Relationship between maternal and fetal immune responses The fetal immune system starts to develop in the third week of gestation and progresses until birth. During this time, it is greatly influenced by maternal events. HIV-exposed uninfected infants (HEU) provide an excellent example in which neonates at birth have multiple immunologic differences compared with HIV-unexposed uninfected infants (HUU) that persist for months to years and, despite the absence of HIV infection, result in an increased risk of severe infection, hospitalization and death. Other maternal medical conditions can also affect the fetal immune system, such as chronic hepatitis B infection, type 1 diabetes and obesity. The mechanism whereby the maternal and fetal immune systems communicate is not known. We hypothesized that the fetal immune system receives instructions from the mother through small molecules or nanoparticles resulting in common immunologic biases between maternal-fetal dyads. In this study, we will investigate maternal metabolites, cytokines/chemokines, placental hormones and extracellular vesicles (EV) as potential mediators of the cross talk between maternal and fetal immune systems. To test this hypothesis, we will leverage paired CBMC and maternal PBMC from women with and without HIV collected in a previous NIAID- funded study. In the current study, we formulated the following Specific Aims: Aim 1. To determine the breadth of the immunologic characteristics shared by mothers with and without HIV and their fetuses. Using high-dimensional flow cytometry, Cite-seq and ATAC-seq we will determine the phenotypic, transcriptomic and epigenetic profiles of maternal and fetal T cells and antigen presenting cells and identify characteristics shared by mother-fetus dyads both unique and common to HEU and HUU dyads. Aim 2. To identify metabolites, maternal hormones, cytokines/chemokines and/or EV shared by maternal-fetal dyads that distinguish HEU from HUU dyads. We will characterize maternal and fetal metabolomic, proteomic, hormonal and EV plasma profiles to identify the factors that are shared between maternal-fetal dyads. We will then identify those that are specific to HEU compared with HUU dyads. Aim 3. To identify mediators of the communication between the maternal and fetal immune systems. Using a transwell system to simulate the placental barrier, we will examine functional and phenotypic changes in the Treg CBMC induced by maternal factors. Impact. We will study maternal-fetal information transfer that programs the HUMAN fetal immune system. This is a critical step in devising interventions to mitigate fetal immune dysfunctions. By leveraging resources from a prior NIAID-funded study, we will be able to use multi-omics to maximize the depth of these pioneering studies.