Known DNA and RNA sensing pathways are not required for responses to c-di-GMP MAVS (also known as IPS-1) is an essential adapter protein in the cytosolic RNA pathway leading to type I IFN induc- Data are means ± SD (n = 3). n.d., not detectable. *, P < 0.01; and **, P < 0.001 as compared with unstimulated cells (Student's t test) . and measured their responsiveness to YAC-1 target cells ex vivo. Because NK cells respond to in vivo injection of poly I:C, a prototypical IFN inducer, we expected that c-di-GMP might also activate NK cells. Indeed, NK cells obtained from B6 mice injected with c-di-GMP responded to YAC-1 cells, as measured by intracellular cytokine staining for IFN- (Fig. 6 B) . However, NK cells obtained from Irf3/7 / mice stimulated with c-di-GMP were nonresponsive to YAC-1 target cells. As a negative control, NK cells stimulated ex vivo with PBS did not produce IFN-. These observations indicate that c-di-GMP can stimulate cytokine and NK cell responses in vivo, and these responses require the IRF3/7 transcription factors.