Ability of rosetting or non-rosetting individual control and inflammatory macrophages to kill Escherichia coli X43 intracellularly

An autoradiographic method combined with a rosette technique was used to assess the bactericidal activity of individual control and inflammatory peritoneal macrophages (PM phi) in the presence or absence of expression of Fc receptor for IgG (FcR). There was a lack of FcR reactivity in a certain percentage of both categories of PM phi exposed to E. coli X43, a bacterium which is readily phagocytosed in the presence of specific antibody. Both rosetting and non-rosetting PM phi were capable of phagocytosing E. coli X43, but inflammatory PM phi showed a marked reduction in their capacity to ingest these bacteria compared with control PM phi. Once ingested the E. coli X43 were killed equally well by non-rosetting and rosetting control and inflammatory PM phi.     

The ability of normal human monocytes to phagocytose IgG-coated red blood cells is related to the number of accessible galactosyl and mannosyl residues in the Fc domain of the anti-red blood cell IgG antibody molecules

The percentage of normal human monocytes (MCs) that are able to form rosettes with, and subsequently phagocytose, IgG-coated red blood cells (RBCs) has been determined in vitro using five batches of anti-RBC IgG antibodies. These antibodies differed from each other by their capacity to bind to lectins recognizing two of the oligosaccharide structures of the Fc domain, namely, peanut agglutinin (PNA) and concanavalin A (ConA) which specifically bind to beta-galactosyl and alpha-mannosyl residues, respectively. The threshold between high (H) and low (L) binding capacities (BC) was arbitrarily fixed at 15% of mean specific binding. For each level of RBC sensitization tested (1500-6000 Ab molecules/one RBC), the percentage of MCs binding at least three IgG-RBCs was similar whatever the IgG Ab preparations used. In contrast, the percentage of MCs capable of phagocytosing at least three IgG-RBCs coated with 3000, 4500 and 6000 IgG/cell, as well as the phagocytosis index (number of IgG-RBCs ingested/100 MCs) of IgG-RBCs coated with 1500, 3000, 4500 and 6000 IgG/cell, were significantly lower (P less than 0.01 at least) using IgG Ab molecules with either [(PNA-H)(ConA-H)] BC, [(PNA-L) (ConA-H)] BC or with [(PNA-L)(ConA-L)] BC than the corresponding values measured using RBCs coated with IgG Ab molecules exhibiting [(PNA-H)(ConA-L)] BC. The binding to MCs of 125I-labelled anti-RBC IgG Ab molecules exhibiting different binding profiles to PNA and to ConA was studied by Scatchard plot analysis. A single class of binding sites was observed in each case. MCs bound a mean of 23,000 IgG molecules with a mean association constant (Ka) for IgG binding of about 1.4 X 10(8) M-1. These data indicate that terminal (and/or accessible) galactosyl and mannosyl residues of IgG Ab molecules play a role in the ingestion of IgG-RBCs by human MCs, despite the fact IgG Ab binding to IgG(Fc) receptors is not significantly affected. Thus, when studying the phagocytosis of IgG-coated RBC by human MC monolayers, the assay should be performed not only using similar RBC/MC ratios and IgG coating values, but also with IgG antibodies having comparable mean PNA and ConA binding capacities.     

Evidence for a role of accessible galactosyl or mannosyl residues of Fc domain in the in vivo clearance of IgG antibody-coated autologous erythrocytes in the rat

