Cytokines govern key cellular processes of pregnancy with both pro- and anti-inflammatory activities. To prevent immune dysregulation at the maternal-fetal interface, cytokines are thought to dictate these seemingly contradictory immune responses through meticulous immune regulation. Yet, there is a gap in knowledge regarding the mechanistic roles of many cytokines in the context of pregnancy, including the versatile cytokine Interleukin 27 (IL-27). Our proposal will address this gap in knowledge, as we have recently established that the IL-27 signaling circuit is active at the maternal-fetal interface and discovered that IL-27 is a protective cytokine during congenital infection. IL-27 serves as a potent regulatory of inflammation, where it can be pro- or anti- inflammatory dependent upon cellular context. Although initially recognized for its proinflammatory activities in promoting immunity, IL-27 was later acknowledged for its profound ability to serve as an anti-inflammatory cytokine during various infections where IL-27 has been found to act on various T subsets cells to limit inflammation during infection. Moreover, IL-27 can directly influence viral infection through induction of antiviral genes in IL-27 responsive cells. Based on literature and our extensive preliminary data, we hypothesize that IL- 27 is both an immunoregulatory and antiviral cytokine during pregnancy. This proposal describes targeted objectives to define the mechanistic features of IL-27 signaling at the maternal-fetal interface that underlie its protective capacity during congenital infection. In Aim 1 we will focus our investigations on defining the immune regulatory functions of IL-27 at the maternal-fetal interface. We will uncover the cellular targets and impacts of IL-27 signaling in dictating activation state of localized T cells. In Aim 2 we will focus our studies on the antiviral functions of IL-27 in the placenta. We will evaluate the transcriptional dynamics of IL-27 signaling and antiviral capacity of IL-27 signaling in mouse and human placental trophoblast organoids. Altogether, outcomes from this proposal will embody new fundamental insights into regulatory cytokine communication at the maternal-fetal interface. Specifically, this proposal will define the mechanisms underlying IL-27 protective immunity during congenital infection, which could ultimately be leveraged to improve maternal and neonatal outcomes.