The characterization and functional significance of plasma membrane Fc Receptors

The Fab portion of immunoglobulin G contributes to its binding to Fcγ receptor III

Most cells active in the immune system express receptors for antibodies which mediate a variety of defensive mechanisms. These receptors interact with the Fc portion of the antibody and are therefore collectively called Fc receptors. Here, using high-speed atomic force microscopy, we observe interactions of human, humanized, and mouse/human-chimeric immunoglobulin G1 (IgG1) antibodies and their cognate Fc receptor, FcγRIIIa. Our results demonstrate that not only Fc but also Fab positively contributes to the interaction with the receptor. Furthermore, hydrogen/deuterium exchange mass spectrometric analysis reveals that the Fab portion of IgG1 is directly involved in its interaction with FcγRIIIa, in addition to the canonical Fc-mediated interaction. By targeting the previously unidentified receptor-interaction sites in IgG-Fab, our findings could inspire therapeutic antibody engineering.

Nonclassical FCGR2C haplotype is associated with protection from red blood cell alloimmunization in sickle cell disease

Red blood cell (RBC) transfusions are of vital importance in patients with sickle cell disease (SCD). However, a major complication of transfusion therapy is alloimmunization. The low-affinity Fcγ receptors, expressed on immune cells, are important regulators of antibody responses. Genetic variation in FCGR genes has been associated with various auto- and alloimmune diseases. The aim of this study was to evaluate the association between genetic variation of FCGR and RBC alloimmunization in SCD. In this case-control study, DNA samples from 2 cohorts of transfused SCD patients were combined (France and The Netherlands). Cases had a positive history of alloimmunization, having received ≥1 RBC unit. Controls had a negative history of alloimmunization, having received ≥20 RBC units. Single nucleotide polymorphisms and copy number variation of the FCGR2/3 gene cluster were studied in a FCGR-specific multiplex ligation-dependent probe amplification assay. Frequencies were compared using logistic regression. Two hundred seventy-two patients were included (130 controls, 142 cases). The nonclassical open reading frame in the FCGR2C gene (FCGR2C.nc-ORF) was strongly associated with a decreased alloimmunization risk (odds ratio [OR] 0.26, 95% confidence [CI] 0.11-0.64). This association persisted when only including controls with exposure to ≥100 units (OR 0.30, CI 0.11-0.85) and appeared even stronger when excluding cases with Rh or K antibodies only (OR 0.19, CI 0.06-0.59). In conclusion, SCD patients with the FCGR2Cnc-ORF polymorphism have over a 3-fold lower risk for RBC alloimmunization in comparison with patients without this mutation. This protective effect was strongest for exposure to antigens other than the immunogenic Rh or K antigens.

Expression Profile of Human Fc Receptors in Mucosal Tissue: Implications for Antibody-Dependent Cellular Effector Functions Targeting HIV-1 Transmission

The majority of new Human Immunodeficiency Virus (HIV)-1 infections are acquired via sexual transmission at mucosal surfaces. Partial efficacy (31.2%) of the Thai RV144 HIV-1 vaccine trial has been correlated with Antibody-dependent Cellular Cytotoxicity (ADCC) mediated by non-neutralizing antibodies targeting the V1V2 region of the HIV-1 envelope. This has led to speculation that ADCC and other antibody-dependent cellular effector functions might provide an important defense against mucosal acquisition of HIV-1 infection. However, the ability of antibody-dependent cellular effector mechanisms to impact on early mucosal transmission events will depend on a variety of parameters including effector cell type, frequency, the class of Fc-Receptor (FcR) expressed, the number of FcR per cell and the glycoslyation pattern of the induced antibodies. In this study, we characterize and compare the frequency and phenotype of IgG (CD16 [FcγRIII], CD32 [FcγRII] and CD64 [FcγRI]) and IgA (CD89 [FcαR]) receptor expression on effector cells within male and female genital mucosal tissue, colorectal tissue and red blood cell-lysed whole blood. The frequency of FcR expression on CD14+ monocytic cells, myeloid dendritic cells and natural killer cells were similar across the three mucosal tissue compartments, but significantly lower when compared to the FcR expression profile of effector cells isolated from whole blood, with many cells negative for all FcRs. Of the three tissues tested, penile tissue had the highest percentage of FcR positive effector cells. Immunofluorescent staining was used to determine the location of CD14+, CD11c+ and CD56+ cells within the three mucosal tissues. We show that the majority of effector cells across the different mucosal locations reside within the subepithelial lamina propria. The potential implication of the observed FcR expression patterns on the effectiveness of FcR-dependent cellular effector functions to impact on the initial events in mucosal transmission and dissemination warrants further mechanistic studies.

