Stimulator of interferon genes (STING) provides insect antiviral immunity by promoting Dredd caspase–mediated NF-κB activation

Abstract

The antiviral cGMP-AMP (cGAMP)-imulator of terferonenes (STING) pathway is well characterized in mammalian cells. However, whether this pathway also plays a role in insect antiviral immunity is unknown. In this study, we found that cGAMP is produced in silkworm () cells infected with nucleopolyhedrovirus (NPV). In searches for STING-related sequences, we identified , a potential cGAMP sensor in We observed that overexpression effectively inhibits NPV replication in silkworm larvae, whereas dsRNA-mediated knockdown resulted in higher viral load. Cleavage and nuclear translocation of BmRelish, a NF-κB-related transcription factor, was also observed when BmSTING was overexpressed and was enhanced by cGAMP stimulation or viral infection of larvae. Moreover, we identified a caspase-8-like protein (BmCasp8L) as a BmSTING-interacting molecule and as a suppressor of BmSTING-mediated BmRelish activation. Interestingly, cGAMP stimulation decreased BmCasp8L binding to BmSTING and increased BmRelish activity. Of note, an interaction between death-related ced-3/Nedd2-like caspase (BmDredd) and BmSTING promoted BmRelish cleavage for efficient antiviral signaling and protection of insect cells from viral infection. Our findings have uncovered BmSTING as a critical mediator of antiviral immunity in the model insect and have identified several BmSTING-interacting proteins that control antiviral defenses.

2’3’-cGAMP triggers a STING and NF-κB dependent broad antiviral response in Drosophila

“…While we detected activity of 3’3’-CDNs, namely of 3’3’-cGAMP and of c-di-AMP, the strongest agonist was 2’,3’-cGAMP, suggesting that an enzyme producing this CDN exists in insects. Indeed, Wang and colleagues recently reported the inducible production of cGAMP in the cytosol of Bombyx mori cells infected with nucleopolyhedrovirus (NPV) 18 . Thus, the production of CDNs in the response to virus infection appears to be ancient, possibly inherited in early eukaryotes from prokaryotes 16, 28 .…”

“…Since most invertebrates lack the IRF family of transcription factors (with the notable exception of mollusks) as well as interferons, the lack of a C-terminal tail (CTT) in invertebrate STING suggests that this recent addition co-evolved with the onset of IRF transcription factors and IFN cytokines 16 . In contrast, the ability of STING to regulate transcription factors of the NF-κB family 17 18 5 or autophagy 19 , seems conserved throughout metazoa. Importantly, these responses are triggered in a CTT-independent manner in mammals 13–15 .…”

p47 licenses activation of the immune deficiency pathway in the tick Ixodes scapularis

“…Noncanonical activation of the IMD pathway occurs in response to certain viral infections (31)(32)(33). Three differing models implicated stimulator of IFN genes (STING) in antimicrobial responses mediated by Relish (34)(35)(36)(37). In Ixodes scapularis ticks, lipid agonists activate the IMD signaling pathway (38).…”

Emerging Mechanisms of Insulin-Mediated Antiviral Immunity in Drosophila melanogaster

“…These transcription factors regulate the induction of Type I IFN responses through the secretion of IFN-α and β (11,19,57). STING-mediated immunity has been heavily studied within mammalian systems; however, it is only recently that STING and its role in innate immunity have been identified in insects and other invertebrates (17,56,(58)(59)(60) (Figure 2B).…”

“…STING-mediated antiviral immunity also occurs through Relish and IKKβ, which regulate expression of the antiviral factor Nazo (58). In the silkmoth, Bombyx mori, STING signaling activates antiviral activity through Dredd and IMD, leading to Relish signaling and induction of antimicrobial peptides against nucleopolyhedrovirus (NPV) (59). Other studies using the Drosophila system have further evaluated STING-mediated immunity by autophagy (17).…”