Decidual-derived RANKL facilitates macrophages accumulation and residence at the maternal-fetal interface in human early pregnancy

A Hypoxia-Decidual Macrophage Regulatory Axis in Normal Pregnancy and Spontaneous Miscarriage

The significance of hypoxia at the maternal-fetal interface is proven to be self-explanatory in the context of pregnancy. During the first trimester, low oxygen conditions play a crucial role in processes such as angiogenesis, trophoblast invasion and differentiation, and immune regulation. Recently, there has been increasing research on decidual macrophages, which contribute to the maintenance of immune tolerance, placental and fetal vascular development, and spiral artery remodeling, to investigate the effects of hypoxia on their biological behaviors. On these grounds, this review describes the dynamic changes in oxygen levels at the maternal-fetal interface throughout gestation, summarizing current knowledge on how the hypoxic environment sustains a successful pregnancy by regulating retention, differentiation and efferocytosis of decidual macrophages. Additionally, we explore the relationship between spontaneous miscarriages and an abnormal hypoxia-macrophage axis, shedding light on the underlying mechanisms. However, further studies are essential to elucidate these pathways in greater detail and to develop targeted interventions that could improve pregnancy outcomes.

Neu5Gc regulates decidual macrophages leading to abnormal embryo implantation

Repeated implantation failure (RIF) is one of the most prominent problems in the field of assisted reproduction. Neu5Gc on the surface of decidual macrophages (dMΦ) leads to different activation patterns of dMΦ, which affects embryo implantation and development. Cmah-/- (Neu5Gc-deficient) mice induced to produce anti-Neu5Gc antibodies in vivo were given a special diet rich in Neu5Gc and their fertility was monitored. The long-term diet rich in Neu5Gc induced the decrease of endometrial receptivity of female mice. The pregnancy rate of female mice fed the normal diet was 63.6% (n = 11) and the average number of embryos was 9.571 ± 1.272, while the pregnancy rate of female mice fed the diet rich in Neu5Gc was 36.4% (n = 11) and the average number of embryos in pregnant mice was 5.750 ± 3.304. The intake of Neu5Gc and the production of anti-Neu5Gc antibody led to M1 polarization of endometrial dMΦ and abnormal embryo implantation.

Macrophage plasticity and function in cancer and pregnancy

As the soil of life, the composition and shaping process of the immune microenvironment of the uterus is worth exploring. Macrophages, indispensable constituents of the innate immune system, are essential mediators of inflammation and tissue remodeling as well. Recent insights into the heterogeneity of macrophage subpopulations have renewed interest in their functional diversity in both physiological and pathological settings. Macrophages display remarkable plasticity and switch from one phenotype to another. Intrinsic plasticity enables tissue macrophages to perform a variety of functions in response to changing tissue contexts, such as cancer and pregnancy. The remarkable diversity and plasticity make macrophages particularly intriguing cells given their dichotomous role in either attacking or protecting tumors and semi-allogeneic fetuses, which of both are characterized functionally by immunomodulation and neovascularization. Here, we reviewed and compared novel perspectives on macrophage biology of these two settings, including origin, phenotype, differentiation, and essential roles in corresponding microenvironments, as informed by recent studies on the heterogeneity of macrophage identity and function, as well as their mechanisms that might offer opportunities for new therapeutic strategies on malignancy and pregnancy complications.

Immune Cell Functionality during Decidualization and Potential Clinical Application

Due to a vast influx in the secretory phase of the menstrual cycle, leukocytes represent 40-50% of the decidua at the time of implantation. Their importance for the implantation, maintenance of pregnancy, and parturition are known yet not fully understood. Thus, in idiopathic infertility, decidual immune-related factors are speculated to be the cause. In this review, the immune cell functions in the decidua were summarized, and clinical diagnostics, as well as interventions, were discussed. There is a rising number of commercially available diagnostic tools. However, the intervention options are still limited and/or poorly studied. In order for us to make big steps towards the proper use of reproductive immunology findings, we need to understand the mechanisms and especially support translational research.

