Co-localization of the neonatal Fc gamma receptor and IgG in human placental term syncytiotrophoblasts

Life-threatening infections in human newborns: Reconciling age-specific vulnerability and interindividual variability

In humans, the interindividual variability of clinical outcome following exposure to a microorganism is immense, ranging from silent infection to life-threatening disease. Age-specific immune responses partially account for the high incidence of infection during the first 28 days of life and the related high mortality at population level. However, the occurrence of life-threatening disease in individual newborns remains unexplained. By contrast, inborn errors of immunity and their immune phenocopies are increasingly being discovered in children and adults with life-threatening viral, bacterial, mycobacterial and fungal infections. There is a need for convergence between the fields of neonatal immunology, with its in-depth population-wide characterization of newborn-specific immune responses, and clinical immunology, with its investigations of infections in patients at the cellular and molecular levels, to facilitate identification of the mechanisms of susceptibility to infection in individual newborns and the design of novel preventive and therapeutic strategies.

The therapeutic age of the neonatal Fc receptor

IgGs are essential soluble components of the adaptive immune response that evolved to protect the body from infection. Compared with other immunoglobulins, the role of IgGs is distinguished and enhanced by their high circulating levels, long half-life and ability to transfer from mother to offspring, properties that are conferred by interactions with neonatal Fc receptor (FcRn). FcRn binds to the Fc portion of IgGs in a pH-dependent manner and protects them from intracellular degradation. It also allows their transport across polarized cells that separate tissue compartments, such as the endothelium and epithelium. Further, it is becoming apparent that FcRn functions to potentiate cellular immune responses when IgGs, bound to their antigens, form IgG immune complexes. Besides the protective role of IgG, IgG autoantibodies are associated with numerous pathological conditions. As such, FcRn blockade is a novel and effective strategy to reduce circulating levels of pathogenic IgG autoantibodies and curtail IgG-mediated diseases, with several FcRn-blocking strategies on the path to therapeutic use. Here, we describe the current state of knowledge of FcRn-IgG immunobiology, with an emphasis on the functional and pathological aspects, and an overview of FcRn-targeted therapy development.

Maternal provisions in type 1 diabetes: Evidence for both protective & pathogenic potential

Maternal influences on the immune health and development of an infant begin in utero and continue well into the postnatal period, shaping and educating the child's maturing immune system. Two maternal provisions include early microbial colonizers to initiate microbiota establishment and the transfer of antibodies from mother to baby. Maternal antibodies are a result of a lifetime of antigenic experience, reflecting the infection history, health and environmental exposure of the mother. These same factors are strong influencers of the microbiota, inexorably linking the two. Together, these provisions help to educate the developing neonatal immune system and shape lymphocyte repertoires, establishing a role for external environmental influences even before birth. In the context of autoimmunity, the transfer of maternal autoantibodies has the potential to be harmful for the child, sometimes targeting tissues and cells with devastating consequences. Curiously, this does not seem to apply to maternal autoantibody transfer in type 1 diabetes (T1D). Moreover, despite the rising prevalence of the disease, little research has been conducted on the effects of maternal dysbiosis or antibody transfer from an affected mother to her offspring and thus their relevance to disease development in the offspring remains unclear. This review seeks to provide a thorough evaluation of the role of maternal microorganisms and antibodies within the context of T1D, exploring both their pathogenic and protective potential. Although a definitive understanding of their significance in infant T1D development remains elusive at present, we endeavor to present what has been learned with the goal of spurring further interest in this important and intriguing question.

Opportunities and Challenges for PBPK Model of mAbs in Paediatrics and Pregnancy

New drugs may in some cases need to be tested in paediatric and pregnant patients. However, it is difficult to recruit such patients and there are many ethical issues around their inclusion in clinical trials. Modelling and simulation can help to plan well-designed clinical trials with a reduced number of participants and to bridge gaps where recruitment is difficult. Physiologically based pharmacokinetic (PBPK) models for small molecule drugs have been used to aid study design and dose adjustments in paediatrics and pregnancy, with several publications in the literature. However, published PBPK models for monoclonal antibodies (mAb) in these populations are scarce. Here, the current status of mAb PBPK models in paediatrics and pregnancy is discussed. Seven mAb PBPK models published for paediatrics were found, which report good prediction accuracy across a wide age range. No mAb PBPK models for pregnant women have been published to date. Current challenges to the development of such PBPK models are discussed, including gaps in our knowledge of relevant physiological processes and availability of clinical data to verify models. As the availability of such data increases, it will help to improve our confidence in the PBPK model predictive ability. Advantages for using PBPK models to predict mAb PK in paediatrics and pregnancy are discussed. For example, the ability to incorporate ontogeny and gestational changes in physiology, prediction of maternal, placental and foetal exposure and the ability to make predictions from in vitro and preclinical data prior to clinical data being available.

Benefits and Risks of IgG Transplacental Transfer

Maternal passage of immunoglobulin G (IgG) is an important passive mechanism for protecting the infant while the neonatal immune system is still immature and ineffective. IgG is the only antibody class capable of crossing the histological layers of the placenta by attaching to the neonatal Fc receptor expressed at the level of syncytiotrophoblasts, and it offers protection against neonatal infectious pathogens. In pregnant women with autoimmune or alloimmune disorders, or in those requiring certain types of biological therapy, transplacental passage of abnormal antibodies may cause fetal or neonatal harm. In this review, we will discuss the physiological mechanisms and benefits of transplacental transfer of maternal antibodies as well as pathological maternal situations where this system is hijacked, potentially leading to adverse neonatal outcomes.

Relevance of the Materno-Fetal Interface for the Induction of Antigen-Specific Immune Tolerance

In humans, maternal IgGs are transferred to the fetus from the second trimester of pregnancy onwards. The transplacental delivery of maternal IgG is mediated by its binding to the neonatal Fc receptor (FcRn) after endocytosis by the syncytiotrophoblast. IgGs present in the maternal milk are also transferred to the newborn through the digestive epithelium upon binding to the FcRn. Importantly, the binding of IgGs to the FcRn is also responsible for the recycling of circulating IgGs that confers them with a long half-life. Maternally delivered IgG provides passive immunity to the newborn, for instance by conferring protective anti-flu or anti-pertussis toxin IgGs. It may, however, lead to the development of autoimmune manifestations when pathological autoantibodies from the mother cross the placenta and reach the circulation of the fetus. In recent years, strategies that exploit the transplacental delivery of antigen/IgG complexes or of Fc-fused proteins have been validated in mouse models of human diseases to impose antigen-specific tolerance, particularly in the case of Fc-fused factor VIII (FVIII) domains in hemophilia A mice or pre-pro-insulin (PPI) in the case of preclinical models of type 1 diabetes (T1D). The present review summarizes the mechanisms underlying the FcRn-mediated transcytosis of IgGs, the physiopathological relevance of this phenomenon, and the repercussion for drug delivery and shaping of the immune system during its ontogeny.

