Fetal–maternal HLA-C mismatch is associated with decidual T cell activation and induction of functional T regulatory cells

Failure of physiologic transformation of spiral arteries, endothelial and trophoblast cell activation, and acute atherosis in the basal plate of the placenta

BACKGROUND

Failure of physiologic transformation of spiral arteries has been reported in preeclampsia, fetal growth restriction, fetal death, and spontaneous preterm labor with intact or ruptured membranes. Spiral arteries with failure of physiologic transformation are prone to develop atherosclerotic-like lesions of atherosis. There are striking parallels between preeclampsia and atherosclerotic disease, and between lesions of atherosis and atherosclerosis. Endothelial activation, identified by intercellular adhesion molecule-1 expression, is present in atherosclerotic-like lesions of heart transplantation, and is considered a manifestation of rejection. Similarly, endothelial activation/dysfunction has been implicated in the pathophysiology of atherosclerosis and preeclampsia. Intercellular adhesion molecule-1-overexpressing-activated endothelial cells are more resistant to trophoblast displacement than nonactivated endothelium, and may contribute to shallow spiral artery trophoblastic invasion in obstetrical syndromes having failure of physiologic transformation.

OBJECTIVE

We sought to determine whether failure of spiral artery physiologic transformation was associated with activation of interstitial extravillous trophoblasts and/or spiral artery endothelium and presence of acute atherosis in the placental basal plate.

STUDY DESIGN

A cross-sectional study of 123 placentas (19-42 weeks' gestation) obtained from normal pregnancies (n = 22), preterm prelabor rupture of membranes (n = 26), preterm labor (n = 23), preeclampsia (n = 27), intrauterine fetal death (n = 15), and small for gestational age (n = 10) was performed. Failure of spiral artery physiologic transformation and presence of cell activation was determined using immunohistochemistry of placental basal plates containing a median of 4 (minimum: 1; maximum: 9) vessels per placenta. Endothelial/trophoblast cell activation was defined by the expression of intercellular adhesion molecule-1. Investigators examining microscopic sections were blinded to clinical diagnosis. Pairwise comparisons among placenta groups were performed with Fisher exact test and Wilcoxon rank sum test using a Bonferroni-adjusted level of significance (.025).

RESULTS

We found that 87% (94/108) of placentas having spiral arteries with failure of physiologic transformation (actin-positive and cytokeratin-negative) in the basal plate, and 0% (0/15) of placentas having only spiral arteries with complete physiologic transformation (cytokeratin-positive and actin-negative), had arterial endothelial and/or interstitial extravillous trophoblasts reactive with the intercellular adhesion molecule-1 activation marker (P < .001). A significant correlation (R² = 0.84) was found between expression of spiral artery endothelial and interstitial extravillous trophoblast intercellular adhesion molecule-1 (P < .001) in activated placentas. Lesions of atherosis were found in 31.9% (30/94) of placentas with complete and/or partial failure of physiologic transformation of spiral arteries that were intercellular adhesion molecule-1-positive, in none of the 14 placentas with failure of physiologic transformation that were intercellular adhesion molecule-1-negative, and in none of the 15 placentas with complete spiral artery physiologic transformation without failure (P = .001). All placentas (30/30, 100%) with atherosis were identified in placentas having concomitant spiral artery endothelial and interstitial extravillous trophoblast activation.

CONCLUSION

Failure of spiral artery physiologic transformation in the placental basal plate is associated with interstitial extravillous trophoblast and arterial endothelial activation along with increased frequency of spiral artery atherosis. These findings may be used to improve the characterization of different disorders of the placental bed such as in refining the existing tools for the early prediction of risk for preterm, preeclamptic, and other abnormal pregnancies.

Cytotoxic potential of decidual NK cells and CD8+ T cells awakened by infections

To establish a healthy pregnancy the maternal immune system must tolerate fetal allo-antigens, yet remain competent to respond to infections. The ability of decidual NK cells (dNK) to promote migration of fetal extravillous trophoblasts (EVT) and placental growth as well as the capacity of EVT to promote immune tolerance are topics of high interest and extensive research. However, the problem of how dNK and decidual CD8+ T cells (CD8+ dT) provide immunity to infections of the placenta and the mechanisms that regulate their cytolytic function has thus far largely been ignored. Fetal EVT are the most invasive cells of the placenta and directly interact with maternal decidual immune cells at this maternal-fetal interface. Besides the expression of non-polymorphic HLA-E and HLA-G molecules that are associated with immune tolerance, EVT also express highly polymorphic HLA-C molecules that can serve as targets for maternal dNK and CD8+ dT responses. HLA-C expression by EVT has a dual role as the main molecule to which immune tolerance needs to be established and as the only molecule that can present pathogen-derived peptides and provide protective immunity when EVT are infected. The focus of this review is to address the regulation of cytotoxicity of dNK and CD8+ dT, which is essential for maternal-fetal immune tolerance as well as recent evidence that both cell types can provide immunity to infections at the maternal-fetal interface. A particular emphasis is given to the role of HLA-C expressed by EVT and its capacity to elicit dNK and CD8+ dT responses.

Myometrial cytokines and their role in the onset of labour

Human labour is an inflammatory event, physiologically driven by an interaction between hormonal and mechanical factors and pathologically associated with infection, bleeding and excessive uterine stretch. The initiation and communicators of inflammation is still not completely understood; however, a key role for cytokines has been implicated. We summarise the current understanding of the nature and role of cytokines, chemokines and hormones and their involvement in signalling within the myometrium particularly during labour.

