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 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 therapeutic studies in preclinical models to show that interleukin-6 (IL-6) neutralization reprograms the glioblastoma TIME to sensitize mouse glioblastoma allografts to ICI and ICI plus radiotherapy. We find rare human glioblastomas that achieve clinical responses to ICI have lower pre-treatment IL-6 compared to glioblastomas that do not respond to ICI. We show that diverse immunostimulatory gene therapies suppress IL-6 in mouse glioblastoma allografts, and that IL-6 from multiple cell types the TIME reduces survival in preclinical models and in patients. IL-6 blockade with a neutralizing antibody sensitizes mouse glioblastoma allografts to ICI by increasing MHCII+ monocytes, CD103+ migratory dendritic cells (DCs), CD11b+ conventional DCs, and effector CD8+ T cells, and by decreasing immunosuppressive Tregs. To translate these findings to a therapeutically relevant combination treatment for patients, we show that IL-6 blockade plus ICI sensitizes mouse glioblastoma allografts to ablative radiotherapy.