Robust transcription of the IFN- gene requires the coordinate activation of several transcription factors, including IRF3 and NF-B (Panne, 2008) . We therefore expected that the c-di-GMP-induced signaling pathway would also require these factors to induce type I IFN. IRF3 activation requires phosphorylation by the TBK1 kinase, which leads to IRF3 dimerization and nuclear translocation (Fitzgerald et al., 2003; Sharma et al., 2003; Hemmi et al., 2004; McWhirter et al., 2004; Perry et al., 2004) . To address whether TBK1 is required for activation of type I IFN by c-di-GMP, we tested including monocytes, macrophages, and granulocytes. c-di-GMP induced dendritic cell maturation and led to the production of various cytokines and chemokines, including TNF, IL-1, IP-10, Rantes, and CXCR4 (Karaolis et al., 2007a) . However, c-di-GMP did not stimulate induction of type I IFNs by plasmacytoid dendritic cells. Impressively, preinjection of 2.5 mg/kg c-di-GMP protected against subsequent lethal challenge with Klebsiella pneumoniae or Staphlyococcus aureus (Karaolis et al., 2005b; Karaolis et al., 2007b) . Despite the impressive in vivo biological effects of c-di-GMP, the mechanism by which c-di-GMP stimulates host immunity remains unknown.