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

Converging cytokine and metabolite networks shape asymmetric T cell fate at the term human maternal-fetal interface

Placentation presents immune conflict between mother and fetus, yet in normal pregnancy maternal immunity against infection is maintained without expense to fetal tolerance. This is believed to result from adaptations at the maternal-fetal interface (MFI) which affect T cell programming, but the identities (i.e., memory subsets and antigenic specificities) of T cells and the signals that mediate T cell fates and functions at the MFI remain poorly understood. We found intact recruitment programs as well as pro-inflammatory cytokine networks that can act on maternal T cells in an antigen-independent manner. These inflammatory signals elicit T cell expression of co-stimulatory receptors necessary for tissue retention, which can be engaged by local macrophages. Although pro-inflammatory molecules elicit T cell effector functions, we show that additional cytokine (TGF-β1) and metabolite (kynurenine) networks may converge to tune T cell function to those of sentinels. Together, we demonstrate an additional facet of fetal tolerance, wherein T cells are broadly recruited and restrained in an antigen-independent, cytokine/metabolite-dependent manner. These mechanisms provide insight into antigen-nonspecific T cell regulation, especially in tissue microenvironments where they are enriched.

Decidual-tissue-resident memory T cells protect against nonprimary human cytomegalovirus infection at the maternal-fetal interface

Congenital cytomegalovirus (cCMV) is the most common intrauterine infection, leading to infant neurodevelopmental disabilities. An improved knowledge of correlates of protection against cCMV is needed to guide prevention strategies. Here, we employ an ex vivo model of human CMV (HCMV) infection in decidual tissues of women with and without preconception immunity against CMV, recapitulating nonprimary vs. primary infection at the authentic maternofetal transmission site. We show that decidual tissues of women with preconception immunity against CMV exhibit intrinsic resistance to HCMV, mounting a rapid activation of tissue-resident memory CD8+ and CD4+ T cells upon HCMV reinfection. We further reveal the role of HCMV-specific decidual-tissue-resident CD8+ T cells in local protection against nonprimary HCMV infection. The findings could inform the development of a vaccine against cCMV and provide insights for further studies of the integrity of immune defense against HCMV and other pathogens at the human maternal-fetal interface.

Antigen-specific decidual CD8+ T cells include distinct effector memory and tissue-resident memory cells

Maternal decidual CD8+ T cells must integrate the antithetical demands of providing immunity to infection while maintaining immune tolerance for fetal and placental antigens. Human decidual CD8+ T cells were shown to be highly differentiated memory T cells with mixed signatures of dysfunction, activation, and effector function. However, no information is present on how specificity for microbial or fetal antigens relates to their function or dysfunction. In addition, a key question, whether decidual CD8+ T cells include unique tissue-resident memory T cells (Trm) or also effector memory T cell (Tem) types shared with peripheral blood populations, is unknown. Here, high-dimensional flow cytometry of decidual and blood CD8+ T cells identified 2 Tem populations shared in blood and decidua and 9 functionally distinct Trm clusters uniquely found in decidua. Interestingly, fetus- and virus-specific decidual CD8+ Trm cells had similar features of inhibition and cytotoxicity, with no significant differences in their expression of activation, inhibitory, and cytotoxic molecules, suggesting that not all fetus-specific CD8+ T cell responses are suppressed at the maternal-fetal interface. Understanding how decidual CD8+ T cell specificity relates to their function and tissue residency is crucial in advancing understanding of their contribution to placental inflammation and control of congenital infections.

Immunobiology of pregnancy: from basic science to translational medicine

Embryo implantation failure and spontaneous abortions represent the main causes of infertility in developed countries. Unfortunately, incomplete knowledge of the multiple factors involved in implantation and fetal development keeps the success rate of medically assisted procreation techniques relatively low. According to recent literature, cellular and molecular mechanisms of 'immunogenic tolerance' towards the embryo are crucial to establish an 'anti-inflammatory' state permissive of a healthy pregnancy. In this review we dissect the role played by the immune system in the endometrial-embryo crosstalk, with a particular emphasis towards the fork-head-box-p3 (Foxp3+) CD4+CD25+ regulatory T (Treg) cells and discuss the most recent therapeutic advances in the context of early immune-mediated pregnancy loss.

Multimodal profiling of term human decidua demonstrates immune adaptations with pregravid obesity

Leukocyte diversity of the first-trimester maternal-fetal interface has been extensively described; however, the immunological landscape of the term decidua remains poorly understood. We therefore profiled human leukocytes from term decidua collected via scheduled cesarean delivery. Relative to the first trimester, our analyses show a shift from NK cells and macrophages to T cells and enhanced immune activation. Although circulating and decidual T cells are phenotypically distinct, they demonstrate significant clonotype sharing. We also report significant diversity within decidual macrophages, the frequency of which positively correlates with pregravid maternal body mass index. Interestingly, the ability of decidual macrophages to respond to bacterial ligands is reduced with pregravid obesity, suggestive of skewing toward immunoregulation as a possible mechanism to safeguard the fetus against excessive maternal inflammation. These findings are a resource for future studies investigating pathological conditions that compromise fetal health and reproductive success.

Gestational Viral Infections: Role of Host Immune System

Viral infections in pregnancy are major causes of maternal and fetal morbidity and mortality. Infections can develop in the neonate transplacentally, perinatally, or postnatally (from breast milk or other sources) and lead to different clinical manifestations, depending on the viral agent and the gestational age at exposure. Viewing the peculiar tolerogenic status which characterizes pregnancy, viruses could exploit this peculiar immunological status to spread or affect the maternal immune system, adopting several evasion strategies. In fact, both DNA and RNA virus might have a deep impact on both innate and acquired immune systems. For this reason, investigating the interaction with these pathogens and the host's immune system during pregnancy is crucial not only for the development of most effective therapies and diagnosis but mostly for prevention. In this review, we will analyze some of the most important DNA and RNA viruses related to gestational infections.

