The Role of Natural IgG and Complement in the Phagocytosis of Type 4 Neisseria gonorrhoeae by Human Polymorphonuclear Leukocytes

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Abstract
The role of human serum components in the phagocytosis of logarithmic-phase type 4 Neisseria gonorrhoeae by human polymorphonuclear leukocytes was investigated. The requirement of fresh normal human serum (FHS) for optimal phagocytosis and the fixation of human immunoglobulin (lgG) and complement (C3) to the gonococcal cell surface suggested that both serum factors participate in the phagocytosis of these organisms. The percentage of neutrophils containing ingested organisms was directly proportional to the concentration of IgG purified from FHS. Absorption studies suggested that this natural IgG binds to a trypsin-sensitive surface protein on type 4 gonococci and cross-reacts with stationary-phase type 2 N. gonorrhoeae, group C Neisseria meningitidis, and Branhamella catarrhalis, but not with logarithmic-phase type 2 gonococci or other Neisseria species. Although complement alone did not promote phagocytosis, it enhanced IgG-mediated ingestion. Studies using C2-deficient serum or serum chelators indicated that the alternative complement pathway participates in this interaction.