Dual PI3Kδ/γ inhibition enhances radiotherapy-induced antitumor immunity via macrophage-dependent cGAS-STING-type I interferon signaling.

Radiotherapy (RT) promotes antitumor immunity by activating the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway and inducing type I interferon (IFN-I); however, its efficacy is often limited by the immunosuppressive tumor microenvironment. Herein, we investigated whether dual inhibition of phosphoinositide 3-kinase (PI3K)δ and PI3Kγ could enhance RT-induced innate immune activation in colorectal cancer. In the syngeneic CT26 model, dual PI3Kδ/γ inhibition (BR101801 or duvelisib), unlike selective PI3Kδ inhibition (idelalisib), synergized with RT (7.5 Gy) to suppress tumor growth and induce durable immune memory. Transcriptional profiling revealed strong upregulation of IFN-I-responsive genes, including Ifnb1 and Cxcl10, in macrophages and tumor-macrophage cocultures rather than in tumor cells. Mechanistically, BR101801 enhanced RT-induced cGAS-STING activation and IFN-I production in macrophages. Furthermore, blockade of the IFN-I receptor abolished CD8+ T cell infiltration and M2-like macrophage suppression, abrogating antitumor efficacy and confirming the requirement for IFN-I signaling. These findings identify macrophage-driven activation of the cGAS-STING-IFN-I axis as a key mechanism by which dual PI3Kδ/γ inhibition potentiates RT, providing a strong scientific rationale for its development as an immunomodulatory radiosensitizer.