Type III IFNs (IFN-λ/IL-28/29) are cytokines with type I IFN-like antiviral activities, which remain poorly characterized. We herein show that most cell types expressed both types I and III IFNs after TLR stimulation or virus infection, whereas the ability of cells to respond to IFN-λ was restricted to a narrow subset of cells, including plasmacytoid dendritic cells and epithelial cells. To examine the role of type III IFN in antiviral defense, we generated IL-28Rα-deficient mice. These mice were indistinguishable from wild-type mice with respect to clearance of a panel of different viruses, whereas mice lacking the type I IFN receptor (IFNAR−/−) were significantly impaired. However, the strong antiviral activity evoked by treatment of mice with TLR3 or TLR9 agonists was significantly reduced in both IL-28RA−/− and IFNAR−/− mice. The type I IFN receptor system has been shown to mediate positive feedback on IFN-αβ expression, and we found that the type I IFN receptor system also mediates positive feedback on IFN-λ expression, whereas IL-28Rα signaling does not provide feedback on either type I or type III IFN expression in vivo. Finally, using bone-marrow chimeric mice we showed that TLR-activated antiviral defense requires expression of IL-28Rα only on nonhemopoietic cells. In this compartment, epithelial cells responded to IFN-λ and directly restricted virus replication. Our data suggest type III IFN to target a specific subset of cells and to contribute to the antiviral response evoked by TLRs.

Antiviral strategies to inhibit Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV2) and the pathogenic consequences of COVID-19 are urgently required. Here, we demonstrate that the NRF2 antioxidant gene expression pathway is suppressed in biopsies obtained from COVID-19 patients. Further, we uncover that NRF2 agonists 4-octyl-itaconate (4-OI) and the clinically approved dimethyl fumarate (DMF) induce a cellular antiviral program that potently inhibits replication of SARS-CoV2 across cell lines. The inhibitory effect of 4-OI and DMF extends to the replication of several other pathogenic viruses including Herpes Simplex Virus-1 and-2, Vaccinia virus, and Zika virus through a type I interferon (IFN)-independent mechanism. In addition, 4-OI and DMF limit host inflammatory responses to SARS-CoV2 infection associated with airway COVID-19 pathology. In conclusion, NRF2 agonists 4-OI and DMF induce a distinct IFN-independent antiviral program that is broadly effective in limiting virus replication and in suppressing the pro-inflammatory responses of human pathogenic viruses, including SARS-CoV2.

The innate immune system is important for control of infections, including herpesvirus infections. Intracellular DNA potently stimulates antiviral IFN responses. It is known that plasmacytoid dendritic cells sense herpesvirus DNA in endosomes via TLR9, and that non-immune tissue cells can sense herpesvirus DNA in the nucleus. However, it remains unknown how and where myeloid cells, like macrophages and conventional dendritic cells, detect infections with herpesviruses. Here we demonstrate that the HSV-1 capsid was ubiquitinated in the cytosol and degraded by the proteasome, hence releasing genomic DNA into the cytoplasm for detection by DNA sensors. In this context, the DNA sensor IFI16 is important for induction of IFN-β in human macrophages after infection with HSV-1 and CMV. Viral DNA localized to the same cytoplasmic regions as IFI16, with DNA sensing being independent of viral nuclear entry. Thus, proteasomal degradation of herpesvirus capsids releases DNA to the cytoplasm for recognition by DNA sensors.

The transcription factor Nrf2 is a critical regulator of inflammatory responses. If and how Nrf2 also affects cytosolic nucleic acid sensing is currently unknown. Here we identify Nrf2 as an important negative regulator of STING and suggest a link between metabolic reprogramming and antiviral cytosolic DNA sensing in human cells. Here, Nrf2 activation decreases STING expression and responsiveness to STING agonists while increasing susceptibility to infection with DNA viruses. Mechanistically, Nrf2 regulates STING expression by decreasing STING mRNA stability. Repression of STING by Nrf2 occurs in metabolically reprogrammed cells following TLR4/7 engagement, and is inducible by a cell-permeable derivative of the TCA-cycle-derived metabolite itaconate (4-octyl-itaconate, 4-OI). Additionally, engagement of this pathway by 4-OI or the Nrf2 inducer sulforaphane is sufficient to repress STING expression and type I IFN production in cells from patients with STING-dependent interferonopathies. We propose Nrf2 inducers as a future treatment option in STING-dependent inflammatory diseases.

SignificanceSeveral chronic inflammatory conditions have recently been shown to depend on abnormally high activity of the signaling protein stimulator of IFN genes (STING). These conditions include examples from systemic lupus erythematosus, Aicardi–Goutiéres syndrome, and STING-associated vasculopathy with onset in infancy. The involvement of STING in these diseases points to an unmet demand to identify inhibitors of STING signaling, which could form the basis of anti-STING therapeutics. With this report, we identify distinct endogenously formed lipid species as potent inhibitors of STING signaling—and propose that these lipids could have pharmaceutical potential for treatment of STING-dependent inflammatory diseases.