The potential role of accessible galactosyl or mannosyl residues of IgG in the clearance of IgG-coated autologous red blood cells (IgG-RBCs) by the spleen and the liver was investigated in the rat using rabbit anti-rat RBC IgG antibody molecules differing from each other by their capacity to bind in vitro to peanut agglutinin (PNA) and concanavalin A (Con A), i.e., two lectins that specifically bind to beta-galactosyl and alpha-mannosyl residues of Fc domain, respectively. Those IgG molecules [IgG(PNA) or IgG(Con A)] were separated from the starting anti-RBC IgG antibody batch [IgG(total)] by affinity chromatography on lectin columns. Each IgG-RBC preparation was labeled with 99mTc, and was reinjected iv with autologous rat RBCs labeled with 111In to correct for 99mTc present in the blood contained in each organ. The mean specific spleen uptakes per gram of the three IgG-RBC preparations increased according to the level of RBC sensitization but were at least 10 times higher in each instance than the mean specific liver uptake per gram. Consistent with the clearance curves of IgG(PNA)-RBCs, the mean specific splenic uptake per gram of those RBCs was significantly increased as compared to the same parameters determined using either IgG(total)-RBCs or IgG(Con A)-RBCs. In contrast, the mean specific liver uptakes per gram of IgG(PNA)-RBCs, of IgG(Con A)RBCs, or of IgG(total)-RBCs were not significantly different under otherwise identical experimental conditions. The increase in the splenic removal of IgG(PNA)-RBCs was C3 independent. Furthermore, splenic macrophages isolated from rats were able to bind in vitro significantly more IgG(PNA)-RBCs than IgG(total)-RBCs or IgG(Con A)-RBCs. These data therefore support the concept that, in the rat, the kinetic of removal of IgG-RBCs from the bloodstream by the Fc receptors of splenic mononuclear phagocytes may vary according to the nature of accessible carbohydrates located in the Fc domain of IgG antibody molecules coating the erythrocytes.     

Low‐affinity FcγR interactions can decide the fate of novel human IgG‐sensitised red blood cells and platelets

G1Δnab is a mutant human IgG1 constant region with a lower ability to interact with FcγR than the natural IgG constant regions. Radiolabelled RBCs and platelets sensitised with specific G1Δnab Abs were cleared more slowly from human circulation than IgG1‐sensitised counterparts. However, non‐destructive splenic retention of G1Δnab‐coated RBCs required investigation and plasma radioactivities now suggest this also occurred for platelets sensitised with an IgG1/G1Δnab mixture. In vitro assays with human cells showed that G1Δnab‐sensitised RBCs did not cause FcγRI‐mediated monocyte activation, FcγRIIIa‐mediated antibody‐dependent cell‐mediated cytotoxicity (ADCC) or macrophage phagocytosis although they did adhere to macrophages. Thus, FcγRII was implicated in the adhesion despite the Δnab mutation reducing the already low‐affinity binding to this receptor class. Additional contacts via P‐selectin enhance the interaction of sensitised platelets with monocytes and this system provided evidence of FcγRII‐dependent activation by G1Δnab. These results emphasise the physiological relevance of low‐affinity interactions: It appears that FcγRII interactions of G1Δnab allowed splenic retention of G1Δnab‐coated RBCs with inhibitory FcγRIIb binding preventing RBC destruction and that FcγRIIb engagement by G1Δnab on IgG1/G1Δnab‐sensitised platelets overcame activation by IgG1. Considering therapeutic blocking Abs, G1Δnab offers lower FcγR binding and a greater bias towards inhibition than IgG2 and IgG4 constant regions.     

In vitro studies on the Fc-receptor function of mononuclear phagocytes in rheumatoid arthritis: Relation between the Fc-receptor blockade and the concanavalin A-binding capacity of autologous immunoglobulin G

The Fc-receptor (Fc-R) function of monocytes isolated from 19 control subjects and from 30 patients presenting with a rheumatoid arthritis (RA) was assessedin vitro by a classical rosette assay using IgG-coated sheep red blood cells. In RA patients, the percentage of monocytes forming rosettes was significantly lower than in controls (34.4±20.4 versus 67.4±4.5%;P<0.001). The blockade observed was reversed by a prior trypsin treatment of RA monocytes, the percentage of recovery being correlated with the IgG plasma levels. Besides, IgG purified from the serum of four RA patients bound a mean of 7.3, 5.2, 1.6, and 1.6 times more than normal IgG did onto concanavalin A (Con A), peanut agglutinin (PNA), phytohemagglutinin (PHA), and pokeweed mitogen (PWM), respectively. Although similar amounts of¹²⁵I-labeled normal and RA IgG were bound to normal monocytes, RA IgG inhibited more efficiently than normal IgG the Fc-R function of normal monocytes, for all concentrations tested (10 to 100 µg/100 µl). A prior treatment of RA IgG by -mannosidase, but not by -galactosidase, significantly reduced their inhibitory properties. The incubation of monocytes withD-mannose or mannan reduced their capacity to form rosettes. The percentage of monocytes forming rosettes in the presence of both mannan and normal IgG was significantly lower than that measured in the presence of normal IgG only. On the contrary, the rosetting capacity of monocytes in the presence of both RA IgG and mannan was the same as that calculated in the presence of RA IgG only. The inhibitory effect of RA IgG was not related to their abnormal circular dichroism. Our data suggest that the greater ability of RA IgG to block the Fc-R function of monocytes probably depends on the presence of a greater number of accessible mannosyl residues on the glycosidic side chains located in the Fc domain of the molecules.     