Fc Characteristics Mediate Selective Placental Transfer of IgG in HIV-Infected Women

The placental transfer of maternal IgG is critical for infant protection against infectious pathogens. However, factors that modulate the placental transfer of IgG remain largely undefined. HIV-infected women have impaired placental IgG transfer, presenting a unique "disruption model" to define factors that modulate placental IgG transfer. We measured the placental transfer efficiency of maternal HIV and pathogen-specific IgG in US and Malawian HIV-infected mothers and their HIV-exposed uninfected and infected infants. We examined the role of maternal HIV disease progression, infant factors, placental Fc receptor expression, IgG subclass, and glycan signatures and their association with placental IgG transfer efficiency. Maternal IgG characteristics, such as binding to placentally expressed Fc receptors FcγRIIa and FcγRIIIa, and Fc region glycan profiles were associated with placental IgG transfer efficiency. Our findings suggest that Fc region characteristics modulate the selective placental transfer of IgG, with implications for maternal vaccine design and infant health.

Expression of the neonatal Fc-receptor in placental-fetal endothelium and in cells of the placental immune system

INTRODUCTION

Starting from the second trimester of pregnancy, passive immunity is provided to the human fetus by transplacental transfer of maternal IgG. IgG transfer depends on the neonatal Fc receptor, FcRn. While FcRn localization in the placental syncytiotrophoblast (STB) has been demonstrated unequivocally, FcRn expression in placental-fetal endothelial cells (pFECs), which are part of the materno-fetal barrier, is still unclear. Therefore, this study aimed to elucidate the spatio-specific expression pattern of FcRn in placental tissue.

METHODS

FcRn expression was investigated by western blotting in term placentas and in isolated human placental arterial and venous endothelial cells (HPAEC, HPVEC) using a validated affinity-purified polyclonal anti-peptide antibody against the cytoplasmic tail of FcRn α-chain. In situ localization of FcRn and IgG was studied by immunofluorescence microscopy on tissue sections of healthy term placentas.

RESULTS

FcRn expression was demonstrated in placental vasculature particularly, in HPAEC, and HPVEC. FcRn was localized in cytokeratin 7⁺ STB and in CD31⁺ pFECs in terminal as well as stem villi in situ. Additionally, CD68⁺ placental macrophages exhibited FcRn expression in situ. Endogenous IgG partially co-localized with FcRn in STB, pFECs, and in placental macrophages.

DISCUSSION

Placental FcRn expression in endothelial cells and macrophages is analogous to the expression pattern in other organs. FcRn expression in pFECs suggests an involvement of FcRn in IgG transcytosis and/or participation in recycling/salvaging of maternal IgG present in the fetal circulation. FcRn expression in placental macrophages may account for recycling of monomeric IgG and/or processing and presentation of immune complexes.

The immunology of the macaque placenta: A detailed analysis and critical comparison with the human placenta

The cynomolgus monkey is increasingly considered in toxicological research as the most appropriate model for humans due to the species' close physiological contiguity, including reproductive physiology. Here, literature on the cynomolgus monkey placenta is reviewed in regards to its similarity to the human placenta and particularly for its immunological role, which is not entirely mirrored in humans. Pertinent original data are included in this article. The cynomolgus monkey placenta is evaluated based on three aspects: first, morphological development; second, the spatial and temporal appearance of maternal and fetal immune cells and certain immune cell products of the innate and adaptive immune systems; and third, the expression of relevant immune tolerance-related molecules including the homologs of anti-human leucocyte antigen, indoleamine 2,3-dioxygenase, FAS/FAS-L, annexin II, and progesterone. Parameters relevant to the immunological role of the placenta are evaluated from the immunologically immature stage of gestational day (GD) 50 until more mature stages close to birth. Selected comparisons are drawn with human and other laboratory animal placentas. In conclusion, the cynomolgus monkey placenta has a high degree of morphological and physiological similarity to the human placenta. However, there are differences in the topographical distribution of cell types and immune tolerance-related molecules. Three basic features are recognized: (1) the immunological capacity of the placenta changes throughout the lifetime of the organ; (2) these immunological changes include multiple parameters such as morphological adaptations, cell type involvement, and changes in immune-relevant molecule expression; and (3) the immune systems of two genetically disparate individuals (mother and child) are functionally intertwined at the maternal-fetal interface.

The Impact of IgG transplacental transfer on early life immunity

Pediatric vaccines have significantly reduced infectious disease-related infant mortality, but as protective immunity often require several infant vaccine doses; maternally-acquired antibodies are critical to protect infants during the first months of life. Consequently, immunization of pregnant women is an important strategy not only to protect mothers from infection, but also to provide immunity to young infants. Nevertheless, maternal immunization can also negatively impact early life immunity. In fact, maternal antibodies can interfere with the development of infant immune responses, though it is unclear if such interference is clinically significant. Moreover, the transplacental transfer of maternal immunoglobulin therapeutics can be harmful to the fetus. Thus, the risk/benefit of maternal immunization for both the mother and the fetus should be carefully weighed. In addition, it is critical to fully understand the mechanisms by which IgG is transferred across the placenta in order to develop optimal maternal and infant immunization strategies.

Factors Affecting the FcRn-Mediated Transplacental Transfer of Antibodies and Implications for Vaccination in Pregnancy

At birth, neonates are particularly vulnerable to infection and transplacental transfer of immunoglobulin G (IgG) from mother to fetus provides crucial protection in the first weeks of life. Transcytosis of IgG occurs via binding with the neonatal Fc receptor (FcRn) in the placental synctiotrophoblast. As maternal vaccination becomes an increasingly important strategy for the protection of young infants, improving our understanding of transplacental transfer and the factors that may affect this will become increasingly important, especially in low-income countries where the burden of morbidity and mortality is highest. This review highlights factors of relevance to maternal vaccination that may modulate placental transfer—IgG subclass, glycosylation of antibody, total maternal IgG concentration, maternal disease, infant gestational age, and birthweight—and outlines the conflicting evidence and questions that remain regarding the complexities of these relationships. Furthermore, the intricacies of the Ab–FcRn interaction remain poorly understood and models that may help address future research questions are described.

Two ultrastructural distribution patterns of immunoglobulin G in human placenta and functional implications

The placenta is known to protect the fetus from infection and maternal rejection. In a previous study, we demonstrated that placental trophoblasts can synthesize immunoglobulin G (IgG). In this study, we investigated the distribution of immunoglobulins (IgG, IgM, and IgA), IgG receptors (FcRn and FcgammaRIII), and complement proteins in placental trophoblasts at the ultrastructural level. In addition, we studied the mRNA expression of IgG1 heavy chain (IGHG1), recombination activating gene 1 (RAG1), RAG2, and activation-induced cytidine deaminase (AID) with nested RT-PCR in primary cultured trophoblasts. The mRNA transcripts of IGHG1, RAG1, RAG2, and AID were all identified in primary trophoblasts, further establishing the IgG-producing capacity of trophoblasts. At the ultrastructural level with colloidal gold-labeled antibodies, IgG was found to be distributed in two distinct locations in syncytiotrophoblasts. For one, it was colocalized with FcRn in endosome displaying low electron density, and for the other it was colocalized with complement C1q in medium-electron density irregular structures that have not been reported previously. This characteristic distribution suggests that IgG is likely processed through two molecular mechanisms in syncytiotrophoblasts: receptor-bound transportation across the syncytiotrophoblast and formation of immune complexes with locally produced IgG. The latter mechanism is probably aimed at neutralizing detrimental maternal anti-paternal major histocompatibility complex antibodies. Our findings support the hypothesis that placenta-produced IgG can selectively react with maternal anti-fetus antibodies and provide a mechanism of fetomaternal tolerance to protect the fetus from maternal immune rejection.

