cGAMP: A tale of two signals

DNA, when inadvertently present in the cytosol, acts as a danger signal alarming the host of infection or cellular damage and triggers a potent innate immune response characterized by the induction of type I IFNs and proinflammatory cytokines. The two major DNA sensors mediating these responses are the cyclic GMP-AMP synthase (cGAS) and the inflammasome forming receptor absent in melanoma 2 (AIM2; Wu and Chen, 2014). cGAS upon DNA binding catalyzes the production of cyclic GMP-AMP (cGAMP), which acts as an endogenous second messenger that triggers type I IFN production through the stimulator of IFN genes (STI NG) pathway (Sun et al., 2013; Wu et al., 2013). AIM2 also directly binds to cytoplasmic dsDNA and assembles an inflammasome complex with the adaptor molecule apoptosis speck-containing protein (ASC) and procaspase-1, leading to activation of caspase-1 and subsequent processing and secretion of proinflammatory cytokines IL-1β and IL-18 (Sharma and Kanneganti, 2016). Although both of these DNA-sensing pathways are important in infections, autoimmune diseases, and cancer, their functional intersection and cross-regulation are less studied (Cai et al., 2014; Man et al., 2016). In this issue, Swanson et al. report the positive cross-talk between the IFN and inflammasome pathways in response to transfected DNA and DNA virus infection where cGAMP mediates both the induction of IFNs as well as the priming and activation of an AIM2–NLRP3–ASC inflammasome complex (see figure). Cytosolic delivery of 2’3′-cGAMP is known to induce IFNβ secretion. In addition to IFNβ secretion, Swanson et al. (2017) found activation of caspase-1 and secretion of inflammasome-dependent cytokines IL-1β and IL-18 from LPSprimed murine bone marrow–derived macrophages (BMDMs) transfected with 2’3′-cGAMP or bacterial cyclic dinucleotides (CDNs). This response is not restricted to murine cells, and transfection of 2’3′-cGAMP or CDNs also induces secretion of IL-1β from primary human macrophages and dendritic cells. Using BMDMs from mice lacking different inflammasome components, the authors identified the role for both AIM2 and the canonical NLRP3 inflammasome in 2’3′-cGAMP–induced IL-1β secretion. In transfected cells, cGAMP localizes with AIM2, NLRP3, ASC, and caspase-1, suggesting the formation of inflammasome complexes containing both the AIM2 and NLRP3 sensors. Association of cGAMP with components of the inflammasome complex supports a direct role for cGAMP in facilitating inflammasome assembly and activation. Interestingly, unlike other inflammasome-activating stimuli, transfection of cGAMP did not induce pyroptosis in BMDMs (Sharma and Kanneganti, 2016). Previous studies have reported concurrent activation of AIM2 and NLRP3 inflammasomes in response to diverse stimuli (Kim et al., 2010; Kalantari et al., 2014; Karki et al., 2015). Whether cGAMP is also involved in assembling the “dual inflammasome complexes” in response to these stimuli and how the assembly and activation of these effector complexes is mediated warrants future studies. To delineate the upstream and downstream components involved in cGAMP-mediated inflammasome activation, Swanson et al. (2017) probed the role of DNA sensor cGAS and the adaptor STI NG. Transfection of dsDNA analogue dA:dT revealed the role of cGAS in enhancing DNA-mediated AIM2 inflammasome activation. Whereas cGAS is dispensable for cGAMP-mediated responses as expected because of its upstream function, STI NG is required for both IFNβ production and optimal inflammasome activation in response to cGAMP. In addition to providing the second signal to induce inflammasome activation, cGAMP also promotes up-regulation of inflammasome components through STI NG-dependent IFNβ production and subsequent IFN feedback loop. Collectively, these observations demonstrated the functional role of cGAMP in providing both the priming and activation signals for inflammasomes. Importantly, 2’3′cGAMP similarly triggers AIM2and NLRP3-dependent inflammasome activation and IL-1β secretion in the lungs after intranasal administration (Swanson et al., 2017). To further underscore the relevance of their findings, the authors explored cGAS-dependent inflammasome activation in mice infected with murine cytomegalovirus (MCMV). In this model, ablation of cGAS impaired inflammasome activation and control of virus replication independently of type I IFNs. Whereas IFN signaling itself is important in limiting MCMV replication, lack of inflammasome activation