Systems biology approach unveils the cellular and molecular mechanisms of formalin-killed vaccine-induced protective immunity against Coxiella burnetii in mice

The formalin-inactivated Coxiella burnetii virulent phase I vaccine (PIV) was demonstrated to be more protective than the avirulent phase II vaccine (PIIV) against virulent C. burnetii challenge in animal models. However, the cellular and molecular mechanism underlying the difference between PIV and PIIV in their ability to induce protective immunity remains unidentified. In this study, we confirmed that regardless single or multiple immunization, PIV and PIIV derived from the axenic (ACCM-D) culture retained their different protectivities, which PIV provided robust protection but PIIV did not. In addition, PIV elicited earlier PI-specific IgM and sustained higher IgG responses than PIIV, suggesting the immunogenicity is different between PIV and PIIV. We also used bulk-RNA-seq and flow cytometry to compare the immune responses between PIV- and PIIV-vaccinated mice. Interestingly, PIV elicited prolonged neutrophil response in the spleen, which accompanied with upregulation of genes involved in neutrophil degranulation, metal sequestration, and TLR-dependent innate immune pathways. In contrast, PIIV-induced neutrophil response was transient, which did not persist until the later stage of vaccination. These findings suggest that PIV-induced protection might be partially dependent on its ability to activate neutrophil-mediated effector functions. Furthermore, depletion of neutrophils in PIV-vaccinated mice before challenging with virulent C. burnetii significantly reduced the ability of PIV to confer protection in mice. Collectively, this study provides novel evidence to support that the ability of PIV to modulate neutrophil’s function may play an important role in PIV-mediated protective immunity against C. burnetii infection.