Food allergens are transferred intact across the rat blood-placental barrier in vivo

We investigated the mechanism of transplacental macromolecular transport in rats on the nineteenth day of pregnancy using tracers, transmission electron microscopy and immunohistochemistry. The blood-placental barrier of full-term rat placentas was composed of a trilaminar layer of trophoblast cells that separates the fetal capillaries from the maternal blood spaces: a layer of cytotrophoblasts lining the maternal blood space and a bilayer of syncytiotrophoblast surrounding the fetal capillaries. Horseradish peroxidase, intravenously injected into the maternal circulation, was found in the maternal blood spaces, the interspaces between the cytotrophoblasts and the syncytiotrophoblast I, many pits and small vesicles in the syncytiotrophoblast I, vesicles of the syncytiotrophoblast II, fetal connective tissue and fetal capillaries. Intravenously injected ovalbumin was detected in the maternal blood spaces, a trilaminar layer and the fetal capillaries. Neonatal Fc receptor (FcRn), a receptor for IgG, was localized at the maternal side of the blood-placental barrier. These results show that the structure of the rat blood-placental barrier is quite similar to the human blood-placental barrier, and non-specific macromolecules and food allergens may penetrate through the blood-placental barrier of the full-term placenta from the maternal to fetal circulation mediated by FcRn.

HFcRn-mediated transplacental immunoglobulin G transport: protection of and threat to the human fetus and newborn

In human newborns, endogenous levels of plasma immunoglobulin G (IgG) begin to rise slowly after birth following exposure to the environment. For immunoprotection during fetal and early neonatal life, maternal IgG is provided by transplacental transport. While cellular immunoprotective IgG effects are mainly triggered by FcγRI, -RII and -RIII, transplacental IgG transfer is mediated by the MHC class I-like neonatal Fc-receptor, hFcRn. This compact review explains the mechanism of hFcRn-mediated IgG transcytosis across the placental barrier - syncytiotrophoblast and fetal endothelial cells. Restrictions of this IgG transport are summarized. These include IgG subclass discrimination and limited IgG transport before the third trimester that can cause insufficient protection from infections of preterm (≤ 35 th week) delivered babies. As hFcRn does not discriminate beneficial from hazardous IgGs, maternal auto- and alloimmune as well as therapeutic antibodies can reach the fetus. The consequences including severe diseases of the newborn are summarized in this article.

Antibody-dependent transplacental transfer of malaria blood-stage antigen using a human ex vivo placental perfusion model

Prenatal exposure to allergens or antigens released by infections during pregnancy can stimulate an immune response or induce immunoregulatory networks in the fetus affecting susceptibility to infection and disease later in life. How antigen crosses from the maternal to fetal environment is poorly understood. One hypothesis is that transplacental antigen transfer occurs as immune complexes, via receptor-mediated transport across the syncytiotrophoblastic membrane and endothelium of vessels in fetal villi. This hypothesis has never been directly tested. Here we studied Plasmodium falciparum merozoite surface protein 1 (MSP1) that is released upon erythrocyte invasion. We found MSP1 in cord blood from a third of newborns of malaria-infected women and in >90% following treatment with acid dissociation demonstrating MSP1 immune complexes. Using an ex vivo human placental model that dually perfuses a placental cotyledon with independent maternal and fetal circuits, immune-complexed MSP1 transferred from maternal to fetal circulation. MSP1 alone or with non-immune plasma did not transfer; pre-incubation with human plasma containing anti-MSP1 was required. MSP1 bound to IgG was detected in the fetal perfusate. Laser scanning confocal microscopy demonstrated MSP1 in the fetal villous stroma, predominantly in fetal endothelial cells. MSP1 co-localized with IgG in endothelial cells, but not with placental macrophages. Thus we show, for the first time, antibody-dependent transplacental transfer of an antigen in the form of immune complexes. These studies imply frequent exposure of the fetus to certain antigens with implications for management of maternal infections during pregnancy and novel approaches to deliver vaccines or drugs to the fetus.

Chapter 4: Multitasking by exploitation of intracellular transport functions the many faces of FcRn

The MHC Class I-related receptor, FcRn, transports antibodies of the immunoglobulin G (IgG) class within and across a diverse array of different cell types. Through this transport, FcRn serves multiple roles throughout adult life that extend well beyond its earlier defined function of transcytosing IgGs from mother to offspring. These roles include the maintenance of IgG levels and the delivery of antigen in the form of immune complexes to degradative compartments within cells. Recent studies have led to significant advances in knowledge of the intracellular trafficking of FcRn and (engineered) IgGs at both the molecular and cellular levels. The engineering of FcRn-IgG (or Fc) interactions to generate antibodies of increased longevity represents an area of active interest, particularly in the light of the expanding use of antibodies in therapy. The strict pH dependence of FcRn-IgG interactions, with binding at pH 6 that becomes essentially undetectable as near neutral pH is approached, is essential for efficient transport. The requirement for retention of low affinity at near neutral pH increases the complexity of engineering antibodies for increased half-life. Conversely, engineered IgGs that have gained significant binding for FcRn at this pH can be potent inhibitors of FcRn that lower endogenous IgG levels and have multiple potential uses as therapeutics. In addition, molecular studies of FcRn-IgG interactions indicate that mice have limitations as preclinical models for FcRn function, primarily due to cross-species differences in FcRn-binding specificity.

Most of diaplacentally transferred allergen is retained in the placenta

BACKGROUND

Transplacental transfer of nutritive and inhalant allergens has been described being potentially responsible for a series of events leading to antigen-specific immune responses in the fetus. As such, cord blood T cell responses appear ubiquitously. However, studies failed to reveal a consistent dose-response relationship between antenatal allergen exposure and allergen-specific cellular reactivity in cord blood.

OBJECTIVE

To examine the transfer process of allergens (ovalbumin (OVA), beta-lactoglobulin (BLG), birch pollen allergen Bet v1) in placental tissue (BeWo cell line, ex vivo placenta model).

METHODS

The choriocarcinoma cell line BeWo was used to study the allergen uptake and transfer experiments in vitro. In the ex vivo placenta model the contribution of different placental compartments was evaluated. For this, immuno-histochemistry, immuno-electronmicroscopy and ELISA techniques were applied using monoclonal antibodies to Bet v1, OVA and -BLG.

