The interaction of C3b bound to pneumococci with factor H (β1H globulin), factor I (C3b/C4b inactivator), and properdin factor B of the human complement system

Pneumococcal cell walls are potent activators of the alternative complement (C) pathway; pneumococcal capsules are not. C3b that is deposited onto the cell walls of encapsulated organisms, however, functions inefficiently in host defense compared to C3b deposited onto capsular polysaccharides. Results of previous studies with guinea pig erythrocytes suggested that C3b deposited onto surfaces that do not activate the alternative pathway is rapidly inactivated. In the presnt study, we examined the interactions of C3b bound to pneumococcal capsules, to pneumococcal cell walls, and to the surface of sheep erythrocytes (E) with the serum control proteins, Factor H (β1H globulin) (H) and Factor I (C3b/4b inactivator) (I), and with Factor B (B) of the alternative C pathway. Conversion of bound C3b to C3bi was assayed by binding of radiolabeled conglutinin in a quantitative binding assay. Neither pneumococcal cell wall C3b nor capsular C3b was converted efficiently to a conglutinin-binding form by serum incubation. Experiments with purified C components showed that, after incubation with H and I, fewer conglutinin-binding sites were created on pneumococci than on E bearing equal numbers of C3b. Molecular analysis demonstrated that this did not result from cleavage of pneumococcal-bound C3b to an unusual, nonconglutinin-binding form of the molcule. Binding studies in which radio-labeled H was used demonstrated that the majority of C3b that is bound to both pneumococcal capsules and cell walls bound H with a lower affinity than did E-bound C3b. Studies of the binding of radiolabeled B demonstrated that C3b that was bound to pneumococcal cell walls and to E demonstrated equal affinity for B. In contrast, the majority of C3b that was fixed to pneumococcal capsules bound B with only 1/30 as high affinity. We conclude that pneumococcal capsules are not alternative pathway activators because the low affinity of capsular C3b for B leads to inefficient formation of an alternative pathway convertase, C3bBb. With regard to H binding, both cell wall- and capsular-bound C3b act as if they were in a 'protected site' and resist degradation by the control proteins.