Effect of Antibody Concentration on Opsonic Requirements for Phagocytosis in Vitro of Streptococcus pneumoniae Types 7 and 19

We investigated the effect of the concentration of type-specific antibody to pneumococcal polysaccharide (PPS) on opsonic requirements for phagocytosis in vitro of Streptococcus pneumonia types 7 and 19. We measured the uptake by human neutrophils of radiolabeled S. pneumoniae opsonized with either complement-intact, complement-depleted (by heat inactivation), or alternative complement pathway-activated (by magnesium dichloride-ethylene glycol tetraacetic acid (MgEGTA) chelation) immune sera with varying concentrations of antibody from individuals immunized with polyvalent PPS vaccine. Increased opsonization was found with increasing concentrations of type-specific antibody in the sera. Higher concentrations of antibody were required to opsonize type 7 than type 19 bacteria, both in the presence and absence of complement activity. Type 19 bacteria were more efficiently opsonized via the alternative complement pathway than type 7. For both types, antibody and the alternative complement pathway provided most of the opsonic activity in sera with lower concentrations of type-specific antibody. At high antibody concentrations, effective opsonization occurred in heat-activated sera, indicating the requirement for complement could be overcome with sufficient amounts of antibody alone.