REVIEW ARTICLE: Toll‐Like Receptors at the Maternal–Fetal Interface in Normal Pregnancy and Pregnancy Disorders

Citation Koga K, Mor G. Toll‐like receptors at the maternal–fetal interface in normal pregnancy and pregnancy disorders. Am J Reprod Immunol 2010 Toll‐like receptors (TLR) form the major family of pattern recognition receptors (PRR) that are involved in innate immunity. Innate immune responses against microorganisms at the maternal–fetal interface may have a significant impact on the success of pregnancy, as intrauterine infections have been shown to be strongly associated with certain disorders of pregnancy. At the maternal–fetal interface, TLRs are expressed not only in the immune cells but also in non‐immune cells such as trophoblasts and decidual cells; moreover, their expression patterns vary according to the stage of pregnancy. Here, we will describe potential functions of TLRs in these cells, their recognition and response to microorganisms, and their involvement in the innate immunity. The impact of TLR‐mediated innate immune response will be discussed via animal model studies, as well as clinical observations.     

Decidual Stromal Cells as Regulators of T‐Cell Access to the Maternal–Fetal Interface

A recent study in the journal Science offers insights into the mechanism behind feto‐maternal tolerance, as evidenced by changes in the immuno‐logical environment of the uterus and decidua. They also provide a rich area of research for the understanding of the regulation of the immune system in other complicated medical conditions, including cancer and pregnancies affected by infection or autoimmunity.     

Expression of Toll‐like Receptors in Genital Tract Tissues from Normal and HIV‐infected Men

Citation Pudney J, Anderson DJ. Expression of Toll‐like receptors in genital tract tissues from normal and HIV‐infected men. Am J Reprod Immunol 2011; 65: 28–43 Problem Cells of the innate immune system use Toll‐like receptors (TLRs) to recognize and respond to invading pathogens. This study was carried out to characterize TLR expression in the human male genital tract, an initial infection site for several sexually transmitted pathogens. Method of Study Immunohistochemistry was used to detect expression of TLRs 1–9 in genital tract tissues from HIV^? and HIV⁺ men. Results In HIV^? men, TLR1⁺ leukocytes were detected throughout the genital tract. Leukocytes in the penile urethra also expressed TLRs2, 3, 5, 7 and 9. Epithelial cells in most tissues did not express TLRs; exceptions were the prostate, where TLRs3 and 8 were observed on the apical surface of luminal epithelial cells, and the penile urethra, where epithelial cells expressed TLR9. In genital tissues from HIV⁺ men with AIDS, few TLR⁺ cells were detected. Conclusion Cells in the male genital tract can express a variety of TLRs. The penile urethra contained the highest number of TLR⁺ cells, indicating that this tissue plays a major role in the innate immune defense of the male genital tract. Overall, TLR expression was reduced in genital tissues from HIV⁺ men.     

Toll‐like Receptor‐2 and Toll‐like Receptor‐4 Expression on Maternal Neutrophils During Pregnancy

Nitsche JF, Jiang S‐W, Brost BC. Toll‐like receptor‐2 and toll‐like receptor‐4 expression on maternal neutrophils during pregnancy. Am J Reprod Immunol 2010; 64: 427–434 Problem Toll‐like receptors (TLR) are an important part of the innate immune system and are present in a variety of human tissues. Work investigating the role of the TLR in pregnancy has thus far focused on placental tissue; however, minimal data is currently available concerning TLR expression in other tissues. Unlike placental tissue, neutrophils are easily retrievable during pregnancy and thus allow assessment of TLR’s prior to delivery. Method of study Using real time quantitative PCR this study investigated whether TLR‐2 and TLR‐4 expression on maternal neutrophils is altered throughout gestation or at the time of labor. A group of 12 non‐pregnant women and two groups of ten pregnant patients were enrolled and followed longitudinally, one group throughout gestation and one group throughout the third trimester. Results Although increased in the placenta, TLR2 and TLR4 expression on maternal neutrophils changes minimally throughout gestation. Conclusion There appears to be very little regulation of TLR2 and TLR4 at the mRNA level during normal pregnancy and labor. However, now that the normal values of TLR expression on maternal neutrophils have been determined it will be possible to compare them to those from pregnancies complicated by such conditions as preeclampsia, preterm labor, or preterm premature rupture of membranes.     

