Characterization of human bactericidal antibodies to Bordetella pertussis

The Bordetella pertussis BrkA protein protects against the bactericidal act ivity of complement and antibody; however, some individuals mount an immune response that overcomes this bacterial defense, To further characterize th is process, the bactericidal activities of sera from 13 adults with differe nt modes of exposure to B. pertussis (infected as adults, occupational expo sure, immunized with an acellular vaccine, or no identified exposure) again st a wild-type strain and a BrkA complement-sensitive mutant were evaluated . All of the sera killed the BrkA mutant, suggesting past exposure to B, pe rtussis or cross-reactive organisms. Several samples had no or minimal acti vity against the wild type. All of the sera collected from the infected and occupationally exposed individuals but not all of the sera from vaccinated individuals had bactericidal activity against the wild-type strain, sugges ting that some types of exposure can induce an immune response that can ove rcome the BrkA resistance mechanism. Adsorbing serum with the wild-type str ain removed the bactericidal antibodies; however, adsorbing the serum with a lipopolysaccharide (LPS) mutant or an avirulent (bvg mutant) strain did n ot always result in loss of bactericidal activity, suggesting that antibodi es to either LPS or bvg-regulated proteins could be bactericidal. All the s amples, including those that lacked bactericidal activity, contained antibo dies that recognized the LPS of B. pertussis, Bactericidal activity correla ted best with the presence of the immunoglobulin G3 (IgG3) antibodies to LP S, the IgG subtype that is most effective at fixing complement.