RESULTS

In vitro transfer studies on a BeWo cell-layer revealed an intracellular allergen uptake and a trans-trophoblastic allergen transfer, which was temperature- and concentration dependent, pH sensitive and asymmetric. Allergen-specific staining of placental tissue after allergen perfusion (BLG) demonstrated bulk of the allergen in the syncytio-trophoblastic cell layer and minor staining in the villous stroma and in the endothelium of fetal vessels. Immunogold staining revealed an accumulation of the perfused allergen in the trophoblastic basement membrane.

CONCLUSION

In vitro/ex vivo trans-trophoblastic and trans-placental allergen transfer is shown with an accumulation of most of the allergen in placental tissues, potentially explaining the missing direct dose-response relationship between prenatal (maternal) allergen exposure and allergen-specific cellular reactivity in cord blood.

Placental expression of glycophosphatidylinositol (GPI)-anchored proteins in paroxysmal nocturnal haemoglobinuria

Paroxysmal nocturnal haemoglobinuria (PNH) is a clonal stem cell disorder in which a defect of glycophosphatidylinositol (GPI)-anchored proteins leads to higher morbidity and mortality because of intravascular haemolysis, haemoglobinuria, pancytopenia and an increased frequency of thrombotic events. We report here the clinical features of a pregnant woman with PNH and present an immunhistochemical analysis of complement regulators, leukocyte activation markers and placental alkaline phosphatase (PALP) on syncytiotrophoblasts and inflammatory cells in her placenta. Placental tissue from normal deliveries served as controls. The patient had severe PNH with haemolysis, thrombosis episodes and signs of bone marrow failure. Placental syncytiotrophoblasts and villous cells of fetal origin in both normal placentas and the placenta from the PNH patient expressed PALP and the complement regulators CD46, CD55 and CD59. Additionally, CD11b-positive leukocytes of presumed maternal origin were negative for CD15 in the PNH placenta, while they stained positive within the villous space and in normal placentas. These findings show that fetally derived cells in the PNH placenta expressed GPI-linked molecules that are known to be of importance for a successful pregnancy outcome.

A variable number of tandem repeats polymorphism influences the transcriptional activity of the neonatal Fc receptor alpha-chain promoter

The neonatal Fc receptor, FcRn, plays a central role in immunoglobulin G (IgG) transport across placental barriers. Genetic variations of FcRn-dependent transport across the placenta may influence antibody-mediated pathologies of the fetus and the newborn. Sequencing analysis of 20 unrelated individuals demonstrated no missense mutation within the five exons of the FcRn gene. However, a variable number of tandem repeats (VNTR) region within the FcRn promoter was observed, consisting of five different alleles (VNTR1-VNTR5). Alleles with two (VNTR2) and three (VNTR3) repeats were found to be most common in Caucasians (7.5 and 92.0%, respectively). Real-time polymerase chain reaction revealed that monocytes from VNTR3 homozygous individuals express 1.66-fold more FcRn transcript than do monocytes from VNTR2/VNTR3 heterozygous individuals (P = 0.002). In reporter plasmid assays, the VNTR3 allele supported the transcription of a reporter gene twice as effectively as did the VNTR2 allele (P = 0.003). Finally, under acidic conditions, monocytes from VNTR3 homozygous individuals showed an increased binding to polyvalent human IgG when compared with monocytes from VNTR2/VNTR3 heterozygous individuals (P = 0.021). These data indicate that a VNTR promoter polymorphism influences the expression of the FcRn receptor, leading to different IgG-binding capacities.

Use of Chitosan for Selective Removal of β-Lactoglobulin from Whey

A method is described for selective removal of undenatured beta-lactoglobulin from cheese whey based on interactions between whey proteins and chitosan. Whey was previously clarified at pH 4.5 with addition of chitosan (25 mg/100 mL), and selective removal of beta-lactoglobulin was studied in the pH interval 4.6 to 6.5. Addition of chitosan caused selective precipitation of beta-lactoglobulin that increased with pH. The content of beta-lactoglobulin in whey decreased as the amount of chitosan added was increased. At pH 6.2, addition of 1.9 to 3.0 mg/mL of chitosan led to complete removal of beta-lactoglobulin, whereas at least 80% of the rest of whey proteins remained in solution. The production of cheese whey without beta-lactoglobulin could help to expand the applications of dairy by-products in food processing, and to isolate hypoallergenic whey protein concentrates.

Placental Alkaline Phosphatase (PLAP) Enzyme Activity and Binding to IgG in Chagas' Disease

Placentas and plasma from women with and without Chagas' disease and cultures of human placental villi with Trypanosoma cruzi, neuraminidase, phospholipase A2 and phospholipase C were analyzed in order to verify if the alterations in placental alkaline phosphatase (PLAP) enzyme activity are caused by T. cruzi as observed in previous works. As IgG receptivity happens to be one of the proposed functions of PLAP, general IgG binding ability of the placentas treated with the mentioned enzymes, which are present on the parasite's surface, were also tested. The phospholipases caused an increase of PLAP's enzyme activity in the supernatant of infected placentas and a decrease of enzyme activity in the membrane of cultured placentas, therefore suggesting the cleavage of PLAP by parasitic enzymes. Desialylation could also partially inhibit PLAP's enzyme activity in supernatant and membrane of placenta culture. Placentas from healthy patients presented higher IgG receptivity than those from patients with Chagas' disease. In vitro infection of healthy placentas with T. cruzi caused no difference in IgG receptivity in placental sections with respect to controls but the phospholipases and neuraminidase increased the IgG receptivity of cultured placentas. The IgG transference index was higher for patients with Chagas' disease than for those without it. Although binding to IgG does not completely inhibit the enzyme activity of PLAP, it interferes with the enzyme activity of PLAP. We concluded that the enzymes on the surface of T. cruzi trypomastigotes can not only affect PLAP's enzyme activity but also increase the IgG binding ability of the placenta and this can be related to the actions of neuraminidase-transsialidase, phospholipase A2 and phospholipase C on the parasite surface. The modification of PLAP from women with Chagas' disease should be considered as a result of multiple factors.

Characterization of basolateral-targeting signals in the neonatal Fc receptor

The neonatal Fc receptor, FcRn, transports proteins through cells, avoiding degradative compartments. FcRn is used in many physiological processes where proteins must remain intact while they move through cells. These contexts include the transport of IgG antibodies from mother to offspring, and the protection of IgG and albumin from catabolism. In polarized cell models, FcRn in the plasma membrane is predominantly at the basolateral surface. This distribution depends on two signals that overlap endocytosis signals. One of these signals resembles a YXXPhi motif, but with a tryptophan in place of the critical tyrosine residue; the other is a DDXXXLL signal. We examined the effects of mutations in and around these signals on the basolateral targeting of rat FcRn in rat inner medullary collecting duct cells. We also studied a second acidic cluster, Glu331/Glu333, some distance from either endocytosis signal. Some amino acid substitutions in the W-2 and W+3 positions disrupted the tryptophan-based basolateral-targeting signal without impairing its function in endocytosis. The tryptophan-based basolateral targeting and endocytosis signals are thus distinct but overlapping, as has been seen for collinear tyrosine-based signals. Surprisingly, the tryptophan-based basolateral-targeting signal required the aspartate pair of the dileucine-based signal. This acidic cluster, separated by two amino acids from the Phi residue of the tryptophan signal, is therefore a component of both of the basolateral-targeting signals. The acidic cluster Glu-331/Glu333 was not required for basolateral targeting, but its replacement reduced endocytosis.

