The Fc receptor for IgG expressed in the villus endothelium of human placenta is Fc gamma RIIb2

CD32 captures committed haemogenic endothelial cells during human embryonic development

During embryonic development, blood cells emerge from specialized endothelial cells, named haemogenic endothelial cells (HECs). As HECs are rare and only transiently found in early developing embryos, it remains difficult to distinguish them from endothelial cells. Here we performed transcriptomic analysis of 28- to 32-day human embryos and observed that the expression of Fc receptor CD32 (FCGR2B) is highly enriched in the endothelial cell population that contains HECs. Functional analyses using human embryonic and human pluripotent stem cell-derived endothelial cells revealed that robust multilineage haematopoietic potential is harboured within CD32+ endothelial cells and showed that 90% of CD32+ endothelial cells are bona fide HECs. Remarkably, these analyses indicated that HECs progress through different states, culminating in FCGR2B expression, at which point cells are irreversibly committed to a haematopoietic fate. These findings provide a precise method for isolating HECs from human embryos and human pluripotent stem cell cultures, thus allowing the efficient generation of haematopoietic cells in vitro.

Immunomodulation of Antibody Glycosylation through the Placental Transfer

Establishing an immune balance between the mother and fetus during gestation is crucial, with the placenta acting as the epicenter of immune tolerance. The placental transfer of antibodies, mainly immunoglobulin G (IgG), is critical in protecting the developing fetus from infections. This review looks at how immunomodulation of antibody glycosylation occurs during placental transfer and how it affects fetal health. The passage of maternal IgG antibodies through the placental layers, including the syncytiotrophoblast, stroma, and fetal endothelium, is discussed. The effect of IgG subclass, glycosylation, concentration, maternal infections, and antigen specificity on antibody transfer efficiency is investigated. FcRn-mediated IgG transport, influenced by pH-dependent binding, is essential for placental transfer. Additionally, this review delves into the impact of glycosylation patterns on antibody functionality, considering both protective and pathological effects. Factors affecting the transfer of protective antibodies, such as maternal vaccination, are discussed along with reducing harmful antibodies. This in-depth examination of placental antibody transfer and glycosylation provides insights into improving neonatal immunity and mitigating the effects of maternal autoimmune and alloimmune conditions.

Evidence for the Role of a Second Fc-Binding Receptor in Placental IgG Transfer in Nonhuman Primates

Transplacental transfer of maternal antibodies provides the fetus and newborn with passive protection against infectious diseases. While the role of the highly conserved neonatal Fc receptor (FcRn) in transfer of IgG in mammals is undisputed, recent reports have suggested that a second receptor may contribute to transport in humans. We report poor transfer efficiency of plant-expressed recombinant HIV-specific antibodies, including engineered variants with high FcRn affinity, following subcutaneous infusion into rhesus macaques close to parturition. Unexpectedly, unlike those derived from mammalian tissue culture, plant-derived antibodies were essentially unable to cross macaque placentas. This defect was associated with poor Fcγ receptor binding and altered Fc glycans and was not recapitulated in mice. These results suggest that maternal-fetal transfer of IgG across the three-layer primate placenta may require a second receptor and suggest a means of providing maternal antibody treatments during pregnancy while avoiding fetal harm. IMPORTANCE This study compared the ability of several human HIV envelope-directed monoclonal antibodies produced in plants with the same antibodies produced in mammalian cells for their ability to cross monkey and mouse placentas. We found that the two types of antibodies have comparable transfer efficiencies in mice, but they are differentially transferred across macaque placentas, consistent with a two-receptor IgG transport model in primates. Importantly, plant-produced monoclonal antibodies have excellent binding characteristics for human FcRn receptors, permitting desirable pharmacokinetics in humans. The lack of efficient transfer across the primate placenta suggests that therapeutic plant-based antibody treatments against autoimmune diseases and cancer could be provided to the mother while avoiding transfer and preventing harm to the fetus.

Contribution of the ex vivo placental perfusion model in understanding transplacental immunoglobulin G transfer

INTRODUCTION

The acquisition of humoral immunity in utero is essential for the fetus. The crucial protein, which is responsible for this part of immunity, is immunoglobulin-G (IgG). Immune functions of IgGs are mediated via the interaction of the crystallizable fragment (Fc) region of IgG with specific Fc γ receptors (FcγRs). However, an atypical FcγR, the neonatal Fc receptor (FcRn), is a key regulator of IgG transfer across the human placenta. During the last four decades ex vivo placental perfusion studies have contributed significantly to the study of mechanisms of IgG transfer across the multicellular placental barrier.

METHOD

A PubMed search was conducted by using specific keywords: placenta, perfusion and IgG to review manuscripts using human placental perfusion to study the transplacental transfer of IgG. Relevant studies found in reference lists of these manuscripts were also added to the review, and references were included that supported or gave nuance to the discussion of the mechanisms of IgG kinetics in the placenta.

RESULTS AND DISCUSSION

We found twenty publications on the study of transplacental transfer of IgG using human ex vivo placental perfusion, by research groups with partly different settings. This review summarizes knowledge about placental IgG transfer, with a strong focus on the contributions from ex vivo placental perfusion studies.

HIV-associated alterations of the biophysical features of maternal antibodies correlate with their reduced transfer across the placenta

BACKGROUND

Human Immunodeficiency Virus (HIV) infection during pregnancy is associated with reduced transplacental transfer of maternal antibodies and increased risk of severe infections in children who are exposed and uninfected with HIV (CHEU). The basis of this reduced transfer of maternal immunity has not yet been defined but could involve modifications in the biophysical features of antibodies.

OBJECTIVE

To assess the impact of maternal HIV infection on the biophysical features of serum IgG and transplacental antibody transfer.

METHODS

Maternal serum IgG subclass levels, Fc glycosylation, Fc Receptor (FcR) binding, and transplacental transfer of pathogen-specific maternal IgG were measured in pregnant women living with HIV (WWH) and pregnant women testing negative for HIV (WNH) in Cape Town, South Africa.

RESULTS

Maternal antibody profiles were strikingly different between pregnant WWH and WNH. Antibody binding to FcγR2a and FcγR2b, IgG1 and IgG3 antibodies, and agalactosylated antibodies were all elevated in WLHIV, whereas digalactosylated and sialylated antibodies were reduced as compared to pregnant WNH. Antibody features that were elevated in WWH were also correlated with reduced transplacental transfer of vaccine antigen-specific antibodies.

CONCLUSION

HIV infection is associated with marked alterations of biophysical features of maternal IgG and reduced placental transfer-potentially impairing antimicrobial immunity.

