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

The antiviral cGMP-AMP (cGAMP)-stimulator of interferon genes (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 (Bombyx mori) cells infected with nucleopolyhedrovirus (NPV). In searches for STING-related sequences, we identified BmSTING, a potential cGAMP sensor in B. mori We observed that BmSTING overexpression effectively inhibits NPV replication in silkworm larvae, whereas dsRNA-mediated BmSTING 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 B. mori 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 B. mori and have identified several BmSTING-interacting proteins that control antiviral defenses.

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