Innate immune response to Coxiella burnetii

Intracellular bacteria pose a unique set of challenges for the immune system. Many have evolved highly specialized virulence factors to evade intracellular detection and created a replicative niche. Coxiella burnetii is a zoonotic intracellular pathogen capable of causing severe acute and chronic Q fever in humans. C. burnetii grows in the lysosome and persists in cells for more than 10 days. Since C. burnetii can replicate inside cells for a prolonged period, we hypothesized that C. burnetii evolved multiple mechanisms to subvert innate immune detection. To test this hypothesis, we tested the ability of the C. burnetii Nine Mile Phase II clone 4/RSA439 strain to activate innate immune responses in restrictive C57BL/6 bone marrow-derived macrophages, specifically: detection by surface and cytosolic immune sensors; activation of the inflammasome; and production of Type I interferon. We found C. burnetii does not robustly stimulate cytosolic sensors, activate the inflammasome, nor induce robust levels of Type I interferon. These pathways do not play a significant role in mediating cell-intrinsic control of bacterial replication. We found TLR detection, specifically TLR2 and TLR4, primarily mediates innate immune detection and restriction of C. burnetii in C57BL/6 macrophages. We further implicated both TLR adaptors MYD88 and TRIF in this response. Building upon these findings, we demonstrated soluble TNF induced by TLR signaling plays a major role in the cell-intrinsic restriction of C. burnetii. We further examined TNF-dependent mechanisms of restriction: cell death, reactive nitrogen (RNS) and reactive oxygen (ROS) species, and guanylate binding proteins (GBPs). We found cell death is not induced in macrophages. Although expression of GBPs are induced upon C. burnetii infection, we ruled out five GBPs as mediators of bacterial restriction. Finally, we found RNS is induced by C. burnetii in a TNF-dependent manner, and ROS is also induced and plays a role in the cell-intrinsic control of C. burnetii replication. Together, these studies demonstrate that the C. burnetii Nine Mile phase II strain is able to evade many arms of the innate immune system in C57BL/6 macrophages and implicated a specific molecule, TNF, in mediating the cell-intrinsic control of this poorly understood bacterium