cGAS is activated by DNA-induced oligomerization and has pivotal roles in antiviral defense and immune adjuvant effects (INM6P.412)

The presence of DNA in the cytoplasm of mammalian cells is a danger signal that alerts the host to microbial infection. We have recently identified cyclic GMP-AMP synthase (cGAS) as a cytosolic DNA sensor that triggers innate immune responses. Here, we further investigate the mechanism of cGAS activation by DNA and the function of cGAS in vivo through a combination of structural, biochemical and genetic approaches. Surprisingly, in contrast to several recent structural studies, which described the formation of a 1:1 complex between cGAS and DNA through a single interaction site, our results reveal that cGAS interacts with DNA through two distinct binding sites, forming a complex composed of dimeric cGAS bound to two molecules of DNA. Functional analyses of cGAS mutants demonstrate that both the dimeric interface and the two DNA binding surfaces are critical for cGAS activation. We also show that multiple cell types from cGAS-deficient mice failed to produce type I interferons and other cytokines in response to DNA virus infection. cGas-/- mice were more susceptible to lethal infection with herpes simplex virus 1 (HSV1) than wild-type mice. In addition, cGAS product cGAMP is an adjuvant that boosts antigen-specific T cell activation and antibody production in mice. These results not only provide new insights into the mechanism of DNA sensing by cGAS, but also demonstrate that cGAS is a nonredundant and general cytosolic DNA sensor that activates type I interferon pathway.