Immune-Mediated Effects of Microplanar Radiotherapy with a Small Animal Irradiator

SIMPLE SUMMARY: Half of all cancer patients receive radiation therapy during their course of treatment. The destructive effects of radiation on normal tissue causes side effects that significantly affect the patient’s quality of life. Microplanar radiation therapy (MRT) is a novel, promising method that, in animal models, has shown fewer effects on normal tissue and better therapeutic efficacy. MRT does not expose the entire tissue to intense radiation, but has micron-scale “valleys” between the peaks of radiation. We have previously developed an accessible method to apply MRT. Using our approach, here, we show that MRT greatly enhanced the response of the immune system to the tumor by stimulating specific signaling molecules. Moreover, we show that combining MRT with immune checkpoint therapy was even more effective in reducing the treated tumors, possibly pointing towards using similar approaches in the clinic. ABSTRACT: Spatially fractionated radiotherapy has been shown to have effects on the immune system that differ from conventional radiotherapy (CRT). We compared several aspects of the immune response to CRT relative to a model of spatially fractionated radiotherapy (RT), termed microplanar radiotherapy (MRT). MRT delivers hundreds of grays of radiation in submillimeter beams (peak), separated by non-radiated volumes (valley). We have developed a preclinical method to apply MRT by a commercial small animal irradiator. Using a B16-F10 murine melanoma model, we first evaluated the in vitro and in vivo effect of MRT, which demonstrated significant treatment superiority relative to CRT. Interestingly, we observed insignificant treatment responses when MRT was applied to Rag(−/−) and CD8-depleted mice. An immuno-histological analysis showed that MRT recruited cytotoxic lymphocytes (CD8), while suppressing the number of regulatory T cells (Tregs). Using RT-qPCR, we observed that, compared to CRT, MRT, up to the dose that we applied, significantly increased and did not saturate CXCL9 expression, a cytokine that plays a crucial role in the attraction of activated T cells. Finally, MRT combined with anti-CTLA-4 ablated the tumor in half of the cases, and induced prolonged systemic antitumor immunity.