IMMU-17. INDUCING IMMUNE RESPONSE WITH FLASH AND CONVENTIONAL DOSE-RATE RADIATION IN DIFFUSE MIDLINE GLIOMA (DMG)

Abstract Diffuse Midline Glioma is a universally fatal inoperable pediatric brain tumor associated with H3K27M mutation and median survival of 9-15 months. Radiation therapy (RT) is the primary treatment, extending life by only three months. Conventional dose-rate RT (2Gy/min, CONV) can induce an immune response with type 1 interferon (IFN1) in multiple cancers including adult brain tumors. Ultra-fast dose-rate RT (90Gy/second, FLASH) has comparable tumor control to CONV with decreased toxicity, however, evaluation of immune response is limited. Using an orthotopic syngeneic model of brainstem DMG, we performed single-cell RNA sequencing (scRNA-seq) on CD45+ immune cells isolated from tumors four days post 15Gy FLASH or CONV RT, and compared to unirradiated tumor. We performed flow cytometry at day 4 and day 10 post-RT. ScRNA-seq reveals 33,308 immune cells in 17 unique clusters. Microglia (MG) make up 73.8% and non-resident myeloid cells make up 7.9% of cells and include macrophages (MAC), dendritic cells (DC) and Monocytes (MONO). Flow at day 4 and day 10 show no significant difference in proportion of immune cell subtypes comparing FLASH, CONV and tumor. With differential gene expression, FLASH and CONV provoke a similar inflammatory and IFN1 response in MG while in MACs and DCs IFN1 pathway expression is increased in CONV compared to FLASH (p< 0.001 for both), confirmed by flow at day 4 using IFNAR1 expression (MACs p< 0.05, DCs p< 0.01). Flow at day 10 post-RT shows increased IFNAR1 expression in MACs in both CONV and FLASH compared to tumor (p< 0.0001) while day 10 IFNAR1 in DCs is similar in all three groups. In summary, while immune proportions are similar comparing different dose rates of RT, IFN1 is differentially expressed in CONV compared to FLASH at day 4 in MACs and DCs, while this response at day 10 is similar comparing dose rates.