Purification of a functional 40 kD human placental Fc gamma-receptor using a monoclonal antibody

Suppression of immunoglobulin secretion by soluble annexin II

Annexin II (AII) belongs to a family of glycoproteins that bind negatively charged phospholipids in the presence of calcium. The annexins exert various biological functions. We have previously shown that soluble AII suppresses mitogen-induced lymphoproliferation in vitro. In this study we address the question of whether soluble AII may also affect immunoglobulin secretion. Mononuclear cells from peripheral blood were stimulated with pokeweed mitogen in vitro and immunoglobulin-secreting cells were quantified using an ELISPOT assay. Retroplacental serum and soluble AII significantly inhibited secretion of IgG and IgM when added at concentrations that did not affect lymphoproliferation or cell viability. The inhibitory effect was dose- and time dependent. Significant suppression was observed when soluble AII was added at concentrations of 0.1 and 0.01 microg/ml. The strongest inhibition was observed when soluble AII or retroplacental serum was added initially. The data demonstrate that soluble AII can suppress immunoglobulin secretion in vitro. AII seems to be a potent immunosuppressive substance. The presence of high levels of soluble AII in retroplacental serum may indicate a possible immunomodulatory role in normal pregnancy.

Binding of human IgG and F(ab')2 and Fc fragments to cultured trophoblast cells from human term placenta

Trophoblast cells, dispersed by trypsin digestion of human term placental villi and purified on Percoll gradient, were maintained in serum-containing medium as monolayer cultures up to 7 days. The initially mononucleated cells, probably cytotrophoblasts, differentiated in culture within 90 h to multinucleated syncytiotrophoblast-like cells. The enigmatic binding of human immunoglobulin G (hIgG) to these cells was studied and compared to the well-known binding of hIgG to cultured human monocytes. Binding of hIgG to cultured trophoblasts at 4 degreesC reached steady state by 0.5-1 h, increased about two- to threefold after 90 h in culture and was linear throughout all concentrations tested (0.00067-132 microM). Fc fragments and even F(ab')2 fragments were found to bind to a similar extent to trophoblasts as the complete hIgG molecules. In contrast, in experiments with cultured monocytes, saturation of hIgG binding could be demonstrated. The binding of complete hIgG molecules and of Fc fragments to monocytes was similar whereas binding of F(ab')2 fragments to monocytes was significantly lower. Our results suggest that, despite morphological and at least partially functional differentiation of trophoblast cells in primary culture, no measurable amounts of functional Fc receptor for monomeric hIgG was expressed.

Human placental Fc receptors and the trapping of immune complexes

Monomeric maternal antibodies are transmitted to the fetus, but most immune complexes are trapped in the placental stroma. The receptors responsible for trapping immune complexes appear to be Fc gamma RIa, Fc gamma RIIa, and Fc gamma RIIIa on stromal macrophages, and Fc gamma RII on fetal capillary endothelial cells.

Human placental Fc receptors and the transmission of antibodies from mother to fetus

During human pregnancy, maternal IgG is transported across the placenta to the fetus. On the way, some maternal antibodies against fetal antigens are removed as immune complexes. The placenta contains several known Fc receptors and also other proteins that bind immunoglobulins. A consideration of the binding properties and distribution of these proteins suggests that the neonated Fc receptor (FcRn) transports IgG across the syncytiotrophoblast, and possibly the fetal blood vessel endothelium. Fc gamma RI, Fc gamma RII and Fc gamma RIII on Hofbauer cells in the stroma probably clear immune complexes, together with Fc gamma RII on endothelial cells.

Suppressive effect on lymphoproliferation in vitro by soluble annexin II released from isolated placental membranes

PROBLEM

Syncytiotrophoblast microvillous plasma membranes (StMPM) are potent suppressors of lymphoproliferation in vitro. We have previously shown that soluble annexin II (AII) is present at higher levels in retroplacental serum (RPS) than in peripheral serum, and that soluble AII has an immunosuppressive effect. The aims of this study were to determine whether AII can be released from StMPM and whether soluble AII from StMPM exerts any immunosuppressive effect.

METHOD OF STUDY

Isolated StMPM were incubated in growth medium for 18 hr and supernatants were prepared by ultracentrifugation. Soluble AII was detected by immunoblotting. StMPM, StMPM supernatant, and affinity-purified AII were analysed in a lymphoproliferation assay for immunomodulating activity.

