Classical complement pathway activation by antipneumococcal antibodies leads to covalent binding of C3b to antibody molecules

We have examined whether or not a physical relationship exists between antipneumococcal antibodies (Ab) and C3b when Ab activate the classical complement pathway on the surface of pneumococci (Pn). After sensitization with ¹²⁵I-labeled Ab, Pn were sequentially incubated with purified C1, C4, C2, and biotinylated C3. Ab molecules were then eluted from Pn, and C3b-associated molecules were purified on avidin-Sepharose. Both ¹²⁵I-labeled immunoglobulin G (IgG) and [¹²⁵I]IgM bound to C3b; the association was stable to incubation in 1% sodium dodecyl sulfate at 37°C. The association was only partially reversed by incubation in 1 M hydroxylamine-0.5% sodium dodecyl sulfate (pH 10.5), implying that Ab and C3b were linked by amide as well as ester bonds. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of dithiothreitol and NH₂OH eluates from the avidin-Sepharose showed that C3b bound to both heavy and light chains of the Ab. Moreover, the ability to bind to erythrocyte C3b receptors could be transferred to unsensitized Pn by eluates from Pn on which Ab had activated the classical pathway. These covalent complexes of Ab and C3b may be especially important in the opsonization of Pn by Ab and complement.