Listeria monocytogenes cytoplasmic entry induces fetal wastage by disrupting maternal Foxp3+ regulatory T cell-sustained fetal tolerance

CD4+ but not CD8+ T cells are required for protection against severe guinea pig cytomegalovirus infections

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus and the leading cause of infectious disease related birth defects worldwide. How the immune response modulates the risk of intrauterine transmission of HCMV after maternal infection remains poorly understood. Maternal T cells likely play a critical role in preventing infection at the maternal-fetal interface and limiting spread across the placenta, but concerns exist that immune responses to infection may also cause placental dysfunction and adverse pregnancy outcomes. This study investigated the role of CD4+ and CD8+ T cells in a guinea pig model of primary cytomegalovirus infection. Monoclonal antibodies specific to guinea pig CD4 and CD8 were used to deplete T cells in non-pregnant and in pregnant guinea pigs after mid-gestation. CD4+ T cell depletion increased the severity of illness, caused significantly elevated viral loads, and increased the rate of congenital guinea pig cytomegalovirus (GPCMV) infection relative to animals treated with control antibody. CD8+ T cell depletion was comparably well tolerated and did not significantly affect the weight of infected guinea pigs or viral loads in their blood or tissue. However, significantly more viral genomes and transcripts were detected in the placenta and decidua of CD8+ T cell depleted dams post-infection. This study corroborates earlier findings made in nonhuman primates that maternal CD4+ T cells play a critical role in limiting the severity of primary CMV infection during pregnancy while also revealing that other innate and adaptive immune responses can compensate for an absent CD8+ T cell response in α-CD8-treated guinea pigs.

Crossing the Barrier: A Comparative Study of Listeria monocytogenes and Treponema pallidum in Placental Invasion

Vertically transmitted infections are a significant cause of fetal morbidity and mortality during pregnancy and pose substantial risks to fetal development. These infections are primarily transmitted to the fetus through two routes: (1) direct invasion and crossing the placenta which separates maternal and fetal circulation, or (2) ascending the maternal genitourinary tact and entering the uterus. Only two bacterial species are commonly found to cross the placenta and infect the fetus: Listeria monocytogenes and Treponema pallidum subsp. pallidum. L. monocytogenes is a Gram-positive, foodborne pathogen found in soil that acutely infects a wide variety of mammalian species. T. pallidum is a sexually transmitted spirochete that causes a chronic infection exclusively in humans. We briefly review the pathogenesis of these two very distinct bacteria that have managed to overcome the placental barrier and the role placental immunity plays in resisting infection. Both organisms share characteristics which contribute to their transplacental transmission. These include the ability to disseminate broadly within the host, evade immune phagocytosis, and the need for a strong T cell response for their elimination.

Reproductive outcomes after pregnancy-induced displacement of preexisting microchimeric cells

Pregnancy confers partner-specific protection against complications in future pregnancy that parallel persistence of fetal microchimeric cells (FMcs) in mothers after parturition. We show that preexisting FMcs become displaced by new FMcs during pregnancy and that FMc tonic stimulation is essential for expansion of protective fetal-specific forkhead box P3 (FOXP3)-positive regulatory T cells (Treg cells). Maternal microchimeric cells and accumulation of Treg cells with noninherited maternal antigen (NIMA) specificity are similarly overturned in daughters after pregnancy, highlighting a fixed microchimeric cell niche. Whereas NIMA-specific tolerance is functionally erased by pregnancy, partner-specific resiliency against pregnancy complications persists in mothers despite paternity changes in intervening pregnancy. Persistent fetal tolerance reflects FOXP3 expression plasticity, which allows mothers to more durably remember their babies, whereas daughters forget their mothers with new pregnancy-imprinted immunological memories.

Human Listeriosis

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that can cause severe invasive infections upon ingestion with contaminated food. Clinically, listerial disease, or listeriosis, most often presents as bacteremia, meningitis or meningoencephalitis, and pregnancy-associated infections manifesting as miscarriage or neonatal sepsis. Invasive listeriosis is life-threatening and a main cause of foodborne illness leading to hospital admissions in Western countries. Sources of contamination can be identified through international surveillance systems for foodborne bacteria and strains' genetic data sharing. Large-scale whole genome studies have increased our knowledge on the diversity and evolution of L. monocytogenes, while recent pathophysiological investigations have improved our mechanistic understanding of listeriosis. In this article, we present an overview of human listeriosis with particular focus on relevant features of the causative bacterium, epidemiology, risk groups, pathogenesis, clinical manifestations, and treatment and prevention.

A Framework for Understanding Maternal Immunity

This is an alternative and controversial framing of the data relevant to maternal immunity. It argues for a departure from classical theory to view, interrogate and interpret existing data.