Isolation of a rabbit IgG fraction with cytophilic properties

About 15% of rabbit IgG loaded on a column of Con A-Sepharose 4B was found to be specifically bound to the column due to a structural variation in its carbohydrate moiety. The Con A-retained rabbit IgG contained a higher amount of neutral hexoses than the initial IgG but its molecular weight, antigenic structure and half-life were identical or similar. The Con A-retained rabbit IgG has an affinity for the Fc receptor-bearing homologous macrophages which is 10 times higher than that of the initial IgG. The IgG fraction not retained on Con A-Sepharose is practically devoid of binding ability. These results suggest that the Con A-bound IgG may represent the cytophilic fraction of monomeric IgG responsible for the binding of IgG to Fc receptor-bearing cells.     

Receptor specific probes for the study of Fc gamma receptor specific function.

Human leukocytes express three distinct families of receptors for the Fc region of IgG (Fc gamma R). We have prepared erythrocytes (E) coated with monoclonal anti-Fc gamma R antibodies for the study of receptor specific phagocytosis using biotin and streptavidin. In this technique, both the E and the monoclonal antibody (mAb) are biotinylated and coupling of the mAb to the E occurs through the use of streptavidin. The same biotin/streptavidin principle was used to prepare E coated with human IgG. Using this technique, receptor specific probes or probes coated with natural ligand (IgG) can be prepared rapidly with the use of small amounts of mAb or IgG. Finally, we have used these receptor specific probes to demonstrate that all three families of Fc gamma R (Fc gamma RI, Fc gamma RII and Fc gamma RIII) expressed on human monocytes and human macrophages are phagocytic receptors.     

Interaction of immune complexes isolated from hepatitis C virus-infected individuals with human cell lines

We investigated the interaction of immune complexes (IC) isolated from hepatitis C virus (HCV)-infected individuals with several cell lines that differentially express Fc receptors, and analyzed viral infection by the presence of HCV RNA sequences. Monocytic (U937 and Monomac-6) and lymphocytic (MOLT-4 and Jurkat) cell lines were incubated with interferon- plus phorbol myristate acetate to stimulate the expression of Fc receptors before addition of IC. Cell interaction with IC was monitored by flow cytometry. Positive cell fluorescence was detected in U937 and Monomac-6 cells [mean fluorescence intensity (MFI) 10.56±0.8 and 11.60±0.8, respectively]. Incubation of cells with monoclonal antibodies against Fc receptors for IgG before addition of IC decreased MFI in both cell lines (U937 2.1±0.5, Monomac-6 4.4±0.8, P<0.001), indicating that cell-IC interaction through these receptors was inhibited. In particular, the blockage of FcRII was responsible for this effect. No binding of IC with either MOLT-4 or Jurkat cell lines was detected, which correlated with a very low Fc receptor expression. HCV RNA sequences were identified in the cells up to 120 h of post incubation with IC. These results suggest that IC can mediate entry of HCV to both U-937 and Monomac-6 cell lines mainly through the FcRII.     

Human IgG isotypes and activating Fcγ receptors in the interaction of Salmonella enterica serovar Typhimurium with phagocytic cells

Several classes and multiple subclasses of immunoglobulins are produced towards protein and polysaccharide antigens in response to Salmonella infection and play a key role in protection against systemic disease. The targeting of Salmonella to Fc receptors (FcR) on phagocytes is a key step in the antibody-mediated antibacterial functions of host cells. We wished to compare the relative efficiency of different human IgG subclasses, which targeted the Salmonella enterica OmpA surface protein in modulating the interaction of bacteria with human phagocytes. To this end, we developed a novel system by tagging OmpA with a foreign CD52 mimotope (TSSPSAD) and opsonizing the bacteria with a panel of humanized CD52 antibodies that share the same antigen-binding V-region, but have constant regions of different subclasses. Our data revealed that opsonization with all the IgG subclasses increases Salmonella uptake by human phagocytes. IgG3 resulted in the highest level of bacterial uptake and the highest average bacterial load per infected cell, which was closely followed by IgG1, then IgG4 and lastly IgG2. Phagocytosis mediated by IgG1, IgG3 and IgG4 had a higher dependency on FcγRI than FcγRIIA, whereas IgG2-mediated phagocytosis required FcγRIIA more than FcγRI. The results show that IgG binding to OmpA increases the uptake of Salmonella by human phagocytic cells and that the efficiency of this process depends both on the subclass of the IgG and the type of FcR that is available for antibody binding.     

