Serotypic variations among virulent pneumococci in deposition and degradation of covalently bound C3b: Implications for phagocytosis and antibody production

To assess serotypic variability in resistance to phagocytosis and in elicitation of antibody response among virulent pneumococci, we quantitated covalently bound C3b and analyzed the C3b degradation fragments for log-phase pneumococci of serotypes 3, 4, 6A, and 14 after opsonization for 20 and 90 min. In C3/C4-deficient agammaglobulinemic serum reconstituted with [³H]C3, covalent binding ranged from 2.9 x 10⁴ to 1.5 x 10⁵ C3b molecules per organism and increased at least two-fold in the presence of type-specific anticapsular antibody. We compared amide- and ester-linked forms of covalently bound C3b among unencapsulated and encapsulated isolates and differentiated cell wall from capsular binding. Analysis of proteolytic degradation of C3b, covalently bound at the capsular level, revealed C3b, iC3b, and C3d on the capsular surfaces of serotypes 3 and 4, whereas serotypes 6A and 14 displayed only iC3b. We conclude that pneumococcal serotypes, differing in their capsular polysaccharides, differ as well in the amount and site of covalently bound C3b and in its degradative processing to iC3b and C3d. We describe how these differences may explain the well-recognized patterns of serotypic variance in resistance to phagocytosis and in stimulation of antibody production.