Relationship of in vitro phagocytosis of serotype 14 Streptococcus pneumoniae to specific class and IgG subclass antibody levels in healthy adults.

The role of specific IgG2 antibody in the protection against serious infection with Streptococcus pneumoniae is unclear. We therefore decided to investigate the relationship between serum antibody levels and opsonization and phagocytosis of this microorganism. We have measured serum IgM, IgA and IgG subclass antibody specific for pneumococcal capsular polysaccharide and in vitro phagocytosis of serotype 14 pneumococcus by polymorphs, in healthy adults before and after immunization with Pneumovax II. IgM and IgG2 were the predominant anti-pneumococcal antibodies seen, IgA and IgG1 being present at low titre. No significant relationship of phagocytosis with specific IgM and IgA antibodies was found. However, both specific IgG1 and IgG2 antibodies in post-immunization sera correlated significantly with phagocytosis of the pneumococcus in the presence of complement (r = 0.57, P = 0.029 and r = 0.59, P = 0.022 respectively). After heat-inactivation, the remaining opsonic activity of sera correlated only with levels of specific IgG2 antibody (r = 0.61, P = 0.0006). Whereas phagocytosis supported by specific IgG1 and IgG2 antibody to serotype 14 pneumococcus after immunization is mediated by complement activation, IgG2-specific antibody in high titre may also be able to function by complement-independent interaction with Fc gamma receptors on polymorphs.