Fc-receptor-bearing macrophages isolated from hypersensitivity and foreign-body granulomas. Delineation of macrophage dynamics, fc receptor density/avidity and specificity.

Foreign-body and delayed hypersensitivity granulomas were induced in mice; and the dynamics of macrophages isolated from dispersed, 1--4-week-old lesions was delineated. The size and histologic complexity of the lesions increased as shown: adjuvant greater than schistosome egg greater than methylated bovine serum albumin greater than bead. Esterase staining, spreading on glass, and the percentage of Fc-receptor--bearing macrophages present in the various granulomas reflected the same gradient. The Fc receptors were examined by rosetting with rabbit-antibody--SRBC complex (EA). Whereas more than 90% of the population of macrophages of the dermal adjuvant granuloma contained undiminished numbers of receptor-bearing macrophages throughout the 4 weeks, the percentage of macrophages that displayed receptors in pulmonary foreign-body (40%) and delayed hypersensitivity granulomas (70%) peaked at 1 week and subsequently declined. The EA rosetting of the foreign-body and delayed hypersensitivity granuloma macrophages was strongly inhibited by monomeric IgG2a-specific and weakly by aggregated IgG2b-specific mouse myeloma proteins. Also, macrophages of the delayed hypersensitivity granulomas rosetted in higher percentages with SRBCs coupled with monomeric IgC2a than with those coupled with aggregated IgG2b myeloma proteins. Macrophages of the foreign-body lesion did not react with aggregated IgG2b--SRBC. Rosetting with monomeric IgG2a--SRBC or aggregated IgG2b--SRBC could not be cross-inhibited by the myeloma proteins. Both the monomeric IgG2a--SRBC and aggregated IgG2b--SRBC complexes were readily phagocytized. Trypsin treatment of the macrophages inhibited rosetting with EA or myeloma-protein--coupled SRBCs. The display of Fc receptors on the granuloma macrophages seems to be related to the etiology of the lesion and the intensity and duration of the inflammatory reaction.     

Interaction of macrophages with "old" red blood cells from syngeneic mice in vitro and the independence of the recognition process on macrophage Fc receptors

The interaction between peritoneal macrophages and "old" red blood cells (RBC) from syngeneic mice was studied in vitro. It seems that this interaction is not mediated directly via Fc receptors on the macrophage. (1) 2-deoxy-D-glucose did not inhibit the phagocytosis of "old" RBC, but did inhibit the phagocytosis of IgG-coated sheep RBC (SRBC). (2) Immobilization of Fc receptors by plating macrophages on coverslips coated with bovine serum albumin: anti-bovine serum albumin (BSA: anti-BSA) complexes had no effect on the phagocytosis of "old" RBC, but inhibited the phagocytosis of IgG-coated SRBC. The interaction between mouse macrophages and "old" RBC is temperature dependent. At 4 degrees C and 22 degrees C "old" RBC attach to macrophages but are not phagocytized; at 37 degrees C phagocytosis occurs. Fetal calf serum (FCS) is required for optimal phagocytosis. Sialic acid residues on the macrophage surface do not play a role in this recognition and phagocytosis process, as neuraminidase treatment of macrophages did not affect the interaction.     

Receptors for IgG (Fc receptors) on human lymphocytes: re-evaluation by multiple techniques

This study examines whether receptors for IgG (Fc receptors), as identified by different methods, are found on identical human lymphocyte subpopulations, and the relationship of Fc receptors to surface immunoglobulin (SIg) and receptor for complement (C'). Fc receptors were identified by two rosetting techniques (antibody-sensitized human erythrocytes, HuEA, or sheep erythrocytes, ShEA) and two immunofluorescent techniques (heat-aggregated IgG of human origin, HuAgg, or of guinea-pig origin, GPAgg).

