TRAF3 activates STING-mediated suppression of EV-A71 and target of viral evasion

Innate immunity represents one of the main host responses to viral infection.1,2,3 STING(Stimulator of interferon genes),a crucial immune adapter functioning in host cells,mediates cGAS(Cyclic GMP-AMP Synthase)sensing of exogenous and endogenous DNA fragments and generates innate immune responses.4 Whether STING activation was involved in infection and replication of enterovirus remains largely unknown.In the present study,we discovered that human enterovirus A71(EV-A71)infection triggered STING activation in a cGAS dependent manner.EV-A71 infection caused mitochondrial damage and the discharge of mitochondrial DNA into the cytosol of infected cells.However,during EV-A71 infection,cGAS-STING activation was attenuated.EV-A71 ^(pro)teins were screened and the viral ^(pro)tease 2A^(pro) had the greatest capacity to inhibit cGAS-STING activation.We identified TRAF3 as an important factor during STING activation and as a target of 2A^(pro).Supplement of TRAF3 rescued cGAS-STING activation suppression by 2A^(pro).TRAF3 supported STING activation mediated TBK1 phosphorylation.Moreover,we found that 2A^(pro) ^(pro)tease activity was essential for inhibiting STING activation.Furthermore,EV-D68 and CV-A16 infection also triggered STING activation.The viral ^(pro)tease 2A^(pro) from EV-D68 and CV-A16 also had the ability to inhibit STING activation.As STING activation prior to EV-A71 infection generated cellular resistance to EV-A71 replication,blocking EV-A71-mediated STING suppression represents a new anti-viral target.

Ectopic Expression of TRIM25 Restores RIG-I Expression and IFN Production Reduced by Multiple Enteroviruses 3Cpro

Enteroviruses(EVs) 3C proteins suppress type I interferon(IFN) responses mediated by retinoid acid-inducible gene I(RIG-I), while an E3 ubiquitin ligase, tripartite motif protein 25(TRIM25)-mediated RIG-I ubiquitination is essential for RIG-I antiviral activity. Therefore, whether the effect of EVs 3C on RIG-I is associated with TRIM25 expression is worth to be further investigated. Here, we demonstrate that 3C proteins of EV71 and coxsackievirus B3(CVB3) reduced not only RIG-I expression but also TRIM25 expression through protease cleavage activity, while overexpression of TRIM25 restored RIG-I expression and IFN-b production reduced by 3C proteins. Further investigation confirmed that the two amino acids and functional domains in TRIM25 required for RIG-I ubiquitination and TRIM25 structural conformation were essential for the recovery of RIG-I expression. Moreover, we also observed that TRIM25 could rescue RIG-I expression reduced by 3C proteins of CVA6 and EV-D68 but not CVA16. Our findings provide an insightful interpretation of 3C-mediated host innate immune suppression and support TRIM25 as an attractive target against multiple EVs infection.

Unique and complementary suppression of cGAS-STING and RNA sensing- triggered innate immune responses by SARS-CoV-2 proteins

The emergence of SARS-CoV-2 has resulted in the COVID-19 pandemic,leading to millions of infections and hundreds of thousands of human deaths.The efficient replication and population spread of SARS-CoV-2 indicates an effective evasion of human innate immune responses,although the viral proteins responsible for this immune evasion are not clear.In this study,we identified SARS-CoV-2 structural proteins,accessory proteins,and the main viral protease as potent inhibitors of host innate immune responses of distinct pathways.In particular,the main viral protease was a potent inhibitor of both the RLR and cGAS-STING pathways.Viral accessory protein 0RF3a had the unique ability to inhibit STING,but not the RLR response.On the other hand,structural protein N was a unique RLR inhibitor.0RF3a bound STING in a unique fashion and blocked the nuclear accumulation of p65 to inhibit nuclear factor-KB signaling.3CL of SARS-CoV-2 inhibited K63-ubiquitin modification of STING to disrupt the assembly of the STING functional complex and downstream signaling.Diverse vertebrate STINGs,including those from humans,mice,and chickens,could be inhibited by 0RF3a and 3CL of SARS-CoV-2.The existence of more effective innate immune suppressors in pathogenic coronaviruses may allow them to replicate more efficiently in vivo.Since evasion of host innate immune responses is essential for the survival of all viruses,our study provides insights into the design of therapeutic agents against SARS-CoV-2.