell-mediated tumor cell killing than either treatment alone. In vivo, TGF-β inhibition and GITR signaling modulation resulted in a higher fraction of long-term surviving glioma-bearing mice than single-agent treatment. Surviving mice were resistant to tumor re-challenge, suggesting adaptive immunity as an underlying mechanism. These data support the assumption that combined immunotherapeutic strategies may represent a promising approach for the treatment of glioma.