When lymphocytes depleted of cells rosetting with ShEA were compared to HuEA-depleted lymphocytes, the two subpopulations appeared to be significantly different. However, when lymphocytes were depleted of cells rosetting with ShEA which had been sensitized with lower concentrations of antibody, the subpopulation so depleted appeared to be virtually identical to HuEA-depleted lymphocytes. These studies suggest that more than one lymphocyte subpopulation has Fc receptors and that each subpopulation can, in part, be identified and distinguished from the other by the capacity to bind IgG at differing concentrations.

In particular, these experiments may serve to resolve the controversy concerning the presence of Fc receptors on lymphocytes bearing surface immunoglobulin (SIg). Depletion of lymphocytes rosetting with ShEA removed most of the SIg-bearing lymphocytes; depletion of cells rosetting with ShEA which had been sensitized with lower concentrations of IgG antibody, however, failed to deplete SIg-bearing lymphocytes even though other Fc-bearing populations were completely depleted. These results suggest that SIg-bearing lymphocytes (B lymphocytes) do have Fc receptors but that high concentrations of IgG are needed to demonstrate them.

     

Increased concanavalin A-binding capacity of immunoglobulin G purified from sera of patients with rheumatoid arthritis.

A solid phase radioimmunoassay was set up for direct measurement of the binding capacity of human IgG to three lectins recognizing different carbohydrates of the Fc domain, i.e. peanut agglutinin (PNA), Concanavalin A (Con A) and pokeweed mitogen (PWM) which mainly bind to beta-galactose, alpha-mannose and dimers of N-acetyl-beta-glucosamine respectively. The mean specific binding of the 96 normal IgG tested to PNA and to PWM was statistically higher (P less than 0.001) than that to Con A, whereas no significant differences were observed between the mean specific bindings to PNA and to PWM. A statistically significant linear negative correlation could be established only between the relative bindings (expressed in percentage of the total binding to the three lectins) to PNA and to PWM (r = -0.65, P less than 0.001). The mean specific binding of IgG purified from 34 patients suffering from rheumatoid arthritis (RA) to PNA and to Con A was statistically higher (P less than 0.001) than that reached with PWM, whereas no significant differences were noted between their mean binding capacities to PNA and to Con A. When compared to normal IgG, only four out of 34 RA IgG exhibited a significantly higher binding capacity to PNA, whereas all but one RA IgG possessed a significantly higher binding capacity to Con A. Accordingly, the mean specific binding of RA IgG to Con A was significantly higher than that of normal IgG (P less than 0.001). Besides (and contrary to normal IgG), a statistically significant negative linear correlation was noted between the relative bindings of RA IgG to PNA and to Con A (r = -0.89, P less than 0.001). All the five RA IgG tested exhibited an abnormal circular dichroism. Our data suggest that, by altered steric conformation and glycosylation, mannosyl-residues of RA IgG become prominent or terminal or both, and are therefore able to react more effectively with Con A than normal IgG do.     

IgG Fc receptor polymorphisms and association with autoimmune disease

The aim of this study was to investigate whether a genetic polymorphism of Fc gammaRIII exists in mice, which could explain the different susceptibility to pathogenic IgG anti-collagen type II (CII) antibodies in mice carrying the collagen-induced arthritis (CIA)-susceptible H-2q haplotype. The gene for Fc gammaRIII was sequenced in 11 common mouse strains, and the results revealed three different haplotypes of mouse Fc gammaRIII: Fc gammaRIII:V, Fc gammaRIII:H and Fc gammaRIII:T. To study the consequences of this polymorphism, we generated mice carrying the Fc gammaRIII:H haplotype from the CIA-susceptible, H-2q-positive DBA/1 mouse or the Fc gammaRIII:V haplotype from the CIA-resistant, H-2q-positive SWR mouse. After CII immunization or transfer of IgG anti-CII antibodies, Fc gammaRIII:H-expressing mice, but not Fc gammaRIII:V-expressing mice, developed progressively severe arthritis. We also investigated if C5, in addition to Fc gammaRIII polymorphism, could affect the susceptibility to the pathogenic IgG anti-CII antibodies in H-2q-positive mice. Here we show that SWR mice, naturally deficient in C5, can develop CIA when supplemented with C5 and that anti-C5 antibody treatment of Fc gammaRIII:H-expressing mice inhibits arthritis development. These data demonstrate for the first time a genetic polymorphism of Fc gammaRIII in mice that may, together with C5, regulate induction of autoimmune disease.     

