(A) WT and Ythdf1-cKO mice were injected s.c. with B16-OZ cells. Tumor-bearing mice were treated with local IR (20 Gy, 1 dose) when the tumor volume reached 100–200 mm³. Expression of cathepsins A and B in tumor-infiltrating DCs (CD45⁺F4/80^–CD11c⁺MHC-II⁺) was detected via flow cytometry on day 5 after IR (n = 5). (B) BMDCs from WT mice were cocultured with 40 Gy–pretreated B16-OZ cells for 24 hours. Purified CD11c⁺ cells were collected to measure the enrichment of cathepsin A and B mRNA in the YTHDF1-immunoprecipitated RNA fraction (n = 3). (C) BMDCs from WT mice were treated with 2′3′-cGAMP and E64 for 12 hours, and STING expression was detected by Western blotting. (D) WT BMDCs and BMDCs with Ythdf1 deletion were treated with 2′3′-cGAMP for 12 hours, and STING expression was detected by Western blotting. (E) BMDCs from WT mice were stimulated with 2′3′-cGAMP. After immunoprecipitation with STING antibody, the expression of cathepsins A and B in whole-cell lysates was detected by Western blot. (F and G) BMDCs from WT mice were pretreated with E64 for 24 hours and then cocultured with 40 Gy–pretreated or nonirradiated B16-OZ cells for 8 hours. Purified CD11c⁺ cells were incubated for another 2 days. Supernatants were collected to measure IFN-β by ELISA (n = 3) (F), and cells were collected to measure Ifnb mRNA levels (n = 3) (G). (H) B16-OZ tumor–bearing WT mice were treated with local IR (20 Gy, 1 dose) and 50 μM E64 (i.t., daily). Tumor growth was monitored after IR. (I) Mechanism of how YTHDF1 affects STING/IFN-I signaling in DCs. IR-induced YTHDF1 increases the degradation of STING via cathepsins in the lysosomes of DCs, ultimately leading to decreased IFN-I production. Data are represented as the mean ± SEM and are representative of 2 or 3 independent experiments. Two-sided, unpaired Student’s t test (B), 1-way ANOVA with Tukey’s multiple-comparison test (A, F, and G), and 2-way ANOVA with Tukey’s multiple-comparison test (H). ***P < 0.001.