Milestones of Hematopoietic Stem Cell Transplantation - From First Human Studies to Current Developments

Since the early beginnings, in the 1950s, hematopoietic stem cell transplantation (HSCT) has become an established curative treatment for an increasing number of patients with life-threatening hematological, oncological, hereditary, and immunological diseases. This has become possible due to worldwide efforts of preclinical and clinical research focusing on issues of transplant immunology, reduction of transplant-associated morbidity, and mortality and efficient malignant disease eradication. The latter has been accomplished by potent graft-versus-leukemia (GvL) effector cells contained in the stem cell graft. Exciting insights into the genetics of the human leukocyte antigen (HLA) system allowed improved donor selection, including HLA-identical related and unrelated donors. Besides bone marrow, other stem cell sources like granulocyte-colony stimulating-mobilized peripheral blood stem cells and cord blood stem cells have been established in clinical routine. Use of reduced-intensity or non-myeloablative conditioning regimens has been associated with a marked reduction of non-hematological toxicities and eventually, non-relapse mortality allowing older patients and individuals with comorbidities to undergo allogeneic HSCT and to benefit from GvL or antitumor effects. Whereas in the early years, malignant disease eradication by high-dose chemotherapy or radiotherapy was the ultimate goal; nowadays, allogeneic HSCT has been recognized as cellular immunotherapy relying prominently on immune mechanisms and to a lesser extent on non-specific direct cellular toxicity. This chapter will summarize the key milestones of HSCT and introduce current developments.

Placental histological lesions in fetal and neonatal alloimmune thrombocytopenia: A retrospective cohort study of 21 cases

BACKGROUND

Alloimmunization against human platelet antigens (HPAs) can occur prenatally and induce fetal/neonatal alloimmune thrombocytopenia (FNAIT). The aim of this study was to identify placental histological features associated with platelet alloimmunization and their clinical significance.

METHODS

This study examined 21 placentas from FNAIT-affected pregnancies and 42 age-matched control cases, all collected from pathology departments in the Rhône-Alpes region. Clinical and laboratory findings were collected for each FNAIT case. Two pathologists reviewed the placental slides of each FNAIT and control case. Histological features, with special emphasis on chronic inflammatory lesions, were evaluated. Differences between the two groups were calculated with odds ratios (ORs) and assessed with Wald's chi-square.

RESULTS

FNAIT was associated with a significantly higher frequency of chronic chorioamnionitis (CC) (OR 14, 95%CI 1.7-113.8), basal chronic villitis (BCV) (OR 17, 95%CI 2-145.6) and chronic intervillositis (CIV). Chronic villitis (CV) (OR 3.7, 95%CI 0.9-15.2) and chronic deciduitis (CD) (OR 4.7, 95%CI 0.79-28.2) were also more frequent in the FNAIT than the control group, but these differences were not statistically significant.

CONCLUSIONS

FNAIT is significantly associated with CC, BCV, and CIV. This chronic inflammatory reaction is preferentially localized on the maternofetal interface. Anti-HPA alloimmunization may trigger an immunological conflict similar to graft-versus-host disease.

How pregnancy can affect autoimmune diseases progression?

Autoimmune disorders are characterized by tissue damage, caused by self-reactivity of different effectors mechanisms of the immune system, namely antibodies and T cells. Their occurrence may be associated with genetic and/or environmental predisposition and to some extent, have implications for fertility and obstetrics. The relationship between autoimmunity and reproduction is bidirectional. This review only addresses the impact of pregnancy on autoimmune diseases and not the influence of autoimmunity on pregnancy development. Th17/Th1-type cells are aggressive and pathogenic in many autoimmune disorders and inflammatory diseases. The immunology of pregnancy underlies the role of Th2-type cytokines to maintain the tolerance of the mother towards the fetal semi-allograft. Non-specific factors, including hormonal changes, favor a switch to Th2-type cytokine profile. In pregnancy Th2, Th17/Th2 and Treg cells accumulate in the decidua but may also be present in the mother’s circulation and can regulate autoimmune responses influencing the progression of autoimmune diseases.

Immunologic challenges of human reproduction: an evolving story

Characterization of the implanting human fetus as an allograft prompted a field of research in reproductive immunology that continues to fascinate and perplex scientists. Paternal- or partner-derived alloantigens are present in the maternal host at multiple times during the reproductive process. They begin with exposure to semen, continue through implantation and placentation, and may persist for decades in the form of fetal microchimerism. Changes in maternal immune responses that allow allogenic fertilization and survival of semiallogenic concepti to delivery must be balanced with a continued need to respond appropriately to pathogenic invaders, commensals, cell or tissue damage, and any tendency toward malignant transformation. This complex and sophisticated balancing act is essential for survival of mother, fetus, and the species itself. We will discuss concepts of alloimmune recognition, tolerance, and ignorance as they pertain to mammalian reproduction with a focus on human reproduction, maternal immune modulation, and the very earliest events in the reproductive process, fertilization and implantation.

The role of the maternal immune system in the regulation of human birthweight

Human birthweight is subject to stabilizing selection. Large babies are at risk of obstetric complications such as obstructed labour, which endangers both mother and child. Small babies are also at risk with reduced survival. Fetal growth requires remodelling of maternal spiral arteries to provide an adequate maternal blood supply to the placenta. This arterial transformation is achieved by placental trophoblast cells, which invade into the uterine wall. Under-invasion is associated with fetal growth restriction; but if invasion is excessive large babies can result. A growing body of evidence suggests that this process is controlled by interactions between killer-cell immunoglobulin-like receptors (KIRs) expressed on maternal uterine natural killer cells (uNK) and their corresponding human leukocyte antigen-C (HLA-C) ligands on invading trophoblast. Mothers with the KIR AA genotype and a fetus with a paternal HLA-C2 allele tend to have small babies, because this combination inhibits cytokine secretion by uNK. Mothers with the activating KIR2DS1 gene and an HLA-C2 fetus are more likely to have large babies. When KIR2DS1 binds to HLA-C2 this increases secretion of cytokines that enhance trophoblast invasion. We conclude that specific combinations of the highly polymorphic gene systems, KIR and HLA-C, contribute to successful reproduction by maintaining birthweight between two extremes.

Obstetric and perinatal complications in an oocyte donation programme. Is it time to limit the number of embryos to transfer?

