Engineering of genetically detoxified pertussis toxin analogs for development of a recombinant whooping cough vaccine

Pertussis toxin (PT) is an important protective antigen in vaccines against whooping cough, and a genetically detoxified PT analog is the preferred form of the immunogen. Several amino acids of the S1 subunit were identified as functionally critical residues by site-directed mutagenesis, specifically, those at positions 9, 13, 26, 35, 41, 58, and 129. Eighty-three mutated PT operons were introduced into Bordetella parapertussis, and the resultant toxin analogs were screened for expression levels, enzymatic activity, residual toxicity, and antigenicity. While more than half of the mutants were found to be poorly secreted or assembled, the rest were fully assembled and most were highly detoxified. Single mutations resulted in up to a 1,000-fold reduction in both toxic and enzymatic activities, while PT analogs with multiple mutations (Lys-9 Gly-129, Glu-58 Gly-129, and Lys-9 Glu-58 Gly-129) were 10(6)-fold detoxified. Operons coding for stable and nontoxic mutants shown to express a critical immunodominant protective epitope were returned to the chromosome of Bordetella pertussis by allelic exchange. In vivo analysis of the toxin analogs showed a dramatic reduction in histamine sensitization and lymphocytosis-promoting activities, paralleling the reduction in toxic activities. All mutants were protective in an intracerebral challenge test, and the Lys-9 Gly-129 analog was found to be significantly more immunogenic than the toxoid. PT analogs such as those described represent suitable components for the design of a recombinant whooping cough vaccine.