(A and B) Pharmacological inhibition of polyamine metabolism (A) and coculture strategy (B). (C and D) PMN-MDSCs were treated with DFMO (200 μM) for 16 hours. Suppression of T cell proliferation by PMN-MDSCs (C) (n = 5 per group) and expression of eIF5A^Hyp (D) (n = 4 per group). Data are representative of 3 independent experiments. (E) CD11b⁺Ly6C^lo/–Ly6G⁺ cells were isolated from adult mice (AM) and newborn mice (NBM). The expression of eIF5A^Hyp was detected by immunofluorescence (n = 3 per group; 3 fields of each donor were randomly selected for calculating the MFI). Original magnification, ×2.5. (F and G) The expression of eIF5A^Hyp (F) (n = 4 per group) and hypusination rate (G) (n = 4 per group) in CD11b⁺Ly6C^lo/–Ly6G⁺ cells of different ages. Data are representative of 3 independent experiments. (H) Suppression of T cell proliferation by PMN-MDSCs, which were pretreated with DMSO, spermidine (SPD) (0.2 μM), GC7 (10 μM), and SPD plus GC7 for 16 hours (n = 6 per group). Data are representative of 3 independent experiments. (I) Suppression of T cell proliferation by Dhps^fl/fl and Dhps^ΔLysm PMN-MDSCs (n = 6 per group). Data are representative of 3 independent experiments. (J) Bulk RNA sequencing of PMN-MDSCs from the spleen of Dhps^fl/fl and Dhps^ΔLysm littermates. Heatmaps of differentially expressed genes involved in immunosuppressive function. (K and L) ARG1 (n = 5 per group) and cellular PGE₂ production (n = 8 per group) in Dhps^fl/fl and Dhps^ΔLysm PMN-MDSCs. Data are shown as mean ± SEM. Statistical analysis was performed using 1-way ANOVA with Tukey’s multiple-comparison test (C and I), unpaired 2-tailed Student’s t test (D, E, K, and L), and 2-way ANOVA with Šidák’s multiple-comparison test (H). Scale bar: 5 μm. *P < 0.05, **P < 0.01, ***P < 0.001.