The aim of this study is to describe obstetric and perinatal complications in pregnancies from oocyte donation (OD) cycles, delivering in our centre and to determine the impact of maternal age. Retrospective observational study of a 225 singleton pregnancies, 113 multiple pregnancies and 447 live birth. Pearson's χ(2) test or Fisher's exact test were used for the statistical analysis. A higher incidence of obstetric complications was observed in multiple compared to singleton pregnancies with regard to preeclampsia (24.8% versus 8%), premature rupture of membranes (9.7% versus 1.8%), preterm delivery at <37 weeks (54.9% versus 10.2%) and caesarean section (81.4% versus 64%) (p < 0.05). If the age factor is added, the caesarean sections are higher in the single pregnancy group aged ≥40 years than in the group of <40 years (73.5% versus 49.4%) (p < 0.05). A higher incidence is found in multiple versus singleton pregnancies for low birth weight (<2500 g) (61.1% versus 8.2%), admissions to the intensive care unit (15.2% versus 4.7%) and perinatal mortality (13.5‰ versus 0‰) (p < 0.05). It is necessary to consider preconception counselling prior to an OD cycle to inform patients about the incidence complications observed and recommend to transfer only a single embryo.

Role of Paternal Antigen-Specific Treg Cells in Successful Implantation

Maternal lymphocytes recognize fetal antigens, so tolerance is necessary to prevent rejection. Seminal plasma is important for induction of paternal antigen-specific Treg cells in the uterine draining lymph nodes and the pregnant uterus. Elimination of Treg cells during implantation or early pregnancy induces implantation failure or fetal resorption in mice. Immunosuppressive therapy with an anti-TNF antibody or the immunosuppressive agent tacrolimus improves the pregnancy rate in women with repeated implantation failure and recurrent pregnancy loss of unknown etiology, suggesting that Treg cells play an essential role in successful implantation and pregnancy in humans.

The HLA-G cycle provides for both NK tolerance and immunity at the maternal-fetal interface

The interaction of noncytotoxic decidual natural killer cells (dNK) and extravillous trophoblasts (EVT) at the maternal-fetal interface was studied. Confocal microscopy revealed that many dNK interact with a single large EVT. Filamentous projections from EVT enriched in HLA-G were shown to contact dNK, and may represent the initial stage of synapse formation. As isolated, 2.5% of dNK contained surface HLA-G. However, surface HLA-G-negative dNK contained internalized HLA-G. Activation of dNK resulted in the disappearance of internalized HLA-G in parallel with restoration of cytotoxicity. Surface HLA-G was reacquired by incubation with EVT. This HLA-G cycle of trogocytosis, endocytosis, degradation, and finally reacquisition provides a transient and localized acquisition of new functional properties by dNK upon interaction with EVT. Interruption of the cycle by activation of dNK by cytokines and/or viral products serves to ensure the NK control of virus infection at the interface, and is illustrated here by the response of dNK to human cytomegalo virus (HCMV)-infected decidual stromal cells. Thus, the HLA-G cycle in dNK can provide both for NK tolerance and antiviral immunity.

The possible role of virus-specific CD8(+) memory T cells in decidual tissue

The most abundant lymphocyte present in decidual tissue is the CD8(+) T cell. It has been shown that most decidual CD8(+) T cells have an effector-memory phenotype, but expressed reduced levels of perforin and granzyme B compared with the peripheral CD8(+) effector-memory T cells. The specificity of these CD8(+) memory T cells has yet to be determined. One hypothesis is that the decidual memory T cells are virus-specific T cells that should protect the fetus against incoming pathogens. As virus-specific CD8(+) memory T cells can cross-react with human leukocyte alloantigens, an alternative, but not mutually exclusive, hypothesis is that these CD8(+) T cells are fetus-specific. Using virus-specific tetramers, we found increased percentages of virus-specific CD8(+) T cells in decidual tissue compared with peripheral blood after uncomplicated pregnancy. So far, no evidence has been obtained for a cross-reactive response of these virus-specific T cells to fetal human leukocyte antigens. These results suggest that the virus-specific memory T cells accumulate in the placenta to protect the fetus from a harmful infection.

Cellular Regulation of the Uterine Microenvironment That Enables Embryo Implantation

Implantation of the fertilized egg into the maternal uterus is a crucial step in pregnancy establishment. Increasing evidence suggests that its success depends on various cell types of the innate immune system and on the fine balance between inflammatory and anti-inflammatory processes. In addition, it has recently been established that regulatory T cells play a superordinate role in dictating the quality of uterine environment required for successful pregnancy. Here, we discuss the cellular regulation of uterine receptivity with emphasis on the function and regulation of cells from the innate and adaptive immune system.

Interleukin-6 controls uterine Th9 cells and CD8(+) T regulatory cells to accelerate parturition in mice

Interleukin-6 (IL6) is a determinant of the timing of parturition and birth in mice. We previously demonstrated that genetic IL6 deficiency delays parturition by ~24 h, and this is restored by administration of exogenous IL6. In this study, we have investigated whether IL6 influences the number or phenotypes of T cells or other leukocytes in uterine decidual tissue at the maternal-fetal interface. In late gestation, decidual leukocytes in Il6 null mutant (Il6(-/-)) mice exhibit an altered profile, characterized by reduced numbers of cells expressing the monocyte/macrophage marker F4/80 or the T-cell marker CD4, increased cells expressing the natural killer (NK) cell marker CD49b or the dendritic cell marker CD11c, but no change in cells expressing the neutrophil marker Ly6G. These changes are specific to late pregnancy, as similar differences in decidual leukocytes were not evident in mid-gestation Il6(-/-) mice. The IL6-regulated changes in decidual NK and dendritic cells appear secondary to local recruitment, as no comparable changes occurred in peripheral blood of Il6(-/-) mice. When exogenous IL6 was administered to restore normal timing of parturition, a partial reversal of the altered leukocyte profile was observed, with a 10% increase in the proportion of decidual CD4(+) T cells, a notable 60% increase in CD8(+) T cells including CD8(+)CD25(+)Foxp3(+) regulatory T cells and a 60% reduction in CD4(+)IL9(+) Th9 cells. Together these findings suggest that IL6-controlled accumulation of decidual CD4(+) T cells and CD8(+) regulatory T cells, with an associated decline in decidual Th9 cells, is instrumental for progressing parturition in mice.

