Pneumococcal type-associated variability in alternate complement pathway activation

Opsonization of Streptococcus pneumoniae may be mediated by the alternate complement pathway. To study the importance of this interaction to human disease, complement consumption by pneumococci of various serotypes was measured in humwn serum chelated with ethyleneglycoltraacetic acid, a substance that blocks the classic but not the alternate complement pathwway. Serotype I, in contrast to all other types studied, lacked ability to consume complement in this system. The ability for serotypes III, IV, and VIII to activate the alternate pathway could be eliminated by prior serum absorption at O C with they type in question, a condition that would remove antibody but not complement. Types VII, XII, XIV, and XXV readily activated the alternate pathway in unabsorbed and absorbed sera. Differences could not be related to properties of the capsules. It was concluded that types I, III, IV, and VIII lack intrinsic ability to activate and thus be opsonized by the alternate complement pathway, although types III, IV, and VIII can do so in concert with specific antibody. The fact that these same types are especially prominent in human disease suggests that the ability to evade opsonization by the alternate complement pathway in pre-antibody phases of infection may be a virulence factor in pneumococci.