Functional activities of various preparations of human intravenous immunoglobulin against type III group B Streptococcus

Two preparations of human immunoglobulin modified for intravenous use (iv immunoglobulin) by different methods (reduction-and-alkylation, or pH 4 treatment) were evaluated for in vitro and in vivo activity against a strain of type III group B Streptococcus (GBS). Both preparations contained similar amounts of total IgG and specific IgG antibody against the type-specific polysaccharide. In vitro, opsonophagocytic studies revealed that pH 4-treated iv immunoglobulin was significantly more effective than reduced-and-alkylated iv immunoglobulin in supporting neutrophil-mediated killing of the type III GBS strain. In vivo, both preparations resulted in similar levels of serum antibody in newborn rats, but pH 4-treated iv immunoglobulin was significantly more protective against the type III GBS strain. This was demonstrated by the lower magnitude of bacteremia, improved survival, and lower protective dose (PD₅₀) in recipients of pH 4-treated iv immunoglobulin. Thus, reduced-and-alkylated iv immunoglobulin was less effective in vitro and in vivo against the strain of type III GBS than was pH 4-treated iv immunoglobulin. Further studies are needed to elucidate the mechanisms for this apparent discrepancy in functional activities of iv immunoglobulin.