T helper cell mediated-tolerance towards fetal allograft in successful pregnancy

Trophoblast HLA-C antigens from paternal origins, which liken the trophoblast to a semiallograft, could be presented by the maternal APCs to the specific maternal CD4+ T helper cells, which could release various cytokines in response to these alloantigens. On the basis of the cytokines produced, these cells can be classified in Th1, Th2 and Th17 cells. Th1 and Th17 cells, known to be responsible for acute allograft rejection, could be involved in miscarriage and Th2 cells together with regulatory CD4+ T cells, known to be involved in allograft tolerance, could be responsible, at least in part, for the success of pregnancy. In this review we focus the role effector CD4+ T cells Th1, Th2 and Th17 cells on the fetal allograft tolerance.

Human HLA-G+ extravillous trophoblasts: Immune-activating cells that interact with decidual leukocytes

Invading human leukocyte antigen-G+ (HLA-G+) extravillous trophoblasts (EVT) are rare cells that are believed to play a key role in the prevention of a maternal immune attack on foreign fetal tissues. Here highly purified HLA-G+ EVT and HLA-G- villous trophoblasts (VT) were isolated. Culture on fibronectin that EVT encounter on invading the uterus increased HLA-G, EGF-Receptor-2, and LIF-Receptor expression on EVT, presumably representing a further differentiation state. Microarray and functional gene set enrichment analysis revealed a striking immune-activating potential for EVT that was absent in VT. Cocultures of HLA-G+ EVT with sample matched decidual natural killer cells (dNK), macrophages, and CD4+ and CD8+ T cells were established. Interaction of EVT with CD4+ T cells resulted in increased numbers of CD4+CD25(HI)FOXP3+CD45RA+ resting regulatory T cells (Treg) and increased the expression level of the Treg-specific transcription factor FOXP3 in these cells. However, EVT did not enhance cytokine secretion in dNK, whereas stimulation of dNK with mitogens or classical natural killer targets confirmed the distinct cytokine secretion profiles of dNK and peripheral blood NK cells (pNK). EVT are specialized cells involved in maternal-fetal tolerance, the properties of which are not imitated by HLA-G-expressing surrogate cell lines.

Expression of CCL22 and Infiltration by Regulatory T Cells are Increased in the Decidua of Human Miscarriage Placentas

PROBLEM

Regulatory T cells (Treg) are a T-cell subpopulation with suppressive capacities, specifically attracted by CCL22. We aimed to investigate whether CCL22 is expressed in human placentas and whether its presence, together with Treg infiltration, is associated with miscarriage conditions.

METHOD OF STUDY

Paraffin samples were stained for CCL22 and for the Treg-specific transcription factor FoxP3. Expression levels were evaluated in a semi-quantitative manner. Double immunofluorescence was used for the identification of CCL22-producing cells.

RESULTS

In all placentas, trophoblasts expressed CCL22. Interestingly, expression in the decidua was only observed in miscarriage conditions. Maternal stromal cells expressed CCL22. Correlation with a higher presence of Treg in the decidua of abortive tissues was observed.

CONCLUSION

Our results demonstrate that CCL22 is expressed in human placenta. Decidual expression was only observed in miscarriage conditions and correlates with Treg infiltration. Thus, CCL22 plays a role in human pregnancy and may occur as a negative feedback response to pro-inflammatory events during miscarriage conditions.

T cell behavior at the maternal-fetal interface

Understanding the function of T cells at the maternal-fetal interface remains one of the most difficult problems in reproductive immunology. A great deal of work over the last two decades has led to the view that the T cells that populate the decidua have important roles in both normal and pathological pregnancies, but the exact nature of these roles has remained unclear. Indeed, the old assumption that decidual T cells are uniformly threatening to fetal survival because the placenta is fundamentally an 'allograft' has given way to the idea that different T cell subsets contribute in different ways to pregnancy success or failure. Accordingly, some T cells are thought to protect the placenta from immune rejection and facilitate embryo implantation, while others are thought to contribute to pregnancy pathologies such as preeclampsia and spontaneous abortion. Here, we review the current state of information on the behavior of decidual T cells with a focus on both mouse and human studies, and with an emphasis on the many unresolved areas within this overall emerging framework.

The Paternal Contribution to Fetal Tolerance

Recognition of foreign paternal antigens expressed in the semi-allogeneic fetus by maternal immune cells is a requirement for successful pregnancy. However, despite intensive research activity during the last decades, the precise mechanisms contributing to the acceptance of the paternal alloantigens are still puzzling and pregnancy remains a fascinating phenomenon. Moreover, most studies focused on the maternal and fetal contribution to pregnancy success, and relatively little is known about the paternal involvement. In the current review, we address the contribution of paternal-derived factors to fetal-tolerance induction. First, we discuss data suggesting that in both humans and mice, the female body gets prepared for a pregnancy in every cycle, also in regard to male alloantigens delivered at coitus. Then, we provide an overview about factors present in seminal fluid and how these factors influence immune responses in the female reproductive tract. We further discuss ways of paternal alloantigen presentation and identify the immune modulatory properties of seminal fluid-derived factors with a special focus on Treg biology. Finally, we highlight the therapeutic potential of seminal fluid in different clinical applications.

Low molecular weight heparin modulates maternal immune response in pregnant women and mice with thrombophilia

PROBLEM

Thrombophilia is associated with pregnancy complications. Treatment with low molecular weight heparin (LMWH) improves pregnancy outcome, but the underlying mechanisms are not clear.

METHODS OF STUDY

We analyzed Treg frequency in blood from thrombophilic pregnancies treated with LMWH (n = 32) or untreated (n = 33) and from healthy pregnancies (n = 39) at all trimesters. Additionally, we treated pregnant wild-type, heterozygous and homozygous factor-V-Leiden (FVL) mice with LMWH or PBS and determined Treg frequency, pro-/anti-inflammatory cytokine levels and Caspase-3-activity in placenta and decidua.

