No abstract

The role of Toll-like receptors (TLRs) in the recognition of extracellular and facultative intracellular bacteria by the innate immune system has been extensively studied, but their role in the recognition of obligate intracellular organisms remains unknown. Coxiella burnetii, the agent of Q fever, is an obligate intracellular bacterium that specifically inhabits monocytes/macrophages. We showed in this study that C. burnetii LPS is involved in the uptake of virulent organisms by macrophages but not in that of avirulent variants. The uptake of virulent organisms was dependent on TLR4 because it was reduced in macrophages from TLR4−/− mice. In addition, LPS was responsible for filamentous actin reorganization induced by virulent C. burnetii, which was prevented in TLR4−/− macrophages. In contrast, the intracellular fate of C. burnetii was not affected in TLR4−/− macrophages, suggesting that TLR4 does not control the maturation of C. burnetii phagosome and the microbicidal activity of macrophages. These results are consistent with in vivo experiments because the pattern of tissue infection and the clearance of C. burnetii were similar in wild-type and TLR4−/− mice. We also showed that the number of granulomas was decreased in the liver of infected TLR4−/− mice, and the formation of splenic granulomas was only transient. The impaired formation of granulomas was associated with decreased production of IFN-γ and TNF. Taken together, these results demonstrate that TLR4 controls early events of C. burnetii infection such as macrophage phagocytosis, granuloma formation, and cytokine production.

Q fever is a zoonosis caused by Coxiella burnetii and recently has been recognized as a potential agent of bioterrorism. In Q fever, men are symptomatic more often than women, despite equal seroprevalence. We hypothesized that sex hormones play a role in the pathogenesis of C. burnetii infection. When C57/BL6 mice were injected with C. burnetii, bacteria load and granuloma numbers were lower in females than in males. Ovarectomized mice showed increased bacteria load in the spleen and the liver, similar to that found in males. The granuloma number was also increased in ovarectomized mice and reached the levels found in males. Tissue infection and granulomatous response are largely under the control of estrogens: treatment of ovarectomized mice with 17beta-estradiol reduced both bacteria loads and granuloma numbers. These results show that sex hormones control host response to C. burnetii infection and may account for host-dependent clinical presentation of Q fever.

Q fever manifests as primary infection or acute Q fever and may become chronic in patients with underlying valvulopathy. Because Coxiella burnetii infection depends on host response, we measured tumor necrosis factor (TNF), interleukin (IL)-6, IL-12, and IL-10 in patients with different clinical presentations of acute Q fever. Compared with control subjects, patients with uncomplicated acute Q fever exhibited increased release of the 4 cytokines. Their amounts were higher in patients with hepatitis than in patients with fever or pneumonia. In patients with valvulopathy, who exhibited the highest risk of chronic evolution, the amounts of TNF and IL-10 were higher than in patients without valvulopathy. TNF production was specifically enhanced in patients who developed Q fever endocarditis. These results show that acute Q fever is associated with cytokine overproduction. Persistent TNF amounts were associated with the occurrence of endocarditis in patients with valvulopathy, and that may be a marker of chronic evolution of Q fever.

Q fever is caused by Coxiella burnetii, a bacterium that survives in monocytes/macrophages by resisting their natural microbicidal activity. Because the link between bacterial killing and phagosome maturation has yet to be demonstrated, we evaluated responses in monocytes from both immunologically naive control subjects and patients with various manifestations of Q fever. Monocytes from patients with chronic Q fever in evolution, who do not control the infection, exhibited defective phagosome maturation and impaired C. burnetii killing. Both responses were stimulated in patients recovering from Q fever. Phagosome maturation and C. burnetii killing were significantly correlated. Defective phagosome maturation and impaired C. burnetii killing were induced by adding interleukin (IL)-10 to monocytes from convalescent patients and were restored by IL-10 neutralization in chronic Q fever in evolution. We show that phagosome maturation and microbial killing are linked in Q fever and that IL-10 regulates both features of microbicidal activity.