Overexpression of the human neonatal Fc-receptor alpha-chain in trophoblast-derived BeWo cells increases cellular retention of beta2-microglobulin

Major histocompatibility complex (MHC) class I and MHC class I-type molecules such as the neonatal Fcgamma-receptor, FcRn, are heterodimers consisting of a transmembrane alpha-chain non-covalently associated with beta2-microglobulin (beta2m). Human placental villous syncytiotrophoblast (STB) lacks MHC class I molecules, but express hFcRn that mediates materno-fetal transmission of immunoglobulin G (IgG). Trophoblast-derived BeWo cells that are used to study placental IgG transport likewise express beta2m and low levels of hFcRn alpha-chain. The contribution of FcRn alpha-chain in retention and subcellular distribution of beta2m in STB and BeWo cells is unclear. To investigate this issue, we increased expression of hFcRn alpha-chain in BeWo cells (BeWo/hFcRn) by cDNA transfection. Overexpressed hFcRn protein exhibited the characteristic pH-dependent IgG binding and association with beta2m. In comparison to parental BeWo cells, beta2m mRNA levels in BeWo/hFcRn cells were not significantly altered, but total cell-associated beta2m protein was increased by 120%. Treatment of BeWo and BeWo/hFcRn cells with brefeldin A, an inhibitor of the secretory pathway, abrogated this effect, demonstrating that hFcRn alpha-chain expression retained otherwise secreted beta2m. Flow cytometry revealed that beta2m plasma membrane expression was unaffected by alpha-chain overexpression whereas by fluorescence microscopy a preferential staining of beta2m in peripheral endosomes was observed.

MAINTENANCE OF SERUM ANTIBODY LEVELS

In vertebrates, serum antibodies are an essential component of innate and adaptive immunity and immunological memory. They also can contribute significantly to immunopathology. Their composition is the result of tightly regulated differentiation of B lymphocytes into antibody-secreting plasma blasts and plasma cells. The survival of antibody-secreting cells determines their contribution to the immune response in which they were generated and to long-lasting immunity, as provided by stable serum antibody levels. Short-lived plasma blasts and/or plasma cells secrete antibodies for a reactive immune response. Short-lived plasma blasts can become long-lived plasma cells, probably by competition with preexisting plasma cells for occupation of a limited number of survival niches in the body, in a process not yet fully understood. Limitation of the number of long-lived plasma cells allows the immune system to maintain a stable humoral immunological memory over long periods, to react to new pathogenic challenges, and to adapt the humoral memory in response to these antigens.

Neonatal Fc receptors discriminates and monitors the pathway of native and modified immunoglobulin G in placental endothelial cells

In the placenta, immunoglobulin G (IgG) is selectively transported from mother to fetus by a highly regulated transcellular mechanism aimed to achieve fetal humoral immunity. We questioned the role of neonatal Fc receptors (FcRn) in the traffic of IgG in human placental endothelial cells (HPEC). Cells were cultured in a double-chamber system and further exposed to IgG or Fc or to diethylpyrocarbonate-modified IgG or Fc in which the receptor recognition domain of the molecule (CH2-CH3) was altered. We provide evidence that the native IgG/Fc probes are transcytosed or recycled by HPEC, whereas the probes with the altered receptor recognition domain (which do not bind to FcRn) massively accumulate into the endocytic/lysosomal compartments. The results indicate that FcRn distinguishes between the intact and modified IgG and control their cellular traffic: native IgG is salvaged and released out of the cells, whereas modified IgG is retained and sorted to a degradative pathway. The data advance the understanding of the basic mechanism for IgG traffic in human endothelial cells, which may be exploited for the specific transport of antibodies in various immune disorders.

Recognition of the Tryptophan-based Endocytosis Signal in the Neonatal Fc Receptor by the μ Subunit of Adaptor Protein-2

Endocytosis of membrane proteins is typically mediated by signals present in their cytoplasmic domains. These signals usually contain an essential tyrosine or pair of leucine residues. Both tyrosine- and dileucine-based endocytosis signals are recognized by the adaptor complex AP-2. The best understood of these interactions occurs between the tyrosine-based motif, YXXPhi, and the mu2 subunit of AP-2. We recently reported a tryptophan-based endocytosis signal, WLSL, contained within the cytoplasmic domain of the neonatal Fc receptor. This signal resembles YXXPhi. We have investigated the mechanism by which the tryptophan-based signal is recognized. Both interaction assays in vitro and endocytosis assays in vivo show that mu2 binds the tryptophan-based signal. Furthermore, the WLSL sequence binds the same site as YXXPhi. Unlike the WXXF motif, contained in stonin 2 and other endocytic proteins, WLSL does not bind the alpha subunit of AP-2. These observations reveal a functional similarity between the tryptophan-based endocytosis signal and the YXXPhi motif, and an unexpected versatility of mu2 function.

Endocytic and Transcytotic Processes in Villous Syncytiotrophoblast: Role in Nutrient Transport to the Human Fetus

The supply of nutrients to the developing fetus is a major function of the human hemochorial placenta, a placenta type in which the fetal chorion is in direct contact with the maternal blood. At term, nutrients have to be transported across two cell layers in chorionic villi, the syncytiotrophoblast (STB) and fetal endothelial cells. The STB is a continuous syncytium covering the entire surface of chorionic villi. This polarized epithelium is specialized in exchange processes and membrane trafficking between the apical membrane facing the maternal blood and the basal membrane facing the fetal endothelium. To meet placental and fetal requirements, the STB selectively takes up and transports a variety of nutrients, hormones, growth factors and cytokines and also transfers passive immunity to the fetus by receptor-mediated transcytosis. In this review in vivo and in vitro systems currently used to study STB functions are discussed and the potential mechanisms of transplacental IgG, iron, lipoprotein and glucose transport are presented. As revealed in this article, the placenta is a tissue where intensive cell biological research is required to unravel endocytic trafficking pathways in a highly specialized cell such as the STB.

The MHC class I related Fc receptor, FcRn, is expressed in the epithelial cells of the human mammary gland

The major histocompatibility complex (MHC) class I related neonatal Fc receptor (FcRn) plays multiple roles, being involved in transporting immunoglobulin G (IgG) and protecting this antibody class from catabolism. The presence of this receptor was previously demonstrated in the lactating murine mammary gland. In the current study we have investigated FcRn expression in various histologic types of human breast carcinoma and lymph node metastases. We used immunohistochemical methods to demonstrate the presence of FcRn in epithelial cells, whereas this Fc receptor could not be detected in mammary gland endothelial cells. The presence of the receptor was also found in the metastasizing epithelial cells within the lymph nodes, and this provides a useful marker for their identification.