RESULTS

Treg frequencies were increased in second and third trimester in LMWH-treated thrombophilic pregnancies compared to controls. Treg levels were comparable to those of normal pregnancies. Homozygous FVL mice had decreased decidual Tregs compared to wild-type mice. LMWH treatment normalized Tregs and was associated with increased decidual IL-10 mRNA. LMWH diminished Caspase-3-activity in mice of all genotypes.

CONCLUSION

We demonstrated anti-apoptotic and anti-inflammatory effects of LMWH in pregnant FVL mice. LMWH increased Treg levels in mice and humans, which suggests benefits of LMWH treatment for thrombophilic women during pregnancy.

Helios-positive functional regulatory T cells are decreased in decidua of miscarriage cases with normal fetal chromosomal content

Regulatory (Treg) T cells play essential roles in the maintenance of allogeneic pregnancy in mice and humans. Recent data show that Foxp3 expression occurs in both immuno-suppressive Treg and -nonsuppressive effector T (Teff) cells upon activation in humans. Samstein et al. (2012) reported that inducible Treg (iTreg) cells enforce maternal-fetal tolerance in placental mammals. Therefore, we should reanalyze which types of Treg cell play an important role in the maintenance of allogeneic pregnancy. In this study, we studied the frequencies of naïve Treg cells, effector Treg cells, Foxp3(+) Teff cells, Helios(+) naturally occurring Treg (nTreg) cells, and Helios(-) iTreg cells using flow cytometry. The frequencies of effector Treg cells and Foxp3(+) Teff cells among CD4(+)Foxp3(+) cells in the decidua of miscarriage cases with a normal embryo karyotype (n=8) were significantly lower (P=0.0105) and significantly higher (P=0.0258) than those in normally progressing pregnancies (n=11), respectively. However, these frequencies in miscarriages with an abnormal embryo karyotype (n=15) were similar to those in normally progressing pregnancies. The frequencies of these cell populations in the three groups were unchanged in peripheral blood; on the other hand, most of the effector Treg cells in the decidua were Helios(+) nTreg cells and these frequencies were significantly higher than those in peripheral blood, while those among effector Treg and naïve Treg cells in the decidua and peripheral blood were similar among the three groups. These data suggest that decreased Helios(+) effector nTreg might play an important role in the maintenance of pregnancy in humans.

Immune cells in term and preterm labor

Labor resembles an inflammatory response that includes secretion of cytokines/chemokines by resident and infiltrating immune cells into reproductive tissues and the maternal/fetal interface. Untimely activation of these inflammatory pathways leads to preterm labor, which can result in preterm birth. Preterm birth is a major determinant of neonatal mortality and morbidity; therefore, the elucidation of the process of labor at a cellular and molecular level is essential for understanding the pathophysiology of preterm labor. Here, we summarize the role of innate and adaptive immune cells in the physiological or pathological activation of labor. We review published literature regarding the role of innate and adaptive immune cells in the cervix, myometrium, fetal membranes, decidua and the fetus in late pregnancy and labor at term and preterm. Accumulating evidence suggests that innate immune cells (neutrophils, macrophages and mast cells) mediate the process of labor by releasing pro-inflammatory factors such as cytokines, chemokines and matrix metalloproteinases. Adaptive immune cells (T-cell subsets and B cells) participate in the maintenance of fetomaternal tolerance during pregnancy, and an alteration in their function or abundance may lead to labor at term or preterm. Also, immune cells that bridge the innate and adaptive immune systems (natural killer T (NKT) cells and dendritic cells (DCs)) seem to participate in the pathophysiology of preterm labor. In conclusion, a balance between innate and adaptive immune cells is required in order to sustain pregnancy; an alteration of this balance will lead to labor at term or preterm.

Relationship between maternal immunological response during pregnancy and onset of preeclampsia

Maternofetal immune tolerance is essential to maintain pregnancy. The maternal immunological tolerance to the semiallogeneic fetus becomes greater in egg donation pregnancies with unrelated donors as the complete fetal genome is allogeneic to the mother. Instead of being rejected, the allogeneic fetus is tolerated by the pregnant woman in egg donation pregnancies. It has been reported that maternal morbidity during egg donation pregnancies is higher as compared with spontaneous or in vitro fertilization pregnancies. Particularly, egg donation pregnancies are associated with a higher incidence of pregnancy-induced hypertension and placental pathology. Preeclampsia, a pregnancy-specific disease characterized by the development of both hypertension and proteinuria, remains the leading cause of maternal and perinatal mortality and morbidity. The aim of this review is to characterize and relate the maternofetal immunological tolerance phenomenon during pregnancies with a semiallogenic fetus, which are the spontaneously conceived pregnancies and in vitro fertilization pregnancies, and those with an allogeneic fetus or egg donation pregnancies. Maternofetal immune tolerance in uncomplicated pregnancies and pathological pregnancies, such as those with preeclampsia, has also been assessed. Moreover, whether an inadequate maternal immunological response to the allogenic fetus could lead to a higher prevalence of preeclampsia in egg donation pregnancies has been addressed.

Uterine NK cells: active regulators at the maternal-fetal interface

Pregnancy presents an immunological conundrum because two genetically different individuals coexist. The maternal lymphocytes at the uterine maternal-fetal interface that can recognize mismatched placental cells are T cells and abundant distinctive uterine NK (uNK) cells. Multiple mechanisms exist that avoid damaging T cell responses to the fetus, whereas activation of uNK cells is probably physiological. Indeed, genetic epidemiological data suggest that the variability of NK cell receptors and their MHC ligands define pregnancy success; however, exactly how uNK cells function in normal and pathological pregnancy is still unclear, and any therapies aimed at suppressing NK cells must be viewed with caution. Allorecognition of fetal placental cells by uNK cells is emerging as the key maternal-fetal immune mechanism that regulates placentation.

