Coxiella burnetii blocks intracellular IL-17 signaling in macrophages
Coxiella burnetii is an obligate intracellular bacterium and the etiological agent of Q fever. Successful host cell infection requires the Coxiella Type IVB Secretion System (T4BSS), which translocates bacterial effector proteins across the vacuole membrane into the host cytoplasm, where they manipulate a variety of cell processes. To identify host cell targets of Coxiella T4BSS effector proteins, we determined the transcriptome of murine alveolar macrophages infected with a Coxiella T4BSS effector mutant. We identified a set of inflammatory genes that are significantly upregulated in T4BSS mutant-infected cells compared to mock-infected cells or cells infected with wild type (WT) bacteria, suggesting Coxiella T4BSS effector proteins downregulate expression of these genes. In addition, the IL-17 signaling pathway was identified as one of the top pathways affected by the bacteria. While previous studies demonstrated that IL-17 plays a protective role against several pathogens, the role of IL-17 during Coxiella infection is unknown. We found that IL-17 kills intracellular Coxiella in a dose-dependent manner, with the T4BSS mutant exhibiting significantly more sensitivity to IL-17 than WT bacteria. In addition, quantitative PCR confirmed increased expression of IL-17 downstream signaling genes in T4BSS mutant-infected cells compared to WT or mock-infected cells, including the pro-inflammatory cytokines I11a, Il1b and Tnfa, the chemokines Cxcl2 and Ccl5, and the antimicrobial protein Lcn2. We further confirmed that the Coxiella T4BSS downregulates macrophage CXCL2/MIP-2 and CCL5/RANTES protein levels following IL-17 stimulation. Together, these data suggest that Coxiella downregulates IL-17 signaling in a T4BSS-dependent manner in order to escape the macrophage immune response.