What Can Comparative Studies of Placental Structure Tell Us?—A Review

The diversity of placental structures in Eutherian mammals is such that drawing generalizations from the definitive forms is problematic. There are always areas of reduced interhaemal distance whether the placenta is epitheliochorial, synepitheliochorial, endotheliochorial or haemochorial. However, the thinning may be achieved by different means. The presence of a haemophagous area as an iron transport facilitator is generally associated with endotheliochorial placentae but is also found in sheep and goats (synepitheliochorial) and in tenrecs and hyaenas (haemochorial). Although similar chorioallantoic placentae are found within families, structure begins to diverge at the ordinal level and there is little correlation at the supraordinal level of phylogeny. Differences in formation and function of the yolk sac provide additional variation. There would appear to be considerable adaptive pressure for development or retention of the haemochorial type of chorioallantoic placenta. This type of placenta has several possible drawbacks including more ready passage of fetal cells to the maternal organism and, should the haemochorial condition be achieved early, oxidative stress. At any rate no animal larger than the human and gorilla has this type of placenta. The endotheliochorial condition is found in animals as large as the bears, manatee and elephants. In addition to the ungulates, the epitheliochorial condition is present in the largest animals with the longest gestation periods, the whales. Considering the length of time since the early stages of mammalian evolution, it is probable that few unmodified structural features are present in any currently surviving mammal. Nevertheless, more complete studies of divergent types of mammalian placenta should help our understanding of mammalian interrelationships as well as placental function.

Generation of mutated variants of the human form of the MHC class I-related receptor, FcRn, with increased affinity for mouse immunoglobulin G

Much data support the concept that the MHC class I-related receptor FcRn serves to regulate immunoglobulin G (IgG) concentrations in serum and other diverse body sites in both rodents and humans. Previous studies have indicated that the human ortholog of FcRn is endowed with unexpectedly high stringency in binding specificity for IgGs. In contrast to mouse FcRn, which binds promiscuously to IgGs across species, human FcRn does not bind to mouse IgG1 or IgG2a, and interacts weakly with mouse IgG2b. Here, we investigate the molecular basis for this high-level specificity. We have systematically mutated human FcRn residues to the corresponding mouse FcRn residues in the regions that encompass the FcRn-IgG interaction site. Notably, mutation of the poorly conserved residue Leu137 of human FcRn to glutamic acid (L137E) generates a human FcRn mutant that binds to mouse IgG1 and mouse IgG2a with equilibrium dissociation constants of 13.2 microM and 14.4 microM, respectively. From earlier high-resolution structural analyses of the rat FcRn-rat Fc complex, residue 137 of human FcRn is predicted to contact residue 436 of IgG, which can be either His436 (mouse IgG1, mouse IgG2a) or Tyr436 (human IgG1, mouse IgG2b). The simplest interpretation of our data for the L137E mutant is therefore that replacement of the Leu137-Tyr436 (human) by the Glu137-His436 (mouse) pair generates a receptor that can bind to mouse IgG1 and mouse IgG2a. The L137E mutation reduces the affinity of human FcRn for human IgG1 by about twofold, consistent with the introduction of a less favorable Glu137-Tyr436 interaction. However, the analysis of the effects of other mutations on the binding to different IgGs indicates that the contribution to binding of the interaction of FcRn residue 137 with IgG residue 436 can vary. This suggests the existence of distinct docking topologies that are accompanied by variations in contacts between these two residues for different FcRn-IgG pairs. Our observations are of direct relevance to understanding the molecular nature of the human FcRn-IgG interaction. In turn, understanding human FcRn function has significance for the optimization of the serum half-lives of therapeutic and prophylactic antibodies.

Increasing the affinity of a human IgG1 for the neonatal Fc receptor: biological consequences

Many biological functions, including control of the homeostasis and maternofetal transfer of serum gamma-globulins, are mediated by the MHC class I-related neonatal FcR (FcRn). A correlation exists in mice between the binding affinity of IgG1/Fc fragments to FcRn at pH 6.0 and their serum t(1/2). To expand this observation, phage display of mutagenized Fc fragments derived from a human IgG1 was used to increase their affinity to both murine and human FcRn. Ten variants were identified that have a higher affinity toward murine and human FcRn at pH 6.0, with DeltaDeltaG (DeltaG(wild type) - DeltaG(mutant)) from 1.0 to 2.0 kcal/mol and from 0.6 to 2.4 kcal/mol, respectively. Those variants exhibit a parallel increase in binding at pH 7.4 to murine, but not human, FcRn. Although not degraded in blood in vitro, accumulated in tissues, nor excreted in urine, their serum concentration in mice is decreased. We propose that higher affinity to FcRn at pH 7.4 adversely affects release into the serum and offsets the benefit of the enhanced binding at pH 6.0.

beta(2)-Microglobulin is important for cell surface expression and pH-dependent IgG binding of human FcRn

FcRn is a heterodimer of an alpha-chain and beta(2)-microglobulin (beta(2)m) and differs from other IgG Fc receptors in that it is structurally related to MHC class I molecules. Several functions attributed to FcRn are affected in beta(2)-microglobulin (beta(2)m)-deficient mice, suggesting that the alpha-chain needs to assemble with beta(2)m to form a functional receptor. However, the precise role of beta(2)m in FcRn function is not known. Here we expressed the human FcRn alpha-chain alone or in combination with beta(2)m in human melanoma FO-1 cells. We show that beta(2)m is important for cell surface expression of FcRn and that, in the absence of beta(2)m, the receptor is retained in the endoplasmic reticulum. Furthermore, in the absence of beta(2)m, IgG binding is decreased compared with that of native FcRn. Thus, assembly of the FcRn alpha-chain with beta(2)m is important for both transport of FcRn from the ER to the cell surface and efficient pH-dependent IgG binding.

Type III group B streptococcal polysaccharide induces antibodies that cross-react with Streptococcus pneumoniae type 14

Covalent linkage of a bacterial polysaccharide to a protein greatly enhances the carbohydrate's immunogenicity and its binding to solid surfaces in immunoassays. These findings have spurred the development of glycoconjugate vaccines to prevent serious bacterial infections as well as the use of glycoconjugates as coating antigens in bioassays. We evaluated sera from women immunized with unconjugated group B streptococcal (GBS) type III (GBS III) polysaccharide (IIIPS) or with IIIPS covalently linked to tetanus toxoid to assess specificity, sensitivity, and parallelism in dilution curves in two GBS III enzyme-linked immunosorbent assays (ELISAs). One assay used IIIPS mixed with methylated human serum albumin (IIIPS + mHSA) as the coating antigen, and the other used IIIPS covalently linked to HSA (III-HSA). Each coating antigen was associated with a highly specific GBS III bioassay. The sensitivity was higher in the III-HSA ELISA, in which conjugated IIIPS is bound to the plates. Parallelism in titration curves was observed in the III-HSA but not in the IIIPS + mHSA ELISA. The excellent correlation between the concentrations of GBS IIIPS-specific immunoglobulin G (IgG) and the opsonophagocytic activity of these antibodies indicated that the III-HSA assay can predict functionality of vaccine-induced IgG against GBS III disease. The structure of the repeating unit of the capsular polysaccharide of GBS III differs from that of Streptococcus pneumoniae type 14 (Pn14 PS) only by the presence on GBS III of a sialic acid residue at the end of the side chain. The majority of healthy adults responding to GBS III vaccines with a fourfold or greater increase in GBS III-specific IgG antibodies developed antibodies cross-reacting with Pn14 PS (i.e., desialylated GBS IIIPS). The proportion of GBS vaccine responders who developed IgG to the desialylated IIIPS did not depend on whether IIIPS was given in the unconjugated or conjugated form. When present, these vaccine-induced cross-reacting antibodies conferred in vitro antibody-mediated opsonophagocytosis and killing of both GBS III and Pn14, two pathogens that cause invasive disease in young infants.

