(A) Polyamine metabolic pathway. (B–D) The expression of ODC1 (B), DHPS (C), and eIF5A^Hyp (D) in neutrophils (CD11b⁺CD14^–CD15⁺LOX1^–) from healthy adults (n = 9) and PMN-MDSCs (CD11b⁺CD14^–CD15⁺LOX1⁺) from healthy neonates (n = 8) and neonates with inflammation (n = 8) was detected by flow cytometry. Clinical parameters are listed in Supplemental Table 1. (E) The expression of ODC1 and eIF5A^Hyp in neutrophils from healthy adults and PMN-MDSCs from healthy neonates and neonates with inflammation was detected by immunofluorescence (n = 3 per group; 3 fields of each donor were randomly selected for calculating the mean fluorescence intensity [MFI]). Clinical parameters are listed in Supplemental Table 1. (F) Transcriptional profiling of CD11b⁺Ly6C^lo/–Ly6G⁺ cells isolated from the spleen of adult mice (AM) and newborn mice (NBM). Heatmaps depict differentially expressed genes involved in polyamine metabolism and transport. (G and H) The expression of ODC1 in CD11b⁺Ly6C^lo/–Ly6G⁺ cells was detected by flow cytometry (G) (n = 4 per group) and immunofluorescence (H) (n = 3 per group; 3 fields of each donor were randomly selected for calculating the MFI). (I) Cellular polyamines in CD11b⁺Ly6C^lo/–Ly6G⁺ cells were detected by UPLC-MS/MS (n = 4 per group). Data are shown as mean ± SEM. Statistical analysis was performed using 1-way ANOVA with Tukey’s multiple-comparison test (B–E) and unpaired 2-tailed Student’s t test (G–I). Scale bars: 5 μm. Original magnification, ×2.5 (insets, E and H). *P < 0.05, **P < 0.01, ***P < 0.001.