The cyclic dinucleotides, GMP-AMP (cGAMP) and c-di-AMP [bis-(39,59)-cyclic dimeric AMP], are potent type I IFN inducers via STING-TBK1-IRF3 cascade. They are promising adjuvants that promote antigen-specific humoral and cellular immune responses in different preclinical models; however, an optimal outcome of vaccination depends on a balanced immune activation. Here, we characterize the process of IFN-b induction by c-di-AMP and cGAMP in an in vitro model on the basis of primary mouse dendritic cells. Results obtained show decreased IFN-b production upon prolonged cell stimulation. We demonstrate that this effect depends on c-di-AMP/cGAMP-mediated down-regulation of stimulator of IFN gene (STING) protein levels. These results were confirmed by using human peripheral blood mononuclear cell-derived dendritic cells. Studies performed to explore the potential mechanism of STING modulation suggested proteolytic degradation to be a contributing factor to the observed decrease in cellular STING levels. Our work contributes to the elucidation of the molecular mode of action of vaccine constituents, which, in turn, is a prerequisite for the rational design of vaccines with predictable efficacy and safety profiles-Rueckert, C., Rand, U., Roy, U., Kasmapour, B., Strowig, T., Guzmán, C. A. Cyclic dinucleotides (CDNs), such as bis-(39,59)-cyclic dimeric GMP and c-di-AMP [bis-(39,59)-cyclic dimeric AMP], are known to be produced by prokaryotes and to function as second messengers in motility, proliferation, and developmental processes (1-3). Recently, the enzyme, cyclic GMP-AMP (cGAMP) synthase, which synthesizes cGAMP after binding to cytosolic double-stranded DNA in mammalian cells, was identified. The CDN cGAMP acts as a second messenger within the cytoplasm by binding and activating the stimulator of IFN genes (STING) (4-8). STING activation can play a central role in the induction of type I IFNs and inflammatory cytokine genes, which, in turn, are pivotal, for example, to establish cellular antiviral responses (9). Furthermore, type I IFNs also strongly contribute to the induction of innate immune signaling that finally results in generation of cellular and humoral immune responses against invading pathogens (10). Thus, CDNs can act as pathogen-associated molecular patterns that function as immune enhancers in the background of infections (11-16). They have also been reported by us and others to be efficient adjuvants in preclinical models (17)(18)(19)(20). The CDNs c-di-AMP and cGAMP enhance antigen-specific humoral responses, induce balanced T helper (T h )1/T h 2/T h 17-cell responses, and promote activation of cytotoxic T lymphocytes (CTLs) when formulated with an antigen and administered by systemic or mucosal routes (17,20). Such T h and CTL responses are critical to control pathogens that cannot be neutralized by Ab responses. Our previous studies have demonstrated that the T h response profile can be shaped by the selection of an appropriate CDN as adjuvant. In this regard, cGAMP is recommended as an adj...