Emergent roles of maternal microchimerism in postnatal development

Maternal microchimerism (MMc) is the phenomenon that a low number of cells from the mother persists within her progeny. Despite their regular presence in mammalian pregnancies, the overall cell type repertoire and roles of maternal cells, especially after birth, remain unclear. By using transgenic mouse strains and human umbilical blood samples, recent studies have for the first time characterized and quantified MMc cell type repertoires in offspring, identified the cross-generational influence on fetal immunity, and determined possible factors that affect their presence in offspring. This review summarizes new findings, especially on the maternal cell type repertoires and their potential role in utero, in postnatal life, and long after birth.

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.

Epidemiology of Pregnancy Complications Through the Lens of Immunological Memory

In the fifteen minutes it takes to read this short commentary, more than 400 babies will have been born too early, another 300 expecting mothers will develop preeclampsia, and 75 unborn third trimester fetuses will have died in utero (stillbirth). Given the lack of meaningful progress in understanding the physiological changes that occur to allow a healthy, full term pregnancy, it is perhaps not surprising that effective therapies against these great obstetrical syndromes that include prematurity, preeclampsia, and stillbirth remain elusive. Meanwhile, pregnancy complications remain the leading cause of infant and childhood mortality under age five. Does it have to be this way? What more can we collectively, as a biomedical community, or individually, as clinicians who care for women and newborn babies at high risk for pregnancy complications, do to protect individuals in these extremely vulnerable developmental windows? The problem of pregnancy complications and neonatal mortality is extraordinarily complex, with multiple unique, but complementary perspectives from scientific, epidemiological and public health viewpoints. Herein, we discuss the epidemiology of pregnancy complications, focusing on how the outcome of prior pregnancy impacts the risk of complication in the next pregnancy — and how the fundamental immunological principle of memory may promote this adaptive response.

An Update Review on Listeria Infection in Pregnancy

Listeria monocytogenes (LM) is an intracellular, aerobic and facultative anaerobic, Gram-positive bacterium, which is primarily transmitted to humans orally via food. LM could occur in asymptomatic pregnant women; however, fetal infection is a serious condition, entailing premature birth, abortion, sepsis, central nervous system (CNS) involvement, or even death. If a pregnant woman exhibits symptoms, the performance is almost like influenza, such as fever, headache, diarrhea, myalgia, or other digestive-related symptoms. This review collected clinical and empirical results regarding the mechanism, clinical manifestations, obstetrical outcome, diagnosis, treatment, vertical transmission, neonatal infection, and prevention of listeriosi according to articles published in PubMed from January 1, 1980, to March 20, 2021. The early detection and diagnosis of pregnancy-associated listeriosis are significant since sensitive antibiotics are effective at enhancing the prognosis of newborns. Listeriosis can be diagnosed using positive cultures from maternal or neonatal blood, neonatal cerebrospinal fluid (CSF), amniotic fluid, intrauterine mucosa, or the placenta. Two weeks of high-dose intravenous amoxicillin (more than 6 g/day) is recommended for LM pregnant women without allergy. Terminating the pregnancy to save the mother’s life should be considered if maternal and fetal conditions aggravate. Neonatal Listeria infection is primarily transmitted through the placenta, which is a critical illness associated with a high mortality rate. The necessary dietary guidance for pregnant women can reduce the incidence rate of pregnancy-related listeriosis.

Cardiotropic Isolates of Listeria monocytogenes with Enhanced Vertical Transmission Dependent upon the Bacterial Surface Protein InlB

Listeria monocytogenes is a facultative Gram-positive intracellular bacterium that is capable of causing serious invasive infections in pregnant women, resulting in abortion, still-birth, and disseminated fetal infection. Previously, a clinical L. monocytogenes isolate, 07PF0776, was identified as having an enhanced ability to target cardiac tissue. This tissue tropism appeared to correlate with amino acid variations found within internalin B (InlB), a bacterial surface protein associated with host cell invasion. Given that the mammalian receptor bound by InlB, Met, is abundantly expressed by placental tissue, we assessed isolate 07PF0776 for its ability to be transmitted from mother to fetus. Pregnant Swiss Webster mice were infected on gestational day E13 via tail vein injection with the standard isolate 10403S, a noncardiotropic strain, or 07PF0776, the cardiac isolate. Pregnant mice infected with 07PF0776 exhibited significantly enhanced transmission of L. monocytogenes to placentas and fetuses compared to 10403S. Both bacterial burdens and the frequency of placental and fetal infection were increased in mice infected with the cardiac isolate. Strain 07PF0776 also exhibited an enhanced ability to invade Jar human trophoblast tissue culture cells in comparison to 10403S, and was found to have increased levels of InlB associated with the bacterial cell surface. Overexpression of surface InlB via genetic manipulation was sufficient to confer enhanced invasion of the placenta and fetus to both 10403S and 07PF0776. These data support a central role for surface InlB in promoting vertical transmission of L. monocytogenes.

Maternal-neonatal listeriosis

Listeriosis is a rare and severe foodborne infection caused by Listeria monocytogenes. It manifests as septicemia, neurolisteriosis, and maternal-fetal infection. In pregnancy, it may cause maternal fever, premature delivery, fetal loss, neonatal systemic and central nervous system infections. Maternal listeriosis is mostly reported during the 2nd and 3rd trimester of pregnancy, as sporadic cases or in the context of outbreaks. Strains belonging to clonal complexes 1, 4 and 6, referred to as hypervirulent, are the most associated to maternal-neonatal infections. Here we review the clinical, pathophysiological, and microbiological features of maternal-neonatal listeriosis.

