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.

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.

NK cells play an important role in antiviral resistance. The integrin α2, which dimerizes with integrin β1, distinguishes NK cells from innate lymphoid cells 1 and other leukocytes. Despite its use as an NK cell marker, little is known about the role of α2β1 in NK cell biology. In this study, we show that in mice α2β1 deficiency does not alter the balance of NK cell/ innate lymphoid cell 1 generation and slightly decreases the number of NK cells in the bone marrow and spleen without affecting NK cell maturation. NK cells deficient in α2β1 had no impairment at entering or distributing within the draining lymph node of ectromelia virus (ECTV)–infected mice or at becoming effectors but proliferated poorly in response to ECTV and did not increase in numbers following infection with mouse CMV (MCMV). Still, α2β1-deficient NK cells efficiently protected from lethal mousepox and controlled MCMV titers in the spleen. Thus, α2β1 is required for optimal NK cell proliferation but is dispensable for protection against ECTV and MCMV, two well-established models of viral infection in which NK cells are known to be important.

CMV has been proposed to play a role in cancer progression and invasiveness. However, CMV has been increasingly studied as a cancer vaccine vector, and multiple groups, including ours, have reported that the virus can drive antitumor immunity in certain models. Our previous work revealed that intratumoral injections of wild-type murine CMV (MCMV) into B16-F0 melanomas caused tumor growth delay in part by using a viral chemokine to recruit macrophages that were subsequently infected. We now show that MCMV acts as a STING agonist in the tumor. MCMV infection of tumors in STING-deficient mice resulted in normal recruitment of macrophages to the tumor, but poor recruitment of CD8 + T cells, reduced production of inflammatory cytokines and chemokines, and no delay in tumor growth. In vitro, expression of type I IFN was dependent on both STING and the type I IFNR. Moreover, type I IFN alone was sufficient to induce cytokine and chemokine production by macrophages and B16 tumor cells, suggesting that the major role for STING activation was to produce type I IFN. Critically, viral infection of wild-type macrophages alone was sufficient to restore tumor growth delay in STING-deficient animals. Overall, these data show that MCMV infection and sensing in tumor-associated macrophages through STING signaling is sufficient to promote antitumor immune responses in the B16-F0 melanoma model.