The neonatal liver hosts a spontaneously occurring neutrophil population, exhibiting distinct spatial and functional characteristics from adults

The elusive nature of the liver immune system in newborns remains an important challenge, casting a shadow over our understanding of how to effectively treat and prevent diseases in children. Therefore, deeper exploration into the intricacies of neonatal immunology might be crucial for improved pediatric healthcare. Using liver intravital microscopy, we unveiled a significant population of granulocytes in the hepatic parenchyma of fetuses and newborns. Utilizing high-dimensional immunophenotyping, we showed dynamic alterations predominantly in granulocytes during neonatal development. Liver intravital microscopy from birth through adulthood captures real-time dynamics, showing a substantial presence of Ly6G+ cells that persisted significantly up to 2 wk of age. Using time-of flight mass cytometry, we characterized neonatal Ly6G+ cells as neutrophils, confirmed by morphology and immunohistochemistry. Surprisingly, the embryonic liver hosts a distinct population of neutrophils established as early as the second gestational week, challenging conventional notions about their origin. Additionally, we observed that embryonic neutrophils occupy preferentially the extravascular space, indicating their early establishment within the liver. Hepatic neutrophils in embryos and neonates form unique cell clusters, persisting during the initial days of life, while reduced migratory capabilities in neonates are observed, potentially compensating with increased reactive oxygen species release in response to stimuli. Finally, in vivo imaging of acute neutrophil behavior in a newborn mouse, subjected to focal liver necrosis, unveils that neonatal neutrophils exhibit a reduced migratory response. The study provides unprecedented insights into the intricate interplay of neutrophils within the liver, shedding light on their functional and dynamic characteristics during development.