Retina-arrestin specific CD8+ T cells are not implicated in HLA-A29-positive birdshot chorioretinitis

BACKGROUND

HLA-A29-positive birdshot chorioretinitis (BCR) is an inflammatory eye disorder that is generally assumed to be caused by an autoimmune response to HLA-A29-presented peptides from retinal arrestin (SAG), yet the epitopes recognized by CD8+ T cells from patients remain to be identified.

OBJECTIVES

The identification of natural ligands of SAG presented by HLA-A29. To quantify CD8+ T cells reactive to antigenic SAG peptides presented by HLA-A29 in patients and controls.

METHODS

We performed mass-spectrometry based immunopeptidomics of HLA-A29 of antigen-presenting cell lines from patients engineered to express SAG. MHC-I Dextramer technology was utilised to determine expansion of antigen-specific CD8+ T cells reactive to SAG peptides in complex with HLA-A29 in a cohort of BCR patients, HLA-A29-positive controls, and HLA-A29-negative controls.

RESULTS

We report on the naturally presented antigenic SAG peptides identified by sequencing the HLA-A29 immunopeptidome of antigen-presenting cells of patients. We show that the N-terminally extended SAG peptide precursors can be trimmed in vitro by the antigen-processing aminopeptidases ERAP1 and ERAP2. Unexpectedly, no enhanced antigen engagement by CD8+ T cells upon stimulation with SAG peptides was observed in patients or HLA-A29-positive controls. Multiplexed HLA-A29-peptide dextramer profiling of a case-control cohort revealed that CD8+ T cells specific for these SAG peptides were neither detectable in peripheral blood nor in eye biopsies of patients.

CONCLUSIONS

Collectively, these findings demonstrate that SAG is not a CD8+ T cell autoantigen and sharply contrast the paradigm in the pathogenesis of BCR. Therefore, the mechanism by which HLA-A29 is associated with BCR does not involve SAG.

Postpartum hepatitis and host immunity in pregnant women with chronic HBV infection

In order to develop immune tolerant to the fetal, maternal immune system will have some modification comparing to the time before pregnancy. Immune tolerance starts and develops at the maternal placental interface. In innate immunity, decidual natural killer (dNK) cells, macrophages and dendritic cells play a key role in immue tolerance. In adaptive immunity, a moderate increase of number and immune inhibition function of regulatory T cells (Treg) are essential for immune tolerance. The trophoblast cells and immune cells expressing indoleamine 2,3-dioxygenase (IDO), the trophoblast cells expressing HLA-G, and Th1/Th2 shifting to Th2 dominant and Th17/Treg shifting to Treg domiant are in favor of maternal fetal immune tolerance. Steroids (estrogen and progesterone) and human chorionic gonadotropin (HCG) also participate in immune tolerance by inducing Treg cells or upregulating immunosuppressive cytokines. Most of the patients with chronic HBV infection are in the "HBV immune tolerance period" before pregnancy, and the liver disease is relatively stable during pregnancy. In chronic HBV infection women, after delivery, the relative immunosuppression in vivo is reversed, and Th1 is dominant in Th1/Th2 and Th17 is dominant in Th17/Treg balance. After delivery, the number of Treg decrease and NK cells increase in quantity and cytotoxicity in peripheral blood. Liver NK cells may cause liver inflammation through a non-antigen specific mechanism. After delivery, the number of CD8⁺ T cells will increase and HBV specific T cell response recovers from the disfunction in pregnancy. Under the background of postpartum inflammation, the rapid decrease of cortisol after delivery, and especially the enhancement of HBV specific T cell response induced by HBV DNA and cytokines, are the main reasons for postpartum hepatitis. HBeAg positive, especially HBeAg<700 S/CO, and HBV DNA>3-5Log₁₀IU/ml are risk factors for postpartum hepatitis. Antiviral treatment in late pregnancy can reduce the incidence of mother to child transmission (MTCT) in chronic HBV infection women. Chronic HBV infection women have hepatitis both during pregnancy and more often in 12 weeks postpartum. It is generally agreed that postpartum hepatitis is mild symptoms and self-limited. Delaying drug withdrawal to 48 weeks can increase the seroconversion rate of HBeAg in delivery women with elevated alanine aminotransferase (ALT) in pregnancy.

Impaired CD8 T cells in term pregnancy decidua with chronic hepatitis B virus infection

PROBLEM

Hepatitis B virus (HBV) infection is more likely to develop a state of chronicity in early life, particularly mother-to-child transmission (MTCT) of HBV in the fetus during pregnancy. Till now, little is known about the impact of chronic HBV infection on the immune status of the maternal-fetus interface, and the immune profile of placental lymphocytes in MTCT of HBV is poorly understood.

METHOD OF STUDY

Thirteen term pregnant women with chronic HBV infection (HBV-PW) and thirteen normal pregnant women as healthy control (HC-PW) were enrolled. The profile of placental immune cells and paired peripheral blood were analyzed by flow cytometry and immunohistochemistry.

RESULTS

Compared with HC-PW, the frequency of CD8⁺ T cells from the term placenta of HBV-PW was significantly reduced. These cells showed decreased expression of activation molecules CD69 and HLA-DR; thus, decidual CD8⁺ T cells from HBV-PW demonstrated hypofunctional signature as evidenced by significantly reduced production of IFN-γ, as well as compromised ability of degranulation and proliferation.

CONCLUSIONS

These findings supported that hypoactivated decidual CD8⁺ T cells might possess compromised ability in chronically HBV-infected term pregnant women. Our study provides robust evidence for the necessity and importance of antiviral intervention in HBV-PW to prevent MTCT of HBV. This article is protected by copyright. All rights reserved.