Regulatory T Cells Beyond Autoimmunity: From Pregnancy to Cancer and Cardiovascular Disease

The evolution of the full range of functions of regulatory T cells (Treg) coincides with the evolution of mammalian pregnancy. Accordingly, Treg function has been shown to be crucial for maternal-fetal tolerance and implantation. As reproduction is a key point of selective pressure, mammalian pregnancy may represent an evolutionary driver for the development of Treg. Yet beyond the chronological boundaries of mammalian pregnancy, several key physiological and pathological events are being gradually uncovered as involving the immunomodulating functions of Treg cells. These include autoimmunity, age-related inflammation in males and in post-menopausal females, but also oncological and cardiovascular diseases. The latter two sets of diseases collectively compose the main causes of mortality world-wide. Emerging data point to Treg-modulable effects in these diseases, in a departure from the relatively narrower perceived role of Treg as master regulators of autoimmunity. Yet recent evidence also suggests that changes in intestinal microbiota can affect the above pathological conditions. This is likely due to the finding that, whilst the presence and maintenance of intestinal microbiota requires active immune tolerance, mediated by Treg, the existence of microbiota per se profoundly affects the polarization, stability, and balance of pro- and anti-inflammatory T cell populations, including Treg and induced Treg cells. The study of these “novel,” but possibly highly relevant from an ontogenesis perspective, facets of Treg function may hold great potential for our understanding of the mechanisms underlying human disease.

Human labour is associated with altered regulatory T cell function and maternal immune activation

During human pregnancy, regulatory T cell (T_reg ) function is enhanced and immune activation is repressed allowing the growth and development of the feto-placental unit. Here, we have investigated whether human labour is associated with a reversal of the pregnancy-induced changes in the maternal immune system. We tested the hypothesis that human labour is associated with a decline in T_reg function, specifically their ability to modulate Toll-like receptor (TLR)-induced immune responses. We studied the changes in cell number, activation status and functional behaviour of peripheral blood, myometrial (myoMC) and cord blood mononuclear cells (CBMC) with the onset of labour. We found that T_reg function declines and that T_reg cellular targets change with labour onset. The changes in T_reg function were associated with increased activation of myoMC, assessed by their expression of major histocompatibility complex (MHC) class II molecules and CBMC inflammatory cells. The innate immune system showed increased activation, as shown by altered monocyte and neutrophil cell phenotypes, possibly to be ready to respond to microbial invasion after birth or to contribute to tissue remodelling. Our results highlight changes in the function of the adaptive and innate immune systems that may have important roles in the onset of human labour.

Sequelae of Fetal Infection in a Non-human Primate Model of Listeriosis

Listeria monocytogenes (Lm) is a common environmental bacterium that thrives on vegetation and soil matter, but can infect humans if contaminated food products are ingested, resulting in severe disease in immunosuppressed populations, including pregnant women and newborns. To better understand how the unique immunological milieu of pregnancy increases susceptibility to infection, we study listeriosis in cynomolgus macaques, a non-human primate that closely resembles humans in placentation and in the physiology, and immunology of pregnancy. Non-human primates are naturally susceptible to Lm infection, and spontaneous abortions due to listeriosis are known to occur in outdoor macaque colonies, making them ideal models to understand the disease pathogenesis and host-pathogen relationship of listeriosis. We have previously shown that Lm infection in the first trimester has a high rate of miscarriage. This study expands on our previous findings by assessing how the quantity of Lm as well as stage of pregnancy at the time of exposure may influence disease susceptibility. In the current study we inoculated a cohort of macaques with a lower dose of Lm than our previous study and although this did not result in fetal demise, there was evidence of in utero inflammation and fetal distress. Animals that were reinfected with an equivalent or higher dose of the same strain of Lm resulted in approximately half of cases continuing to term and half ending in fetal demise. These cases had inconsistent bacterial colonization of the fetal compartment, suggesting that Lm does not need to directly infect the placenta to cause adverse pregnancy outcomes. Timed surgical collection of tissues following inoculation demonstrated that transmission from mother to fetus can occur as soon as 5 days post-inoculation. Lastly, third trimester inoculation resulted in pregnancy loss in 3 out of 4 macaques, accompanied by characteristic pathology and Lm colonization. Collectively, our studies demonstrate that common laboratory culture tests may not always recover Lm despite known maternal ingestion. Notably, we also find it is possible for maternal infection to resolve in some cases with no discernible adverse outcome; however, such cases had evidence of a sterile intrauterine inflammatory response, with unknown consequences for fetal development.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Immunological Basis for Recurrent Fetal Loss and Pregnancy Complications

