Reagents. c-di-GMP was synthesized as previously described (Kawai et al., 2003) , poly I:C was obtained from GE Biosciences, pdA:dT (poly(dA-dT): poly(dA-dT)) was purchased from Sigma-Aldrich, ppGpp (guanosine-3,5bisdiphosphate) was obtained from TriLink Biotechnologies, pGpG (diguanosine) was purchased from IBA GmbH, GTP and cGMP were obtained from Sigma-Aldrich, purified LPS was purchased from InvivoGen, and Sendai virus was obtained from Charles River Laboratories. Theiler's virus was the gift of M. Brahic (Stanford University, Stanford, CA).