IRF8-driven reprogramming of the immune microenvironment enhances anti-tumor adaptive immunity and reduces immunosuppression in murine glioblastoma

ABSTRACT Background Glioblastoma (GBM) has a highly immunosuppressive tumor immune microenvironment (TIME), largely mediated by myeloid-derived suppressor cells (MDSCs). Here, we utilized a retroviral replicating vector (RRV) to deliver Interferon Regulatory Factor 8 (IRF8), a master regulator of type 1 conventional dendritic cell (cDC1) development, in a syngeneic murine GBM model. We hypothesized that RRV-mediated delivery of IRF8 could “reprogram” intratumoral MDSCs into antigen-presenting cells (APCs) and thereby restore T-cell responses. Methods Effects of RRV-IRF8 on survival and tumor growth kinetics were examined in the SB28 murine GBM model. Immunophenotype was analyzed by flow cytometry and gene expression assays. We assayed functional immunosuppression and antigen presentation by ex vivo T-cell-myeloid co-culture. Results Mice with RRV-IRF8 pre-transduced intracerebral tumors had significantly longer survival and slower tumor growth compared to controls. RRV-IRF8 treated tumors exhibited significant enrichment of cDC1s and CD8+ T-cells. Additionally, myeloid cells derived from RRV-IRF8 tumors showed decreased expression of the immunosuppressive markers Arg1 and IDO1 and demonstrated reduced suppression of naïve T-cell proliferation in e x vivo co-culture, compared to controls. Furthermore, DCs from RRV-IRF8 tumors showed increased antigen presentation compared to those from control tumors. In vivo treatment with azidothymidine (AZT), a viral replication inhibitor, showed that IRF8 transduction in both tumor and non-tumor cells is necessary for survival benefit, associated with a reprogrammed, cDC1- and CD8 T-cell-enriched TIME. Conclusions Our results indicate that reprogramming of glioma-infiltrating myeloid cells by in vivo expression of IRF8 may reduce immunosuppression and enhance antigen presentation, achieving improved tumor control. Key points GBM intra-tumoral myeloid cells are proliferative and targets for RRV therapy. Expression of IRF8 significantly improves survival and slows tumor growth in murine GBM. IRF8 expression in MDSCs reduces immunosuppression and enriches cDC1s in vivo . Importance of the study Recent publications have presented conflicting studies regarding the role of IRF8 in GBM. While some studies showed IRF8 as a negative prognostic factor, others demonstrated the conversion of tumor cells into DCs using IRF8. Here, we show that RRV-mediated delivery of IRF8, a clinically relevant modality, allows for transduction of both tumor and immune cells in vivo . We show that a significant survival effect relies heavily on the infection and modulation of both populations, and that even a modest number of reprogrammed intra-tumoral MDSCs can have a substantial impact on the immunological milieu, significantly enriching and activating cytotoxic T-cells. Further, this work reveals intra-tumoral myeloid cells as a target for other RRV-based gene therapies.