Analogous antigenic alterations elicited in C3 by physiologic binding and by denaturation in the presence of sodium dodecylsulfate.

The expression of three antigenic subsets of C3--the C3(S), the C3(N), and the C3(D) antigens--by soluble and target-bound forms of C3 was studied. The C3(S) subset is stable and is expressed by native as well as denatured C3 (exposure to sodium dodecyl sulphate (SDS) M greater than or equal to 10(-3)). The C3(N) and C3(D) subsets are labile and are expressed by native and denatured C3, respectively. Antisera to native C3, anti-C3(S-N), react with the C3(S) as well as the C3(N) subset. Antisera to isolated C3 subunits react exclusively with the C3(D) subset. A separation of anti-C3(S) and anti-C3(N) antibodies was accomplished by adsorbing the anti-C3(S-N) antiserum with insolubilized, denatured C3, anti-C3(N) antibodies remained unadsorbed. Anti-C3(S) antibodies were adsorbed and subsequently eluted from the denatured C3. Agglutination studies with EAC1423b cells showed significant agglutination with anti-C3(S) and anti-C3(D) antisera but reduced agglutination with anti-C3(N) antisera. Agglutination by anti-C3(D) antisera was unaffected in the presence of EDTA serum containing converted or unconverted C3. These data suggest an antigenic modification of C3b-b' upon binding that mirrors the antigenic transition associated with SDS denaturation of C3.