CONCISE COMMUNICATION Protection against Bacteremic Pneumococcal Infection by Antibody

Pneumolysin is an important virulence factor of Streptococcus pneumoniae. This study ex-amined the hypothesis that human antibody to pneumolysin provides protection against pneu-mococcal infection. At the time of hospital admission, patients with nonbacteremic pneu-mococcal pneumonia had higher levels of serum anti-pneumolysin IgG than did patients with bacteremic pneumococcal pneumonia or uninfected control subjects. IgG levels rose signi®-cantly during convalescence in patients with bacteremic pneumonia, reaching levels observed in nonbacteremic patients. Puri®ed human anti-pneumolysin IgG protected mice against in-traperitoneal challenge with S. pneumoniae types 1 or 4 in a dose-related fashion; mice that received anti-pneumolysin IgG had a greater likelihood of surviving challenge and had neg-ative blood cultures. Pneumolysin damages epithelial cells and inhibits phagocytic function of polymorphonuclear leukocytes. One hypothesis that might explain the study results is that, early in infection, IgG to pneumolysin blocks these effects in the alveoli, thereby protecting the host against bacteremic pneumococcal disease. Strong evidence implicates pneumolysin, a thiol-dependent toxin that is nearly uniformly present in Streptococcus pneu