The effect of the Delta SARS-CoV-2 variant in maternal infection and pregnancy

A greater proportion of pregnant women with COVID-19 have mild disease compared with their non-pregnant counterparts. Paradoxically, however, they are at higher risk of developing severe disease, requiring respiratory support and admission to intensive care. The delta SARS-Cov-2 variant is associated with increased risk of hospitalization and morbidity in unvaccinated pregnant populations. However, it is not known whether the worse pregnancy outcomes associated with the delta variant are due to a direct effect of the virus on the pregnancy, or whether this effect is mediated through more severe maternal infection. Here, we synthesize studies of COVID-19 pregnancies, focusing on the different routes of SARS-CoV-2 infection of lung and placenta, and the mechanisms of syncytial formation for each SARS-CoV-2 variant. To delineate COVID-19 complications in pregnant women, future studies should explore whether the delta variant causes greater placental infection compared to other variants and contributes to increased syncytial formation.

A Novel Role for Plasminogen Activator Inhibitor Type-2 as a Hypochlorite-Resistant Serine Protease Inhibitor and Holdase Chaperone

Plasminogen activator inhibitor type-2 (PAI-2), a member of the serpin family, is dramatically upregulated during pregnancy and in response to inflammation. Although PAI-2 exists in glycosylated and non-glycosylated forms in vivo, the majority of in vitro studies of PAI-2 have exclusively involved the intracellular non-glycosylated form. This study shows that exposure to inflammation-associated hypochlorite induces the oligomerisation of PAI-2 via a mechanism involving dityrosine formation. Compared to plasminogen activator inhibitor type-1 (PAI-1), both forms of PAI-2 are more resistant to hypochlorite-induced inactivation of its protease inhibitory activity. Holdase-type extracellular chaperone activity plays a putative non-canonical role for PAI-2. Our data demonstrate that glycosylated PAI-2 more efficiently inhibits the aggregation of Alzheimer’s disease and preeclampsia-associated amyloid beta peptide (Aβ), compared to non-glycosylated PAI-2 in vitro. However, hypochlorite-induced modification of non-glycosylated PAI-2 dramatically enhances its holdase activity by promoting the formation of very high-molecular-mass chaperone-active PAI-2 oligomers. Both PAI-2 forms protect against Aβ-induced cytotoxicity in the SH-SY5Y neuroblastoma cell line in vitro. In the villous placenta, PAI-2 is localised primarily to syncytiotrophoblast with wide interpersonal variation in women with preeclampsia and in gestational-age-matched controls. Although intracellular PAI-2 and Aβ staining localised to different placental cell types, some PAI-2 co-localised with Aβ in the extracellular plaque-like aggregated deposits abundant in preeclamptic placenta. Thus, PAI-2 potentially contributes to controlling aberrant fibrinolysis and the accumulation of misfolded proteins in states characterised by oxidative and proteostasis stress, such as in Alzheimer’s disease and preeclampsia.

An afucosylated anti-CD32b monoclonal antibody induced platelet-mediated adverse events in a human Fcγ receptor transgenic mouse model and its potential human translatability

To assess the safety and tolerability of NVS32b, a monoclonal, afucosylated, anti-CD32b (FCGR2B) antibody we used a humanized transgenic (Tg) mouse model that expresses all human Fc gamma receptors (FCGRs) while lacking all mouse FCGRs. Prior to its use, we extensively characterized the model. We found expression of all human FCGRs in a pattern similar to humans with some exceptions, such as low CD32 expression on T cells (detected with the pan CD32 antibody but more notably with the CD32b-specific antibody), variation in the transgene copy number, integration of additional human genes, and overall higher expression of all FCGRs on myeloid cells compared to human. Unexpectedly, NVS32b induced severe acute generalized thrombosis in huFCGR mice upon iv dosing. Mechanistic evaluation on huFCGR and human platelets revealed distinct binding, activation and aggregation driven by NVS32b in both species. In huFCGR mice, the anti-CD32b antibody NVS32b binds platelet CD32a via both Fc and/or CDR (complementarity determining region) causing their activation while in human, NVS32b-binding requires platelet pre-activation and interaction of platelet CD32a via the Fc portion and an unknown platelet epitope via the CDR portion of NVS32b. We deemed the huFCGR mice to be over-predictive of the NVS32b-associated human thrombotic risk. Impact: In this study we elucidated the mechanism based on the thrombotic adverse events observed in huFCGR mice upon NVS32B dosing and were able to identify this safety liability which led to program termination. Therefore, this mouse model could be useful in research of immunotherapies targeting or involving FCGRs. Potential biological implications resulting from species differences in the FCGR expression pattern are nevertheless important to consider.

Accelerated Clearance and Degradation of Cell-Free HIV by Neutralizing Antibodies Occurs via FcγRIIb on Liver Sinusoidal Endothelial Cells by Endocytosis

Neutralizing Abs suppress HIV infection by accelerating viral clearance from blood circulation in addition to neutralization. The elimination mechanism is largely unknown. We determined that human liver sinusoidal endothelial cells (LSEC) express FcγRIIb as the lone Fcγ receptor, and using humanized FcγRIIb mouse, we found that Ab-opsonized HIV pseudoviruses were cleared considerably faster from circulation than HIV by LSEC FcγRIIb. Compared with humanized FcγRIIb-expressing mice, HIV clearance was significantly slower in FcγRIIb knockout mice. Interestingly, a pentamix of neutralizing Abs cleared HIV faster compared with hyperimmune anti-HIV Ig (HIVIG), although the HIV Ab/Ag ratio was higher in immune complexes made of HIVIG and HIV than pentamix and HIV. The effector mechanism of LSEC FcγRIIb was identified to be endocytosis. Once endocytosed, both Ab-opsonized HIV pseudoviruses and HIV localized to lysosomes. This suggests that clearance of HIV, endocytosis, and lysosomal trafficking within LSEC occur sequentially and that the clearance rate may influence downstream events. Most importantly, we have identified LSEC FcγRIIb-mediated endocytosis to be the Fc effector mechanism to eliminate cell-free HIV by Abs, which could inform development of HIV vaccine and Ab therapy.

The Neonatal Fc Receptor (FcRn): A Misnomer?

Antibodies are essential components of an adaptive immune response. Immunoglobulin G (IgG) is the most common type of antibody found in circulation and extracellular fluids. Although IgG alone can directly protect the body from infection through the activities of its antigen binding region, the majority of IgG immune functions are mediated via proteins and receptors expressed by specialized cell subsets that bind to the fragment crystallizable (Fc) region of IgG. Fc gamma (γ) receptors (FcγR) belong to a broad family of proteins that presently include classical membrane-bound surface receptors as well as atypical intracellular receptors and cytoplasmic glycoproteins. Among the atypical FcγRs, the neonatal Fc receptor (FcRn) has increasingly gained notoriety given its intimate influence on IgG biology and its ability to also bind to albumin. FcRn functions as a recycling or transcytosis receptor that is responsible for maintaining IgG and albumin in the circulation, and bidirectionally transporting these two ligands across polarized cellular barriers. More recently, it has been appreciated that FcRn acts as an immune receptor by interacting with and facilitating antigen presentation of peptides derived from IgG immune complexes (IC). Here we review FcRn biology and focus on newer advances including how emerging FcRn-targeted therapies may affect the immune responses to IgG and IgG IC.

