BrkA protein of Bordetella pertussis inhibits the classical pathway of complement after C1 deposition

Bordetella pertussis produces a 73-kDa protein, BrkA (Bordetella resistance to killing), which inhibits the bactericidal activity of complement. In th is study we characterized the step in the complement cascade where BrkA act s, using three strains: a wild-type strain, a strain containing an insertio nal disruption of brkA, and a strain containing two copies of the brkA locu s. Following incubation with 10% human serum, killing was greatest for the BrkA mutant, followed by that for the wild-type strain, while the strain wi th two copies of brkA was the most resistant. Complement activation was mon itored by enzyme-linked immunosorbent assay (ELISA) or Western blotting. EL ISAs for SC5b-9, the soluble membrane attack complex, showed that productio n of SC5b-9 was greatest with the brkA mutant, less with the wild type. and least with the strain containing two copies of brkA. Deposition of complem ent proteins on the bacteria was monitored by Western blotting. ii decrease in deposition on the bacteria of C4, C3, and C9 corresponded with decrease d complement sensitivity. Deposition of C1, however, was not affected by th e presence of BrkA. These studies show that BrkA inhibits the classical pat hway of complement activation and prevents accumulation of deposited C3.