Evaluation of proton minibeam radiation therapy on anti-tumor immune responses in a rat model of glioblastoma.

Treating radioresistant tumors like glioblastoma multiforme (GBM) remains a challenge exacerbated by their immunosuppressive nature. Radiation therapy has an immunomodulatory role, both immunosuppressive and immunostimulatory. The nature of the effects depends on the total dose, dose per fraction, dose delivery method and treatment length. Hypofractionation is observed to tip the balance towards immune stimulation. However, the use of hypofractionation is restricted in bulky tumours, i.e. gliomas, due to the high risk of toxicity. Therefore, finding new strategies leading to more favourable immune responses while reducing normal tissue toxicities could improve cancer treatment. Here we examine anti-tumoral immune responses to proton minibeam radiation therapy (pMBRT). However, its immunomodulatory effects are not fully understood. To explore this, we conducted an in-depth characterization of the immune response to a curative dose of pMBRT in a preclinical orthotopic rat model of glioblastoma. Our findings revealed a close association between pMBRT and the immune response. pMBRT increased lymphocyte density in tumors more effectively than conventional proton therapy. Single-cell transcriptomics identified several immune cell types and unique transcriptional changes in tumor immune cells post-pMBRT, including increased antibody production, chemotactic cytokine expression, and interferon responses. These results underscore the critical role of adaptive immunity, specifically T cells, in pMBRT's mechanism. The potential of pMBRT to trigger an anti-tumor immune response in a single radiotherapy session with minimal damage to healthy tissue makes it a promising candidate for future clinical trials and radio-immunotherapy combinations.