Fc Glycan-Mediated Regulation of Placental Antibody Transfer

Despite the worldwide success of vaccination, newborns remain vulnerable to infections. While neonatal vaccination has been hampered by maternal antibody-mediated dampening of immune responses, enhanced regulatory and tolerogenic mechanisms, and immune system immaturity, maternal pre-natal immunization aims to boost neonatal immunity via antibody transfer to the fetus. However, emerging data suggest that antibodies are not transferred equally across the placenta. To understand this, we used systems serology to define Fc features associated with antibody transfer. The Fc-profile of neonatal and maternal antibodies differed, skewed toward natural killer (NK) cell-activating antibodies. This selective transfer was linked to digalactosylated Fc-glycans that selectively bind FcRn and FCGR3A, resulting in transfer of antibodies able to efficiently leverage innate immune cells present at birth. Given emerging data that vaccination may direct antibody glycosylation, our study provides insights for the development of next-generation maternal vaccines designed to elicit antibodies that will most effectively aid neonates.

Reduced FcRn-mediated transcytosis of IgG2 due to a missing Glycine in its lower hinge

Neonatal Fc-receptor (FcRn), the major histocompatibility complex (MHC) class I-like Fc-receptor, transports immunoglobuline G (IgG) across cell layers, extending IgG half-life in circulation and providing newborns with humoral immunity. IgG1 and IgG2 have similar half-lives, yet IgG2 displays lower foetal than maternal concentration at term, despite all known FcRn binding residues being preserved between IgG1 and IgG2. We investigated FcRn mediated transcytosis of V_H-matched IgG1 and IgG2 and mutated variants thereof lacking Fc-gamma receptor (FcγR) binding in human cells expressing FcRn. We observed that FcγR binding was not required for transport and that FcRn transported less IgG2 than IgG1. Transport of IgG1 with a shortened lower hinge (ΔGly236, absent in germline IgG2), was reduced to levels equivalent to IgG2. Conversely, transport of IgG2 + Gly236 was increased to IgG1 levels. Gly236 is not a contact residue between IgG and FcRn, suggesting that its absence leads to an altered conformation of IgG, possibly due to a less flexible Fab, positioned closer to the Fc portion. This may sterically hinder FcRn binding and transport. We conclude that the lack of Gly236 is sufficient to explain the reduced FcRn-mediated IgG2 transcytosis and accounts for the low maternal/fetal IgG2 ratio at term.

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.

Multiple Variables at the Leukocyte Cell Surface Impact Fc γ Receptor-Dependent Mechanisms

Fc γ receptors (FcγR) expressed on the surface of human leukocytes bind clusters of immunoglobulin G (IgG) to induce a variety of responses. Many therapeutic antibodies and vaccine-elicited antibodies prevent or treat infectious diseases, cancers and autoimmune disorders by binding FcγRs, thus there is a need to fully define the variables that impact antibody-induced mechanisms to properly evaluate candidate therapies and design new intervention strategies. A multitude of factors influence the IgG-FcγR interaction; one well-described factor is the differential affinity of the six distinct FcγRs for the four human IgG subclasses. However, there are several other recently described factors that may prove more relevant for disease treatment. This review covers recent reports of several aspects found at the leukocyte membrane or outside the cell that contribute to the cell-based response to antibody-coated targets. One major focus is recent reports covering post-translational modification of the FcγRs, including asparagine-linked glycosylation. This review also covers the organization of FcγRs at the cell surface, and properties of the immune complex. Recent technical advances provide high-resolution measurements of these often-overlooked variables in leukocyte function and immune system activation.

Human IgG lacking effector functions demonstrate lower FcRn-binding and reduced transplacental transport

We have previously generated human IgG1 antibodies that were engineered for reduced binding to the classical Fcγ receptors (FcγRI-III) and C1q, thereby eliminating their destructive effector functions (constant region G1Δnab). In their potential use as blocking agents, favorable binding to the neonatal Fc receptor (FcRn) is important to preserve the long half-life typical of IgG. An ability to cross the placenta, which is also mediated, at least in part, by FcRn is desirable in some indications, such as feto-maternal alloimmune disorders. Here, we show that G1Δnab mutants retain pH-dependent binding to human FcRn but that the amino acid alterations reduce the affinity of the IgG1:FcRn interaction by 2.0-fold and 1.6-fold for the two antibodies investigated. The transport of the modified G1Δnab mutants across monolayers of human cell lines expressing FcRn was approximately 75% of the wild-type, except that no difference was observed with human umbilical vein endothelial cells. G1Δnab mutation also reduced transport in an ex vivo placenta model. In conclusion, we demonstrate that, although the G1Δnab mutations are away from the FcRn-binding site, they have long-distance effects, modulating FcRn binding and transcellular transport. Our findings have implications for the design of therapeutic human IgG with tailored effector functions.

New revelations from an old receptor: Immunoregulatory functions of the inhibitory Fc gamma receptor, FcγRIIB (CD32B)

The Fc gamma receptor IIB (FcγRIIB/CD32B) was generated million years ago during evolution. It is the sole inhibitory receptor for IgG, and has long been associated with the regulation of humoral immunity and innate immune homeostasis. However, new and surprising functions of FcγRIIB are emerging. In particular, FcγRIIB has been shown to perform unexpected activatory roles in both immune-signaling and monoclonal antibody (mAb) immunotherapy. Furthermore, although ITIM signaling is an integral part of FcγRIIB regulatory activity, it is now clear that inhibition/activation of immune responses can occur independently of the ITIM. In light of these new findings, we present an overview of the established and noncanonical functions of FcγRIIB and discuss how this knowledge might be exploited therapeutically.

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.

Zika Fetal Neuropathogenesis: Etiology of a Viral Syndrome

The ongoing Zika virus epidemic in the Americas and the observed association with both fetal abnormalities (primary microcephaly) and adult autoimmune pathology (Guillain-Barré syndrome) has brought attention to this neglected pathogen. While initial case studies generated significant interest in the Zika virus outbreak, larger prospective epidemiology and basic virology studies examining the mechanisms of Zika viral infection and associated pathophysiology are only now starting to be published. In this review, we analyze Zika fetal neuropathogenesis from a comparative pathology perspective, using the historic metaphor of "TORCH" viral pathogenesis to provide context. By drawing parallels to other viral infections of the fetus, we identify common themes and mechanisms that may illuminate the observed pathology. The existing data on the susceptibility of various cells to both Zika and other flavivirus infections are summarized. Finally, we highlight relevant aspects of the known molecular mechanisms of flavivirus replication.