Dysfunction of CCR1+ decidual macrophages is a potential risk factor in the occurrence of unexplained recurrent pregnancy loss

Recurrent pregnancy loss (RPL) puzzles 1-3% of women of childbearing age worldwide. Immunological factors account for more than 60% of cases of unexplained RPL (URPL); however, the underlying mechanism remains unclear. Here, using single-cell sequencing data and functional experiments with clinical samples, we identified a distinct population of CCR1⁺ decidual macrophages (dMφ) that were preferentially enriched in the decidua from normal early pregnancies but were substantially decreased in patients with URPL. Specific gene signatures endowed CCR1⁺ dMφ with immunosuppressive and migration-regulatory properties, which were attenuated in URPL. Additionally, CCR1⁺ dMφ promoted epithelial-to-mesenchymal transition (EMT) to promote trophoblast migration and invasion by activating the ERK1/2 signaling pathway. Decidual stromal cell (DSC)-derived CCL8 was the key regulator of CCR1⁺ dMφ as CCL8 recruited peripheral CCR1⁺ monocytes, induced a CCR1⁺ dMφ-like phenotype, and reinforced the CCR1⁺ dMφ-exerted modulation of trophoblasts. In patients with URPL, CCL8 expression in DSCs was decreased and trophoblast EMT was defective. Our findings revealed that CCR1⁺ dMφ play an important role in immune tolerance and trophoblast functions at the maternal-fetal interface. Additionally, decreased quantity and dysregulated function of CCR1⁺ dMφ result in URPL. In conclusion, we provide insights into the crosstalk between CCR1⁺ dMφ, trophoblasts, and DSCs at the maternal-fetal interface and macrophage-targeted interventions of URPL.

The peripheral and decidual immune cell profiles in women with recurrent pregnancy loss

Recurrent pregnancy loss (RPL) affects 1-2% of couples of reproductive age. Immunological analysis of the immune status in RPL patients might contribute to the diagnosis and treatment of RPL. However, the exact immune cell composition in RPL patients is still unclear. Here, we used flow cytometry to investigate the immune cell profiles of peripheral blood and decidual tissue of women who experienced RPL. We divided peripheral immune cells into 14 major subgroups, and the percentages of T, natural killer T (NKT)-like and B cells in peripheral blood were increased in RPL patients. The decidual immune cells were classified into 14 major subpopulations and the percentages of decidual T, NKT-like cells and CD11c^hi Mφ were increased, while those of CD56^hi decidual NK cells and CD11c^lo Mφ were decreased in RPL patients. The spearmen correlation analysis showed that the proportion of peripheral and decidual immune cells did not show significant correlations with occurrences of previous miscarriages. By using flow cytometry, we depicted the global peripheral and decidual immune landscape in RPL patients. The abnormalities of peripheral and decidual immune cells may be involved in RPL, but the correlations with the number of previous miscarriages need further verification.

Trophoblast-derived interleukin 9 mediates immune cell conversion and contributes to maternal-fetal tolerance

In the maternal-fetal crosstalk, fetal derived trophoblast cells can secret several molecules to regulate immune tolerance such as cytokines and chemokines, besides human leukocyte antigens (HLA) providing. However, the mechanism of these factors in pregnancy is still unknown. Our previous study showed that IL9 could be secreted by trophoblasts and exerted a positive effect on trophoblasts themselves through autocrine signaling. Given the immunoregulatory function of IL9 and its expression in trophoblasts, we hypothesize that IL9 contributes to maternal-fetal tolerance by regulating immune cells, especially CD14+ dendritic cells (DCs) and naïve CD4 + T cells who have essential roles in maternal-fetal immune tolerance. We performed a series of experiments, finding that HTR8/SVneo cells could secrete IL9 in vitro, and this secretion was decreased under hypoxia; both CD14 + DCs and naïve CD4 + T cells expressed IL9 receptors, indicating potential interactions among these cells. In CD14 + DCs, trophoblast-derived IL9 promoted the immature differentiation, and induced the secretion of Th2 cytokines, including IL4 and IL10, shifting the Th1/Th2 ratio to Th2. In naïve CD4 + T cells, IL9 also increased Foxp3 expression and promoted the secretion of Treg cytokines, including TGFβ and IL10, inhibiting pro-inflammatory Th17. Therefore, trophoblasts may act as fetal-derived immune cells to maintain maternal-fetal tolerance by secreting IL9. Given that trophoblast derived IL9 is decreased in preeclampsia, our study provides a new insight into maternal-fetal immunology and immunological disorders in abnormal pregnancy.