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

Bordetella parapertussis 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 B. parapertussis outer-membrane vesicles (OMVs) that proved to be safe and protective in a murine-infection model. Based on protection recently reported for the B. parapertussis O antigen in aqueous solution, we assessed here whether the B. parapertussis O-antigen-containing lipopolysaccharide (BppLPS-O(+)) embedded in the membranes, as present in B. parapertussis-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 B. parapertussis infections. Indeed, the B. parapertussis 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 B. parapertussis whole-cell lysates, which immunocomponents recognized, among others, the O antigen and accordingly conferred protection against B. parapertussis infection, as observed in in-vivo–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 B. parapertussis. 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 B. parapertussis infection. The protective capability of the B. parapertussis 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 B. parapertussis.