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Glioblastoma is the most common and uncompromising primary brain tumour and is characterized by a dismal prognosis despite aggressive treatment regimens. At the cellular level, these tumours are composed of a mixture of neoplastic cells and non-neoplastic cells, including tumour-associated macrophages and endothelial cells. Cerebral oedema is a near-universal occurrence in patients afflicted with glioblastoma and it is almost exclusively managed with the corticosteroid dexamethasone despite significant drawbacks associated with its use. Here, we demonstrate that dexamethasone blocks interleukin-1 production in both bone marrow-derived and brain resident macrophage populations following stimulation with lipopolysaccharide and interferon gamma. Additionally, dexamethasone is shown to inhibit downstream effectors of interleukin-1 signalling in both macrophage populations. Co-culture of bone marrow-derived macrophages with organotypic tumour slices results in an upregulation of interleukin-1 cytokines, an effect that is absent in co-cultured microglia. Genetic ablation of interleukin-1 ligands or receptor in mice bearing RCAS/tv-a-induced platelet-derived growth factor B-overexpressing glioblastoma results in reduced oedema and partial restoration of the integrity of the blood-brain barrier, respectively; similar to results obtained with vascular endothelial growth factor neutralization. We establish that tumours from dexamethasone-treated mice exhibit reduced infiltration of cells of the myeloid and lymphoid compartments, an effect that should be considered during clinical trials for immunotherapy in glioblastoma patients. Additionally, we emphasize that caution should be used when immune profiling and single-cell RNA sequencing data are interpreted from fresh glioblastoma patient samples, as nearly all patients receive dexamethasone after diagnosis. Collectively, this evidence suggests that interleukin-1 signalling inhibition and dexamethasone treatment share therapeutic efficacies and establishes interleukin-1 signalling as an attractive and specific therapeutic target for the management of glioblastoma-associated cerebral oedema. [ABSTRACT FROM AUTHOR]