Pregnancy is a unique physiological condition in which two subjects coexist within one body. The placenta and fetal membranes serve as a physical barrier between the maternal decidua and fetal compartments (Arora et al., 2017, Menon et al., 2019, Romero and Mazor, 1988). This creates two distinct feto-maternal interfaces, between the placenta and decidua basalis (placental interface) and between the fetal membranes and decidua parietalis (choriodecidual interface) (Richardson et al., 2022). These interfaces assist in tolerance of fetal growth, as well as transport of nutrients and immune cells between the two (Kammala et al., 2022). These interfaces also play a role in initiation of parturition (labor and delivery). Parturition is an inflammatory process, and this inflammation is amplified by immune cells found at the interfaces (Nancy and Erlebacher, 2014, Xu et al., 2020, Ander et al., 2019, Lash, 2015, Koga and Mor, 2010, Golos, 2004, Bonney et al., 2016). These inflammatory signals can transition reproductive tissues, such as the myometrium and cervix, from quiescence to activated state leading to labor (Hadley et al., 2018, Menon and Shahin, 2021, Shahin et al., 2021). The placental interface is well studied, but the fetal membrane-decidual interface is not as well understood (Menon and Moore, 2020). This study will focus on improving knowledge of immune signals at the choriodecidual interface.

During pregnancy, the fetus will carry both maternal and paternal antigens, and is therefore seen as a semi-allograft to the maternal system (Zha et al., 2022), so there must be a mechanism of immune tolerance to allow maintenance of pregnancy (Billington, 2003). Immune cell balance at both feto-maternal interfaces allows tolerance of the semi-allogenic fetus (Veenstra van Nieuwenhoven et al., 2003) and helps to maintain pregnancy at every stage of gestation. However, the role of immune cells in physiologic and pathologic pregnancy conditions is still not completely understood (PrabhuDas et al., 2015). In animals, immune cells have been shown to be recruited and activated in reproductive tissues as early as exposure to seminal plasma (Gamliel et al., 2018). The decidua grants the embryo immune privilege to allow implantation to occur, but a pro-inflammatory environment is found at early trophoblast invasion (Kraus et al., 2010). The second and third trimesters have a balanced inflammatory environment to maintain pregnancy and allow fetal growth (Förger and Villiger, 2020, Kraus et al., 2012, Graham et al., 2017, Gomez-Lopez et al., 2018). At the time of parturition, inflammation and endocrine signals drive the labor cascade (Cornish et al., 2020, Vannuccini et al., 2016, Menon and Taylor, 2019).

It is known that the decidua contains resident leukocyte populations, largely made up of macrophages and T cells (Farine et al., 2018, Gomez-Lopez et al., 2014, Yang et al., 2019). In both human and animal studies, pro-inflammatory cytokines are upregulated in decidua during labor (Shynlova et al., 2013, Osman et al., 2003). These cytokines can regulate activation and migration of decidual immune cells (Farine et al., 2017). Homeostatic imbalance is an important driver of parturition (Menon, 2022). Imbalance is often seen in subtypes of immune cells. For example, macrophages are known to switch between pro-inflammatory M1 phenotype and anti-inflammatory M2 phenotype (Svensson-Arvelund and Ernerudh, 2015, Nagamatsu and Schust, 2010, Bulmer et al., 1988, Bonney and Johnson, 2019). T cells are also subdivided into many groups with various behaviors and inflammatory functions. Dysregulation of these groups is often found within pregnancy complications (Sykes et al., 2012, Saito et al., 2010, Zhang et al., 2019), and these immune cells are a popular target for research and therapeutics for adverse pregnancy outcomes, especially preterm birth. There has been a myriad of studies identifying immune cells and their differentiation during labor, but there are challenges in combining these data to formulate their functional contributions due to heterogeneity across studies. For example, it has been reported that local inflammation at the feto-maternal interface and reproductive tissues is more relevant in these complications than peripheral immune cells (Wei et al., 2010), but studies are still split across a variety of intrauterine compartments; such as: amniochorion, choriodecidua, decidua parietalis, decidua basalis, umbilical cord, placental tissue, placental blood, and amniotic fluid. In addition, there is more than one method used to isolate immune cells from decidual tissue, which can result in different immune cell populations being enriched. Also, as flow cytometry technology and our knowledge of immune markers becomes more advanced, the ways that immune cell populations are identified has changed over time. For example, older studies may rely on just CD14 found by immunohistochemistry (IHC) to identify macrophages, although we know now that CD14 can be expressed on multiple myeloid cell types, while newer studies will use multiple markers in flow cytometry to identify macrophages and their subtypes.

The goal of this study is to review the available literature on immune cells at the decidual and placental interfaces to identify knowledge gaps and heterogeneities that hinder the synthesis of conclusive evidence. We utilize our own recently developed approaches to help fill this knowledge gap. This study identified differences in cell types between TL and TNL in subtypes that have not previously been measured. Here, consistent sampling is used with the same methods, antibodies, and analyses. A comprehensive, high dimensional immune cell panel was used to create a big picture of immune cell profiles without heterogeneity between the cells. This will facilitate elucidation of signaling cascades in labor that may play a role in preterm birth, as it is known to involve pathways of inflammation and oxidative stress (Menon, 2014, Menon, 2019). This was also done to allow creation of accurate models of immune cells in pregnancy for additional research.