P02.02.a Il6 Underlies Microenvironment Immunosuppression and Resistance to Therapy in Glioblastoma

Abstract BACKGROUND The glioblastoma (GBM) tumor immune microenvironment (TIME) is an immunosuppressive barrier to therapeutic innovation, and unsurprisingly GBM responses to immune checkpoint inhibition (ICI) are rare. Here, we define therapeutic vulnerabilities and mechanisms underlying GBM resistance to ICI therapy. MATERIAL AND METHODS Integration of spatial and single-cell approaches from human IDH-wildtype GBMs before and after ICI treatment with preclinical genetic, immunologic, and efficacy studies in immunocompetent mouse GBM models. RESULTS Human GBMs with radiographic responses to ICI had lower pre-treatment IL6 levels on spatial profiling than GBMs resistant to ICI. Multiplexed immunofluorescence revealed ICI-responders were enriched in T-cell proteins (CD3, CD4, CD8) and activation markers (CD25) at the time of salvage surgery; meanwhile, non-responders were enriched in myeloid proteins (CD68, CD163, CD11c). Single-cell RNA sequencing analysis of 32,877 cells from human GBMs showed IL6 is predominantly produced by radial glial like cancer stem cells. Interestingly, mice bearing intracranial SB28 GBM allografts universally suppressed intratumor IL6 levels when treated with immunostimulatory gene therapies and mouse genetic experiments confirmed prolonged survival in animals with an IL6-/- background. Mouse survival was improved by combination systemic treatment with anti-PD1 and anti-IL6 but was not improved by either antibody alone or by local anti-IL6 delivery. Mass cytometry revealed combination therapy reprogrammed the GBM TIME by increasing MHCII+ monocytes, CD103+ migratory dendritic cells (DCs), CD11b+ conventional DCs, and effector CD8+ T cells, and by decreasing immunosuppressive Tregs. CONCLUSION Systemic anti-IL6 in combination with systemic ICI reprograms the GBM TIME to sensitize preclinical models to ICI, shedding light on a therapeutic strategy for GBM treatment.