Pregnancy stimulates an elaborate assortment of dynamic changes, allowing intimate approximation of genetically discordant maternal and fetal tissues. Although the cellular and molecular details about how this works remain largely undefined, important clues arise from evaluating how a prior pregnancy influences the outcome of a future pregnancy. The risk of complications is consistently increased when complications occurred in a prior pregnancy. Reciprocally, a prior successful pregnancy protects against complications in a future pregnancy. Here, we summarize immunological perturbations associated with fetal loss, with particular focus on how both harmful and protective adaptations may persist in mothers. Immunological aberrancy as a root cause of pregnancy complications is also considered, given their shared overlapping risk factors and the sustained requirement for averting maternal-fetal conflict throughout pregnancy. Understanding pregnancy-induced immunological changes may expose not only new therapeutic strategies for improving pregnancy outcomes but also new facets of how immune tolerance works that may be applicable to other physiological and pathological contexts.

Pregnancy-related listeriosis

About one in seven cases of listeriosis occurs in pregnant women, and, although listeriosis is rare, it is the third leading cause of death from food-borne infections. Pregnancy-related listeriosis increases the risk for fetal and neonatal mortality by approximately 21%. During pregnancy, infections are more likely to occur in the third trimester (66%) than the first trimester (3%). However, fetal and neonatal adverse effects are less common as gestational age increases or with older gestational age at birth. Pregnant women seem highly susceptible in some listeriosis outbreaks making up a large percentage of cases, whereas others contain very few. Whether this results from differences in strains of L. monocytogenes, exposures, or other factors remains to be determined. Food and Agriculture Organization of United Nations/World Health Organization (FAO/WHO) estimates the human lethal dose for 50% (LD₅₀ ) for fetal/neonatal loss is 1.9 × 10⁶ colony forming units (CFUs) L. monocytogenes. Animal models have been developed for pregnancy-related listeriosis showing similar susceptibility and clinical outcomes as in humans. Nonhuman primate and guinea pig animal models have similar (LD₅₀ ) values to the estimated human LD₅₀ . Additional animal studies are needed to understand the pathways leading to fetal and neonatal listeriosis in humans. More information is needed to understand dose response, to model risk for listeriosis at lower concentrations, and to determine why some pregnant women may be more susceptible than others. To better treat listeriosis during pregnancy, biomarkers for early diagnosis of listeriosis are also needed. Last, pregnant women need to be educated about avoiding high-risk foods, like Mexican-style cheese and ready-to-eat meats. Birth Defects Research 109:324-335, 2017.© 2017 Wiley Periodicals, Inc.

Vertical Transmission of Listeria monocytogenes: Probing the Balance between Protection from Pathogens and Fetal Tolerance

Protection of the developing fetus from pathogens is one of the many critical roles of the placenta. Listeria monocytogenes is one of a select number of pathogens that can cross the placental barrier and cause significant harm to the fetus, leading to spontaneous abortion, stillbirth, preterm labor, and disseminated neonate infection despite antibiotic treatment. Such severe outcomes serve to highlight the importance of understanding how L. monocytogenes mediates infiltration of the placental barrier. Here, we review what is currently known regarding vertical transmission of L. monocytogenes as a result of cell culture and animal models of infection. In vitro cell culture and organ models have been useful for the identification of L. monocytogenes virulence factors that contribute to placental invasion. Examples include members of the Internalin family of bacterial surface proteins such as Interalin (Inl)A, InlB, and InlP that promote invasion of cells at the maternal-fetal interface. A number of animal models have been used to interrogate L. monocytogenes vertical transmission, including mice, guinea pigs, gerbils, and non-human primates; each of these models has advantages while still not providing a comprehensive understanding of L. monocytogenes invasion of the human placenta and/or fetus. These models do, however, allow for the molecular investigation of the balance between fetal tolerance and immune protection from L. monocytogenes during pregnancy.

Animal and Human Tissue Models of Vertical Listeria monocytogenes Transmission and Implications for Other Pregnancy-Associated Infections

Intrauterine infections lead to serious complications for mother and fetus, including preterm birth, maternal and fetal death, and neurological sequelae in the surviving offspring. Improving maternal and child heath is a global priority. Yet, the development of strategies to prevent and treat pregnancy-related diseases has lagged behind progress made in other medical fields. One of the challenges is finding tractable model systems that replicate the human maternal-fetal interface. Animal models offer the ability to study pathogenesis and host defenses in vivo However, the anatomy of the maternal-fetal interface is highly divergent across species. While many tools are available to study host responses in the pregnant mouse model, other animals have placentas that are more similar to that of humans. Here we describe new developments in animal and human tissue models to investigate the pathogenesis of listeriosis at the maternal-fetal interface. We highlight gaps in existing knowledge and make recommendations on how they can be filled.