Detection of function-associated molecules on rat NK cells and their role in target cell lysis

Anti-effector cell mAb 5C6.10.4 (5C6) inhibits cytotoxic activity of fish nonspecific cytotoxic cells (NCC). We now show that 5C6 also inhibits mammalian NK cell activity using fresh and cultured (CRC) leukemic rat NK cells. The inhibitory activity of 5C6 was caused by blocking of conjugate formation between NK cells and YAC-1 targets. Binding studies done by flow cytometry (FCM) showed that mAb 5C6 specifically bound to 8% of unfractionated rat spleen cells. Enrichment by nylon-wool fractionation produced 27.2% specific binding, along with a 3.4-fold enrichment in cytotoxic activity. Tissue distribution studies revealed that the highest number of cells recognized by mAb 5C6 were found in NWNA spleen cells (28.7%), followed by liver (18.9%) and peripheral blood (13.9). Two-color FCM showed that although all 3.2.3 mAb-positive cells were also stained with mAb 5C6, a small percentage of 3.2.3. negative noncytotoxic NWNA spleen T cells were 5C6 positive. Redirected lysis experiments demonstrated that anti-effector mAb-producing myeloma cells could be killed by CRC and NWNA spleen cells. In addition, mAb 5C6 produced specific inhibition of redirected lysis of each myeloma target. Experiments were also conducted to determine the signaling capability of the FAM complex. Binding of the anti-FAM mAbs to NWNA rat spleen cells caused a rapid increase in cytosolic free calcium of approximately 472 nM. Western blot analysis of CRC cell lysates showed that the molecules recognized by anti-FAM mAbs have molecular weights of 38 and 42 kDa. These studies indicate that the anti-effector mAbs recognize a functionally relevant molecule on rat NK cells that is involved in the first steps of cytolysis, i.e., antigen recognition, and which also triggers the activation of signal-transducing events in these cells.

Fc gamma receptors on keratinocytes in psoriasis

Epidermis in cryostat sections of skin biopsy specimens from patients with psoriasis and from healthy individuals bound bovine erythrocytes (E) sensitized with rabbit IgG antibodies (A)(EA). No binding occurred using E or E sensitized with IgM or F(ab')2 fragments of IgG. The binding of EA was inhibited by human IgG and by Fc fragments of IgG, whereas human IgA, IgM, albumin, and F(ab)2 fragments of IgG did not inhibit the binding, indicating the presence of receptors for the Fc part of IgG (FcR). EA bound mainly to stratum spinosum and most strongly above FcR-positive cell infiltrates in dermal papillae. The binding of EA to sections from patients with active psoriasis was stronger than to sections from patients with stationary psoriasis vulgaris. Sections of unaffected skin from patients with psoriasis and healthy individuals also bound EA, but the binding was weaker than to sections of psoriatic lesions. The receptors were sensitive to periodic acid, formaldehyde, and heat. Using immune complexes of horseradish peroxidase (HRP) and rabbit IgG antibodies to HRP, the receptors were localized to the outer aspect of the keratinocytes and to the inflammatory cells in the microabscesses. The strongest binding occurred in the same areas which adhered EA most strongly. FcR on dendritic epidermal cells could not be demonstrated in situ. A monoclonal antibody against FcR also stained the outer aspect of most keratinocytes throughout the epidermis. FcR on keratinocytes support the assumption that these cells contribute to immune reactions in the skin.

