Decidual regulatory T cells in placental pathology and pregnancy complications

Overexpression of regulatory T cells in patients with unexplained recurrent pregnancy loss: friend or foe?

Background

This study aimed to investigate the role of regulatory T cells in patients with unexplained recurrent pregnancy loss (URPL).

Methods

We retrospectively analyzed 136 women who had experienced two or more miscarriages before 24 weeks of gestation for no obvious reason from May 2018 to October 2021. The basic clinical data of the patients and expression of lymphocyte subsets such as regulatory T cells (Tregs) and natural killer cells (NKs) by flow cytometry were collected to explore the risk factors of pregnancy outcome in URPL patients.

Results

A total of 136 URPL patients were enrolled in this study. Eventually, 50 patients attained clinical pregnancy. The median age was 31.8 ± 4.6 years in patients with clinical pregnancy. The univariate and multivariate logistic regression analyses indicated that Tregs was associated with the pregnancy outcomes of patients with URPL (odds ratio 0.63, 95% confidence interval 0.50-0.80). More importantly, a U-shaped association was found between Tregs and pregnancy outcome (p < 0.001), with either higher or lower Tregs levels adversely affecting pregnancy outcome.

Conclusion

Tregs levels that are either too high or too low can harm pregnancy outcomes. It was expected to be a very promising quantitative biomarker for predicting pregnancy outcomes in URPL patients.

Memory regulatory T cells in pregnancy

Pregnancy requires the process of maternal immune tolerance to semi-allogeneic embryos. In contrast, an overreactive maternal immune system to embryo-specific antigens is likely to result in the rejection of embryos while damaging the invading placenta, such that the likelihood of adverse pregnancy outcomes can be increased. Regulatory T cells (Tregs) are capable of suppressing excessive immune responses and regulating immune homeostasis. When stimulating Tregs, specific antigens will differentiate into memory Tregs with long-term survival and rapid and powerful immune regulatory ability. Immunomodulatory effects mediated by memory Tregs at the maternal-fetal interface take on critical significance in a successful pregnancy. The impaired function of memory Tregs shows a correlation with various pregnancy complications (e.g., preeclampsia, gestational diabetes mellitus, and recurrent pregnancy losses). However, the differentiation process and characteristics of memory Tregs, especially their role in pregnancy, remain unclear. In this study, a review is presented in terms of memory Tregs differentiation and activation, the characteristics of memory Tregs and their role in pregnancy, and the correlation between memory Tregs and pregnancy complications. Furthermore, several potential therapeutic methods are investigated to restore the function of memory Tregs in accordance with immunopathologies arising from memory Tregs abnormalities and provide novel targets for diagnosing and treating pregnancy-associated diseases.

Placental Syndromes-A New Paradigm in Perinatology

Placental syndromes include pregnancy loss, fetal growth restriction, preeclampsia, preterm delivery, premature rupture of membranes, placental abruption and intrauterine fetal demise. This paper discusses the common etiopathogenesis of those syndromes and the role of angiogenic biomarkers in their development. Pregnancy implantation, placental development and maternal adaptation are complex processes in which fetal and maternal cells interact. The syncytiotrophoblast, trophoblast, uterine natural killer cells and regulatory T cells interfere and interact in all the above-mentioned processes. The proper angioneogenesis and vasculogenesis of the placenta, as well as maternal circulatory adaptation, are dependent on angiogenic factor expression. Insufficient maternal immunotolerance, dysregulation in uterine natural killer or regulatory T cell function, syncytiotrophoblast and trophoblast ischemia and hypoxia or impaired balance in angiogenic factors are all related to the occurrence of placental syndromes. Differences in the time of impairment onset and its intensity and correlation with other dysfunctions result in the development of a specific syndrome. The clinical manifestations in the form of a combination of specific symptoms determine the diagnosis. However, they are just symptoms of an underlying complex trophoblast disorder.

