Macrophages Drive Ferroptosis Resistance after Radiotherapy

Background: Radiotherapy eliminates most tumor cells but spares persister tumor cells that evade cell death and drive relapse. Increasing evidence suggests that stromal components of the tumor microenvironment influence treatment responses, yet whether macrophages actively reprogram tumor-intrinsic stress responses to promote radioresistance remains unclear. Here, we investigated the mechanisms by which macrophage-tumor cell interactions regulate ferroptosis and tumor survival after irradiation. Methods: We used macrophage-tumor cell coculture systems, Transwell separation assays, and 3D microfluidic models to examine contact-dependent effects on tumor survival following irradiation. Kinome-wide small interfering RNA screening, RNA sequencing, lipidomic profiling, and quantitative proteomic analysis of secretomes were performed to identify signaling pathways and metabolic changes. Genetic and pharmacological perturbation of Ephrin receptor b4 (Ephb4) signaling were evaluated in vitro and in syngeneic mouse tumor models. Clinical relevance was assessed using transcriptomic analyses and immunohistochemical staining of patient tumor specimens. Results: Macrophage contact reduced lipid peroxidation and cell death in irradiated tumor cells in a contact-dependent manner. Kinome screening identified Ephb4 as a key mediator induced by irradiation in tumor cells. Ephb4 engagement with ephrinb2 on macrophages initiated bidirectional signaling that increased expression of ferroptosis-protective genes (solute carrier family 7 member 11 [Slc7a11], solute carrier family 3 member 2 [Slc3a2], and glutathione peroxidase 4 [Gpx4]) in tumor cells while activating the toll-like receptor 2- nuclear factor-kappa B pathway and interleukin-6 (IL-6) production in macrophages. Macrophage-derived IL-6 further sustained ferroptosis resistance in tumor cells, and Ephb4-driven secretion of cathepsin S amplified macrophage IL-6 production through a feedforward loop. Genetic or pharmacological inhibition of Ephb4 restored lipid peroxidation and markedly enhanced radiosensitivity in vitro and in vivo. Analysis of patient datasets demonstrated increased EPHB4 expression following radiotherapy and an association between high EPHB4 expression, reduced ferroptosis signatures, and poor treatment response. Conclusions: These findings identify a macrophage-driven ferroptosis evasion program that enables tumor cell survival after irradiation and demonstrate that Ephb4 coordinates bidirectional tumor-macrophage signaling to sustain this resistance. Targeting the Ephb4-ephrinb2 axis represents a potential strategy to enhance ferroptosis and improve radiotherapy efficacy in resistant tumors.