Maternal-fetal cross talk through cell-free fetal DNA, telomere shortening, microchimerism, and inflammation

There exists a strong correlation between unscheduled inflammation at the maternal-fetal interface and the continuum of pregnancy complications. In normal pregnancy, immunological tolerance is established to protect the semi-allogeneic fetus. There has been extensive research on how the immunity, endovascular trophoblast migration, and hormonal nexus are orchestrated during pregnancy at the maternal-fetal interface to program a normal pregnancy outcome. It is not clear what contributes to the plasticity of uterine immune tolerance, fetal survial, and long-term post-partum health of the mother and the offspring. Old and new concepts have reemerged and emerged that include cell-free fetal DNA (cffDNA), telomere shortening, microchimerism involving bidirectional migration of maternal and fetal cells, and pregnancy as a stress factor. The question is how these pathways converge in a gestational age-dependent manner to contribute to the health of the mother and the offspring later in life and respond to an array of inflammatory challenges. In this Review, we provide pertinent discussion on maternal-fetal cross talk through cffDNA, telomere shortening, and microchimerism in the context of inflammatory and anti-inflammatory settings, particularly how these pathways lead to normal and adverse pregnancy outcomes.

Preconceptual Zika virus asymptomatic infection protects against secondary prenatal infection

Pregnant women, and their fetal offspring, are uniquely susceptible to Zika virus and other microbial pathogens capable of congenital fetal infection. Unavoidable exposure to Zika virus in endemic areas underscores the need for identifying at-risk individuals, and protecting expecting mothers and their fetal offspring against prenatal infection. Here we show that primary Zika virus asymptomatic infection in mice confers protection against re-infection, and that these protective benefits are maintained during pregnancy. Zika virus recovery was sharply reduced in maternal tissues and amongst fetal concepti after prenatal challenge in mothers with resolved subclinical infection prior to pregnancy compared with mice undergoing primary prenatal infection. These benefits coincide with expanded accumulation of viral-specific antibodies in maternal serum and fetal tissues that protect against infection by the identical or heterologous Zika virus genotype strains. Thus, preconceptual infection primes Zika virus-specific antibodies that confer cross-genotype protection against re-infection during pregnancy.

Immunological implications of pregnancy-induced microchimerism

Immunological identity is traditionally defined by genetically encoded antigens, with equal maternal and paternal contributions as a result of Mendelian inheritance. However, vertically transferred maternal cells also persist in individuals at very low levels throughout postnatal development. Reciprocally, mothers are seeded during pregnancy with genetically foreign fetal cells that persist long after parturition. Recent findings suggest that these microchimeric cells expressing non-inherited, familially relevant antigenic traits are not accidental 'souvenirs' of pregnancy, but are purposefully retained within mothers and their offspring to promote genetic fitness by improving the outcome of future pregnancies. In this Review, we discuss the immunological implications, benefits and potential consequences of individuals being constitutively chimeric with a biologically active 'microchiome' of genetically foreign cells.

Limited Colonization Undermined by Inadequate Early Immune Responses Defines the Dynamics of Decidual Listeriosis

The bacterial pathogen Listeria monocytogenes causes foodborne systemic disease in pregnant women, which can lead to preterm labor, stillbirth, or severe neonatal disease. Colonization of the maternal decidua appears to be an initial step in the maternal component of the disease as well as bacterial transmission to the placenta and fetus. Host-pathogen interactions in the decidua during this early stage of infection remain poorly understood. Here, we assessed the dynamics of L. monocytogenes infection in primary human decidual organ cultures and in the murine decidua in vivo A high inoculum was necessary to infect both human and mouse deciduas, and the data support the existence of a barrier to initial colonization of the murine decidua. If successful, however, colonization in both species was followed by significant bacterial expansion associated with an inability of the decidua to mount appropriate innate cellular immune responses. The innate immune deficits included the failure of bacterial foci to attract macrophages and NK cells, cell types known to be important for early defenses against L. monocytogenes in the spleen, as well as a decrease in the tissue density of inflammatory Ly6C^hi monocytes in vivo These results suggest that the infectivity of the decidua is not the result of an enhanced recruitment of L. monocytogenes to the gestational uterus but rather is due to compromised local innate cellular immune responses.

Listeria Placental Infection

The Gram-positive facultative intracellular bacterium Listeria monocytogenes is the causative agent of listeriosis, a severe food-borne infection. Pregnant women are at risk of contracting listeriosis, which can potentially lead to miscarriage, stillbirth, preterm birth, and congenital neonatal infections. While other systemic bacterial infections may result in adverse pregnancy outcomes at comparable frequencies, L. monocytogenes has particular notoriety because fetal complications largely occur in the absence of overt illness in the mother, delaying medical intervention. Here, we briefly review the pathophysiology and mechanisms of maternofetal listeriosis, discussed in light of a recent mBio report on Listeria transplacental infection in a nonhuman primate model.

