Effect of differences in antibody and complement requirements on phagocytic uptake and intracellular killing of ''c'' protein-positive and -negative strains of type II group B streptococci

The influence of antibody and complement on the polymorphonuclear leukocytic uptake and killing of type II group B streptococci (GBS) was examined in 11 adult sera and three type II strains possessing the trypsin-resistant and trypsin-sensitive components (II/TR+TS) of the ''c'' (formerly Ibc) protein or two type II strains lacking both components (II/no c) of the c protein. All tested sera mediated a >1 log₁₀ reduction in colony-forming units (CFUR) of a type II/no c strain, even in the absence of measurable type-specific antibody (<1.08 μg/ml), but only 5 of 11 mediated a >1 log₁₀ CFUR of any type II/TR+TS strain, even in the presence of moderate levels of type-specific antibody. The classical pathway of complement activation appeared to be more important than the alternative pathway, and even absorbed or immunoglobulin G (IgG)-depleted serum (IgG, 10 mg/dl) mediated a >1 log₁₀ CFUR without magnesium ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (magnesium EGTA) chelation. Chelation with magnesium EGTA reduced the CFUR in 4 of 11 test sera and greatly reduced the CFUR in absorbed or IgG-depleted sera for type II/no c strains. Despite variation in the phagocytic killing of two representative strains of type II GBS, both strains were well phagocytized, as measured by radiolabeled bacterial uptake or electron microscopy. This study suggested that poorly killed type II/TR+TS GBS were easily phagocytized but apparently resisted intracellular killing.