Development and Characterization of Monoclonal Antibodies Specific for Mouse and Human Fcγ Receptors

FcγRs are key regulators of the immune response, capable of binding to the Fc portion of IgG Abs and manipulating the behavior of numerous cell types. Through a variety of receptors, isoforms, and cellular expression patterns, they are able to fine-tune and direct appropriate responses. Furthermore, they are key determinants of mAb immunotherapy, with mAb isotype and FcγR interaction governing therapeutic efficacy. Critical to understanding the biology of this complex family of receptors are reagents that are robust and highly specific for each receptor. In this study, we describe the development and characterization of mAb panels specific for both mouse and human FcγR for use in flow cytometry, immunofluorescence, and immunocytochemistry. We highlight key differences in expression between the two species and also patterns of expression that will likely impact on immunotherapeutic efficacy and translation of therapeutic agents from mouse to clinic.

Intraperitoneal Immunization with Cry1Ac Protoxin from Bacillus thuringiensis Provokes Upregulation of Fc-Gamma-II/and Fc-Gamma-III Receptors Associated with IgG in the Intestinal Epithelium of Mice

In humans, intestinal epithelial FcRn is expressed throughout life and mediates the bidirectional transport of IgG, but in mice, it is markedly expressed in neonatal intestine. In adults, its expression is only faintly upregulated after intestinal IgG induction such as that elicited by i.p. immunization with Cry1Ac protoxin (pCry1Ac) Bacillus thuringiensis. This led us to suggest that additional Fcγ receptors (Fcγ-R) may be participating in epithelial IgG uptake. So, first we determined whether CD16/32 [an epitope shared by Fcγ-RII (CD32) and Fcγ-RIII (CD16)] was expressed in the intestinal epithelia of mice. Using confocal microscopy and flow cytometry, we detected co-localization of IgG and CD16/32 in epithelial cells, whose frequency was increased by immunization with pCry1Ac. Western blot and cross-immunoprecipitation results with anti-CD16/32 and IgG antibodies in epithelial cell extracts suggested that epithelial cells bear both Fcγ-RII and Fcγ-RIII and contained IgG associated with Fcγ-RII/RIII. Using anti-CD32 and anti-CD16 antibodies, we confirmed by Western blot, confocal microscopy and flow cytometry that both Fcγ-RII and Fcγ-RIII were expressed and suggested that upregulation occurred upon immunization in intestinal epithelia. Finally, we examined the in vitro effect of anti-CD16/32, anti-CD16 and anti-CD32 antibodies on IgG uptake and transport by intestinal epithelial cells and found that it was partially reduced. Although further studies are still required, our results suggest that Fcγ-RII and Fcγ-RIII might participate in the uptake and/or transport of IgG through the intestinal epithelia of adult mice.

Fc gamma receptor IIb participates in maternal IgG trafficking of human placental endothelial cells

The human placental transfer of maternal IgG is crucial for fetal and newborn immunity. Low-affinity immunoglobulin gamma Fc region receptor IIb2 (FCGR2B2 or FcγRIIb2) is exclusively expressed in an IgG-containing, vesicle-like organelle (the FCGR2B2 compartment) in human placental endothelial cells; thus, we hypothesized that the FCGR2B2 compartment functions as an IgG transporter. In this study, to examine this hypothesis, we performed in vitro bio-imaging analysis of IgG trafficking by FCGR2B2 compartments using human umbilical vein endothelial cells transfected with a plasmid vector containing enhanced GFP-tagged FCGR2B2 (pFCGR2B2-EGFP). FCGR2B2-EGFP signals were detected as intracellular vesicular structures similar to FCGR2B2 compartments in vivo. The internalization and transcytosis of IgG was significantly higher in the pFCGR2B2-EGFP-transfected cells than in the mock-transfected cells, and the majority of the internalized IgG was co-localized with the FCGR2B2-EGFP signals. Furthermore, we isolated FCGR2B2 compartments from the human placenta and found that the Rab family of proteins [RAS-related protein Rab family (RABs)] were associated with FCGR2B2 compartments. Among the RABs, RAB3D was expressed predominantly in placental endothelial cells. The downregulation of RAB3D by small interfering RNA (siRNA) resulted in a marked reduction in the FCGR2B2-EGFP signals at the cell periphery. Taken together, these findings suggest that FCGR2B2 compartments participate in the transcytosis of maternal IgG across the human placental endothelium and that RAB3D plays a role in regulating the intracellular dynamics of FCGR2B2 compartments.

Unraveling the Interaction between FcRn and Albumin: Opportunities for Design of Albumin-Based Therapeutics

The neonatal Fc receptor (FcRn) was first found to be responsible for transporting antibodies of the immunoglobulin G (IgG) class from the mother to the fetus or neonate as well as for protecting IgG from intracellular catabolism. However, it has now become apparent that the same receptor also binds albumin and plays a fundamental role in homeostatic regulation of both IgG and albumin, as FcRn is expressed in many different cell types and organs at diverse body sites. Thus, to gain a complete understanding of the biological function of each ligand, and also their distribution in the body, an in-depth characterization of how FcRn binds and regulates the transport of both ligands is necessary. Importantly, such knowledge is also relevant when developing new drugs, as IgG and albumin are increasingly utilized in therapy. This review discusses our current structural and biological understanding of the relationship between FcRn and its ligands, with a particular focus on albumin and design of albumin-based therapeutics.

On the perplexingly low rate of transport of IgG2 across the human placenta

The neonatal receptor, FcRn, mediates both serum half–life extension as well as active transport of maternal IgG to the fetus during pregnancy. Therefore, transport efficiency and half-life go hand-in-hand. However, while the half-life of the human IgG2 subclass is comparable to IgG1, the placental transport of IgG2 is not, with the neonatal IgG1 levels generally exceeding maternal levels at birth, but not for IgG2. We hypothesized that the unique short-hinged structure of IgG2, which enables its κ-, but not λ-isotype to form at least three different structural isoforms, might be a contributing factor to these differences. To investigate whether there was any preference for either light chain, we measured placental transport of IgG subclasses as well as κ/λ-light chain isotypes of IgG1 and IgG2 in 27 matched mother-child pairs. We also studied the half-life of IgG1 and IgG2 light chain isotypes in mice, as well as that of synthesized IgG2 structural isotypes κA and κB. In order to investigate serum clearance of IgG1 and IgG2 light-chain isotypes in humans, we quantified the relative proportions of IgG1 and IgG2 light chains in hypogammaglobulinemia patients four weeks after IVIg infusion and compared to the original IVIg isotype composition. None of our results indicate any light chain preference in either of the FcRn mediated mechanisms; half-life extension or maternal transport.