Tolerance to noninherited maternal antigens, reproductive microchimerism and regulatory T cell memory: 60 years after 'Evidence for actively acquired tolerance to Rh antigens'

Compulsory exposure to genetically foreign maternal tissue imprints in offspring sustained tolerance to noninherited maternal antigens (NIMA). Immunological tolerance to NIMA was first described by Dr. Ray D. Owen for women genetically negative for erythrocyte rhesus (Rh) antigen with reduced sensitization from developmental Rh exposure by their mothers. Extending this analysis to HLA haplotypes has uncovered the exciting potential for therapeutically exploiting NIMA-specific tolerance naturally engrained in mammalian reproduction for improved clinical outcomes after allogeneic transplantation. Herein, we summarize emerging scientific concepts stemming from tolerance to NIMA that includes postnatal maintenance of microchimeric maternal origin cells in offspring, expanded accumulation of immune suppressive regulatory T cells with NIMA-specificity, along with teleological benefits and immunological consequences of NIMA-specific tolerance conserved across mammalian species.

Cross-Generational Reproductive Fitness Enforced by Microchimeric Maternal Cells

Exposure to maternal tissue during in utero development imprints tolerance to immunologically foreign non-inherited maternal antigens (NIMA) that persists into adulthood. The biological advantage of this tolerance, conserved across mammalian species, remains unclear. Here, we show maternal cells that establish microchimerism in female offspring during development promote systemic accumulation of immune suppressive regulatory T cells (Tregs) with NIMA specificity. NIMA-specific Tregs expand during pregnancies sired by males expressing alloantigens with overlapping NIMA specificity, thereby averting fetal wastage triggered by prenatal infection and non-infectious disruptions of fetal tolerance. Therefore, exposure to NIMA selectively enhances reproductive success in second-generation females carrying embryos with overlapping paternally inherited antigens. These findings demonstrate that genetic fitness, canonically thought to be restricted to Mendelian inheritance, is enhanced in female placental mammals through vertically transferred maternal cells that promote conservation of NIMA and enforce cross-generational reproductive benefits.

CXCR3 blockade protects against Listeria monocytogenes infection-induced fetal wastage

Mammalian pregnancy requires protection against immunological rejection of the developing fetus bearing discordant paternal antigens. Immune evasion in this developmental context entails silenced expression of chemoattractant proteins (chemokines), thereby preventing harmful immune cells from penetrating the maternal-fetal interface. Here, we demonstrate that fetal wastage triggered by prenatal Listeria monocytogenes infection is driven by placental recruitment of CXCL9-producing inflammatory neutrophils and macrophages that promote infiltration of fetal-specific T cells into the decidua. Maternal CD8+ T cells with fetal specificity upregulated expression of the chemokine receptor CXCR3 and, together with neutrophils and macrophages, were essential for L. monocytogenes-induced fetal resorption. Conversely, decidual accumulation of maternal T cells with fetal specificity and fetal wastage were extinguished by CXCR3 blockade or in CXCR3-deficient mice. Remarkably, protection against fetal wastage and in utero L. monocytogenes invasion was maintained even when CXCR3 neutralization was initiated after infection, and this protective effect extended to fetal resorption triggered by partial ablation of immune-suppressive maternal Tregs, which expand during pregnancy to sustain fetal tolerance. Together, our results indicate that functionally overriding chemokine silencing at the maternal-fetal interface promotes the pathogenesis of prenatal infection and suggest that therapeutically reinforcing this pathway represents a universal approach for mitigating immune-mediated pregnancy complications.

Generation of cellular immune memory and B-cell immunity is impaired by natural killer cells

The goal of most vaccines is the induction of long-lived memory T and B cells capable of protecting the host from infection by cytotoxic mechanisms, cytokines and high-affinity antibodies. However, efforts to develop vaccines against major human pathogens like HIV and HCV have not been successful, thereby highlighting the need for novel approaches to circumvent immunoregulatory mechanisms that limit induction of protective immunity. Here we show that mouse natural killer (NK) cells inhibit generation of long-lived virus-specific memory T- and B-cells as well as virus-specific antibody production after acute infection. Mechanistically, NK cells suppressed CD4 T cells and follicular helper T cells (T_FH) in a perforin-dependent manner during the first few days of infection, resulting in a weaker germinal center (GC) response and diminished immune memory. We anticipate that innovative strategies to relieve NK cell-mediated suppression of immunity should facilitate development of efficacious new vaccines targeting difficult-to-prevent infections.

Interleukin 2 in the pathogenesis and therapy of type 1 diabetes

Regulatory T cells (Tregs) play a major role in controlling effector T cells (Teffs) responding to self-antigens, which cause autoimmune diseases. An improper Treg/Teff balance contributes to most autoimmune diseases, including type 1 diabetes (T1D). To restore a proper balance, blocking Teffs with immunosuppressants has been the only option, which was partly effective and too toxic. It now appears that expanding/activating Tregs with low-dose interleukin-2 (IL-2) could provide immunoregulation without immunosuppression. This is particularly interesting in T1D as Tregs from T1D patients are reported as dysfunctional and a relative deficiency in IL-2 production and/or IL-2-mediated signaling could contribute to this phenotype. A clinical study of low-dose IL-2 showed a very good safety profile and good Treg expansion/activation in T1D patients. This opens the way for efficacy trials to test low-dose IL-2 in prevention and treatment of T1D and to establish in which condition restoration of a proper Treg/Teff balance would be beneficial in the field of autoimmune and inflammatory diseases.