Innate Immune System at the Maternal–Fetal Interface: Mechanisms of Disease and Targets of Therapy in Pregnancy Syndromes

The maternal–fetal interface is an immunologically unique site that allows the tolerance to the allogenic fetus and maintains host defense against possible pathogens. Balanced immune responses are required for the maintenance of successful pregnancy. It has been demonstrated that innate immune disturbances may be responsible for some adverse pregnancy outcomes such as preeclampsia (PE); hemolysis, elevated liver enzymes, low platelets (HELLP) syndrome; intrauterine growth restriction (IUGR); and recurrent spontaneous abortion (RSA). Observational studies suggest that immunomodulatory treatments in pregnancy‐specific complications may improve both the hematological/biochemical features in the mother and the perinatal outcomes. The following review will discuss how recent and relevant findings in the field of the innate immunity have advanced our understanding of the role of inflammation and innate immune system in the pathogenesis of pregnancy failure and will discuss the therapeutic outcomes of the existing studies and clinical trials in light of these new insights.     

NK Cell Tolerance and the Maternal–Fetal Interface

Natural killer (NK) cells play a fundamental role in the innate immune response through their ability to secrete cytokines and kill target cells without prior sensitization. These effector functions are central to NK cell anti-viral and anti-tumor abilities. Due to their cytotoxic nature, it is vital that NK cells have the capacity to recognize normal self-tissue and thus prevent their destruction. In addition to their role in host defense, NK cells accumulate at the maternal-fetal interface and are thought to play a critical role during pregnancy. The close proximity of uterine NK (uNK) cells to fetal trophoblast cells of the placenta would seemingly lead to catastrophic consequences, as the trophoblast cells are semi-allogeneic. A fundamental enigma of pregnancy is that the fetal cells constitute an allograft but, in normal pregnancies, they are in effect not perceived as foreign and are not rejected by the maternal immune system. Although the mechanisms involved in achieving NK cell tolerance are becoming increasingly well-defined, further clarification is required, given the clinical implications of this work in the areas of infection, transplantation, cancer and pregnancy. Herein, we discuss several mechanisms of NK cell tolerance and speculate as to how they may apply to uNK cells at the maternal–fetal interface.     

Embryo-Associated Immunosuppressor Factor is Produced at the Maternal-Fetal Interface in Human Pregnancy

PROBLEM : To study whether embryo associated immunosuppressor factor (EASF) is synthesized at the maternal-fetal interface. METHOD : Anti-EASF monoclonal antibody H5D12 was used to identify EASF. Paraffin-embedded sections were prepared from placental and fetal tissues and immunohistochemistry was done by the avidin-biotin-peroxidase technique. EASF was affinity purified using H5D12-Sepharose 4B from culture media of placental villi and analyzed for immunosuppressive activity (by Concanavalin A-induced lymphocyte proliferation assay) and molecular weight identity (by metabolic labeling studies with ³⁵S-methionine followed by immunoprecipitation and SDS-PAGE). RESULTS : Immunohistochemical studies demonstrates intense immunostaining of villous syncytiotrophoblast and cytotrophoblast cells of first trimester placental tissues. Hoffbauer cells and decidual cells stained positive. The same cells in second and third trimester placental tissues stained weakly. However, the endothelium and smooth muscle cells of fetal blood vessels, fetal ovarian stroma and primordial follicles, kidney epithelium, cerebral neurons, and glial cells all stained negative. The affinity-purified EASF from the conditioned media of placental villi (less than 12 wk gestational age) was identified as a 37-kDa molecule with immunosuppressive activity. Metabolic labeling studies revealed that placental villi from early gestational age secretes a major factor of 37-kDa and minor factors of 41-kDa and 47-kDa molecular weight. CONCLUSIONS : Monoclonal antibody H5D12 identifies a factor that is produced by the pre-implantation embryo and also synthesized by decidua and trophoblast cells.     

The role of indoleamine‐2,3‐dioxygenase in normal and pathological pregnancies

The survival of allogeneic fetus during pregnancy contradicts the laws of immune responses. Behind this paradoxical phenomenon, the mechanism is quite complex. Indoleamine‐2,3‐dioxygenase (IDO) is the first and rate‐limiting enzyme of tryptophan catabolism. Emerging evidence shows that IDO is expressed at the maternal‐fetal interface, including trophoblast cells, decidual stroma cells, decidual immune cells (eg, natural killer cells, T cells, and macrophages), and vascular endothelial cells of decidua and chorion. Moreover, the expression and activity of IDO are different among non‐pregnant, normal pregnant, and pathological pregnant conditions. IDO plays important roles in normal pregnancy through immune suppression and regulation of fetal invasion and circulation. However, the abnormal expression and dysfunction of IDO are associated with some pathological pregnancies (including recurrent spontaneous abortion, preeclampsia, preterm labor, and fetal growth restriction).     

