Importance of the IgG isotype, not the state of glycosylation, in determining human rheumatoid factor binding

Immunochemical study of human immunoglobulin G Fc region

Fifteen murine monoclonal antibodies (mAb) were produced that reacted with conformational epitopes present on the Fc portion of all human IgG subclasses (PAN-Fc). Inhibition and sandwich ELISA were used to elucidate overlapping and distinct epitopes. The epitopes were aligned to form a continuous "chain" of overlapping determinants from the CH2-CH3 interface to the direction of hinge. The five more hinge-proximal epitopes demonstrated high lability both in competitive and sandwich assays, being completely or partially destroyed when any of these 15 other mAb bound the IgG first. Heat-inactivation of IgG caused full disruption of these epitopes. In contrast, epitopes situated at the opposite distal of the "chain" were more stable and mAb binding could only be affected by occupying an overlapping epitope. Under heat-inactivation these epitopes were affected, but not completely destroyed. Human IgG class anti-DNA autoantibodies were bound to insolubilized dsDNA and their reaction with PAN-Fc mAb was studied. mAb titration plots on IgG and dsDNA-IgG were compared. Five epitopes proved to be altered by antigen (dsDNA) binding. Two of these were the labile hinge-proximal epitopes and the other three were situated near the CH2-CH3 interface. Cross-reactivity of mAb with xenogeneic IgG was also studied. An "epitope map" of the crystallographic model of human IgG Fc portion drawn was based on these experimental data and printed matter, concerning the location of subclass-specific amino acids and homology regions of human and animal IgG.

Binding of human IgM from a rheumatoid factor to IgG of 12 animal species

The binding of IgM from a rheumatoid factor (RF-IgM) to IgG from 12 animal species was analyzed by an ELISA system. The RF-IgM bound various animal IgG with dissimilar affinities. The binding of RF-IgM to animal IgG was inhibited by addition of protein A, which binds some animal IgG by recognizing the junctional site on CH2-CH3 domains in the Fc region. As previously reported, no significant correlation was observed between the binding of RF-IgM to IgG and the content of galactose-free oligosaccharides, which is increased in IgG of rheumatoid arthritis patients or autoimmune mice. We suggest that the crucial epitope of IgG for RF-IgM binding is not the oligosaccharide structure generated specifically in IgG of autoimmune diseases but that RF-IgM may recognize a certain protein conformation of a region in IgG near the binding site of protein A.

Rheumatoid factor avidity in patients with rheumatoid arthritis: identification of pathogenic RFs which correlate with disease parameters and with the gal(0) glycoform of IgG

The standard ELISA for measuring rheumatoid factor (RF) binding was modified by treatment after the RF-Fc interaction with 2 M guanidine, which allowed a measurement of the avidity of the interaction. Incubation with 4 M guanidine eliminated RF binding. There was a direct correlation (r = 0.99) between the avidity as measured by the modified guanidine ELISA, and the dissociation constant for monoclonal RFs, as measured by competitive ELISA. Of the seropositive rheumatoid arthritis (RA) patients tested, 47% had high-avidity RFs (> or = 8% RF binding remaining after guanidine treatment). Tender joint count scores were significantly higher in the high avidity group (p = 0.05), whereas there was no significant difference in the ages, disease duration, sedimentation rate, RF titer or serum Ig levels compared to those with low-avidity RFs. Additionally 58% of those with high-avidity RFs had subcutaneous nodules, compared to 40% of the low-avidity group. A significantly higher number of nodules was present in the high-avidity RF group compared to those with low-avidity RFs (p = 0.03). Interestingly, the RF avidity was significantly higher in isolated immune complexes (IC), compared to that in circulating IgM RFs (p = 0.01). The RF avidity correlated with the presence of the glycoform of IgG lacking galactose in both circulating and IC-derived IgG (p = 0.003 and 0.009 respectively). Information about the strength of binding to Fc identifies a subgroup of IgM RFs that are likely pathological in patients with RA, as well as a specific glycoform of the target antigen.