Differences in promiscuity for antibody-FcRn interactions across species: implications for therapeutic antibodies

Preclinical tests of therapeutic antibodies are frequently carried out in mice to evaluate pharmacokinetics and efficacy. However, the observation that mouse IgG are cleared rapidly from the human circulation suggests that mice may not always be an ideal model. The Fc receptor, FcRn, regulates the serum half-lives of IgG in mice and most likely has a similar function in humans. In the current study we have carried out an extensive analysis of the interaction of the human or mouse forms of FcRn with IgG from various species using surface plasmon resonance. We show that in contrast to mouse FcRn, human FcRn is surprisingly stringent in its binding specificity for IgG derived from different species. Human FcRn binds to human, rabbit and guinea pig IgG, but not significantly to rat, bovine, sheep or mouse IgG (with the exception of weak binding to mouse IgG2b). In contrast, mouse FcRn binds to all IgG analyzed. The lack of binding of human FcRn to mouse IgG1 has been confirmed using transfectants that have been engineered to express human FcRn on the cell surface. Our results provide a molecular explanation for the enigmatic observation that mouse IgG behave anomalously in humans. These studies have implications for the successful application of therapeutic antibodies.

Apical to basolateral transcytosis and apical recycling of immunoglobulin G in trophoblast-derived BeWo cells: effects of low temperature, nocodazole, and cytochalasin D

The murine neonatal Fc receptor, FcRn, carries out two functions: materno-fetal IgG delivery and maintenance of serum IgG homeostasis. During human pregnancy maternal IgG is transferred across placental syncytiotrophoblasts presumably by the human homolog of FcRn, hFcRn. Trophoblast-derived BeWo cells express hFcRn endogenously and can be considered as a model system to investigate IgG transport in syncytiotrophoblasts. Using a pulse-chase protocol, we here demonstrate that polarized BeWo cells exhibit not only apical to basolateral transcytosis but also apical IgG recycling. Thus, for the first time we demonstrate that epithelial cells can be involved in both materno-fetal IgG transmission and regulation of serum IgG levels. Lowering the temperature from 37 to 16 degrees C reduced, but did not block, IgG recycling and transcytosis. Microtubule-disruption by nocodazole did not influence transcytosis or apical recycling. Disassembly of filamentous actin by cytochalasin D stimulated apical endocytosis and recycling, while transcytosis remained unaffected. In summary, in BeWo cells apically internalized IgG enters both a transcytotic and recycling pathway. While the transcytotic route is temperature-sensitive but independent from microtubules and actin filaments, the apical recycling pathway is temperature-influenced and stimulated by actin disassembly, suggestive for the involvement of distinct endosome subcompartments in transcytosis and recycling.

The MHC class I-related receptor, FcRn, plays an essential role in the maternofetal transfer of gamma-globulin in humans

The transfer of maternal gamma-globulin (IgG) provides the neonate with humoral immunity during early life. In humans, maternal IgG is transported across the placenta during the third trimester of pregnancy. The expression of the MHC class I-related receptor, FcRn, in the human placenta suggests that this Fc receptor might be involved in the delivery of maternal IgG, but direct evidence to support this is lacking. In the current study an ex vivo placental model has been used to analyze the maternofetal transfer of a recombinant, humanized (IgG1) antibody in which His435 has been mutated to alanine (H435A). In vitro binding studies using surface plasmon resonance indicate that the mutation ablates binding of the antibody to recombinant mouse and human FcRn. Relative to the wild-type antibody, the H435A mutant is deficient in transfer across the placenta. Significantly, the mutation does not affect binding to Fc gamma RIII, an FcR that has been suggested in earlier studies to mediate the transfer of maternal IgG. The analyses demonstrate that binding of an IgG to FcRn is a prerequisite for transport across the perfused placenta. FcRn therefore plays a central role in the maternofetal delivery of IgG and this has implications for the use of protein engineering to improve the properties of therapeutic antibodies.

Effects of mutations in potential phosphorylation sites on transcytosis of FcRn

The neonatal Fc receptor, FcRn, transports immunoglobulin G (IgG) across intestinal epithelial cells of suckling rats and mice from the lumenal surface to the serosal surface. In cell culture models FcRn transports IgG bidirectionally, but there are differences in the mechanisms of transport in the two directions. We investigated the effects of mutations in the cytoplasmic domain of FcRn on apical to basolateral and basolateral to apical transport of Fc across rat inner medullary collecting duct (IMCD) cells. Basolateral to apical transport did not depend upon determinants in the cytoplasmic domain. In contrast, an essentially tailless FcRn was markedly impaired in apical to basolateral transport. Using truncation and substitution mutants, we identified serine-313 and serine-319 as phosphorylation sites in the cytoplasmic domain of FcRn expressed in Rat1 fibroblasts. Mutations at Ser-319 did not affect transcytosis across IMCD cells. FcRn-S313A was impaired in apical to basolateral transcytosis to the same extent as tailless FcRn, whereas FcRn-S313D transported at wild-type levels. FcRn-S313A recycled more Fc to the apical medium than the wild-type receptor, suggesting that Ser-313 is required to allow FcRn to be diverted from an apical recycling pathway to a transcytotic pathway.

Tryptophan- and dileucine-based endocytosis signals in the neonatal Fc receptor

The neonatal Fc receptor, FcRn, transports immunoglobulin G across intestinal cells in suckling rats. FcRn enters these cells by endocytosis and is present on the apical and basolateral surfaces. We investigated the roles of aromatic amino acids and a dileucine motif in the cytoplasmic domain of rat FcRn. We expressed mutant FcRn in which alanine replaced Trp-311, Leu-322, and Leu-323, or Phe-340 in the inner medullary collecting duct cell line IMCD. Individual replacement of the aromatic amino acids or the dileucine motif only partially blocked endocytosis of (125)I-Fc, whereas uptake by FcRn containing alanine residues in place of both Trp-311 and the dileucine motif was reduced to the level obtained with the tailless receptor. Leu-314 was required for the function of the tryptophan-based endocytosis signal, and Asp-317 and Asp-318 were required for the dileucine-based signal. Nonvectorial delivery of newly synthesized FcRn to the two cell surfaces was unaffected by loss of the endocytosis signals. However, the steady-state distribution of endocytosis mutants was predominantly apical, unlike wild-type FcRn, which was predominantly basolateral. This shift appeared to arise because the loss of endocytosis signals inhibited apical to basolateral transcytosis of FcRn more than basolateral to apical transcytosis.