Maternofetal transplacental transport of recombinant IgG antibodies lacking effector functions

The neonatal Fc receptor (FcRn) directs the transfer of maternal immunoglobulin G (IgG) antibodies across the placenta and thus provides the fetus and newborn with passive protective humoral immunity. Pathogenic maternal IgG antibodies will also be delivered via the placenta and can cause alloimmunity, which may be lethal. A novel strategy to control pathogenic antibodies would be administration of a nondestructive IgG antibody blocking antigen binding while retaining binding to FcRn. We report on 2 human IgG3 antibodies with a hinge deletion and a C131S point mutation (IgG3ΔHinge) that eliminate complement activation and binding to all classical Fcγ receptors (FcγRs) and to C1q while binding to FcRn is retained. Additionally, 1 of the antibodies has a single point mutation in the Fc (R435H) at the binding site for FcRn (IgG3ΔHinge:R435H). We compared transplacental transport with wild-type IgG1 and IgG3, and found transport across trophoblast-derived BeWo cells and ex vivo placenta perfusions with hierarchies as follows: IgG3ΔHinge:R435H>wild-type IgG1≥IgG3ΔHinge and IgG3ΔHinge:R435H=wild-type IgG1=wild-type IgG3>>>IgG3ΔHinge, respectively. Collectively, IgG3ΔHinge:R435H was transported efficiently from the maternal to the fetal placental compartment. Thus, IgG3ΔHinge:R435H may be a good candidate for transplacental delivery of a nondestructive antibody to the fetus to combat pathogenic antibodies.

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.

The scavenger endothelial cell: a new player in homeostasis and immunity

To maintain homeostasis, the animal body is equipped with a powerful system to remove circulating waste. This review presents evidence that the scavenger endothelial cell (SEC) is responsible for the clearance of blood-borne waste macromolecules in vertebrates. SECs express pattern-recognition endocytosis receptors (mannose and scavenger receptors), and in mammals, the endocytic Fc gamma-receptor IIb2. This cell type has an endocytic machinery capable of super-efficient uptake and degradation of physiological and foreign waste material, including all major classes of biological macromolecules. In terrestrial vertebrates, most SECs line the wall of the liver sinusoid. In phylogenetically older vertebrates, SECs reside instead in heart, kidney, or gills. SECs, thus, by virtue of their efficient nonphagocytic elimination of physiological and microbial substances, play a critical role in the innate immunity of vertebrates. In major invertebrate phyla, including insects, the same function is carried out by nephrocytes. The concept of a dual-cell principle of waste clearance is introduced to emphasize that professional phagocytes (macrophages in vertebrates; hemocytes in invertebrates) eliminate larger particles (>0.5 μm) by phagocytosis, whereas soluble macromolecules and smaller particles are eliminated efficiently and preferentially by clathrin-mediated endocytosis in nonphagocytic SECs in vertebrates or nephrocytes in invertebrates. Including these cells as important players in immunology and physiology provides an additional basis for understanding host defense and tissue homeostasis.

An Immunohistochemical Assay to Detect Trophoblasts in Frozen Feline Placenta

The placenta, a fetal endocrine organ, is composed of subpopulations of trophoblasts, including cytotrophoblasts, and syncytiotrophoblasts. Trophoblastic populations can be distinguished based upon their expression of cytokeratin. The purpose of the current study was to develop an immunohistochemistry (IHC) method to identify trophoblasts selectively in frozen feline placental tissue using antibodies specific for cytokeratin. The mouse monoclonal antibody anti-human pan-cytokeratin AE1/AE3 and a commercial detection system were used. Nonspecific immunoreactivity was encountered that could not be eliminated with altered blocking methods. The nonspecific reactivity was attributed to the goat anti-mouse/rabbit immunoglobulin G (IgG) peroxidase polymer included in the commercial kit. Alternatively, a polyclonal rabbit anti-cow cytokeratin wide spectrum screening antibody with goat anti-rabbit IgG polyclonal secondary antibody was used to detect cytokeratin in feline placental tissue. The IHC procedure eliminated nonspecific immunoreactivity while specifically labeling cytokeratin. This new approach provides an IHC method to identify trophoblasts specifically in feline placenta.

Transfer of PAMAM dendrimers across human placenta: prospects of its use as drug carrier during pregnancy

Dendrimers offer significant potential as nanocarriers for targeted delivery of drugs and imaging agents. The objectives of this study were to evaluate the transplacental transport, kinetics and biodistribution of PAMAM dendrimers ex-vivo across the human placenta in comparison with antipyrine, a freely diffusible molecule, using dually perfused re-circulating term human placental lobules. The purpose of this study is to determine if dendrimers as drug carriers can be used to design drug delivery systems directed at selectively treating either the mother or the fetus. The transplacental transfers of fluorescently (Alexa 488) tagged PAMAM dendrimer (16 kDa) and antipyrine (188 Da) from maternal to fetal circulation were measured using HPLC/dual UV and fluorescent detector (sensitivity of 10 ng/mL for dendrimer and 100 ng/mL for antipyrine respectively). C(max) for the dendrimer-Alexa (DA) in maternal perfusate (T(max)=15 min) was 18 times higher than in the fetal perfusate and never equilibrated with the maternal perfusate during 5.5 h of perfusion (n=4). DA exhibited a measurable but low transplacental transport of 2.26±0.12 μg/mL during 5.5h, where the mean transplacental transfer was 0.84±0.11% of the total maternal concentration and the feto-maternal ratio as percent was 0.073%±0.02. The biochemical and physiological analysis of the placentae perfused with DA demonstrated normal function throughout the perfusion. The immunofluorescence histochemistry confirmed that the biodistribution of DA in perfused placenta was sparsely dispersed, and when noted was principally seen in the inter-villous spaces and outer rim of the villous branches. In a few cases, DA was found internalized and localized in nuclei and cytoplasm of syncytiotrophoblast and inside the villous core; however, DA was mostly absent from the villous capillaries. In conclusion, the PAMAM dendrimers exhibited a low rate of transfer from maternal to the fetal side across the perfused human placenta, which is similar to other investigations of large macromolecules, e.g., IgG. These overall findings suggest that entry of drugs conjugated to polymers, i.e., dendrimers, would be limited in their transfer across the human placenta when compared to smaller drug molecules alone, suggesting novel methods for selectively delivering therapeutics to the pregnant woman without significant transfer to the fetus, especially since the half life of the dendrimer in blood is relatively short.

