Fc fragments of immunoglobulin G are an inductor of regulatory rheumatoid factor and a promising therapeutic agent for rheumatic diseases.

We recently identified rheumatoid factor, the production of which neither predicts nor exacerbates experimental autoimmune disease, but the opposite, namely it is associated with autoimmune disease resistance and remission. We have named it regulatory rheumatoid factor (regRF). The aim of this study was to determine whether rat Fc fragments and human Fc fragments are an antigen for regRF, and to determine the conditions for obtaining them. The presence of an antigenic determinant for regRF on IgG fragments was inferred from the fragments' ability to inhibit the agglutination caused by regRF and to induce regRF production in vivo. It was found that antigenic determinants for both human regRF and rat regRF are absent from native IgG and can be induced in the hinge region of Fc fragments of homologous IgG by papain digestion. The rat Fc fragments are susceptible to spontaneous reconfiguration, which results in loss of the antigenic determinants for regRF. Reconfiguration can be observed by SDS-PAGE. Immunization of arthritic rats with Fc fragments of rat IgG that carry antigenic determinants for rat regRF reduces the symptoms of collagen-induced arthritis. The Fc fragments can be viewed as the basis for a therapeutic vaccine to suppress autoimmune responses.