To drive or be driven: the path of a mouse model of recurrent pregnancy loss

This review is an example of the use of an animal model to try to understand the immune biology of pregnancy. A well-known model of recurrent spontaneous pregnancy loss is put in clinical, historical, and theoretical context, with emphasis on T cell biology.

Pregnancy-associated listeriosis in England and Wales

Listeriosis is a rare but severe foodborne disease with low morbidity and high case-fatality rates. Pregnant women, unborn and newborn babies are among the high-risk groups for listeriosis. We examined listeriosis cases reported to the enhanced surveillance system in England and Wales from 1990 to 2010 to identify risk factors influencing outcome. Cases were defined as pregnancy-associated if Listeria monocytogenes was isolated from a pregnant woman or newborn infants aged <28 days. Of the 3088 cases reported, pregnancy-associated listeriosis accounted for 462 (15%) cases and 315 cases resulted in a live birth. Several factors were identified as affecting the severity and outcome of listeriosis in pregnancy in both mother and child including: presence or absence of maternal symptoms, gestational age at onset of symptoms, and clinical presentation in the infant (meningitis or septicaemia). Deprivation, ethnicity and molecular serotype had no effect on outcome.

Maternal and fetal factors that contribute to the localization of T regulatory cells during pregnancy

PROBLEM

To determine the interplay between fetal antigenicity and local maternal factors in determining reproductive tract T regulatory (Treg) cell accumulation during pregnancy.

METHOD OF STUDY

Examination of maternal Treg composition in the uterus, cervix, and uteroplacental interface (UPI) of murine syngeneic and allogeneic pregnancies and non-pregnant controls by flow cytometry. The impact of fetal antigenicity was defined by either fetal gender in syngeneic pregnancies or by allogeneic paternity. Impact of IL-6 on local Treg composition was determined using syngeneic pregnancies in IL-6(-/-) females.

RESULTS

An increased fraction of CD4(+) T cells in the pregnant uterine lymphocytic infiltrate and draining pelvic lymph nodes are Tregs. Maternal IL-6 decreases Treg accumulation within the uterus and to a greater extent in the cervix in syngeneic pregnancy. Fetal antigenicity is matched by accumulation of Tregs to the UPI. Treg accumulation at the UPI of non-antigenic female fetuses is determined by the intrauterine position relative to male siblings.

CONCLUSION

Reproductive tract tissue Treg composition during pregnancy is influenced by maternal IL-6 and fetal antigenicity.

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.

Cis-acting pathways selectively enforce the non-immunogenicity of shed placental antigen for maternal CD8 T cells

Maternal immune tolerance towards the fetus and placenta is thought to be established in part by pathways that attenuate T cell priming to antigens released from the placenta into maternal blood. These pathways remain largely undefined and their existence, at face value, seems incompatible with a mother's need to maintain a functional immune system during pregnancy. A particular conundrum is evident if we consider that maternal antigen presenting cells, activated in order to prime T cells to pathogen-derived antigens, would also have the capacity to prime T cells to co-ingested placental antigens. Here, we address this paradox using a transgenic system in which placental membranes are tagged with a strong surrogate antigen (ovalbumin). We find that although a remarkably large quantity of acellular ovalbumin-containing placental material is released into maternal blood, splenic CD8 T cells in pregnant mice bearing unmanipulated T cell repertoires are not primed to ovalbumin even if the mice are intravenously injected with adjuvants. This failure was largely independent of regulatory T cells, and instead was linked to the intrinsic characteristics of the released material that rendered it selectively non-immunogenic, potentially by sequestering it from CD8α⁺ dendritic cells. The release of ovalbumin-containing placental material into maternal blood thus had no discernable impact on CD8 T cell priming to soluble ovalbumin injected intravenously during pregnancy, nor did it induce long-term tolerance to ovalbumin. Together, these results outline a major pathway governing the maternal immune response to the placenta, and suggest how tolerance to placental antigens can be maintained systemically without being detrimental to host defense.

Regulatory T cells and the immune pathogenesis of prenatal infection

Pregnancy in placental mammals offers exceptional comprehensive benefits of in utero protection, nutrition, and metabolic waste elimination for the developing fetus. However, these benefits also require durable strategies to mitigate maternal rejection of fetal tissues expressing foreign paternal antigens. Since the initial postulate of expanded maternal immune tolerance by Sir Peter Medawar 60 years ago, an amazingly elaborate assortment of molecular and cellular modifications acting both locally at the maternal-placental interface and systemically have been shown to silence potentially detrimental maternal immune responses. In turn, simultaneously maintaining host defense against the infinite array of potential pathogens during pregnancy is equally important. Fortunately, resistance against most infections is preserved seamlessly throughout gestation. On the other hand, recent studies on pathogens with unique predisposition for prenatal infections have uncovered distinctive holes in host defense associated with the reproductive process. Using these infections to probe the response during pregnancy, the immune suppressive regulatory subset of maternal CD4 T cells has been increasingly shown to dictate the inter-workings between prenatal infection susceptibility and pathogenesis of ensuing pregnancy complications. Herein, the recent literature suggesting a necessity for maternal regulatory T cells (Tregs) in pregnancy-induced immunological shifts that sustain fetal tolerance is reviewed. Additional discussion is focused on how expansion of maternal Treg suppression may become exploited by pathogens that cause prenatal infections and the perilous potential of infection-induced immune activation that may mitigate fetal tolerance and inadvertently inject hostility into the protective in utero environment.