Toward an understanding of allogeneic conflict in pregnancy and transplantation

Pregnancy is recognized as a spontaneously acquired state of immunological tolerance by the mother to her semi-allogeneic fetus, but it is a major cause of allosensitization in candidates for organ transplantation. This sensitization, assessed by the presence of anti-HLA IgG, contributes to sex disparity in access to transplantation and increases the risk for rejection and graft loss. Understanding this dual tolerance/sensitization conundrum may lead to new strategies for equalizing access to transplantation among sexes and improving transplant outcomes in parous women. Here, we review the clinical evidence that pregnancy results in humoral sensitization and query whether T cell responses are sensitized. Furthermore, we summarize preclinical evidence on the effects of pregnancy on fetus-specific CD4⁺ conventional, regulatory, and CD8⁺ T cells, and humoral responses. We end with a discussion on the impact of the divergent effects that pregnancy has upon alloantigen re-encounter in the context of solid organ transplantation, and how these insights point to a therapeutic roadmap for controlling pregnancy-dependent allosensitization.

The New Old CD8+ T Cells in the Immune Paradox of Pregnancy

CD8+ T cells are the most frequent T cell population in the immune cell compartment at the feto-maternal interface. Due to their cytotoxic potential, the presence of CD8+ T cells in the immune privileged pregnant uterus has raised considerable interest. Here, we review our current understanding of CD8+ T cell biology in the uterus of pregnant women and discuss this knowledge in relation to a recently published immune cell Atlas of human decidua. We describe how the expansion of CD8+ T cells with an effector memory phenotype often presenting markers of exhaustion is critical for a successful pregnancy, and host defense towards pathogens. Moreover, we review new evidence on the presence of long-lasting immunological memory to former pregnancies and discuss its impact on prospective pregnancy outcomes. The formation of fetal-specific memory CD8+ T cell subests in the uterus, in particular of tissue resident, and stem cell memory cells requires further investigation, but promises interesting results to come. Advancing the knowledge of CD8+ T cell biology in the pregnant uterus will be pivotal for understanding not only tissue-specific immune tolerance but also the etiology of complications during pregnancy, thus enabling preventive or therapeutic interventions in the future.

Immune Profile of the Normal Maternal-Fetal Interface in Rhesus Macaques and Its Alteration Following Zika Virus Infection

The maternal decidua is an immunologically complex environment that balances maintenance of immune tolerance to fetal paternal antigens with protection of the fetus against vertical transmission of maternal pathogens. To better understand host immune determinants of congenital infection at the maternal-fetal tissue interface, we performed a comparative analysis of innate and adaptive immune cell subsets in the peripheral blood and decidua of healthy rhesus macaque pregnancies across all trimesters of gestation and determined changes after Zika virus (ZIKV) infection. Using one 28-color and one 18-color polychromatic flow cytometry panel we simultaneously analyzed the frequency, phenotype, activation status and trafficking properties of αβ T, γδ T, iNKT, regulatory T (Treg), NK cells, B lymphocytes, monocytes, macrophages, and dendritic cells (DC). Decidual leukocytes showed a striking enrichment of activated effector memory and tissue-resident memory CD4+ and CD8+ T lymphocytes, CD4+ Tregs, CD56+ NK cells, CD14+CD16+ monocytes, CD206+ tissue-resident macrophages, and a paucity of B lymphocytes when compared to peripheral blood. t-distributed stochastic neighbor embedding (tSNE) revealed unique populations of decidual NK, T, DC and monocyte/macrophage subsets. Principal component analysis showed distinct spatial localization of decidual and circulating leukocytes contributed by NK and CD8+ T lymphocytes, and separation of decidua based on gestational age contributed by memory CD4+ and CD8+ T lymphocytes. Decidua from 10 ZIKV-infected dams obtained 16-56 days post infection at third (n=9) or second (n=1) trimester showed a significant reduction in frequency of activated, CXCR3+, and/or Granzyme B+ memory CD4+ and CD8+ T lymphocytes and γδ T compared to normal decidua. These data suggest that ZIKV induces local immunosuppression with reduced immune recruitment and impaired cytotoxicity. Our study adds to the immune characterization of the maternal-fetal interface in a translational nonhuman primate model of congenital infection and provides novel insight in to putative mechanisms of vertical transmission.

Evaluation of endometrial immune status of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is described as a low-grade chronic inflammatory state. However, there are limited studies on the specific endometrial immune status of PCOS patients. Whether this endometrial immune cell pattern is intrinsic to PCOS or the consequence of PCOS-associated obesity is a subject of debate. This study retrospectively included one hundred women diagnosed with PCOS and ninety-five normal fertile controls, which further divided into four groups (normoweight PCOS; overweight PCOS; normoweight control; overweight control) based on body mass index. The percentages of endometrial CD68+ macrophages (1.97 % vs. 1.17 %; P < 0.001), CD163+ M2 macrophages (2.30 % vs. 1.83 %; P = 0.001), CD1a+ iDCs (0.044 % vs. 0.029 %; P = 0.002), CD83+ mDCs (1.72 % vs. 1.07 %; P < 0.001) and CD8+ T cells (2.82 % vs. 1.95 %; P < 0.001) were significantly higher in normoweight PCOS women than normoweight controls. The percentage of CD68+ macrophages (2.09 % vs. 1.15 %; P < 0.001) was significantly higher in overweight PCOS women compared with overweight controls. In multivariant linear regression analysis, participants' PCOS status was the main predictors of endometrial CD68+ macrophages, CD163+ M2 macrophages, CD1a+ iDCs, CD83+ mDCs and CD8+ T cells in the whole study population. Additionally, in PCOS group, positive correlations were found between endometrial CD56+ NK, CD163+ M2 macrophages and QUICKI, indicating there was an association between endometrial immune cells and insulin resistance in PCOS women. Our study suggests that women with PCOS have altered endometrial immune cells, which may reflect a state of chronic low grade inflammation. The chronic inflammation, independent of obesity, may help understand the pathophysiologic mechanisms of intrinsic PCOS.

Interplay of immune components and their association with recurrent pregnancy loss

Maintenance and progression of pregnancy is an intricate process governed by a variety of developmental cues. Recurrent pregnancy loss (RPL) is a complication experienced by expecting mothers that is defined as three or more consecutive pregnancy losses. This review focuses on the dysfunctions of the immune system as one of the key contributors towards RPL. The current data suggests that the alloimmune and autoimmune factors contribute to the loss of fetus. Such causes despite being recognized as a definitive reason for recurrent pregnancy loss, are still under extensive investigation with new parameters being discovered and scrutinized for their association with RPLs. More in-depth and high throughput studies are required for devising better diagnostic tools and management strategies for the affected female so that they can carry their pregnancy to term.

