Physicochemical characteristics of human IgG Fc fragments that expose regulatory rheumatoid factor neoepitopes and may show promise as antirheumatic agents

Abstract

Previously, we showed that immunoglobulin G (IgG) Fc fragments can expose neoepitopes specific to antibodies that were named regulatory rheumatoid factor (regRF). RegRF confers resistance to experimental autoimmune diseases. Immunization of rats with rat Fc fragments exposing neoepitopes recognized by regRF reduces the symptoms of collagen‐induced arthritis. Therefore, IgG Fc fragments that expose neoepitopes recognized by regRF are promising antirheumatic agents and regRF‐producing lymphocytes are potential therapeutic biotargets. The purpose of this study was to elucidate the physicochemical features of human IgG Fc fragments that are associated with the presence of immunosuppressive neoepitopes recognized by regRF. It was found that the acquisition of neoepitopes recognized by regRF is associated with a reduction of the hinge disulfide bonds and conformational changes in the Fc fragment domains. Alkylation of thiol groups in the hinge leads to loss of the epitopes. Therefore, the neoepitopes recognized by regRF may form directly in the hinge when interchain disulfide bonds are reduced or in the region of the CH2 domain as part of the conformational changes caused by the reduction of the interchain disulfide bonds in the hinge. Species‐specificity studies of neoepitopes recognized by regRF revealed that human and rat neoepitopes recognized by regRF are cross‐reactive.

Comparison of the specificity of rheumatoid factor detected by latex fixation with that of regulatory rheumatoid factor

“…We have previously shown that the binding of healthy human sera to tanned IgG‐loaded erythrocytes does not change in the presence of IgG 7,8 . On this basis, we concluded that IgG does not carry neoepitopes recognized by regulatory rheumatoid factor 7,8 .…”

“…We have previously shown that the binding of healthy human sera to tanned IgG-loaded erythrocytes does not change in the presence of IgG. 7,8 On this basis, we concluded that IgG does not carry neoepitopes recognized by regulatory rheumatoid factor. 7,8 In the course of this study, we found that lyophilized ( Figure 2B) and non-lyophilized IgG preparations also did not affect the binding of rheumatoid arthritis sera to tanned IgG-loaded erythrocytes.…”

“…In particular, the classical RF associated with rheumatoid arthritis is specific for cryptic epitopes that are absent from the native IgG molecule and appear as a result of changes in its conformation upon interaction with antigen or heat or chemical treatment 8 . RegRF epitopes are also neoepitopes 7,8 . They are absent from IgG and appear on its papain Fc fragments as a result of the reduction of hinge disulfide bonds 7,8 …”

“…To detect RF, we used the method of agglutination of tanned erythrocytes loaded with homologous IgG. A study of the specificity of the rheumatoid factor detected by this method showed that it is a species of anti‐idiotypic antibodies of varying specificity that are united by the presence of a common paratope specific for the neoepitopes of conformers of IgG Fc fragments 6‐8 . Immunization with conformers of IgG Fc fragments carrying neoepitopes recognized by rheumatoid factor reduces collagen‐induced arthritis in rats 7 .…”

“…At the same time, certain findings regarding the specificity of classical rheumatoid factor and regulatory rheumatoid factor point to the possibility that these antibody populations are one and the same. In particular, the classical RF associated with rheumatoid arthritis is specific for cryptic epitopes that are absent from the native IgG molecule and appear as a result of changes in its conformation upon interaction with antigen or heat or chemical treatment 8 . RegRF epitopes are also neoepitopes 7,8 .…”