Innate Immune Sensing and Signaling of Cytosolic DNA
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In eukaryotic cells, DNA is normally confined within the nucleus and mitochondria. DNA exposed in the cytosol is a danger signal that warns the host of invading microbial pathogens and triggers innate immune responses including the production of type-I interferons (IFNs). Endogenous DNA that is inappropriately cleared can also accumulate in cytosol and drive pathological inflammation and autoimmune diseases such as systemic lupus erythematosus (SLE). It is well known that cytosolic DNA induces IFNs through the STING-TBK1-IRF3 axis. However, how DNA is sensed in the cytosol and how this sensing event leads to the activation of STING remains elusive. Using a cell-free complementation assay, we identified cyclic GMP-AMP (cGAMP), as a novel eukaryotic second messenger generated by DNA stimulated or DNA virus infected cells. cGAMP contains a unique phosphodiester linkage combination (both 2'-5' and 3'-5'), for which we referred to it as 2'3'-cGAMP. 2'3'-cGAMP bound to STING with nanomolar affinity and induced a dramatic conformational change that led to its activation. Through biochemical purification and quantitative mass spectrometry, we identified the enzyme that synthesizes cGAMP in a DNA dependent manner. This enzyme, which we named cyclic GMP-AMP synthase (cGAS), turned out to be the long sought-after cytosolic DNA sensor. Structural and functional studies revealed that cGAS is activated by DNA-induced dimerization. We further generated and characterized a cGAS knockout mouse strain, which failed to produce IFNs and other cytokines in response to DNA stimulation and was more vulnerable to lethal infection by DNA viruses. Together, these results not only elucidate the mechanism of cytosolic DNA sensing, but also uncover a novel second messenger-mediated signaling mechanism in innate immunity.