Cell type-specific responses to cytosolic DNA and c-di-GMP Collectively, these results suggest that there are strong similarities in the signaling pathways triggered by the cytosolic presence of c-di-GMP and other nucleic acids, such as DNA and RNA. However, c-di-GMP is not structurally similar to these nucleic acid ligands. The DNA molecules capable of provoking cytosolic responses are double stranded and >25 deoxyribonucleotides in length Stetson and Medzhitov, 2006a) . In contrast, c-di-GMP is composed of two guanosine ribonucleotides (not deoxyribonucleotides) in an unusual cyclic conformation. c-di-GMP also lacks features consistent with recognition by RIG-I or Mda5, such as 5triphosphate, polyuridine, or double strandedness . Therefore, we considered whether distinct sensors might be responsible for triggering responses to DNA/RNA and c-di-GMP, and if so, whether the responses could be distinguished in certain cell types. macrophage cell line stably transfected with an NF-B luciferase reporter. We observed activation of the NF-B luciferase reporter by transfected c-di-GMP (Fig. 3 C) . Both the gel shift and reporter assays showed that the activation of NF-B by transfected c-di-GMP was not as strong as observed with transfected pdA:dT and pI:C. The subtle difference in NF-B induction between cytosolic DNA and c-di-GMP may suggest that there are distinct signaling components upstream of NF-B in the two pathways. Nonetheless, we concluded that transfected c-di-GMP is capable of activating NF-B. The relatively low levels and late time course of NF-B induction by c-di-GMP (as compared with induction by LPS), as well as the fact that we transfected c-di-GMP into cells, may explain why NF-B induction was not previously observed in response to c-di-GMP (Karaolis et al., 2007a) .