Metastatic melanomas express inhibitory low affinity fc gamma receptor and escape humoral immunity

Our research, inspired by the pioneering works of Isaac Witz in the 1980s, established that 40% of human metastatic melanomas express ectopically inhibitory Fc gamma receptors (FcγRIIB), while they are detected on less than 5% of primary cutaneous melanoma and not on melanocytes. We demonstrated that these tumoral FcγRIIB act as decoy receptors that bind the Fc portion of antimelanoma IgG, which may prevent Fc recognition by the effector cells of the immune system and allow the metastatic melanoma to escape the humoral/natural immune response. The FcγRIIB is able to inhibit the ADCC (antibody dependent cell cytotoxicity) in vitro. Interestingly, the percentage of melanoma expressing the FcγRIIB is high (70%) in organs like the liver, which is rich in patrolling NK (natural killer) cells that exercise their antitumoral activity by ADCC. We found that this tumoral FcγRIIB is fully functional and that its inhibitory potential can be triggered depending on the specificity of the anti-tumor antibody with which it interacts. Together these observations elucidate how metastatic melanomas interact with and potentially evade humoral immunity and provide direction for the improvement of anti-melanoma monoclonal antibody therapy.

Activating and inhibitory Fcgamma receptors in immunotherapy: being the actor or being the target

Membrane Fcgamma receptors (FcgammaRs) can act either as potent activators of effector cell functions or as inhibitors of receptor-mediated cell activation following engagement by IgG antibodies bound to their target molecules. The remarkable ability of activating FcgammaRs to trigger antibody-dependent cellular cytotoxicity, cytokine release and phagocytosis/endocytosis followed by antigen presentation has stimulated the development of a number of therapeutic monoclonal antibodies whose Fc regions have been engineered to optimize their effector functions, mostly their killing activities. Conversely, the demonstration that inhibitory FcgammaRs can block or downmodulate effector functions has led to the concept that targeting these receptors is of interest in a number of pathologies. The use of bispecific antibodies leading to the crosslinking of FcgammaRIIB with activating receptors could induce immunomodulation in autoimmune or allergic diseases. Alternatively, the use of cytotoxic/antagonist anti-FcgammaRIIB antibodies could kill FcgammaRIIB-positive tumor cells or prevent the downmodulation of activating receptors. Thus, antibodies engineered to preferentially target activating or inhibitory FcgammaRs are currently being designed for therapeutic use.

IgE in the human placenta: why there?

Immunoglobulin E (IgE) antibodies are key effector molecules in the allergic inflammatory response and are also involved in the protection against extracellular parasites. Allergic symptoms often develop early in life, and the intrauterine environment has been proposed to play an important role in affecting the risk of later allergy development. The placenta constitutes a selective barrier between the maternal and foetal circulation. Recently, we reported that maternal IgE antibodies are present on foetal macrophages in the villous tissue of the human placenta irrespective of maternal allergy status. This review discusses the presence of IgE antibodies in the human placenta and its possible roles in normal and pathologic pregnancy. It also deals with the relationship between placental IgE and development of allergy during childhood. A better understanding of the role of IgE in placenta could give us clues on how to prevent allergy development in the future generations.

Hypoglycemics: pharmacokinetic considerations during pregnancy

A wide range of physiological and hormonal changes occur during pregnancy. Most begin early in the first trimester and increase by the last trimester. These changes can significantly affect pharmacokinetics and pharmacodynamics of drugs and thus may alter their safety and efficacy. Approximately 5% of pregnant women are affected by some form of diabetes, with gestational diabetes being the most prevalent. Several classes of antidiabetic drugs are currently available for the treatment of diabetes, including human insulin, its short and long analogues, and oral hypoglycemic agents. Maternal and fetal responses to these drugs can be affected by changes in absorption, distribution, and elimination in both the mother and the placental-fetal unit. This can dictate the amount of drug that can cross and the amount that is metabolized or eliminated by the placenta. Further studies are needed on the safety of antidiabetic drugs in pregnancy to clarify the extent of their transplacental passage. Specifically, in vitro placental perfusion studies in combination with controlled trials and cord blood measurements can provide insight in to the pharmacokinetics of drug transport across the placenta. This article reviews common types of antidiabetic drugs, focusing on pharmacokinetic considerations that need to be incorporated into the decision on treatment in pregnancy.

IgG is transported across the mouse yolk sac independently of FcgammaRIIb

While generally accepted that FcRn of the human syncytiotrophoblast and the mouse yolk sac endoderm is the major IgG transporter, the finding of a different Fc receptor FcgammaRIIb (RIIb) in the human placental endothelium has suggested the existence of an additional IgG transporter. Testing our hypothesis in mouse, we found that while RIIb is expressed in the yolk sac vasculature, IgG concentrations in fetuses of wild-type mice (RIIb(+/+)) and mice with a null mutation in the gene encoding RIIb (RIIb(-/-) mice) are not different, and we thus reject our hypothesis that yolk sac RIIb transports IgG in utero in the mouse. However, the capillary bed in the mouse yolk sac is structurally more complex than in human placenta, consisting of three types of cells: an RIIb-negative endothelium, a unique RIIb-bearing cell that also expresses 2 out of 4 macrophage markers but not endothelial cell or pericyte markers, and pericytes. As in the human placenta the b2 isoform of RIIb predominates in the mouse yolk sac. Remarkably only a single capillary channel rather than 2 channels with a loop is found in each yolk sac villus, which, along with intracapillary erythrocytes, suggests that blood flow is peristaltic, mediated by pericytes. It is not clear whether RIIb in the human placental villus might mediate an IgG transport function in light of the mouse yolk sac equivalent failing to do so.

Involvement of Fc receptors in disorders of the central nervous system

Immunoglobulins are proteins with a highly variable antigen-binding domain and a constant region (Fc domain) that binds to a cell surface receptor (FcR). Activation of FcRs in immune cells (lymphocytes, macrophages, and mast cells) triggers effector responses including cytokine production, phagocytosis, and degranulation. In addition to their roles in normal responses to infection or tissue injury, and in immune-related diseases, FcRs are increasingly recognized for their involvement in neurological disorders. One or more FcRs are expressed in microglia, astrocytes, oligodendrocytes, and neurons. Aberrant activation of FcRs in such neural cells may contribute to the pathogenesis of major neurodegenerative conditions including Alzheimer's disease, Parkinson's disease, ischemic stroke, and multiple sclerosis. On the other hand, FcRs may play beneficial roles in counteracting pathological processes; for e.g., FcRs may facilitate removal of amyloid peptides from the brain and so protect against Alzheimer's disease. Knowledge of the functions of FcRs in the nervous system in health and disease is leading to novel preventative and therapeutic strategies for stroke, Alzheimer's disease, and other neurological disorders.