Pregnancy and Tumour: The Parallels and Differences in Regulatory T Cells

Immunological tolerance plays a critical role during pregnancy as semi-allogeneic fetus must be protected from immune responses during the gestational period. Regulatory T cells (Tregs), a subpopulation of CD4+ T cells that express transcription factor Foxp3, are central to the maintenance of immunological tolerance and prevention of autoimmunity. Tregs are also known to accumulate at placenta in uterus during pregnancy, and they confer immunological tolerance at maternal-fetal interface by controlling the immune responses against alloantigens. Thus, uterine Tregs help in maintaining an environment conducive for survival of the fetus during gestation, and low frequency or dysfunction of Tregs is associated with recurrent spontaneous abortions and other pregnancy-related complications such as preeclampsia. Interestingly, there are many parallels in the development of placenta and solid tumours, and the tumour microenvironment is considered to be somewhat similar to that at maternal-fetal interface. Moreover, Tregs play a largely similar role in tumour immunity as they do at placenta- they create a tolerogenic system and suppress the immune responses against the cells within tumour and at maternal-fetal interface. In this review, we discuss the role of Tregs in supporting the proper growth of the embryo during pregnancy. We also highlight the similarities and differences between Tregs at maternal-fetal interface and tumour Tregs, in an attempt to draw a comparison between their roles in these two physiologic and pathologic states.

Fetal inflammatory response at the fetomaternal interface: A requirement for labor at term and preterm

Human parturition at term and preterm is an inflammatory process synchronously executed by both fetomaternal tissues to transition them from a quiescent state t an active state of labor to ensure delivery. The initiators of the inflammatory signaling mechanism can be both maternal and fetal. The placental (fetal)-maternal immune and endocrine mediated homeostatic imbalances and inflammation are well reported. However, the fetal inflammatory response (FIR) theories initiated by the fetal membranes (amniochorion) at the choriodecidual interface are not well established. Although immune cell migration, activation, and production of proparturition cytokines to the fetal membranes are reported, cellular level events that can generate a unique set of inflammation are not well discussed. This review discusses derangements to fetal membrane cells (physiologically and pathologically at term and preterm, respectively) in response to both endogenous and exogenous factors to generate inflammatory signals. In addition, the mechanisms of inflammatory signal propagation (fetal signaling of parturition) and how these signals cause immune imbalances at the choriodecidual interface are discussed. In addition to maternal inflammation, this review projects FIR as an additional mediator of inflammatory overload required to promote parturition.

Immunohistochemical Evaluation of CD3, CD4, CD8, and CD20 in Decidual and Trophoblastic Tissue Specimens of Patients with Recurrent Pregnancy Loss

Recurrent miscarriages affect up to 5% of couples. CD3⁺ (T-lymphocytes), CD4⁺ (helper T-lymphocytes), CD8⁺ (cytotoxic T-lymphocytes), and CD20⁺ (B-lymphocytes) cells may participate in the pathophysiology of recurrent pregnancy loss (RPL). The aim of this study was to investigate the complicity of these molecules in RPL. The experimental specimens were obtained from 20 females who underwent miscarriages in the first gestational trimester, while the control group’s specimens consisted of 20 females who proceeded with voluntary pregnancy termination during the same period. Tissue samples were taken from the decidua basalis, decidua parietalis, and trophoblast (placental chorionic villi) and were studied using immunohistochemical methods. Monoclonal antibodies were used against CD3, CD4, CD8, and CD20 cells. The lymphocyte levels in the decidua parietalis displayed profound disparities among the two groups. The decidua basalis and trophoblast exhibited almost the same disparities regarding positive CD cells. The comparison of CD4⁺ and CD8⁺ cells in the endometrial tissue revealed a significant difference between the two groups of study. The analysis uncovered a strong relationship between RPL and the presence of CD3⁺, CD4⁺, CD8⁺, and CD20⁺ cells in the decidua parietalis tissue. The number of positive T cells was decreased in the decidual basalis and chorionic villi, proving that their absence significantly disrupts the balance of the immunological environment.

Regulatory T Cells in Pregnancy Adverse Outcomes: A Systematic Review and Meta-Analysis

Background

Several studies report the role of Regulatory T-cells (Tregs) in the pathophysiology of pregnancy adverse outcomes.

