Several bacterial pathogens, including L. monocytogenes, L. pneumophila, F. tularensis, M. tuberculosis, and group B Streptococcus, have been reported to induce type I IFNs (Coers et al., 2000; O'Riordan et al., 2002; Opitz et al., 2006; Stetson and Medzhitov, 2006a; Henry et al., 2007; Roux et al., 2007; Stanley et al., 2007; Charrel-Dennis et al., 2008) . The mechanism by which these pathogens induce type I IFN resembles that of c-di-GMP: in all cases, type I IFN induction is independent of MyD88/Trif and TLRs, requires TBK1 and IRF3, and (with one exception; unpublished data; Opitz et al., 2006) is independent of the cytosolic RNA-sensing pathway. It is widely assumed that bacterial DNA, perhaps released after bacterial cell lysis, is the relevant ligand that triggers type I IFNs in response to pathogens. There are no data that eliminate or confirm this possibility, as sensors required for IFN induction in response to cytosolic bacteria have not yet been reported. However, it is noteworthy that in all cases, induction of type I IFNs by cytosolic bacteria requires expression of an auxiliary secretion system, namely the ESX-1 system of M. tuberculosis, the Francisella pathogenicity island-encoded 2007a). Serum was collected from immunized mice 2 wk after a single intraperitoneal injection. Mice immunized with HSA alone did not mount a significant antibody response to HSA. In contrast, mice injected with HSA plus c-di-GMP produced variable but significant titers of HSA-specific IgG1 antibodies, confirming that c-di-GMP can function as an adjuvant in vivo. Importantly, our preliminary data indicated that the adjuvant effect of c-di-GMP depended on IRF3/7, as antibody responses in Irf3/7 / mice immunized with HSA and c-di-GMP were significantly (P < 0.05) reduced as compared with immunized B6 mice (Fig. 6 C) . Further studies will be required to establish the mechanism by which c-di-GMP functions as an adjuvant, but our results are consistent with a recent report indicating that the IRF3/7 kinase, TBK1, is required for adaptive immune responses in a DNA-vaccine model (Ishii et al., 2008) .