FcRn in the yolk sac endoderm of mouse is required for IgG transport to fetus

In adults, the nonclassical MHC class I molecule, FcRn, binds both IgG and albumin and rescues both from a degradative fate, endowing both proteins with high plasma concentrations. FcRn also transports IgG from mother to young during gestation. Anticipating that a detailed understanding of gestational IgG transport in the mouse may give us a useful model to understand FcRn function in the human placenta, we have studied FcRn in the mouse yolk sac placenta in detail. Analyzing day 19-20 fetuses of the three FcRn genotypes resulting from matings of FcRn(+/-) parents, we found that FcRn(-/-) fetuses showed negligible IgG concentrations (1.5 microg/ml), whereas IgG concentrations in FcRn(+/-) fetuses were about a half (176 microg/ml) that of FcRn(+/+) fetuses (336 microg/ml), indicating that FcRn is responsible for virtually all IgG transport from mother to fetus. Immunofluorescence and immunoblotting studies indicated that FcRn is expressed in the endoderm of the yolk sac placenta but not in other cells of the yolk sac placenta or in the chorioallantoic placenta. IgG was found in the endoderm of both FcRn(+/+) and FcRn(-/-) yolk sac placentas and in the mesenchyme of FcRn(+/+) but was missing from the mesenchyme of FcRn(-/-) yolk sac placentas, indicating that IgG enters the endoderm constitutively but is moved out of the endoderm by FcRn. The similarities of these results to human placental FcRn expression and function are striking.

Plasmodium falciparum during pregnancy: a puzzling parasite tissue adhesion tropism

P. falciparum malaria severely affects pregnant women and children. Despite immunity through lifelong exposure to malaria, pregnant women become susceptible to infections causing anaemia, abortions and low birth weight. They experience massive accumulation of infected erythrocytes (IEs) in the placenta. Adhesion of IEs to host endothelial receptors is mediated by members of a large diverse protein family called P. falciparum erythrocyte membrane protein 1 (PfEMP1). Pregnancy malaria is generally associated with the emergence of a distinct subset of parasites expressing a unique PfEMP1 that binds to the host-receptor chondroitin sulfate A (CSA). Resistance to pregnancy malaria is associated with the acquisition of antibodies that block IEs binding to placental CSA. The absence (or rare occurrence) of CSA-binding parasites in malaria patients (children, men and non-pregnant women) suggests that these parasites become virulent only during pregnancy. The molecular mechanisms used by P. falciparum to achieve the timely expression of the Pf-CSA ligand in pregnant women remain puzzling. In this review we will discuss two hypothetical mechanisms by which CSA-binding parasites may arise during pregnancy. The first, a selection process by the placenta of a distinct sub-population of P. falciparum expressing a particular PfEMP1. The second, an induction mechanism that facilitates the expression of a particular PfEMP1 protein by specific host factor(s) present only during pregnancy.

Disruption of var2csa gene impairs placental malaria associated adhesion phenotype

Infection with Plasmodium falciparum during pregnancy is one of the major causes of malaria related morbidity and mortality in newborn and mothers. The complications of pregnancy-associated malaria result mainly from massive adhesion of Plasmodium falciparum-infected erythrocytes (IE) to chondroitin sulfate A (CSA) present in the placental intervillous blood spaces. Var2CSA, a member of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family is the predominant parasite ligand mediating CSA binding. However, experimental evidence suggests that other host receptors, such as hyaluronic acid (HA) and the neonatal Fc receptor, may also support placental binding. Here we used parasites in which var2csa was genetically disrupted to evaluate the contribution of these receptors to placental sequestration and to identify additional adhesion receptors that may be involved in pregnancy-associated malaria. By comparison to the wild-type parasites, the FCR3Δvar2csa mutants could not be selected for HA adhesion, indicating that var2csa is not only essential for IE cytoadhesion to the placental receptor CSA, but also to HA. However, further studies using different pure sources of HA revealed that the previously observed binding results from CSA contamination in the bovine vitreous humor HA preparation. To identify CSA-independent placental interactions, FCR3Δvar2csa mutant parasites were selected for adhesion to the human placental trophoblastic BeWo cell line. BeWo selected parasites revealed a multi-phenotypic adhesion population expressing multiple var genes. However, these parasites did not cytoadhere specifically to the syncytiotrophoblast lining of placental cryosections and were not recognized by sera from malaria-exposed women in a parity dependent manner, indicating that the surface molecules present on the surface of the BeWo selected population are not specifically expressed during the course of pregnancy-associated malaria. Taken together, these results demonstrate that the placental malaria associated phenotype can not be restored in FCR3Δvar2csa mutant parasites and highlight the key role of var2CSA in pregnancy malaria pathogenesis and for vaccine development.

Receptor-mediated endocytosis of immune complexes in rat liver sinusoidal endothelial cells is mediated by FcgammaRIIb2

Liver sinusoidal endothelial cells (LSECs) display a number of receptors for efficient uptake of potentially injurious molecules. The receptors for the Fc portion of immunoglobulin G (IgG) antibodies (FcgammaRs) regulate a number of physiological and pathophysiological events. We used reverse transcription polymerase chain reaction (RT-PCR) and Western blotting to determine the expression of different types of FcgammaRs in LSECs. Biochemical approaches and immunofluorescence microscopy were used to characterize the FcgammaR-mediated endocytosis of immune complexes (ICs). FcgammaRIIb2 was identified as the main receptor for the efficient uptake of ICs in LSECs. The receptor was shown to use the clathrin pathway for IC uptake; however, the association with lipid rafts may slow the rate of its internalization. Moreover, despite trafficking through lysosomal integral membrane protein-II (LIMP-II)-containing compartments, the receptor was not degraded. Finally, it was shown that the receptor recycles to the cell surface both with and without IC.

CONCLUSION

FcgammaRIIb2 is the main receptor for endocytosis of ICs in rat LSECs. Internalized ICs are degraded with slow kinetics, and IC internalization is not linked to receptor downregulation. After internalization, the receptor recycles to the cell surface both with and without ICs. Thus, FcgammaRIIb2 in rat LSECs is used as both a recycling receptor and a receptor for efficient IC clearance.

Pregnancy-associated malaria: Parasite binding, natural immunity and vaccine development

Humans living in areas of high malaria transmission gradually acquire, during the early years of life, protective clinical immunity to Plasmodium falciparum, limiting serious complications of malaria to young children. However, pregnant women become more susceptible to severe P. falciparum infections during their first pregnancy. Pregnancy associated malaria is coupled with massive accumulation of parasitised erythrocytes and monocytes in the placental intervillous blood spaces, contributing to disease and death in pregnant women and developing infants. Indirect evidence suggests that prevention may be possible by vaccinating women of childbearing age before their first pregnancy. This review aims to introduce the reader to the implications of malaria infection during pregnancy and to analyse recent findings towards the identification and characterisation of parasite encoded erythrocyte surface proteins expressed in malaria-infected pregnant women that are likely targets of protective immunity and have potential for vaccine development.

