Project Summary: Over the past five years, the leading cause of death worldwide for children under five has been complications from preterm birth (PTB) (<37 weeks’ gestation). In 2016, almost ten percent of all infants born in the United States were delivered prematurely, and by 2018 the United States was reported to have the most adverse outcomes during childbirth in the developed world. Collectively, PTB is estimated to cost 14 billion dollars annually. Historically, PTB has been challenging to study as the underlying causes are poorly understood, and there is a lack of physiologically relevant animal models. Progress in understanding the drivers of PTB has been further stalled by limited access to human tissue for research purposes, as specimens from preterm deliveries usually require pathological evaluation. Though elusive in mechanism, the placenta is an ideal target for uncovering triggers of PTB as placental pathologies are observed in many cases. One such pathology is the presence of inflammatory lesions. While inflammation is elevated during both healthy term and preterm labor, the literature strongly suggests that differential inflammatory pathways are active in preterm deliveries, and inflammation may overall be increased compared to term births. Our recently published work was the first to document the contribution of immune cells within the placental villi (PV) to intraamniotic inflammation in non- human primates and specifically uncovered that PV T cells are active and inflammatory in this model. In agreement with these findings, our preliminary data show that T cells with memory phenotypes are present in the PV in healthy pregnancy and can elicit inflammatory responses when stimulated with antigens from the uterine environment. Furthermore, preliminary transcriptional data from our group revealed that T cells from preterm PV transcribe more activation markers than term PV counterparts and correlatively preterm PV have greater placental destruction indicated by apoptosis of trophoblast cells. We, therefore, hypothesized that failed restriction of antigen presentation to PV T cells leads to PV T cell overactivation, increased cytokine production, and subsequent destruction of placental architecture resulting in PTB. This proposal will use multi-omic analysis coupled with in vitro functional assays on preterm and term placentas to uncover unique PV T cells: populations, functional capacity, transcriptional machinery, and downstream responses on trophoblasts in preterm birth. We propose: (1) Perform comparative functional analyses of PV T cells in preterm and term pregnancies, including anatomic localization, activation state, and TCR repertoire profiles in Aim 1. (2) Evaluate the consequences of PV T cell activation on placental trophoblast health and function in Aim 2. Our repository of cryopreserved PV samples and ongoing collection of new specimens, expertise in placental biology and mucosal immunology, and a multidisciplinary team...