Antibody-independent and -dependent opsonization of group B Streptococcus requires the first component of complement C1

The role of the classical complement pathway and specifically the first component, C1 in antibody-independent opsonization of type Ia group B Streptococcus (GBS) was investigated. For these studies a radiolabeled bacterial uptake assay was developed that was dependent on time and bacterial concentration and that required an intact classical complement pathway. To directly investigate the role of C1 in opsonization of type Ia GBS, C1 was isolated by chromatography on an immunoglobulin G (IgG) affinity column and further purified by molecular sieve chromatography on an Ultrogel AcA 22 column. When normal human serum was absorbed with 10(9) CFU of type Ia or III GBS, the serum opsonic capacity diminished (33 to 34%) for type Ia GBS compared with unadsorbed serum. Preincubation of the bacteria with purified C1 (10(4)U of C1 per ml) restored the opsonizing capacity of the adsorbed serum. A C1-depleted serum was prepared from the nonadherent fractions of the CH-sepharose 4B IgG column which only contained 5 U of C1 per ml. Substitution of C1-depleted reagent for normal serum in the uptake assay resulted in dramatic decreases in the opsonization of type Ia GBS, but opsonization could be restored by preincubation of the bacteria with purified C1. Heat-inactivated C1 depleted serum did not support opsonization of type Ia GBS, even with the addition of C1. Preincubation of type Ia GBS with heat-inactivated hyperimmune sera did not result in opsonization of type Ia GBS in the presence of C1-depleted serum. However, opsonization could be restored by the addition of C1, and the effects of C1 and antibody were additive. These results indicate the critical role of C1 in direct activation of the classical complement pathway by type Ia GBS and in antibody-mediated opsonization of the bacteria.