Objective

The aim of this systematic review and meta-analysis was to determine whether there is an association between regulatory T cell levels and pregnancy adverse outcomes (PAOs), including pre-eclampsia and preterm birth (PTB).

Method

Literature searches were conducted in PubMed/MEDLINE, Embase, and Cochrane CENTRAL databases. Inclusion criteria were original articles (clinical trials, case-control studies and cohort studies) comparing Tregs, sampled from the decidua or maternal blood, in healthy pregnant women versus women with pre-eclampsia or PTB. The outcome was standardised mean difference (SMD) in Treg numbers. The tau-squared (Tau²), inconsistency index (I²), and chi-squared (χ²) test quantified heterogeneity among different studies. Analyses were performed in RevMan software V.5.4.0 for Mac using a random-effects model with outcome data reported with 95% confidence intervals (CI). This study was prospectively registered with PROSPERO (CRD42020205469). PRISMA guidelines were followed.

Results

From 4,085 unique studies identified, 36 were included in qualitative synthesis, and 34 were included in quantitative synthesis (meta-analysis). In total, there were 1,783 participants in these studies: healthy controls=964, pre-eclampsia=759, PTB=60. Thirty-two studies compared Tregs in healthy pregnant women and women with pre-eclampsia, and 30 of these sampled Tregs from peripheral blood showing significantly higher Treg numbers in healthy pregnancies (SMD; 1.46; 95% CI, 1.03-1.88; I²=92%). Four studies sampled Tregs from the maternal decidua showing higher Tregs in healthy pregnancies (SMD, 0.76; 95% CI, -0.13-1.65; I²=84%). No difference was found in the number of Tregs between early versus late pre-eclampsia (SMD,-1.17; 95% CI, -2.79-0.44; I²=94%). For PTB, two studies compared Tregs sampled from the peripheral blood with a tendency for higher Tregs in healthy pregnancies but this did not reach significance (SMD, 2.18; 95% CI, -1.34-5.70; I²=96%). Subcohort analysis using Treg analysis (flow cytometry vs. qPCR vs. immunofluorescence tissue staining) showed similar associations.

Conclusion

Lower Tregs in pregnancy, sampled from the maternal peripheral blood, are associated with pre-eclampsia. There is a need for further studies to confirm a relationship between low Tregs and PTB. As the precise mechanisms by which Tregs may mediate pre-eclampsia and PTB remain unclear, further fundamental research is necessary to elucidate the underlying processes and highlight the causative link.

Systematic Review Registration

PROSPERO, identifier CRD42020205469.

Cytokine profiling: variation in immune modulation with preterm birth vs. uncomplicated term birth identifies pivotal signals in pathogenesis of preterm birth

OBJECTIVES

To assess deviations in longitudinally measured cytokines with preterm birth (PTB).

METHODS

Prospective longitudinal study targeting 80 subjects. Phlebotomy specimens for broad panel of cytokine analysis were obtained at three time (T) intervals: first trimester (T1: 8-14 weeks' gestation), second trimester (T2: 18-22 weeks' gestation), and third trimester (T3: 28-32 weeks' gestation). Important demographics and outcomes were tracked. Data were stratified and the target groups were analyzed as follows: "Uncomplicated" (delivered ≥37 weeks) or "Preterm Birth" (<37 weeks). Generalized Linear Modeling determined rate of change T1-T3 by outcome.

RESULTS

Complete data replete with phlebotomy at all three visits were obtained on 80 women. Birth outcomes were as follows: 11 Uncomplicated Term Birth (UTB), 28 PTB, 4 low birth weight (LBW), 16 OB complications (OBC), 11 current infections (IFN), and 10 mixed complications (MC=2 or more of the above). 28 PTB were compared to 11 uncomplicated term deliveries. In both groups, T helper type 1 (TH1) cytokine (IL-1β), pleiotrophic pro-inflammatory cytokine (IL-6), and counter-regulatory cytokine (IL-10) responses decreased over gestation, but rates of change in IL-1β, IL-6, and IL-10 were significantly different. Stratification of women by smoking status additionally demonstrated significant variance in immune status over the course of pregnancy.