Both FcγRIV and FcγRIII are essential receptors mediating type II and type III autoimmune responses via FcRγ‐LAT‐dependent generation of C5a

FcγRIV is a relatively new IgG Fc receptor (FcγR) that is reported to contribute to the pathogenesis of autoimmune diseases, although its specific role in relation to FcγRIII, complement and IgG2 subclasses remains uncertain. Here we define FcγRIV on macrophages as a receptor for soluble IgG2a/b complexes but not for cellular bound IgG2a and show that simultaneous activation of FcγRIV and FcγRIII is critical to mediate certain type II/III autoimmune responses. FcγRIII‐deficient mice display compensatory enhanced FcγRIV expression, are protected from lung inflammation after deposition of IgG complexes, and show reduced sensitivity to IgG2a/b‐mediated hemolytic anemia, indicating that increased FcγRIV alone is not sufficient to trigger these diseases in the absence of FcγRIII. Importantly, however, blockade of FcγRIV is also effective in inhibiting phagocytosis and cytokine production in IgG2b‐induced anemia and acute lung injury, processes that display a further dependence on C5a anaphylatoxin receptor. Using gene deletion and functional inhibition studies, we found that FcγRIII and FcγRIV are each essential to trigger an FcRγ‐linker for activation of T‐cell‐dependent signal that drives C5a production in the Arthus reaction. Together, the results demonstrate a combined requirement for FcγRIII and FcγRIV in autoimmune injury, and identify the linker for activation of T cells adaptor as an integral component of linked FcγR and C5a anaphylatoxin receptor activation to generate inflammation.     

IgG Binding Sites on Human FCγ Receptors

The structure for the three human Fcγ receptors classes FCγRI (CD64), FCγRII (CD32) and FCγRIII (CD16) has been well characterized. Here the IgG binding sites on FCγRII and FCγRIII with their responsive FG, BC and C'/E loops on the membrane proximal domains are described in detail. For FCγRI the second extracellular domain is suggested as a key structure of IgG binding. The lower hinge regions of human and murine IgG binding to these Fc receptors and their structural relationship in FcγR-IgG interactions are discussed. The potential of inhibiting the pathophysiological effects of Fcγ receptors by blocking studies are considered for future therapeutic modalities.     

The spontaneous ability of normal human IgG to inhibit the Fc receptors of normal human monocytes is related to their binding capacity to lectins

The lectin-binding capacity of 96 normal human IgG, assessed by solid-phase radioimmunoassay, strikingly varied according to the lectin considered. Indeed, half of the IgG samples exhibited peanut agglutinin (PNA)- and pokeweed mitogen-specific binding capacities superior or equal to 4%, whereas less than 15% of IgG similarly bound to concanavalin A (Con A) and to phytohemagglutinin. The ability of those IgG to inhibit the Fc receptor (Fc-R) function of human monocytes, measured by a classical rosette assay, was inversely correlated to their binding ratios to PNA and Con A only. By affinity chromatography, three groups of IgG were separated: the IgG purified on agarose-PNA columns slightly reduced the Fc-R function (40-45% inhibition); the IgG purified on Sepharose-Con A columns exhibited the highest inhibitory properties (80-85% inhibition); the IgG that did not bind to PNA and Con A columns possessed intermediate inhibitory properties (65-70% inhibition). The different effect of IgG on Fc receptors was conserved when monocytes were first treated by trypsin and was unrelated to their specific binding to human monocytes, to their subclasses, and to their C1q- or protein A-binding capacities. Incubation of monocytes with D-galactose (10 mM) significantly improved their capacity to form IgG rosettes, whereas their incubation with D-mannose (10 mM) significantly reduced the Fc-R function. Scatchard plots of 125I-IgG1 myeloma protein binding to monocytes were linear under basal conditions, as well as after a prior incubation of the cells with D-galactose or D-mannose. Monocytes bound about 16,000 molecules of IgG1 per cell in each instance. In contrast, the mean association constant (Ka) for IgG1 binding was 2.59 +/- 0.50 X 10(8) M-1 under basal conditions, 4.4 +/- 0.75 X 10(8) M-1 after D-galactose incubation, and 1.35 +/- 0.50 X 10(8) M-1 after D-mannose incubation. These data suggest that the level of human monocyte Fc-R function blockade induced by human IgG depends mainly on the presence of "accessible" galactosyl or mannosyl residues in the Fc domain and that the modulation of the Fc-R function induced by these carbohydrates is due to a change in the affinity rather than in the number of single class of high-affinity binding sites.     