Anti-DNA and anti-platelet specificities of SLE-derived autoantibodies: evidence for CDR2H mutations and CDR3H motifs

Although polyreactivity appears to be a characteristic feature of natural autoantibodies, polyreactive anti-DNA autoantibodies can be derived both from patients with autoimmune disease and from normal individuals. It is unclear whether these autoantibodies differ depending on their origin, but previous studies from our laboratory have suggested that polyreactive systemic lupus erythematosus (SLE)-derived platelet-binding anti-DNA autoantibodies have more restricted antigen reactivity and greater functional activity than normal-derived polyreactive autoantibodies. The objective of the present study was to characterize the VH and VL region sequences of 10 human hybridoma anti-DNA autoantibodies derived from peripheral blood lymphocytes of different origins [SLE, rheumatoid arthritis (RA), or normal] to determine whether there are structural differences between these autoantibodies. We show that although some unmutated germline structures (VH and VL) are represented, these are not restricted to anti-DNA autoantibodies from normal individuals and that two normal-derived anti-DNA antibodies showed quite extensively mutated VH genes. However, these mutations, unlike those found in the CDR2H of several of the SLE-derived antibodies, did not appear to be antigen-selected. Three different amino acid motifs, putatively involved in antigen binding specificity, were observed in the CDR3H segments of some of the autoantibodies. One was the previously described YYGSG motif, which was found in a normal-derived anti-DNA autoantibody, while two new potential motifs were observed only in SLE-derived platelet-binding anti-DNA autoantibodies. These data suggest that antigenic and functional differences between SLE-derived and normal-derived platelet-binding anti-DNA autoantibodies may be due to antigen-selected mutations in the CDR2H and specific amino acid motifs in the CDR3H.

Recognition sites on human IgG for Fc gamma receptors: the role of glycosylation

Human IgG subclass proteins exhibit more than 95% primary amino acid sequence homology in their Fc regions, but each has a unique profile for recognition by the 3 human Fc gamma receptors. The Fc gamma Rs are themselves highly homologous members of the immunoglobulin supergene family. Consistent with these data we have proposed that Fc gamma RI, Fc gamma RII and Fc gamma RIII recognise overlapping non-identical interaction sites in the lower hinge region of the CH2 domain of the IgG molecule. Evidence in support was provided by protein engineering effecting single amino acid replacements in the proposed site. Alternatively, we have demonstrated that the primary amino acid sequence alone is not sufficient for IgG molecules to fold with the generation of Fc gamma R interaction sites and that glycosylation of Asn 297 of the CH2 domain is essential. We have further defined a 'core' oligosaccharide structure that provides for the generation of Fc gamma R interaction sites which suggests that the addition of outer-arm sugar residues does not affect this primary activity; although in vivo it could influence other essential biological activities. These findings have opened up a new approach to engineering antibody function--by protein engineering of amino acid residues that form contacts with the oligosaccharide moiety. In the present report we demonstrate that replacement of contact residues for galactose on the alpha(1-6) arm does not affect Fc gamma RI and Fc gamma RII recognition while replacement of Asp 265, a contact for a 'core' N-acetylglucosamine residue, results in a loss of Fc gamma RI and Fc gamma RII recognition.

Rheumatoid-factor-reactive sites on CH2 established by analysis of overlapping peptides of primary sequence

Polyclonal IgM rheumatoid factors (RF) from ten patients with rheumatoid arthritis (RA) isolated on IgG affinity columns were studied for their reactivity with overlapping 7-mer peptides representative of solvent accessible regions of the CH2 domains of IgG using a pin-ELISA assay. A panel of nine monoclonal RF from RA patients' B cells were studied in parallel. Four peptides SVFLFPP (239-245), KFNWYVD (274-280), NSTYRVVSV (297-305) and VLTVLHQNWL (305-314) reacted with most RF. Glycine substitution showed that tryptophanes at 277 and 313, tyrosine at 278, valines at 279 and 305, and leucine 314 represented important residues for RF reactivity. Assays using monoclonal IgM RF produced from RA synovial B cells or peripheral blood B cells frequently showed a restricted spectrum of reactivity for CH2 epitopes, which often were identical to those binding to polyclonal IgM RF. Combinations of synthetic 7-mer peptides representing RF-reactive CH2 or CH3 epitopes produced as much as 60-90% inhibition of RF binding to IgG when peptides were preincubated with RF in free solution before completion of the reaction of RF with IgG on the ELISA plate.

The binding of synovial tissue-derived human monoclonal immunoglobulin M rheumatoid factor to immunoglobulin G preparations of differing galactose content

There are several sites on IgG Fc that have been reported to be the epitopes for binding rheumatoid factors (RF). It is now established that there are alterations in the oligosaccharides on IgG from patients with rheumatoid arthritis and it has been suggested that these changes may enhance immune complex and cryoglobulin formation. We have used a series of IgG preparations differing in their content of oligosaccharide chains lacking galactose from 18 to 86% to determine whether changes in sugar content affect the binding of rheumatoid factor. Five of 16 monoclonal rheumatoid factors prepared from synovial tissue, from patients with juvenile or adult rheumatoid arthritis, bound better to IgG which was deficient in galactose. Six of the 16 rheumatoid factors from the same patients bound independently of the galactose content. Four of the 16 rheumatoid factors could not be absolutely grouped in this manner but seemed to demonstrate a preference for agalactosyl IgG. One rheumatoid factor bound better to fully galactosylated IgG. There was an association between enhanced binding to galactose-deficient IgG and monoreactivity and a very strong association between the functional affinity of the rheumatoid factors and the dependent binding.