CONCLUSIONS

Women delivering PTB demonstrated significant differences in cytokine trajectory over pregnancy; these data further validate key role played by immune regulation in directing pregnancy outcome. Likewise, smoking impacts longitudinal trajectory of cytokines over pregnancy.

Pregnancy Alters Innate and Adaptive Immune Responses to Zika Virus Infection in the Reproductive Tract

Recent outbreaks of Zika virus (ZIKV) have been associated with birth defects, including microcephaly and neurologic impairment. However, the mechanisms that confer potential susceptibility to ZIKV during pregnancy remain unclear. We hypothesized that poor outcomes from ZIKV infection during pregnancy are due in part to pregnancy-induced alteration of innate immune cell frequencies and cytokine expression. To examine the impact of pregnancy on innate immune responses, we inoculated immunocompetent pregnant and nonpregnant female C57BL/6 mice with 5 × 105 focus-forming units of ZIKV intravaginally. Innate immune cell frequencies and cytokine expression were measured by flow cytometry at day 3 postinfection. Compared with nonpregnant mice, pregnant mice exhibited higher frequencies of uterine macrophages (CD68+) and CD11c+ CD103+ and CD11c+ CD11b+ dendritic cells. Additionally, ZIKV-infected pregnant mice had lower frequencies of CD45+ IL-12+ and CD11b+ IL-12+ cells in the uterus and spleen. Next, we measured the frequencies of Ag-experienced CD4 (CD4+ CD11a+ CD49d+) and CD8 (CD8lo CD11ahi) T cells at day 10 postinfection to determine the impact of pregnancy-associated changes in innate cellular IL-12 responses on the adaptive immune response. We found that pregnant mice had lower frequencies of uterine Ag-experienced CD4 T cells and ZIKV-infected pregnant mice had lower frequencies of uterine Ag-experienced CD8 T cells compared with ZIKV-infected nonpregnant mice. These data show that pregnancy results in altered innate and adaptive immune responses to ZIKV infection in the reproductive tract of mice and that pregnancy-associated immune modulation may play an important role in the severity of acute ZIKV infection.

Novel predictive and therapeutic options for better pregnancy outcome in frozen embryo transfer cycles

Since 1978, in the first decades of in vitro fertilization (IVF), the use of ovarian hyperstimulation allowed for the development and transfer of multiple embryos. As IVF technology improved, the number of multiple pregnancies increased, which led to gradual reduction in the number of embryos that were transferred. Embryo freezing (vitrification) was recommended to allow subsequent transfer if the fresh cycle was unsuccessful. However, experimentation has continued to improve pregnancy outcomes. We discuss here the significance of frozen embryo transfer cycle and the impact of uterine and peripheral immunity dominated by NK cells and regulatory T cells and human chorionic gonadotropin on pregnancy outcome in this innovative mode of IVF therapy.

Alternatively Activated Macrophages Are the Primary Retinoic Acid-Producing Cells in Human Decidua

In situ production and metabolism of all-trans retinoic acid (RA) in decidual tissue are critically important for endometrial stromal differentiation, embryo implantation, and healthy placentation. However, the cellular source(s) of RA in this tissue has yet to be determined. To identify the primary RA-producing cells in human term decidua, we isolated cells from decidua basalis of delivered placenta and quantified cellular retinal dehydrogenase (RALDH) activity, a major biosynthetic enzyme whose activity determines the synthesis of RA from retinol, using an Aldefluor assay and flow cytometry. RA production in decidual tissue and sorted cell subpopulations was evaluated by liquid chromatography-tandem mass spectrometry. CD14⁺ cells (macrophages/monocytes) showed > 4-fold higher RALDH activity than stromal cells (CD10⁺), T cells (CD3⁺), or non-T lymphocytes (CD3-negative). CD11c⁺ cells that did not co-express CD14 showed about one-third the RALDH activity of their CD14 co-expressing counterparts. The highest RALDH activity was found in "alternatively activated" M2 macrophages delineated by the simultaneous expression of CD14 and CD163. The greater RA synthesizing capacity of M2 versus CD14⁺CD163-ve (M1) cells was confirmed by direct quantitation of RA biosynthesis from retinol. RA levels in whole decidua were correlated with M2 cell density but not with stromal cell (CD10⁺) number, the major cell type comprising the decidua. These results identified M2 monocyte/macrophages as the primary source of RA in human term decidua. This finding may have implications for certain pregnancy complications that are known to be associated with reduced numbers of decidual M2 cells.