RESULTS

AII heavy chain and its p11 light chain were detected both in StMPM supernatant and in RPS after removal of StMPM particles by ultracentrifugation. StMPM, StMPM supernatant, and purified AII suppressed lymphoproliferation in a dose-dependent manner. Absorption of AII from StMPM supernatant reduced the suppressive activity. The suppressive effect of StMPM supernatant and purified AII was completely reversed by heating at 100 degrees C for 30 min or by adding recombinant interleukin-2 at 100 units/ml. Although StMPM and affinity-purified AII suppressed the proliferation of lymphocytes from all donors tested, StMPM supernatant suppressed the proliferation of lymphocytes from 12 of 23 donors. Six of eight female non-suppressed donors were multiparae, whereas five of five female suppressed donors were nulliparae.

CONCLUSIONS

Annexin II is released by isolated placental membranes in vitro and is present in RPS, indicating in vivo release of AII at the fetomaternal interface, probably as AII heterotetramer. AII has immunosuppressive activity and may be important in fetal allograft survival.

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.

Inhibition of phytohaemagglutinin-induced lymphoproliferation by soluble annexin II in sera from patients with renal cell carcinoma

Annexin II (AII) is a member of a family of glycoproteins which bind negatively charged phospholipids in a calcium-dependent manner. Annexins are membrane-associated proteins, expressed both in normal and malignant cells, but have also been detected as soluble molecules in serum and other body fluids. Because of their adhesive properties, it has been suggested that annexins play a role in the metastatic process. An ELISA was established for quantification of soluble AII. Within-run variation was 5.2-10.4% and run-to-run variation 12.4-15.6%. Soluble AII was detected in all sera studies. A strongly positive serum was arbitrarily given the value 100 AII units and used as reference serum. The mean level in sera from 20 normal blood donors was 49 (SE 5.6) AII units. Sera from peripheral blood of five patients with renal cell carcinoma and sera from blood obtained from the renal vein of the same patients contained 47 (SE 20) and 83 (SE 28) AII units, respectively. In two patients, AII levels were increased in renal vein serum as compared with peripheral blood serum. Interestingly, in both cases, and in none of the three remaining cases, phytohaemagglutinin-stimulated lymphoproliferation was suppressed by renal vein serum as compared with peripheral blood serum. Affinity absorption of AII from the renal vein sera with increased AII levels strongly reduced their immunosuppressive activity. Addition of affinity-purified AII to cell cultures suppressed lymphoproliferation. These data show that the level of AII is markedly increased in renal vein sera from some patients with renal cell carcinoma, suggesting that AII may be locally released in vivo. The study also demonstrates an immunosuppressive effect of soluble AII in vitro. We speculate that soluble AII released by the tumour has immunosuppressive properties. This study identifies soluble AII as a novel immunosuppressive factor in sera from patients with renal cell carcinoma. A further study including a larger number of patients is currently in progress, in order to investigate the pathological significance of this finding.

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.

Surface annexin II on placental membranes of the fetomaternal interface

PROBLEM

The phospholipidbinding membrane protein annexin II has been demonstrated to possess FcR activity for IgG and has been localized to the outer part of the syncytiotrophoblast cell layer. The question has arisen whether annexin II is exposed on the surface of syncytiotrophoblast cells thus enabling it to take part in the transport of IgG across the maternal barrier.

METHOD

Syncytiotrophoblast microvillous plasma membranes were analyzed by flow cytometry for annexin II as well as established surface molecules. Fresh, fixed placental tissue was preincubated with antibodies to annexin II or known trophoblast surface molecules, and analyzed by confocal laser scanning microscopy.

RESULTS

Annexin II and its subunit p11 were expressed on the surface of the syncytiotrophoblast microvillous plasma membranes as were other established surface proteins (CD46, CD59, placental alkaline phosphatase), using both flow cytometry and confocal microscopy. Annexin was not detected on the surface of viable cultured trophoblast cells.

CONCLUSION

Annexin II is exposed on the surface of syncytiotrophoblast cells as a heterotetramer together with its light chain p11. It is exposed to maternal blood and may be instrumental in IgG transport across the placental barrier by binding.

