Human placental membrane receptor for IgG. Purification of the receptor and its subunit structure

IgG transport across trophoblast-derived BeWo cells: a model system to study IgG transport in the placenta

In primates, prenatal transfer of IgG from mother to offspring occurs predominantly across the placenta. Although a number of Fcgamma-receptors and IgG binding proteins have been detected in human placental tissue, an involvement of any of these receptors in IgG transport across the syncytiotrophoblast remains to be demonstrated. Therefore, we investigated the mechanism of IgG transcytosis in trophoblast-derived BeWo cells. BeWo cells were not only found to express the MHC class I-related IgG Fc receptor, human FcRn, but also specifically bound fluorescein isothiocyanate (FITC)-labeled human IgG (FITC-hIgG) at the apical surface at mildly acidic pH. The cells preferentially transcytosed FITC-hIgG from the apical to the basolateral side when compared to the fluid-phase marker FITC-dextran and to FITC-hIgG transcytosis in the opposite direction. However, endocytosis of FITC-hIgG at the apical plasma membrane at physiological pH required the continuous presence of FITC-hIgG at concentrations similar to those present in the maternal circulation. These results suggest a mechanism by which IgG is internalized by BeWo cells via fluid-phase endocytosis. Tight binding of IgG to hFcRn may then occur in acidic endosomes, followed by selective sorting into the transcytotic pathway. Thus, the main function of this receptor is to prevent entry of IgG into the degradative pathway in lysosomes.

Expression of placental alkaline phosphatase does not correlate with IgG binding, internalization and transcytosis

The human homologue of FcRn, an IgG Fc receptor expressed in rat villous syncytiotrophoblasts, might be involved in IgG transfer from the maternal to the fetal circulation. However, because the receptor does not bind IgG at the physiological pH of the maternal blood (pH 7.4), FcRn is probably not involved in the initial uptake of IgG. A role in IgG internalization has been suggested for placental alkaline phosphatase (PLAP), which is highly expressed on the apical surface of syncytiotrophoblasts. To determine whether PLAP does indeed have a role in IgG uptake, we analysed the ability of PLAP to bind, internalize and transcytose IgG in BeWo choriocarcinoma cells endogenously expressing the protein, or in Madin-Darby canine kidney (MDCK) cells transfected with the PLAP cDNA. Although PLAP expression in MDCK cells resulted in increased IgG binding to intact cells, binding was not correlated with the level of PLAP expressed in the different cell lines. Furthermore our findings do not support a role for PLAP in IgG endocytosis or transcytosis.

Human placental Fc gamma-binding proteins in the maternofetal transfer of IgG

Annexin II, a member of the annexin family of Ca2+ and phospholipid binding proteins, is present in human placenta. Placental annexin II has low affinity FcR activity, and is present as a heterotetramere on syncytiotrophoblast apical cell membrane extracellular surface. In addition to annexin II, transmembraneous leukocyte FcRIII is present on syncytiotrophoblast apical membrane. Either one, or both molecules may mediate the binding of IgG and thereby facilitate its transport through the syncytiotrophoblast layer. However, the presence of other maternal plasma proteins in syncytiotrophoblasts that are not transported to the human fetus is suggestive of nonspecific fluid phase endocytosis. The MHC class I like FcR, similar to the receptor found in neonatal rodent intestine, FcRn, is present intracellularly in human syncytiotrophoblasts, as is its light chain beta 2-microglobulin. The hFcRn is not detected on the apical plasma membrane. The placental hFcRn co-localizes with IgG in syncytiotrophoblast granules. It is likely that hFcRn binds and transcytoses IgG through the syncytiotrophoblast. Protected transfer of IgG may occur within syncytiotrophoblast endocytotic vesicles prior to release in the villous stroma and subsequent translocation into the lumen of fetal stem vessels by uptake and transport in endothelial caveolae.

A major histocompatibility complex class I-like Fc receptor cloned from human placenta: possible role in transfer of immunoglobulin G from mother to fetus

The acquisition of maternal antibodies is critical for immunologic defense of the newborn. In humans, maternal IgG is actively transported across the placenta. The receptor responsible for this transport has not been identified definitively. We report the isolation from a placental cDNA library of clones encoding the alpha-chain of an immunoglobulin G (IgG)-Fc receptor (hFcRn) that resembles a class I major histocompatibility complex antigen. The DNA and predicted amino acid sequences are very similar to those of the neonatal rat and mouse intestinal Fc receptors, rFcRn and mFcRn. These receptors mediate transport of maternal IgG from milk to the blood-stream of the suckling rat or mouse. Like rat and mouse FcRn, hFcRn binds IgG preferentially at low pH, which may imply that IgG binds hFcRn in an acidic intracellular compartment during transport across the placenta.