Time‐dependent distinct roles of Toll‐like receptor 4 in a house dust mite‐induced asthma mouse model

House dust mites (HDM s) are a common source of allergens that trigger both allergen‐specific and innate immune responses in humans. Here, we examined the effect of allergen concentration and the involvement of Toll‐like receptor 4 (TLR 4) in the process of sensitization to house dust mite allergens in an HDM extract‐induced asthma mouse model. Intranasal administration of HDM extract induced an immunoglobulin E response and eosinophilic inflammation in a dose‐dependent manner from 2.5 to 30 μg/dose. In TLR 4‐knockout mice, the infiltration of eosinophils and neutrophils into the lung was decreased compared with that in wild‐type mice in the early phase of inflammation (total of three doses). However, in the late phase of inflammation (total of seven doses), eosinophil infiltration was significantly greater in TLR 4‐knockout mice than in wild‐type mice. This suggests that the roles of TLR 4 signaling are different between the early phase and the later phase of HDM allergen‐induced inflammation. Thus, innate immune response through TLR 4 regulated the response to HDM allergens, and the regulation was altered during the phase of inflammation.     

Toll‐like receptor 6 mediated inflammatory and functional responses of zinc oxide nanoparticles primed macrophages

Macrophages are among the most sensitive immune cells because of their phagocytic activity and are prone to become dysfunctional or not able to perform properly if nanoparticle load increases. We have previously reported that zinc oxide nanoparticles (ZNPs) induce inflammatory responses in macrophages that contribute to their death. Recognition of ZNPs by pattern recognition receptors such as toll‐like receptors (TLRs) might be a factor in the initiation of these responses in macrophages. Therefore, in this study we explored the role played by TLR6 and mitogen‐activated protein kinase (MAPKs) pathways in the inflammatory responses of macrophages during ZNPs exposure. ZNPs‐activated macrophages showed enhanced expression of activation and maturation markers (CD1d, MHC‐II, CD86 and CD71). Among various TLRs screened, TLR6 emerged as the most potent activator for ZNPs‐induced inflammatory responses. Downstream signalling proteins myeloid differentiation 88, interleukin‐1 receptor associated kinase and tumour necrosis factor receptor‐associated factor were also enhanced. On inhibiting MAPKs pathways individually, the inflammatory responses such as interleukin‐1β, interleukin‐6, tumour necrosis factor‐α, cyclooxygenase‐2 and inducible nitric oxide synthase were suppressed. TLR6 silencing significantly inhibited the pro‐inflammatory cytokine levels, reactive nitrogen species generation and inducible nitric oxide synthase expression. Also, inhibition of MAPKs in the absence of TLR6 signalling validated the link between TLR6 and MAPKs in ZNPs‐induced inflammatory responses. TLR6 was found to be co‐localized with autophagosomes. Macrophages lacking TLR6 inhibited the autophagosome marker protein‐microtubule‐associated protein1 light chain 3‐isoform II formation and phagocytosis. These results demonstrate that inflammatory responses caused by ZNPs‐activated macrophages strongly depend on TLR6‐mediated MAPK signalling.     

Controlling the cytokine storm in severe bacterial diarrhoea with an oral Toll‐like receptor 4 antagonist

Shigella dysenteriae causes the most severe of all infectious diarrhoeas and colitis. We infected rhesus macaques orally and also treated them orally with a small and non‐absorbable polypropyletherimine dendrimer glucosamine that is a Toll‐like receptor‐4 (TLR4) antagonist. Antibiotics were not given for this life‐threatening infection. Six days later, the clinical score for diarrhoea, mucus and blood was 54% lower, colon interleukin‐8 and interleukin‐6 were both 77% lower, and colon neutrophil infiltration was 75% less. Strikingly, vasculitis did not occur and tissue fibrin thrombi were reduced by 67%. There was no clinical toxicity or adverse effect of dendrimer glucosamine on systemic immunity. This is the first report in non‐human primates of the therapeutic efficacy of a small and orally bioavailable TLR antagonist in severe infection. Our results show that an oral TLR4 antagonist can enable controlled resolution of the infection‐related‐inflammatory response and can also prevent neutrophil‐mediated gut wall necrosis in severe infectious diarrhoeas.