PROJECT SUMMARY Infections, allergy or asthma during the first two trimesters of pregnancy can increase miscarriage, preterm birth, and the offspring’s risk of poor developmental outcomes. These gestational risks are largely mediated by the placenta, a critical interface comprised of maternal and fetal tissues and mediator of immune responses between mother and fetus. Infections and other immune events during pregnancy contribute to risk but very little is known about placental immune cell profiles and their precise responses to maternal immune perturbations. Without this knowledge, developing therapies to reduce the broad spectrum of gestational risks and impacts on offspring will be greatly inhibited. In mice, early gestational immune challenges also negatively impact fetal health and development, allowing us to use the maternal immune activation mouse model. Pregnant mice given an immune challenge that mimics a bacterial infection before embryonic day 13.5 (E13.5) experience pronounced fetal loss and placental damage, but become resistant after E13.5. In this model, the immune challenge leads to placental hemorrhage. Preliminary data have implicated several immune cell types and these observations led to the central hypothesis that maternal immune activation in early pregnancy causes placental cytokine release and immune cell recruitment, resulting in placental damage. The project involves two specific aims: 1) Determining if differential recruitment of maternal immune cells to the placenta mediates injury, and 2) Determining how the maternal immune response varies across gestation. Under the first aim, a mass cytometry panel developed by the Palmer lab will be used to study the immune cell subsets and immune function in the placenta at baseline and following immune challenge at vulnerable (E12.5) and resistant (E14.5) times. Immune cell recruitment to the placenta will be inhibited using chemokine receptor knockout mice to determine whether immune cell infiltration is necessary for placental injury. Under the second aim, immune cell subsets and immune function will be measured in maternal blood, thymus, lymph nodes, and uterine horn at baseline and following immune challenge at vulnerable and resistant times. The findings from this study will show how immune challenges in early pregnancy alter immune cell behavior systemically in the mother and locally in the placenta and will define the mechanisms that underlie placental and fetal vulnerability to the maternal immune response in early pregnancy and relative resilience in late pregnancy. The information obtained from this project will allow us to identify therapeutic targets that prevent placental damage and reduce pregnancy complications.