Gamma heavy chain disease protein CHA: Immunological and structural studies

Heavy chain disease (HCD) protein CHA was found to inhibit specific hemagglutination by an anti-IgG1 antiserum but not by anti-IgG3, and reacted by double immunodiffusion with a specific anti-IgG1 antiserum but not with anti-IgG3. It was retained on a column of Sepharose-bound protein A. In contrast, the NH2-terminal sequence of protein CHA was found, after two initial residues difficult to assign ( Thr Ser > Thr), to be identical with that of an IgG3 beginning after the 47 amino acid extra piece characteristic of the hinge region of γ3 chains. Peptide mapping performed by isoelectric focusing of the cyanogen bromide fragments of protein CHA indicated, by comparison with similar fragments obtained from the Fc piece of monoclonal IgGs of different subclasses, that most of the fragments were homologous with peptides present in IgG1 Fc, but some fragments corresponded to those in the IgG3 Fc molecule. These results suggest that protein CHA represents the first case of a hybrid HCD protein: IgG3 (NH2-terminal part of the molecule) and IgG1.