Regulatory T cells: regulators of life

Pregnancy still represents one of the most fascinating paradoxical phenomena in science. Immediately after conception, the maternal immune system is challenged by the presence of foreign paternal antigens in the semen. This triggers mechanisms of recognition and tolerance that all together allow the embryo to implant and later the fetus to develop. Tolerance mechanisms to maintain pregnancy are of special interest as they defy the classical immunology rules. Several cell types, soluble factors, and immune regulatory molecules have been proposed to contribute to fetal tolerance. Within these, regulatory T cells (Treg) are one of the most studied immune cell populations lately. They are reportedly involved in fetal acceptance. Here, we summarize several aspects of Treg biology in normal and pathologic pregnancies focusing on Treg frequencies, subtypes, antigen specificity, and activity as well as on factors influencing Treg generation, recruitment, and function. This review also highlights the contribution of fetal Treg in tolerance induction and addresses the role of Treg in autoimmune diseases and infections during gestation. Finally, the potential of Treg as a predictive marker for the success of assisted reproductive techniques and for therapeutic interventions is discussed.

Cutting edge: committed Th1 CD4+ T cell differentiation blocks pregnancy-induced Foxp3 expression with antigen-specific fetal loss

Pregnancy stimulates induced Foxp3 expression among maternal CD4(+) T cells with fetal specificity. Although sustained maternal regulatory CD4(+) T cell (Treg) expansion is essential for maintaining fetal tolerance during pregnancy, the necessity for Foxp3(+) cells with fetal specificity remains undefined. In this study, we demonstrate that mitigating Treg differentiation among maternal CD4(+) T cells with a single surrogate fetal specificity elicits Ag-specific fetal loss. Using recombinant Listeria monocytogenes to prime stably differentiated Th1 CD4(+) T cells with fetal I-A(b):2W1S55-68 specificity refractory to pregnancy-induced Foxp3 expression, we show that Ag delivery by cytoplasmic L. monocytogenes causes selective loss of 2W1S(+) offspring through CD4 cell- and IFN-γ-dependent pathways. In contrast, CD4(+) T cells primed by L. monocytogenes restricted from the cell cytoplasm are markedly more plastic for induced Foxp3 expression, with normal pregnancy outcomes. Thus, committed Th1 polarization blocks pregnancy induced Treg differentiation among maternal CD4(+) T cells with fetal specificity and triggers Ag-specific fetal loss.

MHC-dependent inhibition of uterine NK cells impedes fetal growth and decidual vascular remodelling

NK cells express variable receptors that engage polymorphic MHC class I molecules and regulate their function. Maternal NK cells accumulate at the maternal-fetal interface and can interact with MHC class I molecules from both parents. The relative contribution of the two sets of parental MHC molecules to uterine NK cell function is unknown. Here we show that, in mice, maternal and not paternal MHC educates uterine NK cells to mature and acquire functional competence. The presence of an additional MHC allele that binds more inhibitory than activating NK cell receptors results in suppressed NK cell function, compromised uterine arterial remodelling and reduced fetal growth. Notably, reduced fetal growth occurs irrespectively of the parental origin of the inhibitory MHC. This provides biological evidence for the impact of MHC-dependent NK inhibition as a risk factor for human pregnancy-related complications associated with impaired arterial remodelling.

NK cells are involved in remodelling of the uterine vasculature during pregnancy and the extent of this process is influenced by the combination of maternal NK cell receptors and MHC-I of the fetus. Here, the authors provide further insights into how the presence of MHC-I from each parent differentially affects NK cell function.

Fetal intervention increases maternal T cell awareness of the foreign conceptus and can lead to immune-mediated fetal demise

Fetal interventions to diagnose and treat congenital anomalies are growing in popularity but often lead to preterm labor. The possible contribution of the maternal adaptive immune system to postsurgical pregnancy complications has not been explored. We recently showed that fetal intervention in mice increases maternal T cell trafficking into the fetus and hypothesized that this process also may lead to increased maternal T cell recognition of the foreign conceptus and subsequent breakdown in maternal-fetal tolerance. In this study, we show that fetal intervention in mice results in accumulation of maternal T cells in the uterus and that these activated cells can produce effector cytokines. In adoptive transfer experiments, maternal T cells specific for a fetal alloantigen proliferate after fetal intervention, escape apoptosis, and become enriched compared with endogenous T cells in the uterus and uterine-draining lymph nodes. Finally, we demonstrate that such activation and accumulation can have a functional consequence: in utero transplantation of hematopoietic cells carrying the fetal alloantigen leads to enhanced demise of semiallogeneic fetuses within a litter. We further show that maternal T cells are necessary for this phenomenon. These results suggest that fetal intervention enhances maternal T cell recognition of the fetus and that T cell activation may be a culprit in postsurgical pregnancy complications. Our results have clinical implications for understanding and preventing complications associated with fetal surgery such as preterm labor.

HLA-G5 induces IL-4 secretion critical for successful pregnancy through differential expression of ILT2 receptor on decidual CD4⁺ T cells and macrophages

Successful pregnancy in humans has been associated with production of IL-4 by T cells at the feto-maternal interface. Soluble HLA-G5 produced by trophoblasts potentially controls the decidual T cell cytokine profile. We studied the effect of HLA-G5 on the cytokine profile of purified human macrophages and Ag-specific T cells in vitro. We demonstrated that HLA-G5 increased production of IL-12 by purified peripheral blood macrophages. Although IL-12 production by macrophages is known to induce IFN-γ production by CD4(+) T cells, HLA-G5 increased production of IL-4 but not IFN-γ by CD4(+) T cells after Ag presentation by macrophages. We found that this apparent paradox was due to the differential expression of the ILT2 HLA-G5 receptor on activated T cells and macrophages. This receptor was upregulated in the former and downregulated in the latter after Ag presentation and activation of both cell types. This observation was confirmed in situ, where decidual macrophages and T cells are continuously exposed to HLA-G5 produced locally and activated by trophoblast alloantigens. Freshly isolated decidua basalis macrophages expressed lower levels of ILT2 than peripheral blood macrophages from the same pregnant women. They did not spontaneously produce IL-12, whereas freshly isolated decidual CD4(+) T cells expressed high levels of activation markers (CD25, HLA-DR, and CD69) as well as ILT2 and spontaneously produced IL-4 but not IFN-γ. Therefore, HLA-G5 could be responsible, at least in part, via its interaction with ILT2, for decidual T cell IL-4 production, known to be crucial for successful pregnancy.