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

Transfer of maternal IgG through the human placenta furnishes the newborn with passive immunity to a number of infectious agents. The exact mechanism of this transfer is still unknown, but it is agreed that it involves active receptor-mediated transport. The neonatal Fc receptor is a major histocompatibility complex class I-like receptor originally identified in the intestines of newborn rodents. A similar receptor has recently been detected in human placental syncytiotrophoblasts. Using multilabeling fluorescence immunohistochemistry and confocal laser scanning microscopy, we found that the neonatal Fc receptor co-localizes with IgG and beta 2-microglobulin in granules of human placental syncytiotrophoblast. The Fc receptor is not detected on syncytiotrophoblast apical plasma membrane. Localization to the outermost cellular barrier between the fetal and maternal blood further strengthens the role of the Fc receptor in transplacental transport of IgG.

Identification of a soluble Fc gamma-binding molecule (annexin II) in human serum using a competitive ELISA

We have previously produced a monoclonal antibody (mAb), B1D6, reactive with a 37 kD placental IgG Fc-binding molecule (FcR), recently identified as annexin II. Annexin II is an intracellular molecule found in several cell types, including endothelium and monocytes. Since soluble Fc-binding molecules are of importance in the regulation of the immune response, we have now used B1D6 in a competitive ELISA to study levels of soluble annexin II in human sera. Soluble annexin II was detected in all sera studied. The highest levels were observed in patients with infectious mononucleosis. Gel filtration of sera revealed annexin II in fractions corresponding to a molecular weight of 40-60 kD. In Western blot analysis a molecule of approximately 37 kD was found. The pI of soluble annexin II was about 7.5-8 as demonstrated by chromatofocusing. Annexin II belongs to a family of phospholipid-binding molecules involved in anti-inflammatory responses, and elevated levels of annexin II in serum may be important for the suppression of an immune response.

Fc gamma-receptor activity of placental annexin II

We have previously produced a MoAb, B1D6, against a placental FcR. The antigen isolated using F(ab')2-fragments of B1D6 exhibits Fc-binding properties with low affinity for IgG. The antigen is a single-chained glycoprotein with a molecular weight of approximately 37 kDa and a pI of about 7.0-8.5. Amino acid sequences from enzymatic digests of the antigen indicated that it is annexin II. Immunoreactivity using anti-annexin antisera and purified placental annexin II have further established the specificity of B1D6 to annexin II. The B1D6 epitope appears to be intramembraneous and intracellular on placental syncytiotrophoblasts, monocytes and other cells investigated.

Current topic: human placental Fc receptors

Human immunoglobulin G (IgG) Fc receptors are important in the materno-fetal relationship. Three classes of IgG Fc receptors are recognized which generate multiple isoforms, most of which are expressed in different cellular components of human placenta at different times during pregnancy. Although the distinct biological functions of Fc gamma R phenotypes expressed in human placenta are still unknown, recent data provide evidence for an important association between the Fc gamma R phenotype and transcytosis of IgG in the placenta. Selective transfer of maternal IgG across the placenta provides passive immunity to the fetus during the period when its own immune system is gaining protective potential. Furthermore, placenta-specific macrophages may contribute through Fc gamma R-mediated phagocytosis to the protection of the fetus from either infection or maternal immune attack against paternally inherited fetal antigens. Ontogeny and expression of various isoforms of Fc gamma R subtypes may be the key to the elucidation of the transport mechanism of maternal IgG to the fetus, in addition to the determination of the mechanisms of placental protection of the fetus against the maternal immune system.

A dot-immunobinding assay for the demonstration of soluble Fc gamma receptors

We have developed a sensitive dot-immunobinding assay to demonstrate and characterize the functional activity of soluble Fc gamma receptors (FcR). Samples containing soluble FcR were immobilized on a nitrocellulose membrane. Immune complexes of horseradish peroxidase and rabbit IgG antibodies to horseradish peroxidase (HRP) were allowed to react with nitrocellulose-bound FcR, and the immune complexes were visualized by HRP developer. The intensity of the grey dots reflected the amount of immune complex bound. Binding of immune complexes to placental extract containing soluble FcR was inhibited completely by IgG and Fc fragments, but not by F(ab')2 fragments, IgA and IgM. The method was used to characterize the subclass specificity of solubilized placental FcR. Human Fc fragments, and intact IgG1 and IgG3 proteins inhibited the binding whereas preparations of F(ab')2, IgG2 and IgG4 did not. In conclusion, the dot-immunobinding assay described is a rapid and simple method for the demonstration and characterization of functionally active soluble FcR.