Influence of infection during pregnancy on fetal development

Infection by bacteria, viruses, and parasites may lead to fetal death, organ injury, or limited sequelae depending on the pathogen. Here, we consider the role of infection during pregnancy in fetal development including placental development and function, which can lead to fetal growth restriction. The classical group of teratogenic pathogens is referred to as 'TORCH' (Toxoplasma gondii, others like Treponema pallidum, rubella virus, cytomegalovirus, and herpes simplex virus) but should include a much broader group of pathogens including Parvovirus B19, Varicella zoster virus, and Plasmodium falciparum to name a few. In this review, we describe the influence of different infections in utero on fetal development and the short- and long-term outcomes for the neonate. In some cases, the mechanisms used by these pathogens to disrupt fetal development are well known. Bacterial infection of the developing fetal lungs and brain begins with an inflammatory cascade resulting in cytokine injury and oxidative stress. For some pathogens like P. falciparum, the mechanisms involve oxidative stress and apoptosis to disrupt placental and fetal growth. An in utero infection may also affect the long-term health of the infant; in many cases, a viral infection in utero increases the risk of developing type 1 diabetes in childhood. Understanding the varied mechanisms employed by these pathogens may enable therapies to attenuate changes in fetal development, decrease preterm birth, and improve survival.

Illuminating the petite picture of T cell memory responses to Listeria monocytogenes

The ease to culture, moderately less safety constraints in handling, and above all, hurdle free induction of an anticipated infection in mouse rendered Listeria monocytogenes the rank of a model organism for studying a variety of host immune responses. Listeria monocytogenes being an intracellular pathogen evokes potent CD8 T cell response during which CD8 T cells pass through a massive expansion phase. This is generally followed by contraction phase wherein majority of activated cells undergo apoptosis leaving behind a population of memory CD8 T cells that has potential to confer enhanced protection upon reencounter with the same pathogen. Functional attributes of various cytokines, transcription factors, receptors, adaptors, and effectors pertaining to the generation of robust memory T cell response have begun to be unravelled for better understanding of memory and opening avenues to create superior vaccine strategies. This review is an attempt to unveil related discoveries along with updating recent advances on this issue.

Extrauterine listeriosis in the gravid mouse influences embryonic growth and development

Gravid mice and other rodents inoculated with Listeria monocytogenes typically fail to clear an intrauterine infection and either succumb or expel their intrauterine contents. We took advantage of this property to investigate the effects of an extrauterine infection on parameters of pregnancy success. Pregnant mice were selected for our study if they showed no clinical signs of listeriosis following oral inoculation at 7.5 gestational days (gd), and had no detectable intrauterine colony forming units (cfu) at near term (18.5 gd). The range of oral doses employed was 10⁶-10⁸ cfu per mouse for two listerial serotype strains (4nonb and 1/2a). At all doses, inoculation resulted in a decrease in average near-term (18.5 gd) fetal weight per litter compared to sham inoculated controls. Additionally, embryonic death (indicated by intrauterine resorptions) was exhibited by some inoculated mice but was absent in all sham inoculated animals. In parallel experiments designed to detect possible loss of placental function, gravid uteruses were examined histopathologically and microbiologically 96 h after oral inoculation. Placental lesions were associated with high (> 10⁶), but not low (< 10²) or absent intrauterine cfu. In vitro, mouse embryonic trophoblasts were indistinguishable from mouse enterocytes in terms of their sensitivity to listerial exposure. A model consistent with our observations is one in which products (host or bacterial) generated during an acute infection enter embryos transplacentally and influences embryonic survival and slows normal growth in utero.

Physiological and molecular determinants of embryo implantation

Embryo implantation involves the intimate interaction between an implantation-competent blastocyst and a receptive uterus, which occurs in a limited time period known as the window of implantation. Emerging evidence shows that defects originating during embryo implantation induce ripple effects with adverse consequences on later gestation events, highlighting the significance of this event for pregnancy success. Although a multitude of cellular events and molecular pathways involved in embryo-uterine crosstalk during implantation have been identified through gene expression studies and genetically engineered mouse models, a comprehensive understanding of the nature of embryo implantation is still missing. This review focuses on recent progress with particular attention to physiological and molecular determinants of blastocyst activation, uterine receptivity, blastocyst attachment and uterine decidualization. A better understanding of underlying mechanisms governing embryo implantation should generate new strategies to rectify implantation failure and improve pregnancy rates in women.