Opsonic activity of IgG2 against Streptococcus pneumoniae.

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  • 免疫グロブリン製剤に含まれるＩｇＧ２の好中球オプソニン作用

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Four subclasses of human IgG exhibit unique profiles and specific spectra against microbial antigens. Antibody titers of human intravenous immunoglobulin preparation (IVIG) including IgG subclasses against 18 clinical isolates of Streptococcus pneumoniae and pneumococcal polysaccharides were determined by enzyme-immunoassay. IgG2 was predominantly reactive against each antigen in comparison with IgGl, IgG3 and IgG4. Since S. pneumoniae SP-23 was an especially representative strain that is most reactive to IgG2, the opsonic activity of IgG2 was studied by using this strain. Binding of IgG2 to human neutrophils was recognized after incubation with FITC conjugated monoclonal anti-human IgG2 by flow cytometry. This binding was inhibited by the addition of anti-Fc γ RIII monoclonal antibody. Column chromatography using spherical adsorbent of magnesium pyrophosphate resulted in preparation of an IgG2-deficient fraction (IgG2-D) and an approximately 4-fold decrease in IgG2 from native IVIG. Opsonic activities of the two immunoglobulin preparations were subsequently compared after incubation with S. pneumoniae SP-23 and human neutrophils at 37°C. Ingested bacteria in neutrophils were counted by microscopy. IgG2-D showed lower opsonic activity against S. pneumoniae than IVIG at the same IgG concentrations of 0.17 and 0.33 mg/ml. Each of the two immunoglobulin fractions was incubated with S. pneumoniae SP-23 at 37°C for 1 h and inoculated into ICR mice intravenously. Viable bacteria were counted 20 minutes after infection. IgG2-D showed less activity against bacterial clearance in mice than WIG at the same concentrations. It is suggested that phagocytosis and subsequent intracellular killing of bacteria require the binding of IgG2 to S. pneumoniae, namely opsonization.

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