Seminal fluid and the generation of regulatory T cells for embryo implantation

T regulatory (Treg) cells are essential mediators of the maternal immune adaptation necessary for embryo implantation. In mice, insufficient Treg cell activity results in implantation failure, or constrains placental function and fetal growth. In women, Treg cell deficiency is linked with unexplained infertility, miscarriage, and pre-eclampsia. To devise strategies to improve Treg cell function, it is essential to define the origin of the Treg cells in gestational tissues, and the regulators that control their functional competence and recruitment. Male seminal fluid is a potent source of the Treg cell-inducing agents TGFβ and prostaglandin E, and coitus is one key factor involved in expanding the pool of inducible Treg cells that react with paternal alloantigens shared by conceptus tissues. In mice, coitus initiates a sequence of events whereby female dendritic cells cross-present seminal fluid antigens and activate T cells, which in turn circulate via the blood to be sequestered into the endometrium. Similar events may occur in the human genital tract, where seminal fluid induces immune cell changes that appear competent to prime Treg cells. Improved understanding of how seminal fluid influences Treg cells in women should ultimately assist in the development of new therapies for immune-mediated pathologies of pregnancy.

Differential immunoregulation in successful oocyte donation pregnancies compared with naturally conceived pregnancies

In oocyte donation (OD) pregnancies, there is a higher level of antigenic dissimilarity between mother and fetus compared with naturally conceived (NC) pregnancies. We hypothesize that a higher degree and/or a different type of immunoregulation is necessary to maintain an uncomplicated OD pregnancy. Different immunological aspects of successful OD pregnancies (n=28) were compared with those of NC pregnancies (n=51), and non-donor IVF (n=20) pregnancies. Maternal peripheral blood mononuclear cells (mPBMCs) were characterized by flow cytometry; the outcome correlated with the number of mother-child HLA mismatches. The fetus-specific alloreactivity of mPBMCs was measured in a mixed lymphocyte reaction. The percentages of CD4(+)CD25(bright) and CD4(+)CD25(dim) cells were higher in mPBMCs of OD and IVF pregnancies compared with NC pregnancies. The percentage of CD4(+)CD25(dim) cells in mPBMCs of OD pregnancies correlated positively with the number of HLA mismatches. Functional studies showed a lower proliferative response to umbilical cord blood by mPBMCs in OD pregnancies. In conclusion, we found a higher degree of peripheral immunoregulation in OD and IVF pregnancies compared with NC pregnancies. A higher number of HLA mismatches in successful OD pregnancies leads to increased percentages of activated T cells in peripheral blood, but not to a higher alloreactivity to the fetus. These studies show that immunoregulation in OD pregnancy is different from that in NC pregnancies. The antigenic dissimilarity in OD pregnancies may play a role in the pathophysiology of preeclampsia.

The immunological basis of villitis of unknown etiology - review

Villitis of unknown etiology (VUE) represents a common placental inflammatory lesion, primarily, but not exclusively, identifiable T lymphocytes at term. Despite considerable evidence to contest that this simply represents a benign pathological finding, VUE remains a significantly undervalued diagnosis. Given its association with adverse pregnancy outcomes; including fetal growth restriction, preterm birth, and recurrent pregnancy loss, an increased awareness amongst clinician obstetricians is certainly warranted. The underlying immunopathogenesis of VUE remains uncertain. Despite initial theories that this represents an infectious placental lesion of undiagnosed pathogenic source, a more complex sequence of events involving the "breakdown" of maternal-fetal tolerance is emerging. Characterization of a unique inflammatory phenomenon in which both maternal and fetal T lymphocytes and Höfbauer cells interact has captivated particular research interest and has generated analogies to both the problems of allograft rejection and graft-versus-host disease (GvHD). Within the context of VUE, this review evaluates how disruption of the multidimensional immunological mechanisms underlying feto-maternal tolerance may permit abnormal lymphocyte infiltration into placental villi. We shall review the existing evidence for these events in VUE and outline areas of certain future interest.

Preservation of human placenta facilitates multicenter studies on the local immune response in normal and aberrant pregnancies

Our standard procedure for phenotypic and functional analysis of immune cells present in the placenta is to isolate leukocytes from the decidua within five hours of the delivery. However, this results in logistical problems with deliveries at night, weekends or in other medical centers. Collecting placentas after complicated pregnancies is even more difficult owing to the low prevalence and the often unscheduled delivery. The aim was to investigate the possibility of preserving the human placenta before phenotypic and functional analysis of decidual lymphocytes. Placentas were obtained after uncomplicated pregnancy. The tissue was divided into two equal parts: decidual lymphocytes from one part were isolated within five hours according to our standard procedure, whereas the other part was preserved in either Celsior(®), a storage solution for solid organ preservation, or phosphate-buffered saline (PBS) for 24h at 4°C before isolation. Overall, the phenotype and functional capacity of decidual lymphocytes isolated within five hours was comparable to decidual lymphocytes isolated after 24-h preservation in Celsior(®) or PBS. Minor differences were found between decidual lymphocytes isolated within five hours and decidual lymphocytes isolated after 24-h preservation in Celsior(®). The results indicate that PBS is sufficient to preserve the placenta for 24h for phenotypical and functional studies. The ability to preserve the placenta will simplify the procedure for the isolation of decidual lymphocytes and makes it easier to analyze tissue from women who deliver during the night, at weekends or in other hospitals, and possibly even women with complicated pregnancies.

Evidence for a role for the adaptive immune response in human term parturition

PROBLEM

Spontaneous labor at term involves leukocyte recruitment and infiltration into the choriodecidua; yet, characterization of these leukocytes and their immunological mediators is incomplete. The purpose of this study was to characterize the immunophenotype of choriodecidual leukocytes as well as the expression of inflammatory mediators in human spontaneous parturition at term.

