Contribution of serotype-specific IgG concentration, IgG subclasses and relative antibody avidity to opsonophagocytic activity against Streptococcus pneumoniae

Abstract

The contribution of serotype-specific IgG concentration, subclasses, and avidity to opsonophagocytic activity (OPA) against Streptococcus pneumoniae (Pnc) was evaluated in sera of adults and infants immunized with different pneumococcal vaccines. Antibody concentrations and avidities were measured by enzyme immunoassay (EIA) and OPAs by killing assay of Pnc. The most important factor contributing positively to OPA was the specific IgG level. In infants, a tendency to negative correlation was found between the concentration needed for killing of bacteria and avidity, suggesting that less antibodies of high rather than low avidity were required for killing. No such correlation was seen in adults. However, in adults the avidity was high already before vaccination and the variation was narrow. Thus, avidity was probably not a limiting factor influencing OPA. The effect of IgG2/IgG1 ratio on OPA was mostly negative but insignificant.

Fig. 1

Relationship between total IgG concentration and opsonophagocytic activity (OPA) in pre- and post-vaccination sera of adults (n = 42) and in sera of infants (n = 16) taken at 7, 15 and 16 months of age. d, Day of sampling; m, month of sampling.

Fig. 2

Relationship between total IgG concentration required for killing of bacteria and antibody avidity in sera of infants obtained after primary series of immunizations at 7 months of age and after booster vaccination at 16 months of age. Only sera with detectable opsonophagocytic activity (OPA) were included in the comparison. m, Month of sampling.