Session Item

Modern glioblastoma (GBM) research has progressed to the consensus that the mesenchymal subtype is associated with worse prognosis, treatment resistance, and therapeutic failure. Mesenchymal gene expression is composed of GBM tumor cell-intrinsic and stromal contributions. GBM growth outpaces its demands for adequate blood supply by producing tortuous feeder arteries and draining veins via abnormal arterio- and venogenesis.

VEGF-A is intricately involved in these processes, namely by stabilizing and maturing new blood vessels and acting as a potent chemokine for certain immune cells, particularly of the myeloid lineage. The VEGF-A-targeting antibody Bevacizumab improves patients'' quality of life, reduces the need for steroid treatment, and prolongs progression-free survival when combined with radio(chemo)therapy. Additionally, Bevacizumab was very recently reported to reduce normal brain toxicity during reirradiation of GBM.

Here, we elucidate the molecular mechanisms of the above mentioned effects of VEGF-A blockade in an orthotopic mouse GBM model.

Mouse GL261 GBM cells were implanted orthotopically into C57BL/6 mice. Established tumors were subjected to fractionated radiation (RTX) over 2 weeks with 5x 2 Gy per week with or without concomitant anti-VEGF-A treatment. Tumor growth was monitored via contrast enhanced conebeam CT scans, and mice were sacrificed upon reaching pre-defined humane endpoints. Explanted tumor and brain tissues were processed for microarray-based gene expression analyses. Immunogenomic and gene set enrichment analyses (GSEA) were performed using R and Cytoscape. Reference data and gene sets were accessed from public resources.

GL261 cells represent a model system of mesenchymal GBM as extracted from GSEA of intracranial transplants and public data sets. Mesenchymal gene expression is attributable – in part – to the tumor cell-intrinsic transcriptome as observed in vitro. In vivo, myeloid immune cell signatures, including microglial, monocytic, and neutrophilic expression patterns contribute to "mesenchymality". RTX enforces enrichment of mesenchymal and derichment of proneural and classical gene expression patterns, paralleled by enrichment of myeloid immune cell gene signatures. Anti-VEGF-A treatment reduces mesenchymal gene expression and attenuates radiation-induced "mesenchymalization". In normal brain tissue, RTX induces expression of necrosis- and inflammation-associated gene sets which is also attenuated by anti-VEGF-A blockade.

We provide evidence that anti-VEGF-A treatment may offer particular benefits in combination with RTX when applied to GBM of mesenchymal subtype by modulating subtype plasticity. Furthermore, anti-VEGF-A treatment may be neuroprotective – not only – in reirradiation settings. These observations deserve further clinical evaluation.