Disease caused by Bordetella pertussis persists with rates increasing over the past decade in industrialized countries. A hindrance to vaccine development has been the lack of a clear serological correlate of protective immunity. Pertussis toxin (PTx), an AB-type toxin, is one of the bacteria’s major virulence factors and among the lead candidates for potential correlates. Of the numerous monoclonal antibodies (mAbs) binding PTx, the murine IgG2a mAb 1B7 is potently neutralizing in all in vitro assays and in vivo murine models of infection. 1B7 binds an epitope on the enzymatic S1-subunit of PTx with some linear elements but previous work was unable to more precisely define the epitope or determine its exact mechanism of protection. We characterize the epitope bound by 1B7 on PTx-S1 in molecular detail and define energetically important interactions between residues at the interface including six residues on PTx-S1 and six residues on 1B7. Using this information, a model of the 1B7-S1 interaction was developed, indicating a predominantly conformational epitope located on the base of S1 near S4. The location of this epitope is consistent with previous data and is shown to be conserved across several naturally occurring strain variants including PTx-S1A, B, D, and E in addition to the catalytically inactive 9K/129G variant. Using immunofluorescent microscopy, it was determined that 1B7’s unique mode of action lies in its ability to bind to the toxin and co-traffic into target cells. Upon endocytosis, 1B7 protects from PTx intoxication by redirecting its intracellular retrograde trafficking. In order to determine whether antibody responses are differently induced by infection or acellular vaccination, we analyzed sera from 30 adults with confirmed exposure to pertussis and 30 recent vaccinees. Natural infection resulted in significantly higher titers of anti-PTx-S1, 1B7-like, and 11E6-like antibodies, while overall anti-PTx titers were similar to vaccinated samples. We also observed a direct correlation between in vitro protection and the presence of 1B7-like and 11E6-like antibodies. Thus, natural infection elicits higher titers of protective antibodies indicating that the use of detoxified PTx in current acellular vaccines although highly immunogenic results in the elicitation of predominantly non-neutralizing antibodies. / text