Retinoic Acid Is a Negative Regulator of sFLT1 Expression in Decidual Stromal Cells, and Its Levels Are Reduced in Preeclamptic Decidua

sFLT1 (soluble VEGF [vascular endothelial growth factor] receptor-1) levels are increased in preeclampsia-a pathological condition of pregnancy. The mechanism of sFLT1 overexpression by gestational tissues, particularly the decidua, remains unknown. Mass spectrometry measurement of the active retinoid metabolite, all-trans retinoic acid (RA), showed significantly lower levels of RA in preeclamptic versus normotensive decidua. In this study, we investigated the involvement of RA in regulating decidual sFLT1 expression. When decidual stromal cells (DSCs) isolated from the decidua basalis of normotensive and preeclampsia placentas were treated with BMS493-a pan-RAR (RA nuclear receptor) antagonist-upregulation of sFLT1 expression was observed. Conversely, treatment with RA resulted in downregulation of sFLT1 in normotensive DSCs and preeclampsia DSCs. Unlike treatment with cAMP, which induces decidualization while downregulating sFLT1, RA treatment did not alter DSC expression of prolactin-a marker of decidualization-or FOXO1 (forkhead box protein 01)-a transcription factor required for prolactin upregulation. TFAP2A (transcription factor AP-2-alpha [activating enhancer-binding protein 2 alpha]), a different transcription factor was upregulated in normotensive DSCs but not in preeclampsia DSCs after RA treatment. Collectively, our data show that RA suppresses sFLT1 expression in DSCs independently of cellular decidualization. These findings suggest that reduced decidual RA levels may contribute to preeclampsia pathogenesis by allowing sFLT1 accumulation at the maternal-fetal interface.

Lower FOXP3 mRNA Expression in First-Trimester Decidual Tissue from Uncomplicated Term Pregnancies with a Male Fetus

Pregnancies with a male fetus are associated with higher risks of pregnancy complications through maladaptation of the maternal immune system. The pathophysiology of this phenomenon is unknown. A possible pathway could be a fetal sex-dependent maternal immune response, since males have a Y chromosome encoding specific allogenic proteins, possibly contributing to a different response and higher complication risks. To analyze whether fetal sex affects mRNA expression of maternal immune genes in early pregnancy, real-time PCR quantification was performed in the decidual tissue from primigravid pregnancies (n = 20) between 10 and 12 weeks with uncomplicated term outcomes. Early-pregnancy decidual mRNA expression of the regulatory T-cell marker, FOXP3, was sixfold lower (p < 0.01) in pregnancies with a male fetus compared to pregnancies with a female fetus. Additionally, mRNA expression of IFNγ was sixfold (p < 0.05) lower in pregnancies with a male fetus. The present data imply maternal immunologic differences between pregnancies with male and female fetuses which could be involved in different pregnancy pathophysiologic outcomes. Moreover, this study indicates that researchers in reproductive immunology should always consider fetal sex bias.

CXCL10 and IL-6: Markers of two different forms of intra-amniotic inflammation in preterm labor

Problem

To determine whether amniotic fluid (AF) CXCL10 concentration is associated with histologic chronic chorioamnionitis in patients with preterm labor (PTL) and preterm prelabor rupture of the membranes (PROM).

Method of Study

This study included 168 women who had an episode of PTL or preterm PROM. AF interleukin (IL)‐6 and CXCL10 concentrations were determined by immunoassay.

