Functional Differences in Pertussis Toxins from Bordetella pertussis Clinical Isolates as Determined by in vitro and in vivo Assays

Abstract

Whooping cough caused by Bordetella pertussis is a serious disease especially for infants and young children. Detoxified pertussis toxin is a key component of vaccines used in campaigns worldwide for the prevention of the disease. A biochemical assay system, which measures pertussis toxin enzymatic and carbohydrate binding activities, has been developed to measure the residual toxin activities in vaccine matrix. We report here that B. pertussis clinical isolates show differences in pertussis toxin (PTx) activities in the in vitro biochemical assays and these in vitro activities were also positively related to the in vivo toxic activities. In addition, interesting information on the genetic and possible post-translational changes of PTx produced by the strains included in this study is tentatively discussed. Of the six strains studied in details, three low- and three high-activity strains, all the DNA sequences of the ptx gene clusters were found to be identical, except that in the high-activity strains, there was a silent mutation with a single nucleotide change at base 681 in ptxC which coded for Cys199 of PTx subunit S3. Mass spectrometric analysis of tryptic peptides from PTx produced by these strains detected two peptides in subunits S1 and S3, which were present in the low-activity strains but not in the high-activity strains. This suggests that PTx might differ in structure between these strains and this is possibly due to post-translational modification.