Coxiella burnetii type IV secretion-mediated activation of host cell Nrf2 signaling
Coxiella burnetii is the causative agent of human Q fever, which presents with either acute flu-like symptoms or progresses to chronic endocarditis.  Alveolar macrophages engulf C. burnetii after human inhalation, and the pathogen transitions through the phagolysosomal maturation pathway to form a parasitophorous vacuole (PV) for intracellular replication.  PV generation requires a type IV secretion system (T4SS) that serves as a critical C. burnetii modulator of host cell activity.  We previously showed C. burnetii infection activates the antioxidant Nrf2-Keap1 pathway, stabilizing Nrf2, a transcription factor that translocates to the nucleus.  Here, we further investigated the role of Nrf2 during C. burnetii infection of human macrophage-like cells and potential important downstream events that influence infection.  We show Nrf2 is activated in a T4SS-dependent manner and that inhibition of Nrf2 activity negatively impacts PV formation and C. burnetii replication.  Assessment of oxidative stress-related gene expression revealed that nox4, nqo1, sod1, and sod2 expression increase during infection.  Nrf2 inhibition also increases levels of the autophagic proteins LC3, p62, and phosphorylated p62, suggesting dysregulated autophagy is deleterious to infection.  Additionally, RNA sequencing analysis determined that Nrf2 inhibition has a significant impact on gene expression involving multiple pathways including oxidative stress, the unfolded protein response, and cell survival signaling.  Overall, this study advances understanding of Nrf2 importance during C. burnetii infection and the cellular signaling pathways controlled by Nrf2 in response to an intracellular bacterial pathogen. Overall design: we predict that C. burnetii also usurps Nrf2 activity to regulate autophagy and survival signaling.  To investigate our hypotheses, we used chemical inhibitors to reduce Nrf2 stability in human macrophages [27, 28].  We then assessed the effect of Nrf2 on two major signaling events during macrophage infection by C. burnetii: oxidative stress and autophagy.