TLR signalling augments macrophage bactericidal activity through mitochondrial ROS

The stimulation of a subset of surface Toll-like receptors (TLRs), transmembrane proteins of the innate immune system that recognize microbe-derived molecules, is shown to induce production of reactive oxygen for bacterial killing by the mitochondrial pathway. When the 'bacterial' TLRs (TLR1, 2 and 4) bind to a ligand they promote the recruitment of mitochondria to macrophage phagosomes and induce upregulation of mitochondrial reactive oxygen species (mROS). This work implicates mROS as important components of antibacterial responses and further establishes mitochondria as hubs for innate immune signalling. Reactive oxygen species (ROS) are essential components of the innate immune response against intracellular bacteria and it is thought that professional phagocytes generate ROS primarily via the phagosomal NADPH oxidase machinery1. However, recent studies have suggested that mitochondrial ROS (mROS) also contribute to mouse macrophage bactericidal activity, although the mechanisms linking innate immune signalling to mitochondria for mROS generation remain unclear2,3,4. Here we demonstrate that engagement of a subset of Toll-like receptors (TLR1, TLR2 and TLR4) results in the recruitment of mitochondria to macrophage phagosomes and augments mROS production. This response involves translocation of a TLR signalling adaptor, tumour necrosis factor receptor-associated factor 6 (TRAF6), to mitochondria, where it engages the protein ECSIT (evolutionarily conserved signalling intermediate in Toll pathways), which is implicated in mitochondrial respiratory chain assembly5. Interaction with TRAF6 leads to ECSIT ubiquitination and enrichment at the mitochondrial periphery, resulting in increased mitochondrial and cellular ROS generation. ECSIT- and TRAF6-depleted macrophages have decreased levels of TLR-induced ROS and are significantly impaired in their ability to kill intracellular bacteria. Additionally, reducing macrophage mROS levels by expressing catalase in mitochondria results in defective bacterial killing, confirming the role of mROS in bactericidal activity. These results reveal a novel pathway linking innate immune signalling to mitochondria, implicate mROS as an important component of antibacterial responses and further establish mitochondria as hubs for innate immune signalling.