Toll-like receptor 9 (TLR9), its adaptor MyD88, the downstream transcription factor interferon regulatory factor 7 (IRF7), and type I interferons (IFN-I) are all required for resistance to infection with ectromelia virus (ECTV). However, it is not known how or in which cells these effectors function to promote survival. Here, we showed that after infection with ECTV, the TLR9-MyD88-IRF7 pathway was necessary in CD11c+ cells for the expression of proinflammatory cytokines and the recruitment of inflammatory monocytes (iMos) to the draining lymph node (dLN). In the dLN, the major producers of IFN-I were infected iMos, which used the DNA sensor-adaptor STING to activate IRF7 and nuclear factor κB (NF-κB) signaling to induce the expression of IFN-α and IFN-β, respectively. Thus, in vivo, two pathways of DNA pathogen sensing act sequentially in two distinct cell types to orchestrate resistance to a viral disease.
How different pathogen-sensing mechanisms contribute to the expression of IFN-I in vivo is unclear. Sigal and colleagues report that in lymph nodes of ectromelia-virus-infected mice, CD11c+ cells use TLR9-MyD88-IRF7 to recruit inflammatory monocytes (iMos). In turn, infected iMos express IFN-α and IFN-β through STING-IRF7 and STING-NF-κB, respectively.