Microglia induce an interferon-stimulated gene expression profile in glioblastoma and increase glioblastoma resistance to temozolomide

Aims
Glioblastoma is the most malignant primary brain tumour. Even with standard treatment comprising surgery followed by radiation and concomitant temozolomide (TMZ) chemotherapy, glioblastoma remains incurable. Almost all patients with glioblastoma relapse owing to various intrinsic and extrinsic resistance mechanisms of the tumour cells. Glioblastomas are densely infiltrated with tumour-associated microglia and macrophages (TAMs). These immune cells affect the tumour cells in experimental studies and are associated with poor patient survival in clinical studies. The aim of the study was to investigate the impact of microglia on glioblastoma chemo-resistance.

Methods
We co-cultured patient-derived glioblastoma spheroids with microglia at different TMZ concentrations and analysed cell death. In addition, we used RNA sequencing to explore differentially expressed genes after co-culture. Immunostaining was used for validation.

Results
Co-culture experiments showed that microglia significantly increased TMZ resistance in glioblastoma cells. RNA sequencing revealed upregulation of a clear interferon-stimulated gene (ISG) expression signature in the glioblastoma cells after co-culture with microglia, including genes such as IFI6, IFI27, BST2, MX1 and STAT1. This ISG expression signature is linked to STAT1 signalling, which was confirmed by immunostaining. The ISG expression profile observed in glioblastoma cells with enhanced TMZ resistance corresponded to the interferon-related DNA damage resistance signature (IRDS) described in different solid cancers.

Conclusions
Here, we show that the IRDS signature, linked to chemo-resistance in other cancers, can be induced in glioblastoma by microglia. ISG genes and the microglia inducing the ISG expression could be promising novel therapeutic targets in glioblastoma.