Evidence that Fc gamma receptors in rabbit yolk sac endoderm do not depend upon an acid pH to effect IgG binding and transcytosis in vitro

An in vitro culture system has been devised creating apical and basal compartments separated by rabbit visceral yolk sac (VYS) with an intact epithelium. Selective transcytosis and binding of heterologous IgG applied to the apical yolk sac endoderm (YSE) was demonstrated in vitro using double label immunofluorescence. Thus, whilst both human and bovine IgG could be detected in endosomes in YSE, only human IgG could be detected in the basement membrane and vascular mesenchyme. This mirrors what is found in vivo. The Fc fragment of human Ig was transcytosed but not the Fab fragment, indicating that Fc receptors were expressed in the cultured YSE. When VYS was previously chilled to 4 degrees C to prevent endocytosis and treated with rabbit serum albumin to prevent non-specific binding, human IgG, but not bovine IgG, became specifically bound to YSE apical plasma membrane; comparison of binding at pH 6.0, 7.3 (the average pH of rabbit uterine fluid) and 8.0 revealed no obvious difference. Pre-exposure of VYS for up to 5 min in monensin, followed by culture in monensin and immunoglobulin-containing medium, did not prevent the selective transcystosis of human IgG, suggesting that an acidic compartment may not be needed for transcytosis. An acid pH dependent Fc gamma receptor equivalent to that on suckling rat gut jejunal enterocyte plasma membranes could not be isolated from rabbit YSE following exposure of solubilized membrane to affinity matrix bound IgG at pH 6.0 and elution at pH 8.0. These results contradict a recent suggestion that Fc receptors on all IgG transcytosing epithelia require an acid pH to effect IgG binding and selective transcytosis.

Immune recognition at the maternal-fetal interface: overview

Trophoblast antigens at the maternal-fetal interface that are capable of stimulating maternal immune responses have been studied. Candidates are blood group I and P, HLA, Fc gamma-receptors, TLX, and phospholipids. Antigens I and P on trophoblast have been implicated in pregnancy loss but incompatible i,p mothers are rare. HLA-G is expressed on cytotrophoblast; however, no evidence for HLA-G allotypy or maternal responses to these molecules exists, although HLA-G has been implicated in recruitment of suppressor T cells. Receptors for IgG (Fc gamma-RI, Fc gamma-RII and Fc gamma-III) are present on trophoblast but allotypy is limited to the NA1-NA2 antigen system associated with Fc gamma-RIII on neutrophils. Maternal Fc-gamma R blocking antibodies have been linked to pregnancy success. The TLX alloantigen system was described by using xenogeneic antisera. Idiotype-antiidiotype regulated maternal responses to TLX are proposed as necessary for successful pregnancy. Several putative TLX monoclonal antibodies (Mab) recognize a regulator of complement activation called MCP (membrane cofactor protein, or CD46). Mab to MCP do not exhibit allotypy. Syncytial and cytotrophoblastic membranes are rich sources of MCP. Preliminary data suggest that a conformational site induced by C3b (iC3) binding to MCP may be responsible for TLX allotypy. Certain pregnancy loss patients produce antiphospholipid antibodies (aPA). Some investigators believe that aPA recognize a plasma protein cofactor, beta 2 GPI and not phospholipid per se. We produced three Mab specific for beta 2 GPI, one of which fails to recognize beta 2 GPI bound to phospholipid [corrected].(ABSTRACT TRUNCATED AT 250 WORDS)

Placental alkaline phosphatase has a binding site for the human immunoglobulin-G Fc portion

Affinity chromatography of human plasma on placental-alkaline-phosphatase-Sepharose columns (placental alkaline phosphatase, PLAP) yielded consistently a pure protein which was identified as IgG on the basis of electrophoretical and immunological comparisons with authentic human IgG. SDS/PAGE of the protein revealed, under reducing conditions, two polypeptides of 55 kDa and 25 kDa. The N-terminal amino acid sequence (12 residues) of the 55-kDa subunit presented high similarity (83-100%) with known sequences of immunoglobulin gamma chains. The IgG binds by its Fc portion to a fully exposed domain in the plasma-membrane-anchored PLAP. Scatchard analysis of the interaction gave a dissociation constant of 3.68 microM, a value close to those found for haematopoietic cells and syncytiotrophoblast Fc receptors. The latter was affinity purified from human placenta as the major IgG-binding component and presented cross-immunoreactivity with anti-PLAP antibodies, indicating that PLAP and the putative placental Fc receptor could be identical molecules.

