Background: Neonatal sepsis remains a leading cause of infant mortality and is driven in part by the functional immaturity of the hematopoietic system. During early postnatal life, hematopoietic stem cells (HSCs) undergo dynamic changes in self-renewal, quiescence, lineage output, and inflammatory responsiveness. While adult HSCs maintain a stable and tightly regulated state, the developmental timeline by which these properties emerge remains poorly defined. A systematic understanding of HSC maturation is essential for uncovering mechanisms of neonatal immune vulnerability and informing strategies to enhance early-life hematopoietic resilience.

Results: Bone marrow hematopoietic composition across postnatal weeks 1, 2, and adulthood was examined under steady-state conditions. Frequencies of HSCs and multipotent progenitors (MPPs) remained largely unchanged across these developmental stages, but expression of myeloid-biased markers (CD38, CD61, and CD41) was reduced on neonatal HSCs compared to adults, suggesting reduced myeloid bias in early-life HSCs. Among lineage-primed progenitors, we observed a progressive decline in the frequencies of colony-forming unit–erythroid (CFU-E), pre-colony-forming unit-erythroid (pre-CFU-E), and pre-megakaryocyte-erythroid progenitors (pre-MegE) from 1-week-old mice to adulthood, indicative of a developmental restriction in erythroid differentiation potential. Within the myeloid progenitor compartment, monocyte progenitor (MoP), granulocyte-monocyte progenitor (GMP), and granulocyte progenitor (GP) population frequencies peaked in 1-week-old mice, then decreased in 2-week-old mice and adults. At the level of mature and developing neutrophils, the frequencies of mature and immature neutrophils increased progressively with age, while early neutrophil precursors (pre-neutrophils and pro-neutrophils) remained consistent across developmental stages. Competitive transplantation assays showed that HSCs from embryonic day 18 (E18) and 2-week-old mice had higher engraftment potential compared to adults, while cell cycle profiling demonstrated that HSCs achieve adult-like quiescence by week 1, indicating that acquisition of quiescence precedes the decrease of engraftment potential. In the context of infection, our preliminary findings in a neonatal mouse model of Group B Streptococcus (GBS), a major cause of sepsis and neutropenia in neonates, recapitulate this defect, suggesting that failure to initiate emergency myelopoiesis may cause early-life susceptibility to bacterial infection.

Conclusion: Our study defines the postnatal timeline of hematopoietic stem and progenitor cell maturation in the bone marrow and reveals that adult-like properties emerge asynchronously. HSCs acquire adult-like quiescence status by postnatal week 1, a timepoint marked by elevated frequencies of multiple progenitor populations and increased mature myeloid output. In a neonatal GBS infection model, 1-week-old mice fail to mount emergency myelopoiesis, indicating that HSCs at this stage remain functionally immature in their response to inflammatory stress. Engraftment potential declines even later, with HSCs from E18 and 2-week-old mice retaining higher donor-derived peripheral blood reconstitution capacity than adult HSCs following transplantation. These findings support a model in which hematopoietic maturation follows a phased progression, rather than a coordinated transition, toward adult-like function. Together, this developmental framework provides new insight into early-life immune vulnerability and informs strategies to enhance neonatal hematopoietic resilience.

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2025
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