Results

(i) Increased AF CXCL10 concentration was associated with chronic (OR: 4.8; 95% CI: 1.7‐14), but not acute chorioamnionitis; (ii) increased AF IL‐6 concentration was associated with acute (OR: 4.2; 95% CI: 1.3‐13.7) but not chronic chorioamnionitis; and (iii) an increase in AF CXCL10 concentration was associated with placental lesions consistent with maternal anti‐fetal rejection (OR: 3.7; 95% CI: 1.3‐10.4). (iv) All patients with elevated AF CXCL10 and IL‐6 delivered preterm.

Conclusion

Increased AF CXCL10 concentration is associated with chronic chorioamnionitis or maternal anti‐fetal rejection, whereas increased AF IL‐6 concentration is associated with acute histologic chorioamnionitis.

In vivo T-cell activation by a monoclonal αCD3ε antibody induces preterm labor and birth

PROBLEM

Activated/effector T cells seem to play a role in the pathological inflammation associated with preterm labor. The aim of this study was to determine whether in vivo T-cell activation by a monoclonal αCD3ε antibody induces preterm labor and birth.

METHOD OF STUDY

Pregnant B6 mice were intraperitoneally injected with a monoclonal αCD3ε antibody or its isotype control. The gestational age, the rates of preterm birth and pup mortality at birth as well as the fetal heart rate and umbilical artery pulsatility index were determined.

RESULTS

Injection of a monoclonal αCD3ε antibody led to preterm labor/birth (αCD3ε 83 ± 16.97% [10/12] vs isotype 0% [0/8]) and increased the rate of pup mortality at birth (αCD3ε 87.30 ± 8.95% [77/85] vs isotype 4.91 ± 4.34% [3/59]). In addition, injection of a monoclonal αCD3ε antibody decreased the fetal heart rate and increased the umbilical artery pulsatility index when compared to the isotype control.

CONCLUSION

In vivo T-cell activation by a monoclonal αCD3ε antibody in late gestation induces preterm labor and birth.

The role of decidual cells in uterine hemostasis, menstruation, inflammation, adverse pregnancy outcomes and abnormal uterine bleeding

BACKGROUND

Human pregnancy requires robust hemostasis to prevent hemorrhage during extravillous trophoblast (EVT) invasion of the decidualized endometrium, modification of spiral arteries and post-partum processes. However, decidual hemorrhage (abruption) can occur throughout pregnancy from poorly transformed spiral arteries, causing fetal death or spontaneous preterm birth (PTB), or it can promote the aberrant placentation observed in intrauterine growth restriction (IUGR) and pre-eclampsia; all leading causes of perinatal or maternal morbidity and mortality. In non-fertile cycles, the decidua undergoes controlled menstrual bleeding. Abnormal uterine bleeding (AUB) accompanying progestin-only, long-acting, reversible contraception (pLARC) accounts for most discontinuations of these safe and highly effective agents, thereby contributing to unwanted pregnancies and abortion. The aim of this study was to investigate the role of decidual cells in uterine hemostasis, menstruation, inflammation, adverse pregnancy outcomes and abnormal uterine bleeding.

METHODS

We conducted a critical review of the literature arising from PubMed searches up to December 2015, regarding in situ and in vitro expression and regulation of several specific proteins involved in uterine hemostasis in decidua and cycling endometrium. In addition, we discussed clinical and molecular mechanisms associated with pLARC-induced AUB and pregnancy complications with abruptions, chorioamnionitis or pre-eclampsia.

RESULTS

Progestin-induced decidualization of estradiol-primed human endometrial stromal cells (HESCs) increases in vivo and in vitro expression of tissue factor (TF) and type-1 plasminogen activator inhibitor (PAI-1) while inhibiting plasminogen activators (PAs), matrix metalloproteinases (MMPs), and the vasoconstrictor, endothelin-1 (ET-1). These changes in decidual cell-derived regulators of hemostasis, fibrinolysis, extracellular matrix (ECM) turnover, and vascular tone prevent hemorrhage during EVT invasion and vascular remodeling. In non-fertile cycles, progesterone withdrawal reduces TF and PAI-1 while increasing PA, MMPs and ET-1, causing menstrual-associated bleeding, fibrinolysis, ECM degradation and ischemia. First trimester decidual hemorrhage elicits later adverse outcomes including pregnancy loss, pre-eclampsia, abruption, IUGR and PTB. Decidual hemorrhage generates excess thrombin that binds to decidual cell-expressed protease-activated receptors (PARs) to induce chemokines promoting shallow placentation; such bleeding later in pregnancy generates thrombin to down-regulate decidual cell progesterone receptors and up-regulate cytokines and MMPs linked to PTB. Endometria of pLARC users display ischemia-induced excess vasculogenesis and progestin inhibition of spiral artery vascular smooth muscle cell proliferation and migration leading to dilated fragile vessels prone to bleeding. Moreover, aberrant TF-derived thrombin signaling also contributes to the pathogenesis of endometriosis via induction of angiogenesis, inflammation and cell survival.