Human Tissue-Resident Memory T Cells in the Maternal-Fetal Interface. Lost Soldiers or Special Forces?

The immune system plays a critical role during pregnancy, but the specific mechanisms and immune cell function needed to support pregnancy remain incompletely understood. Despite decades of research efforts, it is still unclear how the immune system maintains tolerance of fetal-derived tissues, which include most cells of the placenta and of course the fetus itself, without forfeiting the ability to protect against harmful infections. T cells recognize antigen in the context of major histocompatibility complex (MHC) encoded proteins, but classical MHC class I and II expression are diminished in fetal-derived cells. Can T cells present at the maternal–fetal interface (MFI) protect these cells from infection? Here we review what is known in regard to tissue-resident memory T (Trm) cells at the MFI. We mainly focus on how Trm cells can contribute to protection in the context of the unique features of the MFI, such as limited MHC expression as well as the temporary nature of the MFI, that are not found in other tissues.

Visualizing Dynamic Changes at the Maternal-Fetal Interface Throughout Human Pregnancy by Mass Cytometry

During healthy pregnancy, a balanced microenvironment at the maternal-fetal interface with coordinated interaction between various immune cells is necessary to maintain immunological tolerance. While specific decidual immune cell subsets have been investigated, a system-wide unbiased approach is lacking. Here, mass cytometry was applied for data-driven, in-depth immune profiling of the total leukocyte population isolated from first, second, and third trimester decidua, as well as maternal peripheral blood at time of delivery. The maternal-fetal interface showed a unique composition of immune cells, different from peripheral blood, with significant differences between early and term pregnancy samples. Profiling revealed substantial heterogeneity in the decidual lymphoid and myeloid cell lineages that shape gestational-specific immune networks and putative differentiation trajectories over time during gestation. Uncovering the overall complexity at the maternal-fetal interface throughout pregnancy resulted in a human atlas that may serve as a foundation upon which comprehension of the immune microenvironment and alterations thereof in pregnancy complications can be built.

Viral-Immune Cell Interactions at the Maternal-Fetal Interface in Human Pregnancy

The human decidua and placenta form a distinct environment distinguished for its promotion of immunotolerance to infiltrating semiallogeneic trophoblast cells to enable successful pregnancy. The maternal-fetal interface also successfully precludes transmission of most pathogens. This barrier function occurs in conjunction with a diverse influx of decidual immune cells including natural killer cells, macrophages and T cells. However, several viruses, among other microorganisms, manage to escape destruction by the host adaptive and innate immune system, leading to congenital infection and adverse pregnancy outcomes. In this review, we describe mechanisms of pathogenicity of two such viral pathogens, Human cytomegalovirus (HCMV) and Zika virus (ZIKV) at the maternal-fetal interface. Host decidual immune cell responses to these specific pathogens will be considered, along with their interactions with other cell types and the ways in which these immune cells may both facilitate and limit infection at different stages of pregnancy. Neither HCMV nor ZIKV naturally infect commonly used animal models [e.g., mice] which makes it challenging to understand disease pathogenesis. Here, we will highlight new approaches using placenta-on-a-chip and organoids models that are providing functional and physiologically relevant ways to study viral-host interaction at the maternal-fetal interface.

Mechanisms and evidence of vertical transmission of infections in pregnancy including SARS-CoV-2s

There remain unanswered questions concerning mother‐to‐child‐transmission of SARS‐CoV‐2. Despite reports of neonatal COVID‐19, SARS‐CoV‐2 has not been consistently isolated in perinatal samples, thus definitive proof of transplacental infection is still lacking. To address these questions, we assessed investigative tools used to confirm maternal‐fetal infection and known protective mechanisms of the placental barrier that prevent transplacental pathogen migration. Forty studies of COVID‐19 pregnancies reviewed suggest a lack of consensus on diagnostic strategy for congenital infection. Although real‐time polymerase chain reaction of neonatal swabs was universally performed, a wide range of clinical samples was screened including vaginal secretions (22.5%), amniotic fluid (35%), breast milk (22.5%) and umbilical cord blood. Neonatal COVID‐19 was reported in eight studies, two of which were based on the detection of SARS‐CoV‐2 IgM in neonatal blood. Histological examination demonstrated sparse viral particles, vascular malperfusion and inflammation in the placenta from pregnant women with COVID‐19. The paucity of placental co‐expression of ACE‐2 and TMPRSS2, two receptors involved in cytoplasmic entry of SARS‐CoV‐2, may explain its relative insensitivity to transplacental infection. Viral interactions may utilise membrane receptors other than ACE‐2 thus, tissue susceptibility may be broader than currently known. Further spatial‐temporal studies are needed to determine the true potential for transplacental migration.

Got your mother in a whirl: The role of maternal T cells and myeloid cells in pregnancy

Appropriate development of the placenta is required for healthy pregnancy to occur. After implantation of the fertilized blastocyst, fetal trophoblasts invade the endometrium and myometrium of the mother's uterus to establish placentation. In this process, fetal trophoblasts encounter maternal immune cells. In this review, we focus on the role of maternal T cells and myeloid cells (macrophages, dendritic cells) in pregnancy and their interaction with trophoblasts. To retain immunologic tolerization, trophoblasts evade immune recognition by T cells and produce factors that modulate their phenotype and function. On top of that, the local environment at the maternal-fetal interface favors expansion of regulatory T cells. Macrophages and dendritic cells are essential in maintaining a healthy pregnancy. They produce soluble factors and act as antigen-presenting cells, thereby interacting with T cells. Herein, M2 macrophages, immature dendritic cells, CD4⁺ Th2 cells, and regulatory T cells represent an axis that maintains a local immune tolerant environment. We consider outstanding issues concerning these cell types and their pathways, which need to be addressed in future investigations. Data from recent single-cell sequencing experiments of the placental bed, to study heterogeneity of maternal immune cells and to predict cell-cell interactions, are discussed. Novel ways for long-term culturing of primary trophoblasts allow for cell-cell interaction studies in a functional way. Future directions should include study of the functionality of currently known and newly identified decidual immune cell subsets in healthy and complicated pregnancies, and their interaction with and modulation by trophoblast cells.

