Natural mouse IgG reacts with self antigens including molecules involved in the immune response.

IgG isolated on protein A-Sepharose from pools of normal sera from various mouse strains were examined by immunoblotting for reaction with self antigens. Homogenates of the major mouse organs, i.e. brain, skin, spleen, kidney, adrenals, thymus, heart, muscle and liver were used as the source of autoantigens. IgG stained at least 220 bands on the immunoblots. The antigens corresponding to these bands were tentatively identified by molecular mass estimation and referenced to computerized mouse protein data banks. IgG mainly recognized enzymes but it also stained intracellular structural constituents and surface molecules implicated in the functioning of the immune system. The validity of this identification was confirmed by analyzing purified antigens from mouse or other animal species by immunoblotting and enzyme immunoassays. Furthermore, extracts of 125I-surface-labeled cells were immunoprecipitated with IgG in the liquid phase or immobilized on beads. The proteins precipitated migrated to the same positions as those precipitated by specific monoclonal antibodies (mAb), such as class I alpha chain and beta 2-microglobulin, class II alpha and beta chains, CD3, CD4 and CD8 antigens. The results obtained with several enzyme immunoassay procedures using cell membrane extracts, specific mAb and normal IgG further supported the specific interaction of IgG with Ia, CD4 and CD8 molecules. Affinity chromatography indicated that at least 20% of normal mouse IgG possess polyreactive autoantibody function. Dissociation constants of these IgG were calculated for some autoantigens and found to be in the range of 2 x 10(-6)-7 x 10(-6) M. It is concluded that normal mouse IgG exhibit autoreactivities similar to those previously described for IgM.