IMMU-22. TGFβ inhibition and radiotherapy synergize to enable immune memory response to PD-L1 immune checkpoint inhibition in syngeneic glioblastoma mouse models

Abstract Eradicating glioblastoma (GBM) using immuno-oncology agents would be transformative, but GBM is typically devoid of tumor-infiltrating lymphocytes, limiting the efficacy of immune checkpoint inhibitors (ICIs). Combining ICIs with radiotherapy is an attractive strategy, as standard-of-care radiation not only reduces tumor burden that can enhance antigen presentation, potentially priming the immune system. However, radiation induces activation of transforming growth factor β (TGFβ), which mediates effective DNA repair and immunosuppression that are major barriers to treatment benefit. Based on the hypothesis that activating immunity in GBM requires targeting both the tumor cells and the immunosuppressive tumor microenvironment, we evaluated TGFβ inhibition (TGFβi) in two syngeneic mouse models of GBM: immunogenic GL261 and refractory SB28. GL261 tumors were inherently responsive to anti–PD-L1 therapy, exhibiting durable tumor rejection and long-term survival, including immune memory evidence by rejection upon orthotopic or subcutaneous tumor rechallenge. Notably, the combination of TGFβi and irradiation recapitulated this effect, suggesting that TGFβ suppresses radiation-induced anti-tumor immunity. In contrast, in SB28 tumors, which did not respond to either ICI or TGFβi monotherapies, the combination of TGFβi and 10 Gy only modestly extended survival from 21 to 31 days but did not result in long-term survival. Hypofractionated radiotherapy (5 × 6 Gy) in both models yielded durable responses (defined as twice the survival of controls), though only 25% of survivors exhibited immune memory. In GL261, adding TGFβi increased the proportion of long term survivors with immune memory to 66%, but did not do so in SB28. However, the triple combination of TGFβi, ICI, and fractionated radiotherapy effectively induced long-term survival in SB28 tumor bearing mice, and most mice rejected subsequent tumor rechallenge. These studies indicating that TGFβ is a critical immunosuppressive barrier in GBM demonstrate that its inhibition can unlock effective immune responses when combined with radiotherapy and ICI.