METHOD OF STUDY

Choriodecidual leukocytes were analyzed by FACS, immunohistochemistry, and RT-PCR in three different groups: (i) preterm gestation delivered for medical indications without labor; (ii) term pregnancy without labor; and (iii) term pregnancy after spontaneous labor.

RESULTS

Two T-cell subsets of memory-like T cells (CD3(+) CD4(+) CD45RO(+) and CD3(+) CD4(-) CD8(-) CD45RO(+) cells) were identified in the choriodecidua of women who had spontaneous labor. Evidence for an extensive immune signaling network composed of chemokines (CXCL8 and CXCL10), chemokine receptors (CXCR1-3), cytokines (IL-1β and TNF-α), cell adhesion molecules, and MMP-9 was identified in these cells during spontaneous labor at term.

CONCLUSIONS

The influx of memory-like T cells in the choriodecidua and the evidence that they are active by producing chemokines and cytokines, and expressing chemokine receptors, cell adhesion molecules, and a matrix-degrading enzyme provides support for the participation of the adaptive immune system in the mechanisms of spontaneous parturition at term.

CD8+ effector T cells at the fetal-maternal interface, balancing fetal tolerance and antiviral immunity

During pregnancy CD8+ effector T cells need optimal immune regulation to prevent a detrimental response to allogeneic fetal cells while providing immune protection to infections. A significant proportion of (prospective) mothers carry naïve or memory CD8+ T cells with a TCR that can directly bind to paternal MHC molecules. In addition, a high percentage of pregnant women develop specific T cell responses to fetal minor histocompatibility antigens (mHags). Under normal conditions, fetal-maternal MHC and mHag mismatches lead to elevated lymphocyte activation but do not induce pregnancy failure. Furthermore, viral infections alter the maternal CD8+ T cell response by changing the CD8+ T cell repertoire and increasing the influx of CD8+ T cells to decidual tissue. The normally high T cell activation threshold at the fetal-maternal interface may prevent efficient clearance of viral infections. Conversely, the increased inflammatory response due to viral infections may break fetal-maternal tolerance and lead to pregnancy complications. The aim of this review is to discuss the recent studies of CD8+ T cells in pregnancy, identify potential mechanisms for antigen-specific immune recognition of fetal extravillous trophoblast (EVT) cells by CD8+ T cells, and discuss the impact of viral infections and virus-specific CD8+ T cells during pregnancy.

Decidual regulatory T cells in placental pathology and pregnancy complications

Regulatory T cells (Tregs) have been identified as immunomodulatory cells, which induce tolerance. Evidence from numerous recent studies implicates these cells as critical for maternal tolerance to the fetal-derived placenta, proper functioning of which is required for normal gestation and growth and development of the fetus in utero. This review focuses on the recent studies of Tregs at the maternal-fetal interface in pregnancy complications, as well as their identification in specific inflammatory lesions in the placenta.

Immunology of the maternal-fetal interface

The immune cells that reside at the interface between the placenta and uterus are thought to play many important roles in pregnancy. Recent work has revealed that the composition and function of these cells are locally controlled by the specialized uterine stroma (the decidua) that surrounds the implanted conceptus. Here, I discuss how key immune cell types (natural killer cells, macrophages, dendritic cells, and T cells) are either enriched or excluded from the decidua, how their function is regulated within the decidua, and how they variously contribute to pregnancy success or failure. The discussion emphasizes the relationship between human and mouse studies. Deeper understanding of the immunology of the maternal-fetal interface promises to yield significant insight into the pathogenesis of many human pregnancy complications, including preeclampsia, intrauterine growth restriction, spontaneous abortion, preterm birth, and congenital infection.

Natural killer cells promote immune tolerance by regulating inflammatory TH17 cells at the human maternal-fetal interface

Natural killer (NK) cells accumulate at the maternal-fetal interface in large numbers, but their exact roles in successful pregnancy remain poorly defined. Here, we provide evidence that T(H)17 cells and local inflammation can occur at the maternal-fetal interface during natural allogenic pregnancies. We found that decidual NK cells promote immune tolerance and successful pregnancy by dampening inflammatory T(H)17 cells via IFN-γ secreted by the CD56(bright)CD27(+) NK subset. This NK-cell-mediated regulatory response is lost in patients who experience recurrent spontaneous abortions, which results in a prominent T(H)17 response and extensive local inflammation. This local inflammatory response further affects the regulatory function of NK cells, leading to the eventual loss of maternal-fetal tolerance. Thus, our data identify NK cells as key regulatory cells at the maternal-fetal interface by suppressing T(H)17-mediated local inflammation.

Specific infiltration pattern of FOXP3+ regulatory T cells in chronic histiocytic intervillositis of unknown etiology

INTRODUCTION

Chronic histiocytic intervillositis of unknown etiology (CIUE) is a rare placental lesion characterized by an intervillous mononuclear inflammatory infiltrate of maternal origin. Although the mechanism and origin of these lesions are currently not understood, they appear to be related to an immune conflict between mother and fetus cells.

AIM

To clarify the inflammatory cell profile and evaluate the T regulatory lymphocyte (Treg) status in CIUE.

MATERIALS AND METHODS

All cases of CIUE that occurred over an 8-year period were analyzed using immunohistochemistry.

RESULTS

The inflammatory profile of CIUE was characterized by a clearly predominant component of histiocytic cells (80% ± 6.9) associated with some T cells (24% ± 5.7). The ratio of CD4+ versus CD8+ T cells was close to 1. This profile differs from infectious disease and chronic histiocytic villitis, the main differential diagnoses of CIUE. As for normal pregnancies most regulatory T cells were localized in the decidua basalis. Nevertheless, their appearance was also noted in the intervillous space. In both the intervillous space and the deciduas the number of Tregs gradually increased from grade 1 to 3.

CONCLUSION

We found that CIUE is associated with an increase in Treg lymphocytes in the decidua basalis and the intervillous space. Contrary to previously published data on human miscarriage, this result appears to be specific to CIUE and would support the hypothesis of an immunopathological disorder for CIUE.