Dual blockade of PD-1 and CTLA-4 generates long-lasting immunity against irradiated glioblastoma.

Radiation therapy (RT) can release pro-inflammatory signals to jumpstart an anti-tumor immune response. However, glioblastoma (GBM) often recurs, suggesting that RT might not act as an immune adjuvant in this disease. A possible explanation for the lack of immune stimulation is the use of irradiation regimens that do not effectively stimulate anti-tumor immunity against GBM. Here, we tested the ability of various RT schedules to elicit type I interferon (IFN-I) response and explored its synergy with immunotherapy (IT) to trigger anti-tumor immunity against GBM. Using three murine GBM models, we show in vitro that single dose radiation ranging from 0Gy to 20Gy and fractionated radiation schedules (i.e. 3 daily fractions of 8Gy; 3 × 8Gy and 5 daily fractions of 6Gy; 5 × 6Gy) accumulates double stranded DNA and release IFN-I related cytokines in a dose-dependent fashion; with fractionated schedules being superior in triggering cancer-cell intrinsic IFN-I responses. Side-by-side comparison of various radiation regimen in vivo revealed that 5 × 6Gy better control GBM across the three GBM models tested. However, the addition of anti-PD1 or anti-CTLA4 to an immunogenic radiation schedule (i.e. 5 × 6Gy) did not prolong survival of irradiated mice. Surprisingly, only the dual blockade of PD-1 and CTLA4 promoted the expansion of proliferative T cells and conveyed immunological memory against irradiated GBM. Overall, this study demonstrates that an immunogenic radiation regimen is not sufficient to mount an anti-tumor immune response when combine with IT as monotherapy and highlights the need to combine an immunogenic irradiation with multiple IT to overcome immunosuppression of GBM.