Single Amino Acid Polymorphisms of Pertussis Toxin Subunit S2 (PtxB) Affect Protein Function

<div><p>Whooping cough due to <i>Bordetella pertussis</i> is increasing in incidence, in part due to accumulation of mutations which increase bacterial fitness in highly vaccinated populations. Polymorphisms in the pertussis toxin, <i>ptxA</i> and <i>ptxB</i> genes, and the pertactin, <i>prn</i> genes of clinical isolates of <i>Bordetella pertussis</i> collected in Cincinnati from 1989 through 2005 were examined. While the <i>ptxA</i> and <i>prn</i> genotypes were variable, all 48 strains had the <i>ptxB2</i> genotype; <i>ptxB1</i> encodes glycine at amino acid 18 of the S2 subunit of pertussis toxin, while <i>ptxB2</i> encodes serine. We investigated antigenic and functional differences of PtxB1 and PtxB2. The S2 protein was not very immunogenic. Only a few vaccinated or individuals infected with <i>B</i>. <i>pertussis</i> developed antibody responses to the S2 subunit, and these sera recognized both polymorphic forms equally well. Amino acid 18 of S2 is in a glycan binding domain, and the PtxB forms displayed differences in receptor recognition and toxicity. PtxB1 bound better to the glycoprotein, fetuin, and Jurkat T cells <i>in vitro</i>, but the two forms were equally effective at promoting CHO cell clustering. To investigate <i>in vivo</i> activity of Ptx, one μg of Ptx was administered to DDY mice and blood was collected on 4 days after injection. PtxB2 was more effective at promoting lymphocytosis in mice.</p>