PROJECT SUMMARY Approximately 70% of preterm births are preceded by preterm labor, which is primarily associated with non- infectious etiologies. However, little is known about the non-acute-inflammatory causes of preterm birth. This project builds on extensive published and preliminary data indicating that T cells and the cytokine IL-9 at the maternal-fetal interface play an important role in preterm labor and birth. The overall goals of this project are to define the mechanisms by which the cytokine IL-9 at the maternal-fetal interface promotes preterm birth and negative consequences for offspring, to determine the mechanisms by which vitamin D can prevent these outcomes, and to identify new T cell subtypes that contribute to preterm labor in humans. Aim 1 is to elucidate the mechanisms by which IL-9 promotes preterm labor and birth. This aim tests the hypothesis that IL-9 targets both hematopoietic and non-hematopoietic cells at the maternal-fetal interface in mice. Aim 1 will also explore whether IL-9 signaling is required within the maternal or fetal tissues to promote preterm birth. Additionally, the mechanisms whereby vitamin D prevents IL-9-induced preterm birth in mice will be determined. Aim 2 is to define the impact on offspring of IL-9 at the maternal-fetal interface. This aim tests the hypothesis that maternal IL-9 that enters the amniotic cavity causes adverse fetal and neonatal outcomes. Specifically, this aim will examine the fetal lung, intestine, and placenta and neonatal airway and gut immunity. Additionally, the extent to which prenatal vitamin D supplementation can mitigate the impact of in utero IL-9 exposure will be investigated. A hypothesis-generating Aim 3 will identify T-cell subsets at the maternal-fetal interface in women with preterm labor and birth. This project will make use of an advanced mouse model of IL-9-induced preterm birth, imaging mass cytometry, spatial transcriptomics, single-cell RNA sequencing of human placenta samples from term and preterm pregnancies, and functional testing on T cells isolated from patients with term or preterm birth. The innovative fusion of immunology and omics approaches used here will unveil non-infectious causes of preterm labor and birth and may lead to development of a treatment – such as vitamin D, which is FDA-approved during pregnancy – to reduce the risk of preterm birth. Additionally, the new immune targets identified could lead to development of novel strategies to reduce preterm birth and its sequelae. This project is directly relevant to the call for research “exploring the contribution of immune dysregulation to gestational disorders and adverse pregnancy outcomes” to improve reproductive health.