NK and T Cell Differentiation at the Maternal-Fetal Interface in Sows During Late Gestation

The phenotype and function of immune cells that reside at the maternal-fetal interface in humans and mice have been, and still are, extensively studied with the aim to fully comprehend the complex immunology of pregnancy. In pigs, information regarding immune cell phenotypes is limited and mainly focused on early gestation whereas late gestation has not yet been investigated. We designed a unique methodology tailored to the porcine epitheliochorial placenta, which allowed us to address immune phenotypes separately in the maternal endometrium (ME) and fetal placenta (FP) by flow cytometry. In-depth phenotyping of NK cells, non-conventional and conventional T cells within maternal blood (mBld), ME, FP, and fetal spleen (fSpln) revealed major differences between these anatomic sites. In both maternal compartments, all NK cells were perforin⁺ and had NKp46-defined phenotypes indicative of late-stage differentiation. Likewise, T cells with a highly differentiated phenotype including CD2⁺CD8α⁺CD27^dim/–perforin⁺ γδ T cells, CD27^–perforin⁺ cytolytic T cells (CTLs), and T-bet⁺ CD4⁺CD8α⁺CD27^– effector memory T (Tem) cells prevailed within these compartments. The presence of highly differentiated T cells was also reflected in the number of cells that had the capacity to produce IFN-γ. In the FP, we found NK cells and T cell populations with a naive phenotype including CD2⁺CD8α^–CD27⁺perforin^– γδ T cells, T-bet^–CD4⁺CD8α^–CD27⁺ T cells, and CD27⁺perforin^– CTLs. However, also non-naive T cell phenotypes including CD2⁺CD8α⁺CD27⁺perforin^– γδ T cells, T-bet⁺CD4⁺CD8α⁺CD27^– Tem cells, and a substantial proportion of CD27^–perforin⁺ CTLs resided within this anatomic site. Currently, the origin or the cues that steer the differentiation of these putative effector cells are unclear. In the fSpln, NKp46^high NK cells and T cells with a naive phenotype prevailed. This study demonstrated that antigen-experienced immune cell phenotypes reside at the maternal-fetal interface, including the FP. Our methodology and our findings open avenues to study NK and T cell function over the course of gestation. In addition, this study lays a foundation to explore the interplay between immune cells and pathogens affecting swine reproduction.

Functional significance of lymphocytes in pregnancy and lymphocyte immunotherapy in infertility: A comprehensive review and update

During pregnancy, the fetal-maternal interface underlies several dynamic alterations to permit the fetus to be cultivated and developed in the uterus, in spite of being identifies by the maternal immune system. A large variety of decidual leukocyte populations, including natural killer cells, NKT cells, innate lymphoid cells, dendritic cells, B cells, T cells, subpopulations of helper T cells play a vital role in controlling the trophoblast invasion, angiogenesis as well as vascular remodeling. In contrast, several regulatory immunosuppressive mechanisms, including regulatory T cells, regulatory B cells, several cytokines and mediators are involved in maintain the homeostasis of immune system in the fetal-maternal interface. Nonetheless, aberrant alterations in the balance of immune inflammatory or immunosuppressive arms have been associated with various pregnancy losses and infertilities. As a result, numerous strategies have been developed to revers dysregulated balance of immune players to increase the chance of successful pregnancy. Lymphocyte immunotherapy has been developed through utilization of peripheral white blood cells of the husband or others and administered into the mother to confer an immune tolerance for embryo's antigens. However, the results have not always been promising, implying to further investigations to improve the approach. This review attempts to clarify the involvement of lymphocytes in contributing to the pregnancy outcome and the potential of lymphocyte immunotherapy in treatment of infertilities with dysregulated immune system basis.

Investigation of mucosal-associated invariant T (MAIT) cells expressing immune checkpoint receptors (TIGIT and CD226) in early-onset preeclampsia

OBJECTIVE

During our work, we examined the possible contribution of MAIT cells in the pathogenesis of the clinical phase of early-onset preeclampsia and how this could be influenced by TIGIT and CD226 immune checkpoint molecules.

STUDY DESIGN

37 pregnant women diagnosed with early-onset preeclampsia and 36 healthy, age-matched control women were involved in this study. Peripheral blood mononuclear cells were isolated by density gradient and frozen. After thawing, cells were stained with monoclonal antibodies to characterize MAIT, MAIT-like, and non-MAIT cells. Flow cytometric analyses were used to measure TIGIT, CD226, intracellular perforin, and granzyme B expression.

RESULTS

MAIT (CD3+ CD8+ Vα7.2+ CD161++), MAIT-like (CD3+ CD8+ Vα7.2+ CD161+) and non-MAIT (CD3+ CD8+ Vα7.2+ CD161-) cell population were identified based on their CD161 receptor positivity. MAIT cells markedly differed in proportion, TIGIT expression, granzyme B, and perforin content compared to MAIT-like and non-MAIT cells. A significant difference was determined in TIGIT expression by non-MAIT cells and in CD8/CD226 positive relationship between the preeclamptic and healthy condition.

CONCLUSIONS

Considering that we did not detect a notable difference between early-onset preeclampsia and healthy pregnancy, we suppose that peripheral MAIT cells expressing TIGIT and CD226 immune checkpoint molecules have a marginal role in the pathogenesis of early-onset preeclampsia.

Decidual CD8+T cells exhibit both residency and tolerance signatures modulated by decidual stromal cells

BACKGROUND

During early pregnancy, tolerance of the semi-allogeneic fetus necessitates comprehensive modifications of the maternal immune system. How decidual CD8+T (CD8+dT) cells balance maternal tolerance of the fetus with defense from invading pathogens remains undefined.

