Summary Circulating natural killer (NK) cells help protect the host from lympho-hematogenous acute viral diseases by rapidly entering the draining lymph node (dLN) to curb virus dissemination. Here we identified a highly choreographed mechanism underlying this process. Using footpad infection with ectromelia virus, a pathogenic DNA virus of mice, we show that TLR9/MyD88 sensing induces NKG2D ligands in virus-infected, skin-derived migratory dendritic cells (mDCs) to induce production of IFN-γ by classical NK cells and other types of Group 1 innate lymphoid cells (ILC) already in the dLN, via NKG2D. Uninfected inflammatory monocytes, also recruited to the dLN by mDCs in a TLR9/MyD88 dependent manner, respond to this IFN-γ by secreting CXCL9 for optimal CXCR3-dependent recruitment of circulating NK cells. This work unveils a TLR9/MyD88-dependent mechanism whereby in the dLN, three cells types -mDCs, Group 1 ILC (mostly NK cells), and inflammatory monocytes-coordinately recruit protective circulating NK cells to the dLN.

Cells sensing infection produce Type I interferons (IFN-I) to stimulate Interferon Stimulated Genes (ISGs) that confer resistance to viruses. During lympho-hematogenous spread of the mouse pathogen ectromelia virus (ECTV), the adaptor STING and the transcription factor IRF7 are required for IFN-I and ISG induction and resistance to ECTV. However, it is unknown which cells sense ECTV and which pathogen recognition receptor (PRR) upstream of STING is required for IFN-I and ISG induction. We found that cyclic-GMP-AMP (cGAMP) synthase (cGAS), a DNA-sensing PRR, is required in bone marrow-derived (BMD) but not in other cells for IFN-I and ISG induction and for resistance to lethal mousepox. Also, local administration of cGAMP, the product of cGAS that activates STING, rescues cGAS but not IRF7 or IFN-I receptor deficient mice from mousepox. Thus, sensing of infection by BMD cells via cGAS and IRF7 is critical for resistance to a lethal viral disease in a natural host.

SUMMARY During disseminating viral infections, a swift innate immune response (IIR) in the draining lymph node (dLN) that restricts systemic viral spread is critical for optimal resistance to disease. However, it is unclear how this IIR is orchestrated. We show that after footpad infection of mice with ectromelia virus, dendritic cells (DCs) highly expressing major histocompatibility complex class II (MHC class IIhi DCs), including CD207+ epidermal Langerhans cells (LCs), CD103+CD207+ double-positive dermal DCs (DP-DCs), and CD103−CD207− double-negative dermal DCs (DN-DCs) migrate to the dLN from the skin carrying virus. MHC class IIhi DCs, predominantly LCs and DP-DCs, are the first cells upregulating IIR cytokines in the dLN. Preventing MHC class IIhi DC migration or depletion of LCs, but not DP-DC deficiency, suppresses the IIR in the dLN and results in high viral lethality. Therefore, LCs are the architects of an early IIR in the dLN that is critical for optimal resistance to a disseminating viral infection.

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Type I interferons (IFN-I) are antiviral cytokines that signal through the ubiquitous IFN-I receptor (IFNAR). Following footpad infection with ectromelia virus (ECTV), a mouse-specific pathogen, C57BL/6 (B6) mice survive without disease, while B6 mice broadly deficient in IFNAR succumb rapidly. We now show that for survival to ECTV, only hematopoietic cells require IFNAR expression. Survival to ECTV specifically requires IFNAR in both natural killer (NK) cells and monocytes. However, intrinsic IFNAR signaling is not essential for adaptive immune cell responses or to directly protect non-hematopoietic cells such as hepatocytes, which are principal ECTV targets. Mechanistically, IFNAR-deficient NK cells have reduced cytolytic function, while lack of IFNAR in monocytes dampens IFN-I production and hastens virus dissemination. Thus, during a pathogenic viral infection, IFN-I coordinates innate immunity by stimulating monocytes in a positive feedback loop and by inducing NK cell cytolytic function.