Host and Bacterial Factors Control Susceptibility of Drosophila melanogaster to Coxiella burnetii Infection

Abstract

Coxiella burnetii is the causative agent of Q fever, a zoonotic disease that threatens both human and animal health. Due to the paucity of experimental animal models, little is known about how host factors interface with bacterial components and affect pathogenesis. Here, we used Drosophila melanogaster, in conjunction with the biosafety level 2 (BSL2) Nine Mile phase II (NMII) clone 4 strain of C. burnetii, as a model to investigate host and bacterial components implicated in infection. We demonstrate that adult Drosophila flies are susceptible to C. burnetii NMII infection and that this bacterial strain, which activates the immune deficiency (IMD) pathway, is able to replicate and cause mortality in the animals. We show that in the absence of Eiger, the only known tumor necrosis factor (TNF) superfamily homolog in Drosophila, Coxiella-infected flies exhibit reduced mortality from infection. We also demonstrate that the Coxiella type 4 secretion system (T4SS) is critical for the formation of the Coxiella-containing vacuole and establishment of infection in Drosophila. Altogether, our data reveal that the Drosophila TNF homolog Eiger and the Coxiella T4SS are implicated in the pathogenesis of C. burnetii in flies. The Drosophila/NMII model mimics relevant aspects of the infection in mammals, such as a critical role of host TNF and the bacterial T4SS in pathogenesis. Our work also demonstrates the usefulness of this BSL2 model to investigate both host and Coxiella components implicated in infection.KEYWORDS Q fever, innate immunity, IMD, TNF, Eiger, NMII, pathogenesis, T4SS, tumor necrosis factor C oxiella burnetii is an obligate intracellular Gram-negative bacterium and the causative agent of the zoonosis Q fever (1). Acute C. burnetii infection in humans is characterized primarily by influenza-like symptoms and pneumonia. Domestic ruminants act as reservoir hosts of C. burnetii and have been implicated in several outbreaks of Q fever worldwide (1-3). Based on morbidity, low infectious dose, and the environmental stability of the organism, the U.S. Centers for Disease Control and Prevention (CDC) has designated C. burnetii a category B biological weapon agent (4). C. burnetii presents two antigenic forms: a pathogenic phase I variant and an attenuated phase II variant that has a truncated O chain in its lipopolysaccharide (5, 6). C. burnetii phase I is associated with Q fever, whereas phase II does not cause disease in immunocompetent hosts (7-9). The Nine Mile phase II (NMII) clone 4/RSA439 is an attenuated strain of C. burnetii derived from the virulent Nine Mile phase I (NMI) strain through repeated passages in embryonated eggs (5). Although attenuated in immunocompetent hosts, the NMII strain has been shown to be virulent to SCID mice (10) and to cause fever in gamma interferon knockout (IFN-␥ Ϫ/Ϫ ) and Toll-like receptor 2 knockout (TLR2 Ϫ/Ϫ ) mice (11). Because C. burnetii NMI and NMII strains present similar replication kinetics in tissue culture models, the NMII strain has been used as a safer o...

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