In situ demonstration of Fc gamma-receptors in human chronic marginal and apical periodontitis

The distribution of receptors for the Fc portion of IgG (Fc gamma R) in chronic marginal and apical periodontitis were studied by using a monoclonal antibody against placental Fc gamma R and soluble immune complexes as indicators. Cryostat sections were used in indirect immunofluorescence technique. Fc gamma R were detected on varying numbers of cells in the inflammatory cell infiltrates, on endothelial cells of certain vessels and in fibrous tissue. In chronic marginal periodontitis Fc gamma R were also observed on cells within the oral gingival epithelium (OGE) and the pocket epithelium (PE). There was a distinct fluorescence in stratum spinosum and occasionally in stratum basale of OGE and in the coronal portion of the PE. Fc gamma R on Langerhans cells could not be demonstrated. In apical periodontitis Fc gamma R were also detected on cells within the epithelium. In some cases epithelium in periapical cysts was positive. Soluble immune complexes bound to morphologically similar, but fewer cells compared to the monoclonal antibody against Fc gamma R. The results indicate that Fc gamma R are generally expressed on cells in inflamed tissue. Thus, this presence of Fc gamma R on certain specialized cells such as endothelial cells and keratinocytes, may endow these cells with functions previously thought to be restricted to cells of the lymphoreticular system.

Demonstration of Fc gamma receptors on human peripheral nerve fibres

Cryostat sections of human peripheral nerves adsorbed sheep erythrocytes (E) sensitized with rabbit IgG antibodies (A) (EA). No adsorption occurred using unsensitized E or E sensitized with IgM or F(ab')2 fragments of IgG. The binding of EA was inhibited by IgG and by Fc fragments of IgG, but not by IgA, IgM, F(ab')2 fragments of IgG or albumin, and thus indicated the presence of receptors for the Fc part of IgG (FcR). IgG1 and IgG3 subclasses inhibited the haemadsorption. IgG4 was less inhibitory than IgG1 and IgG3, whereas IgG2 did not inhibit the binding of EA. The receptors were sensitive to periodic acid, formaldehyde and heat. FcR were present in sections of both unmyelinated and myelinated nerves, indicating that the binding was not to myelin. FcR were also demonstrated in sections of fetal peripheral nerves at a gestational age of approximately 21 weeks. Using immune complexes of horseradish peroxidase (HRP) and rabbit IgG antibodies to HRP and a monoclonal antibody against FcR, the receptors were shown on the nerve fibres, probably on the Schwann cell membrane (neurilemma).

Fc gamma receptors and HLA-DR antigens on thymus cells in myasthenia gravis

Receptors for the Fc part of IgG (Fc gamma R) and HLA-DR antigens were detected in thymus tissue from patients with myasthenia gravis (MG) using monoclonal antibodies (B1D6 and OKIa1) in indirect immunofluorescence. The amount of Fc gamma R and HLA-DR antigens was increased on epithelial reticular cells and on interdigitating reticular cells in hyperplastic thymus as compared to normal thymus. In thymomas from MG patients the neoplastic epithelial cells expressed Fc gamma R, whereas only a few cells had HLA-DR antigens. Neither thymocytes nor B lymphocytes in the hyperplastic thymuses and in the thymomas were stained by B1D6.

Binding and processing of immunostimulatory Fc gamma 1 fragments by the murine macrophage cell line P388D1

Previous data from this laboratory indicated that human Fc gamma fragments induce murine B cells to proliferate and that the induction is macrophage-dependent. To further investigate the role of macrophages in this phenomenon, biologically active Fc gamma fragments from a human IgG1 myeloma protein and the murine macrophage-like cell line P388D1 were utilized. Fc gamma 1 fragments bound specifically and to a single class of receptor on P388D1 cells with a Ka value of 4 X 10(6) M-1 and to approximately 2.4 X 10(5) binding sites/cell. The binding was not effectively inhibited by two immunostimulatory Fc gamma 1 subfragments that were macrophage independent, i.e., pFc' fragments approximating the C gamma 3 domain of IgG1 and synthetic peptides representing residues 335-357 in IgG1. P388D1 cells were able to process Fc gamma 1 fragments but not intact IgG1 into subfragments that were able to induce lymphocyte proliferation in the absence of macrophages. The processing was rapid and resulted in active subfragments of several size classes. These findings not only further document the molecular and cellular events in these systems but underscore the usefulness of the P388D1 cell line in future studies on Fc fragment-induced lymphocyte regulation.

