Gamma Irradiation Triggers Immune Escape in Glioma-Propagating Cells

Glioblastoma multiforme is the most common and devastating form of brain tumor for which only palliative radio- and chemotherapy exists. Although some clinical studies on vaccination approaches have shown promising efficacy due to their potential to generate long-term immune surveillance against cancer cells, the evasion mechanisms preventing therapy response are largely uncharacterized. Here, we studied the response of glioblastoma-propagating cells (GPCs) to clinically relevant doses of γ radiation. GPCs were treated with 2.5 Gy of γ radiation in seven consecutive cellular passages to select for GPCs with increased colony-forming properties and intrinsic or radiation-induced resistance (rsGPCs). Quantitative proteomic analysis of the cellular signaling platforms of the detergent-resistant membranes (lipid rafts) in GPCs vs. rsGPCs revealed a downregulation of the MHC class I antigen-processing and -presentation machinery. Importantly, the radio-selected GPCs showed reduced susceptibility towards cytotoxic CD8+ T-cell-mediated killing. While previous studies suggested that high-dose irradiation results in enhanced antigen presentation, we demonstrated that clinically relevant sub-lethal fractionated irradiation results in reduced expression of components of the MHC class I antigen-processing and -presentation pathway leading to immune escape.