Suppression of mitogen-induced lymphoproliferation by soluble IgG Fc receptors in retroplacental serum in normal human pregnancy

A competitive ELISA was used to quantify soluble IgG Fc receptors (FcR) in retroplacental serum (RPS) and peripheral serum (PS) from 10 women after uncomplicated full-term deliveries. The RPS contained significantly higher amounts of soluble FcR than did PS from the same individuals. RPS suppressed phytohaemagglutinin-stimulated lymphoproliferation as compared with PS, and a positive correlation (r = 0.66) was found between the degree of suppression and the difference in soluble FcR level between RPS and PS. Absorption of sera with Sepharose 4B coupled with heat-aggregated IgG strongly reduced the immunosuppressive activity, whereas absorption with Sepharose coupled with IgG F(ab')2 fragments did not. When IgG-binding material was eluted from Sepharose beads and added to cell cultures, the immunosuppressive activity was restored. The data indicate that soluble FcR at physiological levels have immunosuppressive properties. FcR-mediated immunosuppression may be of importance for maintenance of local immunosuppression during pregnancy.

Membrane-spanning Fc gamma receptor III isoform expressed on human placental trophoblasts

PROBLEM

Fc receptor for immunoglobulin (Fc gamma R) is an important mediator of immunological functions in the feto-maternal relationship. We have demonstrated by immunohistochemical means that three distinct classes of Fc gamma Rs are expressed in the different cell components of the human placenta.

METHOD

In this study, Fc gamma RIII isoform expressed on placental trophoblasts (PTs) was investigated by indirect immunofluorescence and cDNA cloning. PTs, isolated from human term placenta by digestion with proteolytic enzyme, were reacted with monoclonal antibodies (MAb) against the Fc gamma Rs and other surface markers of leukocytes and subjected to flow cytometric analysis.

RESULTS

PTs were positively stained with 3G8 and Leu1 1b against Fc gamma RIII, partially stained with MAb against MHC class I, but not with 32.2 (Fc gamma RI), IV3 (Fc gamma RII), or MAbs against CD4, CD19, or CD56, indicating that only low affinity receptor, Fc gamma RIII, is expressed on PTs. The DNA sequence of cloned Fc gamma RIII CDNA from PTs by PCR was identical to that of natural killer (NK) cell isoform, including the position of the stop codon that differs from the granulocyte isoform by several nucleotide substitutions. We further analyzed the susceptibility of PTs against phosphatidylinositol specific phospholipase C (PI-PLC) to determine the structural topology of PT isoform. While the reactivity with 3G8 on PTs was not influenced by treatment with PI-PLC, that on granulocytes was significantly diminished with PI-PLC.

CONCLUSIONS

This result confirmed that Fc gamma RIII on PTs is a membrane-spanning molecule, and that it is distinctive from PI anchoring Fc gamma RIII on granulocytes.

IgG Fc receptor heterogeneity in human peripheral nerves

The presence of Fc-receptors for IgG, FcRI (CD64), FcRII (CD32) and FcRIII (CD16) in human peripheral nerves was studied by indirect immunofluorescence staining of cryostat sections with monoclonal antibodies (mabs). The mabs 32.2 against FcRI, IV.3 and C1KM5 against FcRII, B1D6 against a 40 kD FcR with low affinity for IgG, and Leu-11b and 3G8 against FcRIII stained a few scattered cells in the endoneurium, apparently monocytes/macrophages. In addition, B1D6 and Leu11b gave a membranous staining of Schwann cells, endothelial cells and perineurial cells. Axons and fibroblasts were not stained by any of the mabs. The FcR may be important in binding immune complexes, in phagocytosis, in the release of lymphokines as well as cytotoxic and inflammatory mediators and in local immunoregulation.

