Molecular Mechanisms of Coxiella burnetii Formalin Fixed Cellular Vaccine Reactogenicity

Local and systemic reactogenic responses to Q-VAX® have prevented licensing of this vaccine outside of Australia. These reactogenic responses occur in previously sensitize individuals and have not been well defined at the cellular level, in part because many studies have been done in guinea pigs that have limited molecular tools. We previously characterized a mouse model of reactogenicity where local reactions sites showed an influx of CD8+ and IFNγ-expressing IL17a+ CD4+ T cells consistent with a Th1 delayed-type hypersensitivity. In this study we determined using depletion and adoptive transfer experiments that both anti- Coxiella antibodies and CD4+ T cells were essential for localized reactions at the site of vaccination. Furthermore, IFNγ depletion showed significant histological changes at the local reaction sites demonstrating the essential nature of this cytokine to reactogenicity. In addition to the cells and cytokines required for this response, we determined WCV material remained at the site of vaccination for at least 26 weeks post-injection. Transmission electron microscopy of these sites demonstrated intact rod-shaped bacteria at 2 weeks post-injection and partially degraded bacteria within macrophages at 26 weeks post-injection. Finally, since SCVs are an environmentally stable form, we determined that local reactions were more severe when the WCV material was prepared with higher levels of SCVs compared to typical WCV or with higher levels of LCV. These studies support the hypothesis that antigen persistence at the site of injection contributes to this reactogenicity and that anti- Coxiella antibodies, CD4+ T cells, and IFNγ each contribute to this process.