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

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 outer-membrane vesicles (OMVs) that proved to be safe and protective in a murine-infection model. Based on protection recently reported for the O antigen in aqueous solution, we assessed here whether the O-antigen-containing lipopolysaccharide (BppLPS-O) embedded in the membranes, as present in -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 infections. Indeed, the 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), ( < 0.001). We detected that the OMVs(Bpp-LPS-O) induced IgG antibodies against whole-cell lysates, which immunocomponents recognized, among others, the O antigen and accordingly conferred protection against infection, as observed in -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 . 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 infection. The protective capability of the O antigen was also detected in formulations containing both the OMVs derived from and purified BppLPS-O. This combined formulation protected mice against along with .