Project 2: Identify the mechanisms that regulate the maturation of HSCs from fetal to adult function.

Fetal hematopoietic stem cells (HSC) are functionally distinct from adult HSCs with regards to the two cardinal features of stemness: self-renewal and multi-lineage potential. Fetal HSCs are less quiescent, more metabolically active, and have a higher engraftment potential than adult HSCs, with their lineage output skewed towards erythropoiesis and away from myelopoiesis.

In preliminary work, we have observed that the transition of fetal HSCs to adult levels of quiescence occurs rapidly in the first week of post-natal life and is uncoupled from the acquisition of adult engraftment levels that does not switch until between the 3rd and 4th weeks of life, suggesting that maturation of distinct HSC functions occurs discontinuously. Unanswered questions include when during the neonatal and juvenile period do each of the functions of HSCs mature from fetal-like to adult-like, how is this maturation regulated, and what are the consequences of dysregulation of this maturation (such as might be experienced by critically ill neonates in the NICU).

The post-natal period is one of dynamic change as the neonate adapts to an environment that is far more microbially and metabolically complex than it experienced in utero. Several lines of evidence point to microbiome-derived signals as drivers of immune system maturation.

In both neonatal and adult mice, depletion of the gut microbiota results in fewer myeloid cells and increased susceptibility to bacterial infection, mirroring the immune landscape of the fetus and neonate. Appropriately timed microbial colonization during a window of development elicits a “weaning reaction” that functions as a rheostat to program immune responses later in life, driven by dietary metabolites regulated by the intestinal microbiome. Adult mice colonized with a neonatal-like gut microbiome have an immature mucosal immune system and increased susceptibility to Salmonella infection.

Based on these observations, we hypothesize that post-natal HSC maturation is driven by the inter-related microbial and metabolic transitions that occur during the neonatal developmental window, and that delaying or accelerating the kinetics of these transitions will delay or accelerate the maturation of HSCs.

To answer these questions, we employ antibiotic depletion strategies, germ-free mice and early introduction of the microbiome and/or metabolites by gavage, paired with extensive immunophenotyping of the fetal HSPC compartment using multi-parameter flow cytometry and single cell RNA sequencing, and in vitro and in vivo functional assays of HSPC function.

These studies will inform our understanding of the impact of important clinical interventions (i.e., antibiotic therapy, neonatal nutrition) on hematopoietic function in the perinatal period and beyond, with implications for the parameters that govern current clinical practice in the NICU.