METHODS

We investigated the distribution patterns of CD8+T cells and their heterogeneity in paired peripheral blood and decidual tissue in the first trimester of pregnancy using flow cytometry and mRNA-Seq. Gene Set Enrichment Analysis was utilized to determine the transcriptional features of CD8+dT cells. Moreover, we examined activation of T cells when they were cocultured with trophoblasts, in addition to the effect of the fetal-maternal environment on peripheral CD8+T (CD8+pT) cells.

RESULTS

We found that, compared with CD8+pT cells, CD8+dT cells consisted mainly of effector memory cells (TEM) and terminally differentiated effector memory cells (TEMRA). Both TEM and TEMRA subsets contained increased numbers of CD27+CD28- cells, which have been shown to possess only partial effector functions. In-depth analysis of the gene-expression profiles of CD8+dT cells revealed significant enrichment in T cell exhaustion-related genes and core tissue residency signature genes that have been found recently to be shared by tissue resident memory cells and tumor-infiltrating lymphocytes (TILs). In accordance with gene expression, protein levels of the exhaustion-related molecules PD-1 and CD39 and the tissue resident molecules CD103 and CXCR3 were increased significantly with almost no perforin secretion in CD8+dT cells compared with CD8+pT cells. However, the levels of granzyme B, IFN-γ, and IL-4 in CD8+dT cells were increased significantly compared with those in CD8+pT cells. Both CD8+dT and CD8+pT cells were not activated after being cocultured with autologous trophoblast cells. Moreover, the production of granzyme B in CD103+CD8+dT cells decreased significantly compared with that in their CD103- counterparts. Coculture with decidual stromal cells and trophoblasts upregulated CD103 expression significantly in CD8+pT cells.

CONCLUSIONS

Our findings indicate that the selective silencing of effector functions of resident CD8+dT cells may favor maternal-fetal tolerance and that the decidual microenvironment plays an important role in promoting the residency of CD8+T cells and their tolerance-defense balance.

Maternal and fetal T cells in term pregnancy and preterm labor

Pregnancy is a state of immunological balance during which the mother and the developing fetus must tolerate each other while maintaining sufficient immunocompetence to ward off potential threats. The site of closest contact between the mother and fetus is the decidua, which represents the maternal-fetal interface. Many of the immune cell subsets present at the maternal-fetal interface have been well described; however, the importance of the maternal T cells in this compartment during late gestation and its complications, such as preterm labor and birth, has only recently been established. Moreover, pioneer and recent studies have indicated that fetal T cells are activated in different subsets of preterm labor and may elicit distinct inflammatory responses in the amniotic cavity, leading to preterm birth. In this review, we describe the established and proposed roles for maternal T cells at the maternal-fetal interface in normal term parturition, as well as the demonstrated contributions of such cells to the pathological process of preterm labor and birth. We also summarize the current knowledge of and proposed roles for fetal T cells in the pathophysiology of the preterm labor syndrome. It is our hope that this review provides a solid conceptual framework highlighting the importance of maternal and fetal T cells in late gestation and catalyzes new research questions that can further scientific understanding of these cells and their role in preterm labor and birth, the leading cause of neonatal mortality and morbidity worldwide.

The Dual Role of HLA-C in Tolerance and Immunity at the Maternal-Fetal Interface

To establish a healthy pregnancy, maternal immune cells must tolerate fetal allo-antigens and remain competent to respond to infections both systemically and in placental tissues. Extravillous trophoblasts (EVT) are the most invasive cells of extra-embryonic origin to invade uterine tissues and express polymorphic Human Leucocyte Antigen-C (HLA-C) of both maternal and paternal origin. Thus, HLA-C is a key molecule that can elicit allogeneic immune responses by maternal T and NK cells and for which maternal-fetal immune tolerance needs to be established. HLA-C is also the only classical MHC molecule expressed by EVT that can present a wide variety of peptides to maternal memory T cells and establish protective immunity. The expression of paternal HLA-C by EVT provides a target for maternal NK and T cells, whereas HLA-C expression levels may influence how this response is shaped. This dual function of HLA-C requires tight transcriptional regulation of its expression to balance induction of tolerance and immunity. Here, we critically review new insights into: (i) the mechanisms controlling expression of HLA-C by EVT, (ii) the mechanisms by which decidual NK cells, effector T cells and regulatory T cells recognize HLA-C allo-antigens, and (iii) immune recognition of pathogen derived antigens in context of HLA-C.

Immune Cells at the Fetomaternal Interface: How the Microenvironment Modulates Immune Cells To Foster Fetal Development

Immune cells adapt their phenotypic and functional characteristics in response to the tissue microenvironment within which they traffic and reside. The fetomaternal interface, consisting of placental trophoblasts and the maternal decidua, is a highly specialized tissue with a unique and time-limited function: to nourish and support development of the semiallogeneic fetus and protect it from inflammatory or immune-mediated injury. It is therefore important to understand how immune cells within these tissues are educated and adapt to fulfill their biological functions. This review article focuses on the local regulatory mechanisms ensuring that both innate and adaptive immune cells appropriately support the early events of implantation and placental development through direct involvement in promoting immune tolerance of fetal alloantigens, suppressing inflammation, and remodeling of maternal uterine vessels to facilitate optimal placental function and fetal growth.

The appropriate frequency and function of decidual Tim-3+CTLA-4+CD8+ T cells are important in maintaining normal pregnancy

Maternal decidual CD8⁺ T (dCD8⁺ T) cells must integrate the antithetical demands of maternal–fetal tolerance and anti-viral immunity to establish a successful pregnancy. T-cell immunoglobulin mucin-3 (Tim-3) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) are two important co-inhibitory molecules that regulating CD8⁺ T cells responses during infection and tumor. In the present study, we examined the co-expression of Tim-3 and CTLA-4 on CD8⁺ T cells during pregnancy and found the higher frequency of Tim-3⁺CTLA-4⁺dCD8⁺ T cells in response to trophoblasts. This Tim-3⁺CTLA-4⁺dCD8⁺ T cells subset showed an active status and produced more anti-inflammatory cytokines. Furthermore, the decreased number and altered function of Tim-3⁺CTLA-4⁺dCD8⁺ T cells correlated to miscarriage. Combined blocking Tim-3 and CTLA-4 pathways were highly effective in inhibiting the production of anti-inflammatory cytokines and were detrimental to the maintenance of pregnancy. Together, these findings supported that Tim-3 and CTLA-4 pathways might play positive roles in the establishment and/or maintenance of maternal–fetal tolerance so to promote the maintenance of normal pregnancy. So the reproductive safety must be considered, especially when anti-Tim-3/CTLA-4 antibody (and other immune checkpoint inhibitors) are used in pregnancy.

