CXCR5⁺ monocyte emigration impairs the radiation-induced antitumor immune response

A substantial portion of patients experience radioresistance, which impedes clinical benefit. The radiation-induced 'protumor' immune response is previously demonstrated to limit antitumor efficacy. However, the detailed mechanism remains to be explored. In this study, we observe CXCR5⁺ monocytes are enriched in tumor upon radiation. CXCR5 expression on monocytes in host is induced by tumor-derived VEGF through PI3K/mTOR/HIF-1α axis. Local radiation enhances CXCL13 expression from tumor cells, a specific ligand of CXCR5, which leads to the recruitment of CXCR5⁺ monocytes. Tumor-infiltrating CXCR5⁺ monocytes induce radioresistance by inhibiting CD8⁺ T cells through PD-1/PD-L1 interaction. Moreover, radiation-induced GM-CSF promotes the differentiation of CXCR5⁺ monocytes toward M2-like macrophages. In contrast, inhibiting VEGFR signaling, neutralizing CXCL13 and GM-CSF, or blocking PD-L1 facilitates radiation-induced tumor control by abrogating CXCR5⁺ monocyte-mediated immunosuppression. Furthermore, the CXCR5⁺ and CD14⁺ populations are increased in patients with cancer following radiotherapy. Monocyte is increased in the peripheral blood of patients with progressive disease following radiotherapy. These findings suggest potential strategies for blocking the CXCR5/CXCL13 axis to improve radiotherapy efficacy.