Molecular mechanisms underlying the abscopal effect induced by radiotherapy and its synergistic translational potential with immunotherapy

Abscopal immunity—the regression of distant, non-irradiated lesions after localized radiotherapy (RT)—signals conversion of focal DNA damage into systemic antitumor immunity. This review advances a unifying three-stage framework—initiation, amplification, and reinforcement—explaining how RT can be leveraged to elicit durable systemic control. In initiation, immunogenic cell death and cytosolic DNA activate cGAS–STING (with TLR3–interferon (IFN)-I as a compensatory axis), driving dendritic cell recruitment and cross-priming in tumor-draining lymph nodes. Amplification entails chemokine-guided trafficking and expansion of CXCR3 + cytotoxic T cells, together with stromal and vascular remodeling that enable infiltration at out-of-field sites. Reinforcement reflects the balance between memory formation and adaptive resistance (PD-L1 upregulation, myeloid/Treg accrual, adenosine, and metabolic checkpoints), defining actionable targets for combinatorial intervention. We critically appraise clinical data showing that RT paired with immune-checkpoint inhibition can increase out-of-field control in selected settings, whereas heterogeneous or negative trials underscore the importance of dose and fractionation, field design/target coverage, RT-immune checkpoint inhibitor sequencing, and sparing of lymphoid structures. We outline emerging levers—including spatially fractionated RT, FLASH RT, proton therapy, myeloid- and adenosine-axis blockade, and nanotechnology-enabled in situ vaccination—and candidate biomarkers (interferon-response signatures, circulating tumor DNA kinetics, T-cell clonotypes). Operationalizing these principles points toward making the abscopal effect a predictable, clinically actionable endpoint rather than a rarity.