Endothelial Expression of Fc Gamma Receptor IIb in the Full-term Human Placenta

In the third trimester, human placental endothelial cells express Fc gamma receptor IIb (FcgammaRIIb). This expression is unique because FcgammaRIIb is generally expressed on immune cells and is typically undetectable in adult endothelial cells. Recently, we found a novel FcgammaRIIb-defined, IgG-containing organelle in placental endothelial cells; this organelle may be a key structure for the transcytosis of IgG across the endothelial layer. In this study, we verify the expression of FcgammaRIIb in endothelial placenta cells and use reverse transcriptase-polymerase chain reaction (RT-PCR) and sequencing analyses to define the expressed FCGR2B mRNA transcript variant. We also investigated the distribution of FCGR2B mRNA and protein within the vascular tree of the full-term human placenta by RT-PCR and quantitative microscopy. The mRNA sequence of FCGR2B expressed specifically in placental endothelial cells is that of transcript variant 2. FcgammaRIIb expression and synthesis occur throughout the placental vascular tree but do not extend into the umbilical cord. This study provides additional information on FcgammaRIIb expression in the human placenta.

Fc gamma receptors and cancer

FcgammaRs are a family of heterogeneous molecules that play opposite roles in immune response and control the effector functions of IgG antibodies. In many cancers, IgG antibodies are produced that recognize cancer cells, form immune complexes and therefore, activate FcgammaR. The therapeutic efficacy of monoclonal IgG antibodies against hematopoietic and epithelial tumors also argue for an important role of IgG antibodies in anti-tumor defenses. Since the 1980s, a series of lines of evidence in experimental models and in humans strongly suggest that FcgammaR are involved in the therapeutic activity of monoclonal IgG antibodies by activating the cytotoxic activity of FcgammaR-positive cells such as NK cells, monocytes, macrophages and neutrophils and by increasing antigen presentation by dendritic cells. Since many cell types co-express activating and inhibitory FcgammaR, the FcgammaR-dependent effector functions of IgG anti-tumor antibodies are counterbalanced by the inhibitory FcgammaRIIB. In addition, some tumor cells express FcgammaR either constitutively, such as B cell lymphomas or ectopically, such as 40% of human metastatic melanoma. The tumor FcgammaR isoform is preferentially FcgammaRIIB, which is functional at least in human metastatic melanoma. This review summarizes these data and discusses how FcgammaRIIB expression may influence the anti-tumor immune reaction and how beneficial or deleterious this expression could be for the efficiency of therapeutics based on monoclonal anti-tumor antibodies.

Plasmodium falciparum: Chondroitin sulfate A is the major receptor for adhesion of parasitized erythrocytes in the placenta

Plasmodium falciparum parasites that sequester in the placenta bind to the molecule chondroitin sulfate A (CSA). Women become resistant to malaria during pregnancy as they acquire antibodies that inhibit parasite adhesion to CSA, suggesting that a vaccine against placental malaria is feasible. Hyaluronic acid (HA) and non-immune IgG have also been proposed as receptors for P. falciparum adhesion in the placenta, but evidence for their roles is inconclusive. In this study, CSA, HA, and IgG were simultaneously assessed for their relative contributions to placental adhesion. Placental parasites collected in Tanzania uniformly adhered to the molecule CSA, and soluble CSA completely inhibited adhesion of most samples to placental cryosections. Three of 46 placental parasite samples also adhered to immobilized HA, but HA failed to inhibit adhesion of any placental parasites to placental cryosections. Similarly, non-immune IgG and protein A failed to inhibit adhesion of parasite samples to placental cryosection. P. falciparum adhesion in the placenta appears to be a non-redundant process that requires CSA as a receptor. Vaccines that elicit functional antibodies against CSA-binding parasites may confer resistance to pregnancy malaria.

Immunohistochemistry in human placental tissue--pitfalls of antigen detection

Because incongruous controversial staining results are a common phenomenon in the placenta, methodical investigations are important to prevent researchers from obtaining misleading results. While investigating dendritic cells (DC) at the human fetomaternal interface, we observed staining of endothelial cells (EC) in chorionic villi for CD83. Given the high specificity of this antigen for DC, this did not seem credible. Previous studies had revealed the same surprising staining pattern with human leukocyte antigen (HLA)-G antibodies. We therefore analyzed human placental EC staining more closely. Both CD83 and HLA-G antibodies were of the same mouse IgG2b isotype. We also observed EC staining with a panel of control antibodies of the IgG2b isotype. This suggests a high affinity of human placental capillaries for mouse IgG2b. Several commonly used techniques for blocking nonspecific binding of antibodies could not prevent this nonspecific EC staining. A new preincubation step with purified human IgG was introduced. This abolished any placental EC staining with CD83, HLA-G, and IgG2b isotype control antibodies, presumably by blocking Fc receptors, whereas specific staining patterns remained unchanged. Mouse antibody of the IgG2b isotype are bound nonspecifically by vascular endothelial cells in human placenta and this can be overcome by blocking with purified human IgG. This blocking procedure could also be appropriate for frozen tissues other than placenta in which Fc receptors are expressed.

A Novel FcγR-Defined, IgG-Containing Organelle in Placental Endothelium

Placental transfer of IgG from maternal circulation to that of the fetus is crucial for fetal and newborn immunity. This process requires that IgG broach two cellular layers of the placenta. IgG transport across the first layer, the syncytiotrophoblast, is almost certainly mediated by the MHC-related FcR for IgG, FcRn. The second layer, the villus endothelium, was until recently thought to allow IgG movement nonspecifically by constitutive transcytosis in caveolae. However, we recently showed that villus endothelium expressed a separate FcR for IgG, the inhibitory motif-bearing Fc gammaRIIb2 seen most notably on macrophages and as a minor fraction of the Fc gammaRIIb expressed on B cells. Now, by quantitative microscopy, we find Fc gammaRIIb2 to be expressed abundantly in an unidentifiable and likely novel organelle of the villus endothelium, unassociated with caveolae. About half of these Fc gammaRIIb2 organelles contain IgG; the remainder lack IgG. The majority fraction (approximately 80%) of IgG-containing organelles is associated with Fc gammaRIIb. No IgG-containing organelles are associated with caveolin. These findings are compatible with Fc gammaRIIb-mediated transfer of IgG across the villus endothelium, independent of caveolae.