PROJECT SUMMARY (of the Parent Award) Spontaneous pregnancy loss is the most common early obstetric complication, affecting 1 in 4 women worldwide. While ~50% of cases are due to fetal chromosomal abnormalities, the causes for the other ~50% remain unclear, although there is increasing evidence that a dysfunctional maternal endometrium may play a role. Bacterial and viral infections have been associated with ~15% of early pregnancy losses, as well as later gestational complications such as preterm birth and preeclampsia. However, how the maternal endometrium intrinsically responds to infection remains unclear, highlighting a major gap in knowledge. Endometrial stromal cells (EnSCs) are the major cellular component of the endometrium and they undergo a specialized differentiation process, termed decidualization, monthly in anticipation of pregnancy. If pregnancy ensues, EnSCs play key roles in regulating blastocyst implantation and placental trophoblast invasion at the maternal-fetal interface. Thus, appropriate EnSC function is crucial to the establishment of a healthy pregnancy. EnSCs are likely to affect trophoblast function through a combination of soluble factor secretion and production of extracellular vesicles (EVs) which mediate intercellular communication through their protein, nucleic acid, and lipid cargos. This research proposes to take a focused mechanistic approach to investigate how a viral infection affects EnSC decidualization, EV production and interactions with trophoblasts using viral dsRNA [Poly(I:C)] as a model. In preliminary studies, we found that Poly(I:C) reduced EnSC decidualization whilst increasing senescence and EV production. Poly(I:C)-exposed EnSCs also had reduced capacity to chemoattract trophoblasts. Based on this, our central hypothesis is that viral infection prevents adequate EnSC decidualization and accelerate senescence, leading to increased EV production by EnSCs. Together, this results in altered EnSC interactions with trophoblasts, impairing implantation and placentation. Our specific aims are to: examine the mechanisms by which Poly(I:C) affects EnSC decidualization and senescence (Aim 1); determine the effects of early Poly(I:C) exposure on pregnancy progression and outcome in vivo (Aim 2); and determine how Poly(I:C) affect EnSC EV production and subsequent effects on trophoblasts (Aim 3). This project enables the candidate to combine her backgrounds in reproductive immunology, early placental development and endometrial biology with her interests in EVs as a novel mode of intercellular communication at the maternal-fetal interface. These studies are significant because they will provide novel mechanistic insights into how infection impacts EnSC decidualization and their subsequent interactions with placental trophoblasts, establishing a strong link between infection, endometrial dysfunction, implantation failure and the myriad of obstetric complications associated with impaired placentation. Thus, suc...