Sequential Activation of Two Pathogen-Sensing Pathways Required for Type I Interferon Expression and Resistance to an Acute DNA Virus Infection

• TLR9, MyD88, STING, and IRF7 are required for IFN-I expression in the dLN
• Inflammatory monocytes are the major producers of IFN-I in the dLN
• TLR9, MyD88, and IRF7 are required in CD11c+ cells for iMo recruitment to dLNs
• STING-IRF7 are required in iMos for IFN-α and STING-NF-κB for IFN-β expression in iMos

SummaryToll-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.