Characterizing the Potential of a Ferric Siderophore Receptor as a Putative B. pertussis Vaccine Antigen

Abstract

Pertussis (whooping cough), the disease caused by the bacterium Bordetella pertussis, has been resurging despite widespread vaccination, and this is mainly attributed to the switch to acellular pertussis vaccines (aPV) from whole cell pertussis vaccines (wPV). This correlation, as well as the emergence of strains lacking antigens in the current aPV formulations, suggests that novel Bp antigens should be explored to engineer a next-generation aPV. Looking at another aspect of B. pertussis pathogenesis that is not addressed by the current aPV formulations, siderophores are necessary for survival and proliferation to scavenge ferric iron cations (Fe3+) in the iron-deficient environment of the host body, making iron-bound (ferric) siderophore complex uptake receptors potential useful targets for the immune system. In addition, previous screening experiments for novel T cell antigens showed the immunological potential of BfrD and BfrG, two extracellular ferric siderophore receptor proteins found on multiple strains of B. pertussis. In this study, we first show that BfrD and BfrG can be overexpressed in and purified from an E. coli mutant that lacks endotoxic lipopolysaccharide (LPS) via bacterial transformation so that LPS-removal measures that significantly reduce protein amount do not have to be taken. Second, we determined that the mutant LPS truly was not immunogenic, and so removal from the final protein solution was not necessary for use in murine models of infection. Finally, this study suggests immunization and boosting with BfrD adjuvated with alum (the adjuvant in the current aPV formulations) generates robust opsonizing capability, as well as possibly inducing a Th1 immune response and reducing bacterial burden in the nasal cavity.