PROJECT SUMMARY/ABSTRACT Maternal infections during pregnancy lead to adverse outcomes for the offspring, including short-term outcomes such as intrauterine fetal demise, preterm delivery and neonatal sepsis, and long-term sequelae such as neurodevelopmental impairment. A key factor contributing to fetal/neonatal susceptibility to infection is the failure to robustly produce innate immune cells such as neutrophils that form a first line of defense against infection. We recently made the novel observation that fetal hematopoietic stem and progenitor cells (HSPCs) are extrinsically restricted from activating the pathways that would lead to rapid production of neutrophils, termed emergency myelopoiesis (EM), by maternal interleukin-10 (IL-10), although how maternal IL-10 restricts fetal EM is unknown. Altered placental function is proposed to be the critical central mediator linking the effects of maternal inflammation on adverse outcomes in the offspring. In particular, maternal inflammation increases placental synthesis of serotonin that is released into the fetal circulation, reaching the fetal brain and disrupting the growth of serotonergic axons. We have found that fetal HSPCs express serotonin receptors and their proliferation is suppressed in vitro by serotonin, supporting the hypothesis that placental serotonin may also regulate fetal hematopoiesis. This proposal will test the hypothesis that maternal IL-10 restricts fetal emergency myelopoiesis via a multi-layered cascade of events involving a network of maternal decidual immune cells that trigger placental serotonin synthesis which is then released into the fetal circulation and reaches the fetal liver where it regulates fetal HSPCs in either a direct manner or indirectly by regulating the stromal and mature hematopoietic cells of the fetal liver niche. Our approach will be to 1) identify the IL-10-responsive cell in the maternal-fetal milieu and determine the function of that cell that results in restriction of fetal emergency myelopoiesis, and 2) determine how maternal inflammation is sensed in the fetal liver microenvironment and whether this signal, which we predict to be serotonin, regulates fetal HSPC function directly or indirectly. We predict that maternal IL-10 restricts fetal EM via its baseline function of establishing a tolerogenic profile in maternal decidual macrophages and that in its absence, the exaggerated inflammatory response of alternatively-primed macrophages negatively impacts placental function resulting in increased synthesis of serotonin that regulates fetal HSPCs directly and indirectly. This project will systematically identify the mechanisms by which maternal inflammation is translated across the placenta and sensed in the fetal liver to modulate fetal HSPC emergency myelopoiesis, uncovering potential nodes along the cascade that might be amenable to therapeutic targeting, and ultimately boosting neonatal immune function and improving neonatal outcomes.