Attenuated Coxiella burnetii Phase II Causes a Febrile Response in Gamma Interferon Knockout and Toll-Like Receptor 2 Knockout Mice and Protects against Reinfection

Coxiella burnetii is a highly infectious obligate intracellular bacterium. The phase I form is responsible for Q fever, a febrile illness with flu-like symptoms that often goes undiagnosed. The attenuated C. burnetii phase II (having a truncated "O" chain of its lipopolysaccharide) does not cause disease in immunocompetent animals; however, phase II organisms remain infectious, and we questioned whether disease could be produced in immunodeficient mice. To study C. burnetii phase II infections, febrile responses in gamma interferon knockout (IFN-γ⁻/⁻), BALB/c, Toll-like receptor 2 knockout (TLR2⁻/⁻), and C57BL/6 mice were measured using the Nine Mile phase II (NMII) strain of C. burnetii. Immunocompetent mice showed minimal febrile responses, unlike those obtained with IFN-γ⁻/⁻ and TLR2⁻/⁻ mice, which showed elevated rectal temperatures that were sustained for ~15 days with transient increases in splenic weights. Reinfection of IFN-γ⁻/⁻ and TLR2⁻/⁻ mice with C. burnetii NMII 30 days after primary infection protected mice as evident by reduced febrile responses and a lack of splenic inflammation. Although minimal detection of Coxiella in TLR2⁻/⁻ mouse spleens was observed, greater colonization was evident in the IFN-γ⁻/⁻ mice. Cytokine analysis was performed on infected peritoneal macrophages isolated from these mice, and immunocompetent macrophages showed robust tumor necrosis factor alpha, IFN-γ, and granulocyte-macrophage colony-stimulating factor (GM-CSF) but no interleukin-12 (IL-12) responses. IFN-γ⁻/⁻ macrophages produced elevated levels of IL-6, IL-10, and IL-12, while TLR2⁻/⁻ macrophages produced GM-CSF, IL-12, and minimal IL-10. To distinguish immunity conferred by innate or adaptive systems, adoptive transfer studies were performed and showed that immune lymphocytes obtained from immunocompetent mice protected against a subsequent challenge with NMII, indicating that adaptive immunity mediates the observed protection. Thus, our data show that NMII is capable of eliciting disease in immunocompromised mice, which may help in evaluation of vaccine candidates as well as the study of host-pathogen interactions.