IgG3 reactive rheumatoid factor in rheumatoid arthritis: etiologic and pathogenic considerations

Rheumatoid factor (RF) is a polyclonal autoantibody directed against the Fc portion of IgG. Although the role of RF in patients with rheumatoid arthritis (RA) is unclear, immune complexes that form between RF and IgG can activate the classical complement (C) pathway, leading to pathogenic outcomes involving inflammatory events and tissue damage. The specificity of serum RF and RF produced by rheumatoid synovial cells (RSC) is different. Serum RF has specificity for rabbit IgG and human IgG subclasses IgG1, 2, and 4, but binds poorly to IgG3. The affinity of serum RF for IgG Fc is low, having an association constant of 10(4)-10(5) M-1. RSC RF, however, has specificity for human IgG and high avidity for IgG3. Because of this greater specificity and avidity for IgG3, and because RSC RF may be pathogenically more important than serum RF, an important role for IgG3-reactive RF in RA may exist. Binding of RF to IgG may be dependent on the allotype and glycosylation of IgG. Infectious agents present in RA patients may directly or indirectly induce the production of certain RF. In this communication, we review and expand on several observations examining the role of IgG3-reactive RF in RA including: 1) binding differences between RF derived from RSC and serum; 2) glycosylation characteristics of IgG and its interaction with RF; 3) apparent allotype dependent binding of IgG3-reactive RF; and 4) possible relationship between infectious agents and the production of IgG3-reactive RF. Taken together, these observations suggest an important role for IgG3-reactive RF in better understanding the etiology and pathogenesis of RA.

Monospecific but not polyreactive human hybridoma rheumatoid factors exhibit preferential binding specificities for IgG3 and IgG4

Among 38 human hybridoma-derived monoclonal rheumatoid factors (RFs) generated from patients with either rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE), two groups of RFs can be identified. Monospecific RFs were derived primarily from patients with RA and are characterized by a binding specificity for IgG3 and/or IgG4. Polyreactive RFs were derived largely from patients with SLE and show a broader pattern of reactivity to all four isotypes of IgG. Neither population of RFs was exclusive to either disease. The binding specificities identified appear to be different from the RFs isolated from patients with mixed cryoglobulinemia and may reflect a different antigen selection mechanism.

Restricted immunoglobulin variable region gene usage by hybridoma rheumatoid factors from patients with systemic lupus erythematosus and rheumatoid arthritis

Rheumatoid factors (RFs) are autoantibodies that are produced by approximately 75% of patients with rheumatoid arthritis (RA). Their role in pathogenesis is not well understood. In this study of 81 human hybridoma IgM antibodies derived from unstimulated peripheral blood B-cells of patients with RA and systemic lupus erythematosus (SLE), we have demonstrated that idiotypes associated with RFs derived from patients with mixed cryoglobulinemia were expressed by approximately 60% of RFs and 6% of IgM antibodies lacking RF activity. The specificity of the RFs for the Fc portion of IgG only (monospecificity) or for Fc and additional self antigens (polyreactivity) was found to correlate with the expression of specific heavy chain associated idiotypes. The VH3 associated RF idiotypes, D12 and B6, were expressed by 0/16 (0%) of monospecific RFs compared with 6/22 (27%) of polyreactive RFs. The predominant use of VH3 was verified by analysis of the expressed Ig with VH family specific anti-peptide antibodies. The light chains expressed by both populations of IgM RFs were found to be predominantly VKIII, both by detection of specific epitopes/idiotypes and V family analysis. This non-random gene usage of both the heavy and light chains suggests that there is a selective expression of V regions in the RF producing B-cells in patients with RA and SLE. We suggest that different antigen-driven, clonal selection events may occur which result in either monospecific RFs or polyreactive RFs.

Galactose terminating oligosaccharides of IgG in patients with primary Sjögren's syndrome

Using a simple but novel ELISA, we have screened 40 serum samples from patients with primary Sjögren's syndrome and 34 normal controls for IgG glycosylation deficiencies, identified by their specific ricin binding. Elevated levels of asialylated IgG were detected in 24 patients. The extent of asialylation was significantly higher in the patients with extraglandular manifestations than in the others. Interestingly, the correlation of asialylated IgG was apparent only with Raynaud's phenomenon and arthritis, and not other extraglandular manifestations. Strong correlations (P less than 0.01) were noted between asialylated IgG and rheumatoid factor or IgA-containing immune complexes.