Memory T Cells in Pregnancy

Adaptations of the maternal immune response are necessary for pregnancy success. Insufficient immune adaption is associated with pregnancy pathologies such as infertility, recurrent miscarriage, fetal growth restriction, spontaneous preterm birth, and preeclampsia. The maternal immune system is continuously exposed to paternal-fetal antigens; through semen exposure from before pregnancy, through fetal cell exposure in pregnancy, and through microchimerism after pregnancy. This results in the generation of paternal-fetal antigen specific memory T cells. Memory T cells have the ability to remember previously encountered antigens to elicit a quicker, more substantial and focused immune response upon antigen reencounter. Such fetal antigen specific memory T cells could be unfavorable in pregnancy as they could potentially drive fetal rejection. However, knowledge on memory T cells in pregnancy has shown that these cells might play a favorable role in fetal-maternal tolerance rather than rejection of the fetus. In recent years, various aspects of immunologic memory in pregnancy have been elucidated and the relevance and working mechanisms of paternal-fetal antigen specific memory T cells in pregnancy have been evaluated. The data indicate that a delicate balance of memory T cells seems necessary for reproductive success and that immunologic memory in reproduction might not be harmful for pregnancy. This review provides an overview of the different memory T cell subtypes and their function in the physiology and in complications of pregnancy. Current findings in the field and possible therapeutic targets are discussed. The findings of our review raise new research questions for further studies regarding the role of memory T cells in immune-associated pregnancy complications. These studies are needed for the identification of possible targets related to memory mechanisms for studies on preventive therapies.

New Paradigm in the Role of Regulatory T Cells During Pregnancy

Semi-allogenic fetuses are not rejected by the maternal immune system because feto-maternal tolerance induced by CD4⁺CD25⁺FoxP3⁺ regulatory T (Treg) cells is established during pregnancy. Paternal antigen-specific Treg cells accumulate during pregnancy, and seminal plasma priming plays an important role in expanding paternal antigen-specific Treg cells in mouse models. Although paternal-antigen specific Treg cells have not been identified in humans, recent studies suggest that antigen-specific Treg cells exist and expand at the feto-maternal interface in humans. Studies have also revealed that reduction of decidual functional Treg cells occurs during miscarriage with normal fetal chromosomal content, whereas insufficient clonal expansion of decidual Treg cells is observed in preeclampsia. In this review, we will discuss the recent advances in the investigation of mechanisms underlying Treg cell-dependent maintenance of feto-maternal tolerance.

Cross-Reactivity of Virus-Specific CD8+ T Cells Against Allogeneic HLA-C: Possible Implications for Pregnancy Outcome

Heterologous immunity of virus-specific T cells poses a potential barrier to transplantation tolerance. Cross-reactivity to HLA-A and -B molecules has broadly been described, whereas responses to allo-HLA-C have remained ill defined. In contrast to the transplant setting, HLA-C is the only polymorphic HLA molecule expressed by extravillous trophoblasts at the maternal-fetal interface during pregnancy. Uncontrolled placental viral infections, accompanied by a pro-inflammatory milieu, can alter the activation status and stability of effector T cells. Potential cross-reactivity of maternal decidual virus-specific T cells to fetal allo-HLA-C may thereby have detrimental consequences for the success of pregnancy. To explore the presence of cross-reactivity to HLA-C and the other non-classical HLA antigens expressed by trophoblasts, HLA-A and -B-restricted CD8+ T cells specific for Epstein-Barr virus, Cytomegalovirus, Varicella-Zoster virus, and Influenza virus were tested against target cells expressing HLA-C, -E, and -G molecules. An HLA-B^*08:01-restricted EBV-specific T cell clone displayed cross-reactivity against HLA-C^*01:02. Furthermore, cross-reactivity of HLA-C-restricted virus-specific CD8+ T cells was observed for HCMV HLA-C^*06:02/TRA CD8+ T cell lines and clones against HLA-C^*03:02. Collectively, these results demonstrate that cross-reactivity against HLA-C can occur and thereby may affect pregnancy outcome.

Interferons and Proinflammatory Cytokines in Pregnancy and Fetal Development

Successful pregnancy requires carefully-coordinated communications between the mother and fetus. Immune cells and cytokine signaling pathways participate as mediators of these communications to promote healthy pregnancy. At the same time, certain infections or inflammatory conditions in pregnant mothers cause severe disease and have detrimental impacts on the developing fetus. In this review, we examine evidence for the role of maternal and fetal immune responses affecting pregnancy and fetal development, both under homeostasis and following infection. We discuss immune responses that are necessary to promote healthy pregnancy and those that lead to congenital disorders and pregnancy complications, with a particular emphasis on the role of interferons and cytokines. Understanding the contributions of the immune system in pregnancy and fetal development provides important insights into the pathogenesis underlying maternal and fetal diseases and sheds insights on possible targets for therapy.

