771 Mitigating feedback immunosuppression through IDO inhibition during oncolytic virotherapy of glioblastoma

<h3>Background</h3> Glioblastoma (GBM) is the most frequent brain malignant tumor with poor prognosis. Currently, standard-of-care (SOC) treatments cannot completely eradicate the tumor and patient survival remains dismal. Recently, oncolytic virus (OV) treatment provides a new immunotherapy direction for solid tumor therapy, which acts by both direct lysis of tumor cells and activating the host anti-tumor immune response. HSV-1 derived oncolytic viruses (oHSVs) are the only type of viruses currently approved by the FDA in the USA. We recently discovered that disabling tumor-intrinsic PKR signaling (oHSV-shPKR) could increase oHSV infection, replication, and oncolysis in both oHSV-sensitive and -resistant GBM cells, but also increase adaptive anti-tumor immune response. However, we also find that disabling tumor-intrinsic PKR signaling could induce feedback immunosuppression, including activation of the indolamine 2,3-dioxegenase (IDO) signaling pathway. <h3>Methods</h3> We analyzed IDO signaling activation during oHSV therapy of GBM using both <i>in vitro</i> coculture and <i>in vivo</i> GBM xenograft model in immunocompetent mice and tested whether the combination of oHSV with IDO inhibitor (indoximod) could block the feedback immunosuppression. <h3>Results</h3> Flow cytometry analysis and immunofluorescence staining showed that both GBM development and oHSV intratumor therapy increase IDO+CD11c+ dendritic cells infiltration in tumor. <i>In vitro</i> coculture of oHSV-infected human GBM neurospheres with CD14+ monocytes-derived dendritic cells (MoDCs) dramatically increased IDO signaling activation in MoDCs, partially through GBM-mediated cGAS-STING activation. Combination with indoximod in the coculture not only significantly increased human MoDCs activation including upregulation of costimulatory molecules (CD80 and HLA DR), but also increased immunostimulatory cytokine secretion including IL12. In i<i>n vivo</i> murine 005-OVA GBM model, combination treatment with indoximod and oHSV not only significantly inhibited tumor growth and prolonged tumor-bearing mice survival, but also induced antigen specific CD8+ T cell activation. <h3>Conclusions</h3> The results demonstrated that IDO signaling inhibition through indoximod could significantly block feedback immunosuppression during oncolytic virotherapy of glioblastoma.