Identification of IgG rheumatoid factors by a novel method utilizing immunoblotting

Monoclonal IgG rheumatoid factors (RFs) are identified using a novel immunoblot assay which detects RFs that bind to SDS-denatured Fc on nitrocellulose. Recognition of these self-associating antibodies is by the use of F(ab')2 fragments of light chain-specific antisera. In this way, IgG RFs can be easily identified and the precise binding characteristics to different isotypes of IgG, or other antigens, further specified. The assay can also be used to detect other classes of RFs such as IgM RFs. Although less sensitive than the standard ELISA, the use of this immunoblot RF assay (IRFA) will identify IgG RFs and their target antigen with precision.

Changes in normal glycosylation mechanisms in autoimmune rheumatic disease

To investigate potential mechanisms controlling protein glycosylation we have studied the interrelationship between lymphocytic galactosyltransferase (GTase) activity and serum agalactosylated immunoglobulin G levels (G(0)) in healthy individuals and patients with rheumatoid arthritis and non-autoimmune arthritis. In RA there was reduced GTase activity and increased G(0). A positive linear correlation between B and T cell GTase was found in all individuals. The relationship between GTase and G(0) was found to be positive and linear in the control population and negative and linear in the RA population. Sulphasalazine therapy maintained normal levels of GTase and caused a reduction in G(0) in the RA population. IgG anti-GTase antibodies (abs) were significantly increased in the RA population, whereas IgM anti-GTase abs were significantly decreased in both the RA and the non-autoimmune arthritis groups. These data describe a defect in RA lymphocytic GTase, with associated abnormal G(0) changes, which is corrected by sulphasalazine. A possible regulatory mechanism controlling galactosylation in normal cells is suggested, in which there is parallel control of B and T cell GTase. IgM anti-GTase abs may be integrated into this normal regulatory process. This is disrupted in RA, where the positive feedback between GTase and G(0) is lost and there is an associated increase in IgG anti-GTase abs, which may result from isotype switching as IgM anti-GTase abs are reduced. We suggest that these mechanisms are of relevance to the pathogenesis of RA, and that their manipulation may form part of a novel therapeutic approach.

Mapping rheumatoid factor binding sites using genetically engineered, chimeric IgG antibodies

We are using chimeric IgG antibodies consisting of murine variable regions joined to human constant regions as rheumatoid factor (RF) binding substrates to localize and map IgM RF binding sites on IgG. Using chimeric antibodies in a modified RF ELISA, we showed that RFs from rheumatoid arthritis (RA) and Waldenstrom's macroglobulinemia (WMac) patients differ in their binding specificities for IgG3, although some of these RFs share common specificity for IgG1, IgG2, and IgG4. By shuffling constant region domains between IgG3 and IgG4, we showed that sequence variation in the CH3 domain is responsible for WMac-derived RF differentiation of IgG3 and IgG4. By making site-directed mutations in the wild-type IgG3 or IgG4 human gamma constant genes, we showed that His-435 is an essential residue in RF binding to IgG for most WMac RFs. The allotypic polymorphism in IgG3 at 436 is not responsible for differences in previous reports of high-frequency IgG3 binding by WMac RFs. A amino acid loop in the CH2 domain of IgG4 proximal to the CH2-CH3 interface is important in WMac RF binding to IgG; a more distal CH2 loop in CH2 has a more variable effect on WMac RF binding. To evaluate the contribution of the N-linked carbohydrate moiety at Asn-297 to RF binding sites on IgG, we measured RF binding to aglycosylated IgG antibodies produced by mutating the glycosylation signal Asn-297 to another amino acid. Of all four IgG subclasses, only aglycosylated IgG3 was a better RF binding substrate than its glycosylated subclass counterpart.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

Agalactosyl IgG: an aid to differential diagnosis in early synovitis

Sixty consecutive patients presenting with early-onset synovitis were studied by measuring rheumatoid factor (RF) titers and the percentage of oligosaccharide chains attached to the C gamma 2 domain of IgG that lack galactose (GAL[0]). After 2 years of followup, 39 patients (65%) had developed rheumatoid arthritis (RA), and 21 had developed a variety of other inflammatory joint diseases. A combination of RF positivity and GAL(0) levels above the age-corrected mean gave a positive predictive value for a diagnosis of RA in 94% of these patients. These observations may well have clinical utility.