Expression of functionally active FcRn and the differentiated bidirectional transport of IgG in human placental endothelial cells

The mechanism of selective transport of the immunoglobulins G from the placental stroma to the lumen of the fetal blood vessels has not been elucidated yet. It was postulated that the specific transport as well as the regulation of IgG level in the blood, involves the MHC class I related receptor FcRn for the Fc domain of IgG. We questioned whether human placental endothelial cells (HPEC) express FcRn and, if present, whether it is in a functionally active form. The experiments were performed on cultured HPEC and as positive control, human trophoblastic (JEG3) and mouse endothelial cells (SVEC) were used. Expression of FcRn, was demonstrated by indirect immunofluorescence and RT-PCR. The role of FcRn was assessed by quantifying the transcellular transport of [(125)I]-hIgG or [(125)I]-rF(ab')(2) fragments from the apical to basolateral surface, and in the reverse direction of HPEC grown on filters in a double chamber system. The intracellular pathway of FcRn or IgG was examined by electron microscopy using the proteins adsorbed to 5 nm and 20 nm colloidal gold particles, respectively. The results showed that: (a) FcRn is expressed by human placental endothelial cells, in a functionally active form; (b) transcytosis of IgG in HPEC is a time-dependent process that takes place preferentially from the basolateral to the apical compartment; and (c) both IgG and FcRn colocalize in an intracellular endocytic compartment, chloroquine sensitive. Together these data suggest that the regulation of IgG level by endothelial cells may result from interplay between salvaging, exocytosis, and transcytosis of the molecules. One can assume that IgG that does not bind to FcRn may be destined for destruction, and this would explain the mechanism by which IgG homeostasis is maintained.

Up-regulation of CCR5 expression in the placenta is associated with human immunodeficiency virus-1 vertical transmission

The role of placenta in vertical transmission is not yet fully understood. A protective role of the placenta during gestation is suggested by the finding that caesarian sections reduce the risk of transmission of human immunodeficiency virus (HIV)-1 from mother to child three- to fourfold. Here we investigated whether the immunological milieu of the placenta might be important in HIV-1 transmission. In situ imaging of immunohistochemically stained placenta sections and reverse transcriptase-polymerase chain reaction demonstrated a fourfold increase in CCR5:CXCR4 expression ratio in placentae from transmitting women compared to placentae from nontransmitting women. This chemokine receptor repertoire was consistent with an up-regulation of interleukin-4 and interleukin-10 expression in placentae from nontransmitting placentae compared to transmitting placentae. In situ imaging demonstrated that CCR5 and CXCR4 were expressed on placental macrophages and lymphocytes but not in trophoblasts. Simultaneous immunofluorescence/ultrasensitive in situ hybridization for HIV-1 gag-pol mRNA revealed that HIV-1 infects primarily CXCR4-expressing cells in placentae from nontransmitting women whereas predominantly CCR5-expressing cells were infected in placentae from transmitting women. These data are consistent with transmission of a homogeneous population of nonsyncytium-inducing HIV-1 isolates that use CCR5 as co-receptor.

Human cytomegalovirus infection of placental cytotrophoblasts in vitro and in utero: implications for transmission and pathogenesis

Human cytomegalovirus (CMV) is the leading cause of prenatal viral infection. Affected infants may suffer intrauterine growth retardation and serious neurologic impairment. Analysis of spontaneously aborted conceptuses shows that CMV infects the placenta before the embryo or fetus. In the human hemochorial placenta, maternal blood directly contacts syncytiotrophoblasts that cover chorionic villi and cytotrophoblasts that invade uterine vessels, suggesting possible routes for CMV transmission. To test this hypothesis, we exposed first-trimester chorionic villi and isolated cytotrophoblasts to CMV in vitro. In chorionic villi, syncytiotrophoblasts did not become infected, although clusters of underlying cytotrophoblasts expressed viral proteins. In chorionic villi that were infected with CMV in utero, syncytiotrophoblasts were often spared, whereas cytotrophoblasts and other cells of the villous core expressed viral proteins. Isolated cytotrophoblasts were also permissive for CMV replication in vitro; significantly, infection subsequently impaired the cytotrophoblasts' ability to differentiate and invade. These results suggest two possible routes of CMV transmission to the fetus: (i) across syncytiotrophoblasts with subsequent infection of the underlying cytotrophoblasts and (ii) via invasive cytotrophoblasts within the uterine wall. Furthermore, the observation that CMV infection impairs critical aspects of cytotrophoblast function offers testable hypotheses for explaining the deleterious effects of this virus on pregnancy outcome.

Cloning and analysis of the gene encoding the human neonatal Fc receptor

The neonatal Fc receptor, FcRn, is expressed in human placental syncytiotrophoblast, capillary endothelium, intestinal epithelium, and other tissues. By analogy with its role in the mouse, human FcRn is expected to transport maternal IgG to the foetus, and protect circulating IgG from catabolism. The larger subunit of FcRn is homologous to the alpha chains of the major histocompatibility complex (MHC) class I proteins, but is encoded outside the MHC on chromosome 19. We report the isolation of clones encoding the alpha chain of human FcRn from chromosome 19-specific libraries. The sequence revealed a similar organization to classical and non-classical MHC, and MHC-related genes. Compared with classical MHC class I genes, the human FcRn alpha chain gene has expanded by acquiring many repetitive sequences in its introns, including multiple Alu elements in the fourth intron. Primer extension analysis showed that there are two transcription initiation sites in the upstream flanking sequence.

Multiple roles for the major histocompatibility complex class I- related receptor FcRn

Multiple functions have recently been identified for the neonatal Fc receptor FcRn. In addition, a human homolog of the rodent forms of FcRn has been identified and characterized. This major histocompatibility complex class I-related receptor plays a role in the passive delivery of immunoglobulin (Ig)Gs from mother to young and the regulation of serum IgG levels. In addition, FcRn expression in tissues such as liver, mammary gland, and adult intestine suggests that it may modulate IgG transport at these sites. These diverse functions are apparently brought about by the ability of FcRn to bind IgGs and transport them within and across cells. However, the molecular details as to how FcRn traffics within cells have yet to be fully understood, although in vitro systems have been developed for this purpose. The molecular nature of the FcRn-IgG interaction has been studied extensively and encompasses residues located at the CH2-CH3 domain interface of the Fc region of IgG. These Fc amino acids are highly conserved in rodents and man and interact with residues primarily located on the alpha2 domain of FcRn. Thus, it is now possible to engineer IgGs with altered affinities for FcRn, and this has relevance to the modulation of IgG serum half-life and maternofetal IgG transport for therapeutic applications.