Identification of an Oolemmal IgG Fc Receptor: Its Role in Promoting Binding of Antibody-Labelled Human Sperm to Zona-Free Hamster Eggs

ABSTRACT: Antisperm antibodies (ASAs) present in sera of infertile men and women have been shown either to promote or inhibit penetration of zona-free hamster eggs by antibody-labelled human spermatozoa. Increased numbers of oolemmal-bound sperm have been noted in association with increased sperm penetration frequencies, following antibody labelling, when compared with antibody-free sperm. The promotion of adherence of ASA-labelled sperm to the oolemma could be mediated through the binding of antibodies to common epitopes present on the sperm and egg surfaces or through Fc-mediated binding to an oolemmal Fc receptor. In support of the latter hypothesis, we report that zona-free hamster eggs bind aggregated human IgG and IgG Fc fragments. The presence of an oolemmal IgG Fc receptor has been confirmed using a rat monoclonal antibody (2.4G2) directed against a murine IgG Fc receptor (Fc γ R_II) as judged both by indirect immunofluorescence and by immunobead binding. In addition, the pre-incubation of zona-free hamster eggs with IgG Fc diminished both adhesion to and penetration of the oolemma by human spermatozoa.     

Fcγ receptor expression on splenic macrophages in adult immune thrombocytopenia

Splenic macrophages play a key role in immune thrombocytopenia (ITP) pathogenesis by clearing opsonized platelets. Fcγ receptors (FcγR) participate in this phenomenon, but their expression on splenic macrophages and their modulation by treatment have scarcely been studied in human ITP. We aimed to compare the phenotype and function of splenic macrophages between six controls and 24 ITP patients and between ITP patients according to the treatments they received prior to splenectomy. CD86, human leucocyte antigen D‐related (HLA‐DR) and FcγR expression were measured by flow cytometry on splenic macrophages. The major FcγR polymorphisms were determined and splenic macrophage function was assessed by a phagocytosis assay. The expression of the activation markers CD86 and HLA‐DR was higher on splenic macrophages during ITP compared to controls. While the expression of FcγR was not different between ITP and controls, the phagocytic function of splenic macrophages was reduced in ITP patients treated with intravenous immunoglobulin (IVIg) within the 2 weeks prior to splenectomy. The FCGR3A (158V/F) polymorphism, known to increase the affinity of FcγRIII to IgG, was over‐represented in ITP patients. Thus, these are the first results arguing for the fact that the therapeutic use of IVIg during human chronic ITP does not modulate FcγR expression on splenic macrophages but decreases their phagocytic capabilities.     

Possible role of microbial IgG Fc-binding proteins in rheumatoid arthritis

IgG Fc binding substances (receptors) are widespread among pathogenic microorganisms. The receptors fromStaphylococcus aureus, streptococci of group A, C and G as well asHerpes-infected cells bind to the interface between the CH2 and CH3 domains i.e. to His 435, Tyr 436 and possibly also His 433 and/or 310. Most rheumatoid factors (RF) from patients with rheumatoid arthritis show a similar binding pattern. Hence, it has been shown that antibodies to microbial IgG Fc receptors (S. aureus and group A streptococci type M15) and RF are idiotypic — anti-idiotypic antibody partners i.e. that RF are the internal images of microbial IgG Fc binding proteins. Group A streptococci possessing IgG Fc receptors elicit higher titres of RF when injected in rabbits as compared to group A streptococci without IgG Fc receptors. The streptococcal IgG Fc receptors exhibit a diversity of preferences for subclasses of human IgG, some of them showing allotype preferences. Such allotypes are also recognized by certain RF. IgG RF are able to self-associate thereby forming immune complexes which can activate the complement cascade as well as stimulate release of prostaglandins and (probably) interleukin-1. Since these factors have been assigned an important pathogenic role in rheumatoid arthritis, self-aggregating IgG RF, proposed to be induced by microbial IgG Fc receptors might be an important pathogenic factor in rheumatoid arthritis because rheumatoid arthritis is the only known condition where synthesis of RF takes place in the synovia.