A human IgG 3 is opsonic in vitro against type III group B streptococci.

Preparations of IgG 3 isolated by absorption of IgG 1, IgG 2, and IgG 4 from a human iv immunoglobulin with protein A-Sepharose were evaluated for their opsonic activities against type III group B streptococcal (GBS) strains. The resulting preparations were free of IgG 1 and IgG 2 and contained only trace amounts of IgG 4 (less than 2% of total IgG). These IgG 3 preparations exhibited excellent opsonic activities against type III GBS strains, similar to those of the unfractionated iv immunoglobulin (based on total IgG concentrations in the opsonic assays). In contrast, preparations of IgG 1, 2, and 4 eluted from protein A-Sepharose with 2 M acetic acid and 7 M urea were significantly less effective in enhancing phagocytosis and killing of type III GBS than IgG 3 preparations or iv immunoglobulin. The reasons for excellent opsonic activity of IgG 3 preparations as well as for decreased opsonic activity of IgG 1, 2, and 4 preparations are not clear. Perhaps alteration of IgG by lower pH and high concentrations of urea may have impaired the functional activity of IgG 1, 2, and 4 preparations. The significant finding of this study is the first demonstration of the excellent opsonic activity of IgG 3, emphasizing the importance of having intact IgG 3 in commercial immunoglobulin preparations used in prophylaxis or treatment of GBS infections.