Tissue-resident neutrophils serve homeostatic and immunological functions in embryos.

Development of neutrophils in the bone marrow and their crucial role in first-line defense are well understood in adults, but remarkably little is known about fetal neutrophils. Here, we analyze the production, distribution, and functions of neutrophils during embryonic development in the mouse. We discover that multiple non-hematopoietic steady-state organs harbor substantial numbers of immature and mature neutrophils, many of which are localized in the tissue parenchyma outside the blood vasculature. Using single-cell transcriptomic analyses, we reveal the presence of neutrophil progenitors and precursor cells in the blood and even in non-hematopoietic tissues in fetal and newborn mice. Embryonic tissue-resident neutrophils are transcriptionally different from embryonic blood-borne neutrophils and adult neutrophils. We demonstrate, through functional analyses, that embryonic neutrophils proliferate actively, have a high glycolytic capacity, and undergo partial diurnal rhythmicity. Embryonic neutrophils display lineage-specific innate immune effector functions and are responsive to maternal immunostimulation and immunosuppression. Using a genetic embryonic neutrophil depletion model, we discover that neutrophils may impact homeostatic processes in the testis. Collectively, our data provides a cross-tissue atlas of the fetal neutrophil landscape and assesses their responses in steady-state.