Coxiella burnetii Delays Apoptosis and Causes Lytic Cell Death in Human Neutrophils In Vitro

Coxiella burnetii is an obligate intracellular Gram-negative bacterium that causes acute and chronic Q fever in humans. Neutrophils are considered the major innate cells with a primary role in controlling extracellular bacteria. Despite the obligate intracellular lifestyle of C. burnetii, our previous study demonstrated that neutrophils are crucial for host defense against C. burnetii infection in mice. Our recent study also showed that virulent C. burnetii Nine Mile phase I (NMI) and avirulent NM phase II (NMII) strains differentially modulate autophagy and apoptosis in murine neutrophils. However, the interaction between human neutrophils and Coxiella remains unexplored. In this study, we investigated how human neutrophils interact with C. burnetii. The results indicated that NMII bacteria were able to infect and reside in autophagolysosomal compartments of human neutrophils. Additionally, NMII bacteria inhibited the expression of ANNEXIN-V and the activation of caspase-3, resulting in delayed apoptosis in neutrophils. Live imaging showed that NMII bacteria actively swarm and recruit human neutrophils to form localized neutrophil clusters, which deploy neutrophil extracellular traps (NETs) to restrain bacteria. The deployment of slow-diffusing NETs in localized clusters was observed with NMII-infected human neutrophils in a dose-dependent manner. Furthermore, the observation of Coxiella colocalization with the deployed NETs demonstrates the ability of human neutrophils to immobilize and contain C. burnetii. We also found that the neutrophil's decision to undergo constitutive apoptosis or NETosis depends on the activation of the PI3K-autophagic pathway. Collectively, this study provides novel information for understanding the mechanism of human neutrophil interaction with Coxiella.