Cross-reactive Dengue virus-specific CD8+ T cells protect against Zika virus during pregnancy

As Zika virus (ZIKV) emerges into Dengue virus (DENV)-endemic areas, cases of ZIKV infection in DENV-immune pregnant women may rise. Here we show that prior DENV immunity affects maternal and fetal ZIKV infection in pregnancy using sequential DENV and ZIKV infection models. Fetuses in ZIKV-infected DENV-immune dams were normal sized, whereas fetal demise occurred in non-immune dams. Moreover, reduced ZIKV RNA is present in the placenta and fetuses of ZIKV-infected DENV-immune dams. DENV cross-reactive CD8⁺ T cells expand in the maternal spleen and decidua of ZIKV-infected dams, their depletion increases ZIKV infection in the placenta and fetus, and results in fetal demise. The inducement of cross-reactive CD8⁺ T cells via peptide immunization or adoptive transfer results in decreased ZIKV infection in the placenta. Prior DENV immunity can protect against ZIKV infection during pregnancy in mice, and CD8⁺ T cells are sufficient for this cross-protection. This has implications for understanding the natural history of ZIKV in DENV-endemic areas and the development of optimal ZIKV vaccines.

Mixed signature of activation and dysfunction allows human decidual CD8+ T cells to provide both tolerance and immunity

Understanding how decidual CD8⁺ T cell (CD8⁺ dT) cytotoxicity is regulated and how these cells integrate the competing needs for maternal-fetal tolerance and immunity to infection is an important research and clinical goal. Gene-expression analysis of effector-memory CD8⁺ dT demonstrated a mixed transcriptional signature of T cell dysfunction, activation, and effector function. High protein expression of coinhibitory molecules PD1, CTLA4, and LAG3, accompanied by low expression of cytolytic molecules suggests that the decidual microenvironment reduces CD8⁺ dT effector responses to maintain tolerance to fetal antigens. However, CD8⁺ dT degranulated, proliferated, and produced IFN-γ, TNF-α, perforin, and granzymes upon in vitro stimulation, demonstrating that CD8⁺ dT are not permanently suppressed and retain the capacity to respond to proinflammatory events, such as infections. The balance between transient dysfunction of CD8⁺ dT that are permissive of placental and fetal development, and reversal of this dysfunctional state, is crucial in understanding the etiology of pregnancy complications and prevention of congenital infections.

Lymphocytes in Placental Tissues: Immune Regulation and Translational Possibilities for Immunotherapy

Immune modulation at the fetomaternal interface is crucial to ensure that the fetal allograft is not rejected. In the present review, the focus is to describe basic functions of lymphocyte populations and how they may contribute to fetomaternal immune regulation, as well as determining what proportions and effector functions of these cells are reported to be present in placental tissues in humans. Also explored is the possibility that unique cell populations at the fetomaternal interface may be targets for adoptive cell therapy. Increasing the understanding of immune modulation during pregnancy can give valuable insight into other established fields such as allogeneic hematopoietic stem cell transplantation and solid organ transplantation. In these settings, lymphocytes are key components that contribute to inflammation and rejection of either patient or donor tissues following transplantation. In contrast, an allogeneic fetus eludes rejection by the maternal immune system.

Up-and-down immunity of pregnancy in humans

One part of the human placenta in early pregnancy is particularly important for local immunity: the decidua basalis, which is transformed endometrium located at the site of embryo implantation . This placental bed tissue contains both maternal uterine immune cells, including decidual natural killer (NK) cells, the dominant leukocyte population exhibiting a unique phenotype, and fetal extravillous trophoblast which comes into direct contact with maternal decidual cells . To establish a successful placental development and healthy pregnancy outcome, the maternal immune system must tolerate paternal antigens expressed by trophoblast cells yet remain efficient for clearing any local pathogen infection. This review deals mainly with decidual NK cells. A key element, among others, to achieve such dual functions is the direct interaction between activating and inhibitory receptors expressed by decidual NK cells and their specific ligands presented by trophoblast or other decidual cells. Depending whether maternal decidual cells and trophoblast are infected by viruses, the balance between activating and inhibitory receptor signals mediated by decidual NK cell-trophoblast cross-talk results in tolerance (healthy pregnancy) or specific killing (pathogen-infected cells).

MAIT cells accumulate in placental intervillous space and display a highly cytotoxic phenotype upon bacterial stimulation

During pregnancy, the maternal immune system must tolerate the developing foetus, and yet retain a potent antimicrobial response to prevent infections. Mucosal associated invariant T (MAIT) cells recognize microbial-derived vitamin B metabolites presented on the MR1 molecule, but their presence and function at the foetal-maternal interface is not known. We here isolated mononuclear cells from paired samples of peripheral blood (PB), intervillous blood (IVB), and decidua parietalis (DP) following uncomplicated term pregnancies. Interestingly, MAIT cells were highly enriched in IVB compared to PB and DP. The activation status of IVB MAIT cells was similar to that of PB MAIT cells, except for a lower expression of PD-1. Both IVB MAIT cells and conventional T cells were more dominated by an effector memory phenotype compared to PB MAIT cells and T cells. IVB MAIT cells also responded more vigorously with expression of IFN-γ, granzyme B, and perforin in response to Escherichia coli stimulation compared to PB. MR1 was not expressed in syncytiotrophoblasts, but in placental villous and decidual macrophages. These data indicate that maternal MAIT cells accumulate in the intervillous space of the placenta and that they are highly armed to quickly respond if bacteria are encountered at the foetal-maternal interface.

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.

Immune Regulation in Pregnancy: A Matter of Perspective?

The maternal immune system is complex and governed by multiple hormonal and metabolic factors, including those provided to the mother via the fetus. Understanding of the balance between maternal tolerance and protection of the fetus may require thinking from multiple theoretical approaches to the general problem of immune activation and tolerance. This article provides a brief review of the immune system, with aspects relevant to pregnancy. The references include reviews that expand on the elements discussed. The article also uses different models of immune system activation and tolerance to provide a theoretical understanding of the problem of maternal tolerance.

Immune Regulation in Pregnancy: A Matter of Perspective?

The maternal immune system is complex and governed by multiple hormonal and metabolic factors, including those provided to the mother via the fetus. Understanding of the balance between maternal tolerance and protection of the fetus may require thinking from multiple theoretical approaches to the general problem of immune activation and tolerance. This article provides a brief review of the immune system, with aspects relevant to pregnancy. The references include reviews that expand on the elements discussed. The article also uses different models of immune system activation and tolerance to provide a theoretical understanding of the problem of maternal tolerance.