Activation of C3 via the alternative complement pathway results in fixation of C3b to the pneumococcal cell wall.

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Abstract The present study was performed in order to determine the identity of the pneumococcal surface structure to which C3b binds when it has been activated via the alternative pathway. The binding of C3b was assessed by incubating the desired pneumococcal strain or subcellular component in C4-deficient guinea pig serum to which radiolabeled C3 had been added. Nascent C3b was able to bind to two different strains of unencapsulated pneumococci, devoid of detectable capsular polysaccharide, equally as well as to six different serotypes of encapsulated pneumococci. Removal of the capsule from a type 3 pneumococcus, using a specific decapsulating enzyme, neither enhanced nor diminished the binding of C3b to the organism. Preparations of crude and purified cell walls, derived from an unencapsulated pneumococcus, were also able to bind nascent C3b. Although the preceding experiments demonstrate that C3b activated via the alternative pathway is able to bind to the pneumococcal cell wall, they do not exclude the possibility that the C3b also binds to the capsule of the intact encapsulated organism. However, when radiolabeled C3b was fixed to encapsulated type 3 pneumococci, enzymatic removal of over 90% of the capsule did not remove any of the C3b. These experiments demonstrate that C3b, activated via the alternative pathway, fixes to the cell wall of both unencapsulated and encapsulated pneumococci.

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