IL-6 underlies microenvironment immunosuppression and resistance to therapy in glioblastoma

The glioblastoma tumor immune microenvironment (TIME) is an immunosuppressive barrier to therapy that encumbers glioblastoma responses to immune checkpoint inhibition (ICI). Immunosuppressive cytokines, pro-tumor macrophages and myeloid cells, and exhausted T-cells are all hallmarks of the glioblastoma TIME. Here we integrate spatial and single-cell analyses of patient-matched human glioblastoma samples before and after ICI with genetic, immunologic, single-cell, and pharmacologic studies in preclinical models to show that interleukin-6 (IL-6) neutralization reprograms the glioblastoma TIME to sensitize mouse glioblastoma allografts to ICI and radiotherapy. We find rare human glioblastomas that achieve clinical responses to ICI have lower pre-treatment IL-6 levels compared to glioblastomas that do not respond to ICI. Our data show that diverse immunostimulatory gene therapies suppress local IL-6 levels in mouse glioblastoma allografts, and that IL-6 from glioblastoma cells and the tumor microenvironment is associated with reduced survival in preclinical models and in patients. We show that IL-6 blockade with a neutralizing antibody transiently sensitizes mouse glioblastoma allografts to ICI by decreasing immunosuppressive Tregs, and by increasing MHCII+ monocytes, CD103+ migratory dendritic cells (DCs), CD11b+ conventional DCs, and effector CD8+ T cells. To translate these findings to a combination treatment strategy that could be used for patients, we show that IL-6 blockade plus ICI more durably sensitizes mouse glioblastoma allografts to immunostimulatory ablative radiotherapy.