Irradiated dendritic cell subsets retain key immune capabilities and anti-tumor immunity is not strictly dependent on cDC1

Radiotherapy can enhance anti-tumor immunity by modulating the tumor microenvironment. However, its direct effects on dendritic cells (DCs) as key antigen-presenting cells remain poorly studied. Although DCs are considered relatively radioresistant, their functional capacity following irradiation is controversial and likely subset-dependent. In this study, we systematically analyzed the radiosensitivity and post-irradiation functional capacity of murine bone marrow-derived conventional type 1 DCs (cDC1s), which play a pivotal role in initiating CD8+ T cell-mediated anti-tumor immunity, conventional type 2 DCs (cDC2s), and monocyte-derived DCs (MoDCs) in vitro. After irradiation, DC viability and surface marker expression were assessed. To evaluate functional competence, DCs were stimulated with high hydrostatic pressure-inactivated tumor cells in combination with the adjuvants poly I:C and α-CD40 as a model system for therapeutic tumor vaccination, and the phagocytic activity, activation, and cytokine/chemokine secretion of the DCs were analyzed. DC radiosensitivity depended strongly on both the subset and maturation state, with mature DCs showing higher radioresistance than their immature counterparts, and with immature cDC1s being particularly more susceptible. Nevertheless, surviving DCs retained their ability to phagocytose tumor cells and, upon stimulation, upregulate activation- and migration-associated markers and secrete cytokines and chemokines. In vivo, radio-immunotherapy promoted DC migration to the tumor-draining lymph node and induced DC activation, as indicated by elevated concentrations of DC-related cytokines in the peripheral blood. Within a fixed radiotherapy plus checkpoint inhibition regimen, tumor growth delay was largely preserved in Batf3−/− mice, lacking the cDC1 subset. Collectively, irradiation does not abrogate DC function, and anti-tumor immunity in radio-immunotherapy does not strictly require cDC1s.