Antigens of complement factor C3 involved in the interactions with factors I and H.

This study explores the relationship between defined antigens of the C3 molecule and those surface structures that are involved in interaction with factors I and H. Methylamine treatment at pH 11 followed by neutralization converts C3 into a modified state in which the molecule is optimally susceptible to cleavage by factor I in the presence of factor H. The modified C3 is characterized by an antigenic profile with expression of antigens of the C3(N), C3(S), and C3(D) subsets. These antigenic properties closely mirror those of physiologically bound C3b, suggesting that modification of antigenic expression upon denaturation of C3 reflects a regulatory mechanism for I and H function. Immunochemical studies of the alkaline-denatured C3 suggested that factor H interacts with surfaces of C3 that are situated within the C3c fragment and that are defined by C3(SN) antigens, while factor I predominantly interacts with C3(SN) antigens associated with the C3d fragment and with C3(D) antigens hidden in native C3. Therefore the continuing immunochemical study of this and related problems will require polyclonal and monoclonal antibody reagents reactive not only with the C3(S) and C3(N) but also the C3(D) antigenic subsets.