Maternal Monocytes Respond to Cell-Free Fetal DNA and Initiate Key Processes of Human Parturition

Abstract

Throughout gestation, the maternal immune system is tightly modulated to allow growth of a semiallogeneic fetus. During the third trimester, the maternal immune system shifts to a proinflammatory phenotype in preparation for labor. What induces this shift remains unclear. Cell-free fetal DNA (cffDNA) is shed by the placenta and enters maternal circulation throughout pregnancy. Levels of cffDNA are increased as gestation progresses and peak before labor, coinciding with a shift to proinflammatory maternal immunity. Furthermore, cffDNA is abnormally elevated in plasma from women with complications of pregnancy, including preterm labor. Given the changes in maternal immunity at the end of pregnancy and the role of sterile inflammation in the pathophysiology of spontaneous preterm birth, we hypothesized that cffDNA can act as a damage-associated molecular pattern inducing an inflammatory cytokine response that promotes hallmarks of parturition. To test this hypothesis, we stimulated human maternal leukocytes with cffDNA from primary term cytotrophoblasts or maternal plasma and observed significant IL-1β and CXCL10 secretion, which coincides with phosphorylation of IFN regulatory factor 3 and caspase-1 cleavage. We then show that human maternal monocytes are crucial for the immune response to cffDNA and can activate bystander T cells to secrete proinflammatory IFN-γ and granzyme B. Lastly, we find that the monocyte response to cffDNA leads to vascular endothelium activation, induction of myometrial contractility, and PGE2 release in vitro. Our results suggest that the immune response to cffDNA can promote key features of the parturition cascade, which has physiologic consequences relevant to the timing of labor.

Fetal DNA Causes Sex-Specific Inflammation From Human Fetal Membranes

“…It is known that cffDNA can stimulate inflammation in several tissues and cell types ( Goldfarb et al, 2018 ; Kazemi et al, 2021 ; Yeganeh Kazemi et al, 2021 ) but its effects have not yet been studied on the human FM. In addition to this, it is also known that the sex of the fetus can influence both the magnitude and the resultant cytokine profile of the inflammatory response, depending on the specific circumstances ( Burns et al, 2015 ; Mitchell et al, 2017 ).…”

“…Increased circulating levels of cffDNA have also been associated with adverse pregnancy outcomes and pathologies such as; preeclampsia ( Martin et al, 2014 ; Kwak et al, 2020 ), gestational diabetes ( Hopkins et al, 2020 ), and preterm birth ( Gomez-Lopez et al, 2020 ). cffDNA from the trophoblast is also known to increase by sterile inflammation, via HMGB1 ( Yaganeh Kazemi et al, 2021 ), but its ability to generate inflammation itself is contentious. Some studies have clearly shown that it is able to interact with toll-like receptor 9 (TLR9) ( Goldfarb et al, 2018 ) and that immune cells respond to it by producing inflammatory cytokines ( Yeganeh Kazemi et al, 2021 ).…”

“…cffDNA from the trophoblast is also known to increase by sterile inflammation, via HMGB1 ( Yaganeh Kazemi et al, 2021 ), but its ability to generate inflammation itself is contentious. Some studies have clearly shown that it is able to interact with toll-like receptor 9 (TLR9) ( Goldfarb et al, 2018 ) and that immune cells respond to it by producing inflammatory cytokines ( Yeganeh Kazemi et al, 2021 ). However, others have not been able to confirm this ( van Boeckel et al, 2020 ).…”

Human placental exosomes induce maternal systemic immune tolerance by reprogramming circulating monocytes

“…The exact mechanisms by which pregnancy-induced monocyte functional changes are unknown. It has been suggested that the factors released by trophoblasts such as cytokines [ 8 ], fetal DNAs [ 9 ], or hormones contribute to the changes in monocytes [ 6 ]. Extracellular vesicles are a newly found mode of intercellular communications.…”

“…Successful pregnancy is associated with the functional regulation of monocytes [ 9 , 11 ]. We hypothesized that the pEXO facilitates the establishment of maternal immune tolerance via regulating monocyte phenotype in early pregnancy.…”