Differential mRNA expression of three distinct classes of Fc gamma receptor at the feto-maternal interface

Heterogeneous expression of three classes of Fc gamma receptor (Fc gamma RI, IIa, IIb, and III) in the human placenta and decidua was examined by Northern blot hybridization and cDNA amplification analysis by polymerase chain reaction. Messenger RNA of Fc gamma RI, IIa and III genes were consistently expressed in the human placenta in all trimesters of gestation. The transcripts of the Fc gamma RIIb gene, on the other hand, dramatically increased in placentae at the second and third trimesters. This characteristic expression of Fc gamma RIIb after 20 gestational weeks was confirmed by sequential cDNA amplification analysis. Fc gamma RI, IIa and III mRNAs, but not Fc gamma RIIb, were also detected in the human decidua. Interestingly, while Fc gamma R mRNA could be induced in uterine endometrium by pseudopregnancy therapy using estrogen and progesterone, there was no detectable mRNA in hormone-unprimed normal endometrium. These findings suggest that Fc gamma Rs expressed at the feto-maternal interface can be transcriptionally regulated by sex steroid hormones as multifunctional molecules. In addition, the Fc gamma RIIb molecule is predominantly produced by placental tissues after the mid-trimester of gestation and possibly plays an important role in the transport of IgG molecules from mother to fetus.

Fc gamma receptor heterogeneity in the human placenta

Fc receptors for IgG, FcRI (CD64), FcRII (CD32), and FcRIII (CD16) in human placenta were studied by indirect immunohistochemistry using avidin-biotin peroxidase complexes for the staining of cryostat sections. The MoAb 32.2 against FcRI stained cells in the loose connective tissue of the placental villi. The MoAb IV3 (FcRII) and C1KM5 (FcRII) also stained stromal cells and in addition stained the endothelium of the placental villi. The MoAb anti-Leu-11b against FcRIII and B1D6 against a 40-kDa FcR from placenta stained both stromal cells and endothelium as well as the fetal trophoblasts lining the villi. The MoAb 3G8 (FcRIII) also stained trophoblasts and stromal cells but did not stain the endothelium. The heterogeneity of FcR expression on human placenta is established. The function of the different receptors is still unclear.

Properties of solubilized Fc gamma-receptors from psoriatic scales

Extracts from psoriatic scales, prepared using Tris-HCl buffer containing ethylenediaminetetraacetatic acid (EDTA) and 2-mercaptoethanol (ME), agglutinated erythrocytes (E) sensitized with IgG antibodies (A) (EA), but not E or E sensitized with F(ab')2-fragments of IgG. The agglutination was inhibited by IgG and Fc fragments of IgG, but not by IgA, IgM or F(ab')2-fragments of IgG. Partially reduced and alkylated IgG did not inhibit the agglutination, indicating that an inter-heavy-chain disulphide-linked Fc region is required for binding of FcR. The extracts inhibited EA, but not E or EAC rosette formation with mononuclear cells. The results strongly indicated that the extract contained functionally active FcR. The agglutinating activity of the extract was not affected by treatment with periodic acid or formaldehyde, whereas heat reduced the activity. Using a monoclonal antibody (B1D6) a functionally active 40 kDa FcR with low affinity for native IgG was purified from the scale extract. The extracts also contained FcR activity not recognized by B1D6.

The ontogenesis of Fc gamma receptors and complement receptors CR1 in human peripheral nerve

The ontogenesis of Fc gamma receptors (FcR) and C3b/C4b receptors (CR1) was studied in peripheral nerves from ten fetuses aged from 20 to 38 weeks using immunohistochemical and functional assays. Monoclonal antibodies (mAbs) against FcR and CR1 stained nerve fibers at 10 weeks of gestation and the staining intensity increased during nerve maturation. FcR and CR1 are probably expressed on Schwann cells and are early markers during the development of peripheral nerves. Functional FcR activity was detected in nerve sections before initiation of myelination, which occurs at approximately 18-19 weeks, whereas functional CR1 activity was found in the sections after myelination. Functional CR1 activity may, therefore, be related to myelin. The ontogenesis of FcR and CR1 was also studied on Schwann cells in culture from three fetuses aged 14, 16 and 19 weeks, using immunofluorescence technique with mAbs. The FcR and CR1 are lost on cultured Schwann cells. This suggests that the receptors are not intrinsic to the cells or that Schwann cells require axonal contact for the expression of FcR and CR1.