An alternative role for specific antibody in neutrophil bactericidal activity against highly pathogenic group B streptococci.

An investigation was undertaken into the role of antibody and complement in neutrophil phagocytosis and killing of group B streptococci of low and high pathogenicity. Phagocytosis of both types of organism appears to be a nonspecific event requiring only nonspecific antibody or complement. However, neutrophil bactericidal activity is mediated by the pathogenicity of the infecting organism. Neutrophils alone can kill some streptococci of low pathogenicity, but their killing ability is considerably increased in the presence of specific antibody or complement. An active role for the alternative pathway of complement in the killing process was demonstrated for organisms of low pathogenicity. Neutrophils did not kill the highly pathogenic organisms in the absence of antibody and complement, and required specific antibody, but not complement, to kill these bacteria. The alternative complement pathway is not involved in killing of highly pathogenic organisms. Addition of specific antibody to neutrophils containing ingested bacteria stimulated the neutrophils to kill the intracellular bacteria, suggesting an alternative role for specific antibody in the killing process other than as an opsonin. It is suggested that activation of Fc receptors on the neutrophil surface initiates the bactericidal action of the neutrophils.