CONCLUSION

Perivascular decidualized HESCs promote endometrial hemostasis during placentation yet facilitate menstruation through progestational regulation of hemostatic, proteolytic, and vasoactive proteins. Pathological endometrial hemorrhage elicits excess local thrombin generation, which contributes to pLARC associated AUB, endometriosis and adverse pregnancy outcomes through several biochemical mechanisms.

Is Infant Immunity Actively Suppressed or Immature?

Almost 7 million children under the age 5 die each year, and most of these deaths are attributable to vaccine-preventable infections. Young infants respond poorly to infections and vaccines. In particular, dendritic cells secrete less IL-12 and IL-18, CD8^pos T cells and NK cells have defective cytolysis and cytokine production, and CD4^pos T cell responses tend to bias towards a Th2 phenotype and promotion of regulatory T cells (T_regs). The basis for these differences is not well understood and may be in part explained by epigenetic differences, as well as immaturity of the infant's immune system. Here we present a third possibility, which involves active suppression by immune regulatory cells and place in context the immune suppressive pathways of mesenchymal stromal cells (MSC), myeloid-derived suppressor cells (MDSC), CD5^pos B cells, and T_regs. The immune pathways that these immune regulatory cells inhibit are similar to those that are defective in the infant. Therefore, the immune deficiencies seen in infants could be explained, in part, by active suppressive cells, indicating potential new avenues for intervention.

Natural killer cells and regulatory T cells in early pregnancy loss

Survival of the allogeneic embryo in the uterus depends on the maintenance of immune tolerance at the maternal-fetal interface. The pregnant uterus is replete with activated maternal immune cells. How this immune tolerance is acquired and maintained has been a topic of intense investigation. The key immune cells that predominantly populate the pregnant uterus are natural killer (NK) cells. In normal pregnancy, these cells are not killers, but rather provide a microenvironment that is pregnancy compatible and supports healthy placentation. In placental mammals, an array of highly orchestrated immune elements to support successful pregnancy outcome has been incorporated. This includes active cooperation between maternal immune cells, particularly NK cells, and trophoblast cells. This intricate process is required for placentation, immune regulation and to remodel the blood supply to the fetus. During the past decade, various types of maternal immune cells have been thought to be involved in cross-talk with trophoblasts and in programming immune tolerance. Regulatory T cells (Tregs) have attracted a great deal of attention in promoting implantation and immune tolerance beyond implantation. However, what has not been fully addressed is how this immune-trophoblast axis breaks down during adverse pregnancy outcomes, particularly early pregnancy loss, and in response to unscheduled inflammation. Intense research efforts have begun to shed light on the roles of NK cells and Tregs in early pregnancy loss, although much remains to be unraveled in order to fully characterize the mechanisms underlying their detrimental activity. An increased understanding of host-environment interactions that lead to the cytotoxic phenotype of these otherwise pregnancy compatible maternal immune cells is important for prediction, prevention and treatment of pregnancy maladies, particularly recurrent pregnancy loss. In this review, we discuss relevant information from experimental and human models that may explain the pregnancy disrupting roles of these pivotal sentinel cells at the maternal-fetal interface.

Maternal floor infarction/massive perivillous fibrin deposition: a manifestation of maternal antifetal rejection?

