Although substantial progress has been achieved concerning neonatal sepsis, its lethality remains considerably high, and further insights into peculiarities and malfunctions of neonatal immunity are needed. This study aims to contribute to a better understanding of the role of human neonatal granulocyte subpopulations and calgranulin C (S100A12). For this purpose, we gathered 136 human cord blood (CB) samples. CD66b⁺ CB low-density granulocytes (LDG) and CB normal-density granulocytes were isolated and functionally and phenotypically compared with healthy adult control granulocytes. We could identify CB-LDG as CD66b^bright CD64^high CD16^low CD35^low CD10^low S100A12^med-low and, based on these markers, recovered in whole CB stainings. Consistent with flow cytometric findings, microscopic imaging supported an immature phenotype of CB-LDG with decreased S100A12 expression. In CB serum of healthy neonates, S100A12 was found to be higher in female newborns when compared with males. Additionally, S100A12 levels correlated positively with gestational age independently from sex. We could solidify functional deficits of CB-LDG concerning phagocytosis and generation of neutrophil extracellular traps. Our study reveals that previously described suppressive effects of CB-LDG on CD4⁺ T cell proliferation are exclusively due to phagocytosis of stimulation beads used in cocultures and absent when using soluble or coated Abs. In conclusion, we characterize CB-LDG as immature neutrophils with functional deficits and decreased expression and storage of S100A12. Concerning their cross-talk with the adaptive immunity, we found no direct inhibitory effect of LDG. Neonatal LDG may thus represent a distinct population that differs from LDG populations found in adults.

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