Abstract 5612: Targeting the non-enzymatic function of IDO1 in glioblastoma immunotherapy
Authors: Zhai Lijie , Ladomersky Erik , Qian Jun , Nyugen Brenda , Bell April , Lauing Kristen , Wainwright Derek 
Published: September 2020

Abstract
Purpose: More effective immunotherapies against GBM are urgently needed considering current failures in several phase III clinical trials. Indoleamine 2, 3-dioxygenase 1 (IDO1) has been shown to mediate tumor-induced immunosuppression through its tryptophan catabolic activity. Surprisingly, pharmacological IDO1 enzyme inhibitors have not demonstrated any survival benefit in cancer patients either mono-therapeutically or in combination with other therapies. Previously we confirmed that tumor IDO1 mRNA expression is significantly upregulated in GBM patients compared to those from patients with lower grades gliomas. Furthermore, knockdown of IDO1 in mouse GBM cells significantly increased mouse survival and decreased tumor infiltrating regulatory T cells (Treg) but did not affect tumor local tryptophan catabolism. Consistently, inhibition of IDO1 enzymatic activity on GBM-bearing mice did not yield any survival benefit. Collectively, these findings prompt us to test the hypothesis whether GBM cell derived IDO1 can suppress immune response through a non-enzymatic pathway.
Methods: IDO1-deficient mouse GBM cells were created and modified to stably express either wild-type (WT) or enzyme-null IDO1. Tumor infiltrating lymphocytes (TILs) and survival rate were analyzed from mice engrafted with these modified cells. Ex vivo co-culture of splenic monocytes with the modified GBM cells was also employed. To interrogate potential IDO1-regulated gene(s) in GBM, gene expression profile in human GBM cell lines under either IDO1 knockdown or overexpression conditions were investigated by microarray analysis and verified by real-time RT-PCR.
Result: Overexpression of WT IDO1 in GBM cells led to significantly increased Treg accumulation in GBM and much poorer survival in comparison to IDO1-deficent tumor cells. Interestingly, ablation of IDO1 enzyme activity in the enzyme-null IDO1 overexpressing tumor cells showed similar Treg accumulation as in the WT IDO1-overexpressing tumor cells and poor survival rate. Moreover, overexpression of both WT and enzyme-null IDO1 in GBM cells induced higher level of mature macrophages (CD11c-CD11b+Ly6c+Ly6glow) than IDO1-deficient GBM cells. Finally, an IDO1 regulated gene encoding a secreted protein was identified in the microarray analysis, which also demonstrates high correlation with IDO1 in many different tumor types.
Conclusions: Results from this study strongly imply an enzyme independent pathway associated with IDO1-driven immunosuppression in mouse GBM. Current investigation of the newly identified IDO1 regulated gene in the setting of human GBM will provide novel perspective in our understanding of the non-enzyme function of IDO1 in tumor immunity and enlighten the strategic design for future IDO1-based GBM immunotherapy.
Citation Format: Lijie Zhai, Erik Ladomersky, Jun Qian, Brenda Nyugen, April Bell, Kristen Lauing, Derek Wainwright. Targeting the non-enzymatic function of IDO1 in glioblastoma immunotherapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5612.