Binding and uptake of rabbit IgG complexes by diploid fibroblasts via plasma membrane Fc receptors

The existence of IgG receptors on the plasma membrane of diploid human fibroblasts is demonstrated. The receptors specifically bind heat-aggregated rabbit IgG as well as rabbit IgG within antigen-antibody complexes. Monomeric rabbit IgG were only poor ligands of the receptor. Competition experiments with Fab and Fc fragments of IgG revealed that the receptor specifically recognizes the Fc domain of the IgG molecule. Heat-aggregated IgG or antigen-antibody IgG complexes are specifically bound to the receptors, endocytosed and subsequently degraded. The receptors do not seem to be recycled because protein synthesis is a prerequisite for further cycles of endocytosis.

The measurement of specific cell: cell interactions by dual-parameter flow cytometry

The Fc receptor-mediated aggregation of antibody-coated spleen cells with cells from the P388D1 mouse macrophage line was followed using a novel flow cytometric technique. P388D1 and spleen cells were directly labeled with green-emitting (fluorescein isothiocyanate) and red-emitting (substituted rhodamine isothiocyanate) fluorophores, respectively. They were mixed, incubated in suspension at 4 degrees C, and analyzed for aggregation with a dual laser flow cytometer. Unconjugated cells appeared as particles which were either red or green, while conjugates were detected as particles which were both red and green. Using this assay procedure, 5 X 10(4) cells were analyzed in 2-3 min for the percentages of conjugates, free spleen cells, and free P388D1 cells. Intercellular aggregation required both antibody on the spleen cells and free Fc receptors on the P388D1 cells; nonspecific aggregates accounted for 1% or less of the total particles analyzed. Measurements of the fluorescence distributions within conjugates indicated that the majority of conjugates contained a single P388D1 cell bound to 1-3 spleen cells, and that only heterophilic aggregation occurred. The flow cytometric technique described here should be applicable for the measurement of the initial events of intercellular aggregation in other systems as well.

Factors influencing immune complex localisation

In systemic immune complex (IC) diseases such as SLE and rheumatoid vasculitis, IC accumulate in a number of tissues, either after deposition from the circulation or from in situ formation. The tissue localisation of IC depends on a delicate balance between the production of IC and the ability of the mononuclear phagocytic system (MPS) to remove them from blood. At times IC are cleared inefficiently, persist in the circulation and subsequently localise in tissues. This review evaluates the role of local tissue factors - anatomical, physiological, physical and immunological - in this process. We report on our studies examining the significance of C3b and IgG Fc receptors in tissues subject to IC deposition. No evidence for such receptors was found with the exception of a C3b receptor in human glomeruli. Our negative findings may be due to methodological difficulties in the identification of in situ receptors. Alternatively, immune receptors may not be present at these extra-glomerular sites and would therefore be unlikely to participate in IC localisation.

Inflammatory myopathy: a review of etiologic and pathogenetic factors

Human idiopathic inflammatory myopathy is an acquired disorder with an annual incidence of two to five cases per million. A genetic influence on host susceptibility may also play a role. With the marked heterogeneity of the disease one of the major challenges is to identify subsets that might share a more uniform pathogenesis and manifest a less diverse profile of clinical findings, histopathological abnormalities, and natural history. Dermatomyositis can be distinguished by clinical appearance and pathological changes, but the recognition of additional disease subsets remains very inexact. Current evidence suggests that dermatomyositis occurs as a result of a vasculopathy, but immune mechanisms involved in other categories of idiopathic inflammatory myopathy may also involve cell-mediated immunity and possibly multiple mechanisms. Even though viral-induced muscle inflammation occurs in humans, there is no convincing evidence for a viral cause of idiopathic inflammatory myopathy. Experimental allergic myositis may be produced by the injection of animals with skeletal muscle homogenates and complete Freund's adjuvant, but the myositogenic factor is unknown and the parallels between experimental allergic myositis and human idiopathic inflammatory myopathy are limited.

