Presence of IgG-CD4 complexes in the circulation

Many proteins of various origins are able to form complexes with Immunoglobulin G using reaction sites formed by residues of constant domains. Studies of IgG complexes of non-immune nature are important as biologically active proteins could escape from the circulation forming complexes with IgG as well as with other serum proteins whose concentration in serum is high. A quantitative characterization of circulating complexes in various diseases could give valuable information on the development of pathological processes. Immunoglobulins G are widely used for the treatment of a number of diseases and it is essential to understand what proteins are present in the preparations beside IgG. In the present study CD4 T-cell membrane glycoprotein was found in complexes with human IgG molecules isolated from donor sera as well as from sera of SLE patients via a sensitive quantitative dot-blot assay. According to immunoblotting experiments the CD4 part of the complexes had a molecular mass of about 50 kDa and is composed from all four extracellular domains. The CD4 content of the complexes varied among the studied human sera. There was no difference in IgG-CD4 complex concentration between the SLE patients and healthy controls. The data support the assumption that IgG molecules are able to act as scavengers and eliminate various proteins from the circulation including soluble CD4 protein.