Placental alkaline phosphatase is related to human IgG internalization in HEp2 cells

The biological function(s) of placental alkaline phosphatase has not yet been unraveled. The low catalytic activity of the enzyme at physiological pH, and the lack of "natural substrates", bring about the necessity of a more structure-related conception of its role. We have observed an interaction between placental alkaline phosphatase and human IgG. In this report we show that this isozyme is the major membrane protein able to bind IgG in a IgG-internalizing cell line (HEp2). Pretreatment of these cells with Fab fragments of anti placental alkaline phosphatase antibodies blocks the internalization of IgG without perturbing the endocytosis of other ligands. Our results indicate that placental alkaline phosphatase has the ability not only to bind human IgG, but also to promote its internalization in HEp2 cells.

Proteins associated with activity of Fc receptors on isolated human placental syncytiotrophoblast microvillous plasma membranes

To identify Fc receptors from human placental microvilli, proteins that were liberated by detergents from human placental synctiotrophoblast microvillous membranes (StMPM) were characterized by their abilities to bind human IgG in immune complexes with sheep or goat anti-human IgG and to monomeric rabbit anti-dinitrophenol (DNP) IgG bound to DNP-lysine Sepharose. Three placental IgG-binding proteins coprecipitated with immune complexes (Mr = 68,000, 52,000-56,000, 40,000) and were designated pIBP68, pIBP56 and pIBP40, respectively. Of the three proteins only pIBP56 bound to immobilized monomeric rabbit IgG. It was isolated from detergent lysates of StMPM and LDS/phenol glycoprotein extracts of placental plasma membranes suggesting that pIBP56 was a glycoprotein FcR previously reported (Mikulska et al, 1982). The binding specificities of pIBP56 and pIBP40 appeared to be detergent dependent. Photoaffinity crosslinking of StMPM surface proteins in situ to monomeric rabbit derivatized with N-succinimidyl(4-azidophenyl)-I, 3-dithiopropionate identified IgG-binding proteins identical in size to pIBP56 and pIBP40. Crosslinking further suggested that monomeric IgG covalently bound to a complex of StMPM proteins with a total size of 110,000-120,000 Mr. The findings suggest that pIBP68, pIBP56 and pIBP68 are responsible for IgG binding activity of placental StMPM.

Isolation from human chronic-lymphocytic-leukaemia cells of membrane glycoproteins associated with Fc-receptor functions. Physical parameters and production of polyclonal antibodies

Two membrane glycoproteins that bound immune complexes and inhibited Fc-receptor- (FcR-)mediated functions in vitro were purified from human FcR+ chronic-lymphocytic-leukaemia cells. A multi-step purification was developed, consisting essentially in: (i) Tween 40 extraction of crude cell membranes; (ii) solubilization of membrane fragments by Renex-30; (iii) isolation of glycoproteins by affinity chromatography on Lens culinaris haemagglutinin-Sepharose; (iv) papain treatment of the eluted glycoproteins followed by gel-filtration chromatography; (v) purification by polyacrylamide-gel electrophoresis of two molecular species from the protein-size fraction enriched for immune-complex-binding activity. The two electrophoretically isolated components displayed apparent molecular masses of 70 and 45 kDa by SDS/polyacrylamide-gel electrophoresis and restricted charge heterogeneity by two-dimensional analysis. Two-dimensional peptide mapping revealed the presence of many peptides in common between the two proteins and the absence of a number of peptides in the 45 kDa component. These two polypeptides were used as immunogens to produce polyclonal antibodies that cross-reacted with both proteins and specifically inhibited FcR-mediated reactions in vitro. Furthermore, FcR-related components from detergent-extracted lysates of the human K562 and U937 cell lines or human placental membranes were revealed by the putative anti-FcR antibodies adsorbed on Protein A-Sepharose.

A micromethod for the determination of an unbound radioactive iodine in radiolabeled glycoproteins by means of uranyl acetate

A method for determination of an unbound radioactive iodine in radiolabeled glycoproteins is based on coprecipitation of these macromolecules with uranyl and phosphate ions. Under these conditions unbound 125I is not precipitated. The method is simple, rapid, and very sensitive.