OBJECTIVE

Massive perivillous fibrin deposition (MPFD) and maternal floor infarction (MFI) are related placental lesions often associated with fetal death and fetal growth restriction. A tendency to recur in subsequent pregnancies has been reported. This study was conducted to determine whether this complication of pregnancy could reflect maternal antifetal rejection.

METHODS

Pregnancies with MPFD were identified (n = 10). Controls consisted of women with uncomplicated pregnancies who delivered at term without MPFD (n = 175). Second-trimester maternal plasma was analyzed for panel-reactive anti-HLA class I and class II antibodies. The prevalence of chronic chorioamnionitis, villitis of unknown etiology, and plasma cell deciduitis was compared between cases and controls. Immunohistochemistry was performed on available umbilical vein segments from cases with MPFD (n = 4) to determine whether there was evidence of complement activation (C4d deposition). Specific maternal HLA-antibody and fetal HLA-antigen status were also determined in paired specimens (n = 6). Plasma CXCL-10 concentrations were measured in longitudinal samples of cases (n = 28 specimens) and controls (n = 749 specimens) by ELISA. Linear mixed-effects models were used to test for differences in plasma CXCL-10 concentration.

RESULTS

(i) The prevalence of plasma cell deciduitis in the placenta was significantly higher in cases with MPFD than in those with uncomplicated term deliveries (40% versus 8.6%, P = 0.01), (ii) patients with MPFD had a significantly higher frequency of maternal anti-HLA class I positivity during the second trimester than those with uncomplicated term deliveries (80% versus 36%, P = 0.01); (iii) strongly positive C4d deposition was observed on umbilical vein endothelium in cases of MPFD, (iv) a specific maternal antibody against fetal HLA antigen class I or II was identified in all cases of MPFD; and 5) the mean maternal plasma concentration of CXCL-10 was higher in patients with evidence of MPFD than in those without evidence of MFPD (P < 0.001).

CONCLUSION

A subset of patients with MPFD has evidence of maternal antifetal rejection.

Detection of anti-HLA antibodies in maternal blood in the second trimester to identify patients at risk of antibody-mediated maternal anti-fetal rejection and spontaneous preterm delivery

PROBLEM

Maternal anti-fetal rejection is a mechanism of disease in spontaneous preterm labor. The objective of this study was to determine whether the presence of human leukocyte antigen (HLA) panel-reactive antibodies (PRA) during the second trimester increases the risk of spontaneous preterm delivery.

METHODS OF STUDY

This longitudinal case-control study included pregnant women with spontaneous preterm deliveries (n = 310) and control patients with normal term pregnancies (n = 620), matched for maternal age and gravidity. Maternal plasma samples obtained at 14-16, 16-20, 20-24, and 24-28 weeks of gestation were analyzed for HLA class I and class II PRA positivity using flow cytometry. The fetal HLA genotype and maternal HLA alloantibody epitope were determined for a subset of patients with positive HLA PRA.

RESULTS

(i) Patients with spontaneous preterm delivery were more likely to exhibit HLA class I (adjusted OR = 2.54, P < 0.0001) and class II (adjusted OR = 1.98, P = 0.002) PRA positivity than those delivering at term; (ii) HLA class I PRA positivity for patients with spontaneous preterm delivery between 28 and 34 weeks (adjusted OR = 2.88; P = 0.001) and after 34 weeks of gestation (adjusted OR = 2.53; P < 0.0001) was higher than for those delivering at term; (iii) HLA class II PRA positivity for patients with spontaneous preterm delivery after 34 weeks of gestation was higher than for those delivering at term (adjusted OR = 2.04; P = 0.002); (iv) multiparous women were at a higher risk for HLA class I PRA positivity than nulliparous women (adjusted OR = 0.097, P < 0.0001 for nulliparity); (v) nulliparous women had a higher rate of HLA class I PRA positivity with advancing gestational age (P = 0.001); and (vi) 78% of women whose fetuses were genotyped had alloantibodies specific against fetal HLA class I antigens.

CONCLUSION

Pregnant women with positive HLA class I or class II PRA during the second trimester are at an increased risk of spontaneous preterm delivery due to antibody-mediated maternal anti-fetal rejection.