Activation of human complement by the pneumococcal toxin pneumolysin

Highly purified pneumolysin (at a concentration of 10 micrograms/ml) caused significant activation of human complement, as measured by conversion of C3. Complement activation in the presence of pneumolysin was not observed in sera chelated with a combination of Mg2+ and ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid, and activation was only slight in C2-deficient sera. This suggests that the toxin is capable of activating the classical complement pathway. Treatment of normal human serum with pneumolysin also significantly reduced its opsonic activity for Streptococcus pneumoniae.