Nitro-fatty acids are formed in response to virus infection and are potent inhibitors of STING palmitoylation and signaling.

The adaptor molecule stimulator of IFN genes (STING) is central to production of type I IFNs in response to infection with DNA viruses and to presence of host DNA in the cytosol. Excessive release of type I IFNs through STING-dependent mechanisms has emerged as a central driver of several interferonopathies, including systemic lupus erythematosus (SLE), Aicardi-Goutières syndrome (AGS), and stimulator of IFN genes-associated vasculopathy with onset in infancy (SAVI). The involvement of STING in these diseases points to an unmet need for the development of agents that inhibit STING signaling. Here, we report that endogenously formed nitro-fatty acids can covalently modify STING by nitro-alkylation. These nitro-alkylations inhibit STING palmitoylation, STING signaling, and subsequently, the release of type I IFN in both human and murine cells. Furthermore, treatment with nitro-fatty acids was sufficient to inhibit production of type I IFN in fibroblasts derived from SAVI patients with a gain-of-function mutation in STING. In conclusion, we have identified nitro-fatty acids as endogenously formed inhibitors of STING signaling and propose for these lipids to be considered in the treatment of STING-dependent inflammatory diseases.

Development of nitroalkene-based inhibitors to target STING-dependent inflammation.

Stimulator of Interferon Genes (STING) is essential for the inflammatory response to cytosolic DNA. Despite that aberrant activation of STING is linked to an increasing number of inflammatory diseases, the development of inhibitors has been challenging, with no compounds in the pipeline beyond the preclinical stage. We previously identified endogenous nitrated fatty acids as novel reversible STING inhibitors. With the aim of improving the specificity and efficacy of these compounds, we developed and tested a library of nitroalkene-based compounds for in vitro and in vivo STING inhibition. The structure-activity relationship study revealed a robustly improved electrophilicity and reduced degrees of freedom of nitroalkenes by conjugation with an aromatic moiety. The lead compounds CP-36 and CP-45, featuring a ß-nitrostyrene moiety, potently inhibited STING activity in vitro and relieved STING-dependent inflammation in vivo. This validates the potential for nitroalkene compounds as drug candidates for STING modulation to treat STING-driven inflammatory diseases, providing new robust leads for preclinical development.

Nrf2 Negatively Regulates Type I Interferon Responses and Increases Susceptibility to Herpes Genital Infection in Mice.

Herpes simplex virus-2 (HSV-2) is a leading cause of sexually transmitted infections for which no effective vaccines or prophylactic treatment currently exist. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor involved in the detoxification of reactive oxygen species (ROS) and has been more recently shown to regulate inflammatory and antiviral responses. Here, we evaluated the importance of Nrf2 in the control of HSV-2 genital infection, and its role in the regulation of HSV-induced innate antiviral immunity. Comparison of antiviral gene expression profile by RNA-sequencing analysis of wild type and Nrf2-mutant (Nrf2AY/AY ) murine macrophages showed an upregulation at the basal level of the type I interferon-associated gene network. The same basal increased antiviral profile was also observed in the spleen of Nrf2-/- mice. Interestingly, the lack of Nrf2 in murine cells was sufficient to increase the responsiveness to HSV-derived dsDNA and protect cells from HSV-2 infection in vitro. Surprisingly, there was no indication of an alteration in STING expression in murine cells as previously reported in cells of human origin. Additionally, genetic activation of Nrf2 in Keap1-/- mouse embryonic fibroblasts increased HSV-2 infectivity and replication. Finally, using an in vivo vaginal herpes infection model, we showed that Nrf2 controlled early innate immune responses to HSV-2 without affecting STING expression levels. Nrf2-/- mice exhibited reduced viral replication that was associated with higher level of type I interferons in vaginal washes. Nrf2-/- mice also displayed reduced weight loss, lower disease scores, and higher survival rates than wild type animals. Collectively, these data identify Nrf2 as a negative regulator of the interferon-driven antiviral response to HSV-2 without impairing STING mRNA and protein expression levels in murine cells.

Nrf2 negatively regulates STING indicating a link between antiviral sensing and metabolic reprogramming.

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.

2017 Keystone Symposia at the Fairmont Banff Springs: Exploring new concepts in innate immunity and interferon signaling at the haunted castle.

At the 2017 Keystone Symposia meeting, new research was presented in the fields of innate immunity and type I interferon regulation. Gathering experts from these research communities offered a unique opportunity to discuss new concepts and formulate novel approaches to modulate pathological mechanisms in human inflammatory diseases.