PROJECT SUMMARY/ABSTRACT Coxiella burnetii is an obligate intracellular bacterium and the etiological agent of Q fever. During natural infection Coxiella targets alveolar macrophages, where the bacterium promotes formation of a phagolysosome-like vacuole called the Coxiella Containing Vacuole (CCV). Successful host cell infection requires the Type IVB Secretion System (T4BSS), which translocates bacterial effector proteins across the CCV membrane into the host cytoplasm, where they manipulate a variety of cell processes. We recently demonstrated that the Coxiella T4BSS downregulates expression of IL-17 target genes as well as IL-17-stimulated chemokine secretion. Further, the Coxiella T4BSS confers protection against IL-17 mediated killing by the macrophage. IL-17 is a pro- inflammatory cytokine that has a key role in the innate immune response against pulmonary pathogens. Upon IL-17 binding, the macrophage surface IL-17 receptor activates several intracellular signaling pathways through the E3-ubiquitin ligase ACT1. Unphosphorylated ACT1 ubiquitinates TRAF6, triggering transcriptional activation of IL-17 target genes, while phosphorylated ACT1 recruits TRAF2/5, binds to the 3’ mRNA of IL-17 target genes and stabilizes the mRNA for translation. The proposed experiments will test our hypothesis that Coxiella!T4BSS effector proteins downregulate intracellular IL-17 signaling pathway(s) in order to evade the host innate immune response and promote bacterial pathogenesis. Aim 1 will determine whether Coxiella targets the ACT1-TRAF6 pathway to downregulate IL-17 target gene transcription. Aim 2 will test whether Coxiella destabilizes the mRNA of IL-17 target genes through ACT1-TRAF2/5. Completion of these studies will not only reveal a specific host innate immune response used against C. burnetii, but also a novel strategy employed by pathogens to escape the immune response during the initial stages of infection.