Coxiella burnetii actively blocks IL-17-induced oxidative stress in macrophages

Coxiella burnetii is a highly infectious pathogen that causes Q fever, a leading cause of culture-negative endocarditis. Coxiella first targets alveolar macrophages and forms a phagolysosome-like compartment called the Coxiella-Containing Vacuole (CCV). Successful host cell infection requires the Type 4B Secretion System (T4BSS), which translocates bacterial effector proteins across the CCV membrane into the host cytoplasm, where they manipulate numerous cell processes. Our prior transcriptional studies revealed that Coxiella T4BSS blocks IL-17 signaling in macrophages. Given that IL-17 is known to protect against pulmonary pathogens, we hypothesize that C. burnetii T4BSS downregulates intracellular IL-17 signaling to evade the host immune response and promote bacterial pathogenesis. Using a stable IL-17 promoter reporter cell line, we confirmed that Coxiella T4BSS blocks IL-17 transcription activation. Assessment of the phosphorylation state of NF-{kappa}B, MAPK, and JNK revealed that Coxiella downregulates IL-17 activation of these proteins. Using ACT1 knockdown and IL-17RA or TRAF6 knockout cells, we next determined that IL17RA-ACT1-TRAF6 pathway is essential for the IL-17 bactericidal effect in macrophages. In addition, macrophages stimulated with IL-17 generate higher levels of reactive oxygen species, which is likely connected to the bactericidal effect of IL-17. However, C. burnetii T4SS effector proteins block the IL-17-mediated oxidative stress, suggesting that Coxiella blocks IL-17 signaling to avoid direct killing by the macrophages. ImportanceBacterial pathogens are constantly evolving mechanisms to modulate the hostile host environment encountered during infection. Coxiella burnetii, the causative agent of Q fever, is a fascinating example of intracellular parasitism. Coxiella survives in a phagolysosome-like vacuole and uses the Dot/Icm type IVB secretion system (T4BSS) to deliver bacterial effector proteins into the host cell cytoplasm to manipulate several host cell functions. We recently demonstrated that Coxiella T4BSS blocks the IL-17 signaling in macrophages. Here, we found that Coxiella T4BSS inhibits IL-17 activation of the NF-{kappa}B and MAPK pathways and blocks IL-17-mediated oxidative stress. These findings reveal a novel strategy employed by intracellular bacteria to escape the immune response during initial stages of infection. Further identification of virulence factors involved in this mechanism will bring to light new therapeutic targets to prevent Q fever development into a chronic life-threatening endocarditis.