Potentiation of virulence by group B streptococcal polysaccharides.

The ability of group B streptococcal (GBS) capsular polysaccharides to potentiate virulence was examined. Incubation of soluble type Ia or III polysaccharide in hypogammaglobulinemic human serum inhibited classical complement-dependent opsonophagocytic killing of type Ia strain 515. When functional complement components were measured, C1 activity increased in sera with added polysaccharide whereas C4 activity decreased 80%-90%. Incubation of purified C1 with type Ia polysaccharide inhibited lysis of EAC4 cells in a C1 transfer assay. In a mouse lethality model, tail-vein injection of 50 micrograms of type Ia or III polysaccharide decreased the 50% lethal dose (LD50) from 3.2 X 10(6) to 2.2 X 10(5). Total hemolytic complement levels in mice immediately after polysaccharide injection were increased over levels in control mice, but this difference was not seen 30 min later. The LD50 in mice depleted of C3 (with cobra venom factor) was 6 X 10(5) and simultaneous injection of polysaccharide did not further lower the LD50.