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

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.