Localization of Fc gamma receptors in the human central nervous system

Immune complexes of horseradish peroxidase (HRP) and rabbit IgG antibodies to HRP were used to study the Fc gamma receptors in the human central nervous system (CNS). The peroxidase activity was demonstrated with 3,3'-diaminobenzidine tetrahydrochloride and H2O2. The majority of the pia and arachnoid cells of the leptomeninges, the stroma cells of the arachnoid granulations and the adventitial cells in the perivascular spaces of the nervous tissue were stained. The villi of the choroid plexus were also stained. By electron microscopy the reaction products were localized to the plasma membranes of the stroma cells and at the basal aspects of the epithelial cells in the choroid villi. In addition, reaction product was demonstrated on pericytes of some of the brain capillaries. The immune complexes did not bind to the brain parenchyma.

Multiple sclerosis serum and cerebrospinal fluid immunoglobulin binding to Fc receptors of oligodendrocytes

Suspensions of bovine oligodendrocytes were used to study the immunofluorescent staining properties of sera from multiple sclerosis (MS) patients and normal individuals. All sera (14 MS patients, 8 patients with other neurological diseases, and 11 normal individuals) showed positive oligodendrocyte staining by indirect immunofluorescence. Staining persisted after extensive absorption of sera with bovine liver to remove nonspecific binding. Similar findings were obtained for cerebrospinal fluid from all 5 MS patients as well as all 5 patients with other neurological diseases. In additional studies to determine if binding is mediated by the Fc fragment of IgG, results were: (1) ultracentrifuged normal and MS sera failed to react with oligodendrocytes, (2) positive staining with oligodendrocytes was observed after heat aggregation of IgG, (3) ox red blood cells, complexed with antibody, reacted with oligodendrocytes to produce strong rosette formation, and (4) the rosette formation could be blocked by prior treatment of oligodendrocytes with heat-aggregated IgG. The studies fail to support a previous claim for specific binding of immunoglobulins to oligodendrocytes in MS. However, they confirm and extend previous findings that the binding of IgG to oligodendrocytes may not necessarily be an antigen-antibody reaction. Therefore, adherence reactions with a putative antibody must exclude Fc-mediated binding.

Immunological characterization of sural nerve biopsies from patients with Guillain-Barré syndrome

Among eight sural nerve biopsies from patients with the Guillain-Barré syndrome (GBS), demyelination was observed in five and endoneural mononuclear cell infiltrates in three. Receptors for the activated third component of complement (C3b) were detected within the nerve fascicles. The receptor activity was reduced in five biopsies, and in vivo deposition of C3 within the endoneurium occurred in four. Immunoglobulins were found in four biopsies and appeared to be localized along the myelin sheaths. Both T and B lymphocytes could be detected in the mononuclear cell infiltrates, but the relative proportions of these cells could not be determined. Macrophages present in two biopsies were found to possess IgG Fc receptors. The results indicate that the complement receptors may play a role in the binding of complement-containing complexes in the nerve during the GBS disease process.

Mouse spleen cells are sensitized sheep red blood cells: an Fc-rosette-forming system allowing the detection of "activated" Fc structures

From a variety of Fc receptor-bearing cell/sensitized red blood cell combinations, mouse spleen cells, and sensitized SRBC were selected as an Fc-specific EA rosette assay system because only this mixture combined a high percentage (about 50%) of rosette-forming cells with complete absence of spontaneous rosettes and showed no influence of complement on the rosette formation. From studies on the minimal structural requirement of IgG both for mediation and inhibition of EA rosettes using IgG and several well-defined fragments, it appeared that both the CH2 and the CH3 domain of Fc are needed for optimal interaction with the lymphocyte Fc receptor. Finally, it was demonstrated that the assay system is able to detect "activated" Fc structures (here: heat-aggregated IgG) and to differentiate between varying amounts of such structures.