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Pregnancy represents a period when the mother undergoes significant immunological changes to promote tolerance of the fetal semi‐allograft. Such tolerance results from the exposure of the maternal immune system to fetal antigens (Ags), a process that has been widely investigated at the maternal‐fetal interface and in the adjacent draining lymph nodes. However, the peripheral mechanisms of maternal‐fetal crosstalk are poorly understood. Herein, we hypothesized that specific innate immune cells interact with fetal Ags in the maternal circulation. To test this hypothesis, a mouse model was utilized in which transgenic male mice expressing the chicken ovalbumin (OVA) Ag under the beta‐actin promoter were allogeneically mated with wild‐type females to allow for tracking of the fetal Ag. Fetal Ag‐carrying Ly6G+ and F4/80+ cells were identified in the maternal circulation, where they were more abundant in the second half of pregnancy. Such innate immune cells displayed unique phenotypes: while Ly6G+ cells expressed high levels of MHC‐II and CD80 together with low levels of pro‐inflammatory cytokines, F4/80+ cells up‐regulated the expression of CD86 as well as the anti‐inflammatory cytokines IL‐10 and TGF‐β. In vitro studies using allogeneic GFP+ placental particles revealed that maternal peripheral Ly6G+ and F4/80+ cells phagocytose fetal Ags in mid and late murine pregnancy. Importantly, cytotrophoblast‐derived particles were also engulfed in vitro by CD15+ and CD14+ cells from women in the second and third trimester, providing translational evidence that this process also occurs in humans. Collectively, this study demonstrates novel interactions between specific maternal circulating innate immune cells and fetal Ags, thereby shedding light on the systemic mechanisms of maternal‐fetal crosstalk. [ABSTRACT FROM AUTHOR]

Preeclampsia, defined as new‐onset hypertension accompanied by proteinuria occurring at 20 weeks of gestation or later, is a leading cause of perinatal morbidity and mortality worldwide. The pathophysiology of this major multi‐systemic syndrome includes defective deep placentation, oxidative stress, endothelial dysfunction, the presence of an anti‐angiogenic state, and intravascular inflammation, among others. In this review, we provide a comprehensive overview of the cellular immune responses involved in the pathogenesis of preeclampsia. Specifically, we summarize the role of innate and adaptive immune cells in the maternal circulation, reproductive tissues, and at the maternal‐fetal interface of women affected by this pregnancy complication. The major cellular subsets involved in the pathogenesis of preeclampsia are regulatory T cells, effector T cells, NK cells, monocytes, macrophages, and neutrophils. We also summarize the literature on those immune cells that have been less characterized in this clinical condition, such as γδ T cells, invariant natural killer T cells, dendritic cells, mast cells, and B cells. Moreover, we discuss in vivo studies utilizing a variety of animal models of preeclampsia to further support the role of immune cells in this disease. Finally, we highlight the existing gaps in knowledge of the immunobiology of preeclampsia that require further investigation. The goal of this review is to promote translational research leading to clinically relevant strategies that can improve adverse perinatal outcomes resulting from the obstetrical syndrome of preeclampsia. [ABSTRACT FROM AUTHOR]