Outer-Membrane-Vesicle-Associated O Antigen, a Crucial Component for Protecting Against Bordetella parapertussis Infection

Abstract

<i>Bordetella parapertussis</i> is a respiratory-disease pathogen producing symptomatology similar to that of pertussis but of underestimated incidence and with no specific vaccine existing. We recently designed a vaccine candidate from <i>B. parapertussis</i> outer-membrane vesicles (OMVs) that proved to be safe and protective in a murine-infection model. Based on protection recently reported for the <i>B. parapertussis</i> O antigen in aqueous solution, we assessed here whether the <i>B. parapertussis</i> O-antigen-containing lipopolysaccharide (BppLPS-O⁺) embedded in the membranes, as present in <i>B. parapertussis</i>-derived OMVs (OMVs(Bpp-LPS-O⁺)), was the component responsible for that previously observed protection by OMVs. By performing a comparative study with OMVs from a human strain with undetectable O antigen (OMVs(Bpp-LPS-O⁻)), we demonstrated that the OMVs(Bpp-LPS-O⁺), but not the OMVs(Bpp-LPS-O⁻), protected mice against sublethal <i>B. parapertussis</i> infections. Indeed, the <i>B. parapertussis</i> loads were significantly reduced in the lungs of OMVs(Bpp-LPS-O⁺) -vaccinated animals, with the CFUs recovered being decreased by 4 log units below those detected in the non-immunized animals or in the animals treated with the OMVs(Bpp-LPS-O⁻), (p < 0.001). We detected that the OMVs(Bpp-LPS-O⁺) induced IgG antibodies against <i>B. parapertussis</i> whole-cell lysates, which immunocomponents recognized, among others, the O antigen and accordingly conferred protection against <i>B. parapertussis</i> infection, as observed in <i>in-vivo</i>-passive-transfer experiments. Of interest was that the OMVs(Bpp-LPS-O⁺) -generated sera had opsonophagocytic and bactericidal capabilities that were not detected with the OMVs(Bpp-LPS-O⁻)-induced sera, suggesting that those activities were involved in the clearance of <i>B. parapertussis</i>. Though stimulation of cultured spleen cells from immunized mice with formulations containing the O antigen resulted in gamma interferon (IFN-γ) and interleukin-17 production, spleen cells from OMVs(Bpp-LPS-O⁺) -immunized mice did not significantly contribute to the observed protection against <i>B. parapertussis</i> infection. The protective capability of the <i>B. parapertussis</i> O antigen was also detected in formulations containing both the OMVs derived from B. pertussis and purified BppLPS-O⁺. This combined formulation protected mice against B. pertussis along with <i>B. parapertussis</i>.