Epitope specificity of three anti-pertussis toxin monoclonal antibodies with dissimilar effects in assays of toxin neutralizing activity

The epitope specificity of two monoclonal antibodies against the S1 subunit (A4, A12) and one MAb against the S3 subunit (B9) of pertussis toxin, all protective in the mouse aerosol model of B. pertussis infection, but with different effects in assays of toxin-neutralizing activity, was examined in competitive binding enzyme immunoassays using biotinylated anti-pertussis toxin monoclonal antibodies or biotinylated goat anti-pertussis toxin polyclonal antibody after preincubation with unlabelled antibody. Biotinylated A4 was blocked by A4, A12, and B9; A12 was blocked by A4, A12, and B9. In contrast, biotinylated B9 was blocked by B9 and A4, but not by A12. All three monoclonal antibodies successfully blocked the anti-pertussis toxin polyclonal antibody; a mixture of the three anti-pertussis toxin monoclonal antibodies was more effective than any monoclonal antibody alone P less than or equal to 0.01). These data suggest that these three anti-pertussis toxin monoclonal antibodies recognize separate, but closely linked epitopes on pertussis toxin, and that epitopes on the S1 subunit and B-oligomer may induce protective immunity.