Absolute requirement for complement in monoclonal IgM antibody-mediated protection against experimental infection with type III group B streptococci.

The role of complement in the protective and opsonic activity of monoclonal IgM antibody to type III group B streptococci (GBS) was examined in a neonatal rat model of infection and in vitro with human sera as the complement source. C3 levels in uninfected neonatal rats were less than 50% of those in adult rats, similar to the low complement levels observed in human neonates. The monoclonal type III-specific IgM antibody provided protection to neonatal rats (with unaltered complement levels) that were infected intra-peritoneally or intranasally with type III GBS. In contrast, neonatal rats depleted of complement by administration of cobra venom factor were not protected by IgM antibody. In vitro, classical complement pathway activity was adequate in sera from well, term neonates and GBS-infected neonates in the presence of higher concentrations of the monoclonal IgM antibody. At lower IgM levels, however, the alternative complement pathway was less efficient in both neonatal sera and adult sera.