Marek's Disease Virus RLORF4 Inhibits Type I Interferon Production by Antagonizing NF-κB Activation.

Marek's disease virus (MDV), which causes T cell lymphomas in chickens, is economically important and has contributed to knowledge of herpesvirus-associated oncogenicity. The DNA-sensing pathway induces innate immune responses against DNA virus infection, and nuclear factor κB (NF-κB) signaling is critical for the establishment of innate immunity. Here, we report that RLORF4, an MDV-specific protein directly involved in viral attenuation, is an inhibitor of the DNA-sensing pathway. The results showed that ectopically expressed RLORF4 blocked beta interferon (IFN-β) promoter activation induced by cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING). RLORF4 selectively inhibited the activation of NF-κB but not IFN-regulatory factor 7. RLORF4 was found to bind the endogenous NF-κB subunits p65 and p50, and it also bound to the Rel homology domains of these subunits. Furthermore, RLORF4 suppressed the nuclear translocation of p65 and p50 mediated by tumor necrosis factor alpha and interferon-stimulatory DNA. Finally, deletion of RLORF4 from the MDV genome promoted IFN-β and interleukin-6 (IL-6) production in vitro and in vivo In the absence of RLORF4, the host cellular immunity was significantly increased, and reduced viral titers were observed during infection of chickens. Our results suggest that the RLORF4-mediated suppression of the host antiviral innate immunity might play an important role in MDV pathogenesis.IMPORTANCE Marek's disease virus (MDV) RLORF4 has been shown to be directly involved in the attenuation of MDV upon serial passages in vitro; however, the exact function of this protein during viral infection was not well characterized. This study demonstrated that RLORF4 significantly inhibits cGAS-STING-mediated NF-κB activation by binding to the Rel homology domains of the NF-κB subunits p65 and p50, interrupting their translocation to the nuclei and thereby inhibiting IFN-β production. Furthermore, RLORF4 deficiency promoted the induction of IFN-β and downstream IFN-stimulated genes during MDV infection in chickens. Our results suggest that the contribution of RLORF4 to MDV virulence may stem from its inhibition of viral DNA-triggered IFN-β responses.

Liu Y ，Gao L ，Xu Z ，Luo D ，Zhang Y ，Gao Y ，Liu C ，Zhang Y ，Qi X ，Cui H ，Li K ，Wang X ... -  《-》

Inhibition of DNA-Sensing Pathway by Marek's Disease Virus VP23 Protein through Suppression of Interferon Regulatory Factor 7 Activation.

The type I interferon (IFN) response is the first line of host innate immune defense against viral infection; however, viruses have developed multiple strategies to antagonize host IFN responses for efficient infection and replication. Here, we report that Marek's disease virus (MDV), an oncogenic herpesvirus, encodes VP23 protein as a novel immune modulator to block the beta interferon (IFN-β) activation induced by cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) in chicken fibroblasts and macrophages. VP23 overexpression markedly reduces viral DNA-triggered IFN-β production and promotes viral replication, while knockdown of VP23 during MDV infection enhances the IFN-β response and suppresses viral replication. VP23 selectively inhibits IFN regulatory factor 7 (IRF7) but not nuclear factor κB (NF-κB) activation. Furthermore, we found that VP23 interacts with IRF7 and blocks its binding to TANK-binding kinase 1 (TBK1), thereby inhibiting IRF7 phosphorylation and nuclear translocation, resulting in reduced IFN-β production. These findings expand our knowledge of DNA sensing in chickens and reveal a mechanism through which MDV antagonizes the host IFN response.IMPORTANCE Despite widespread vaccination, Marek's disease (MD) continues to pose major challenges for the poultry industry worldwide. MDV causes immunosuppression and deadly lymphomas in chickens, suggesting that this virus has developed a successful immune evasion strategy. However, little is known regarding the initiation and modulation of the host innate immune response during MDV infection. This study demonstrates that the cGAS-STING DNA-sensing pathway is critical for the induction of the IFN-β response against MDV infection in chicken fibroblasts and macrophages. An MDV protein, VP23, was found to efficiently inhibit the cGAS-STING pathway. VP23 selectively inhibits IRF7 but not NF-κB activation. VP23 interacts with IRF7 and blocks its binding to TBK1, thereby suppressing IRF7 activation and resulting in inhibition of the DNA-sensing pathway. These findings expand our knowledge of DNA sensing in chickens and reveal a mechanism through which MDV antagonizes the host IFN response.

Gao L ，Li K ，Zhang Y ，Liu Y ，Liu C ，Zhang Y ，Gao Y ，Qi X ，Cui H ，Wang Y ，Wang X ... -  《-》

Avian oncogenic herpesvirus antagonizes the cGAS-STING DNA-sensing pathway to mediate immune evasion.

Li K ，Liu Y ，Xu Z ，Zhang Y ，Luo D ，Gao Y ，Qian Y ，Bao C ，Liu C ，Zhang Y ，Qi X ，Cui H ，Wang Y ，Gao L ，Wang X ... -  《-》

Herpes Simplex Virus 1 UL24 Abrogates the DNA Sensing Signal Pathway by Inhibiting NF-κB Activation.

Cyclic GMP-AMP synthase (cGAS) is a newly identified DNA sensor that recognizes foreign DNA, including the genome of herpes simplex virus 1 (HSV-1). Upon binding of viral DNA, cGAS produces cyclic GMP-AMP, which interacts with and activates stimulator of interferon genes (STING) to trigger the transcription of antiviral genes such as type I interferons (IFNs), and the production of inflammatory cytokines. HSV-1 UL24 is widely conserved among members of the herpesviruses family and is essential for efficient viral replication. In this study, we found that ectopically expressed UL24 could inhibit cGAS-STING-mediated promoter activation of IFN-β and interleukin-6 (IL-6), and UL24 also inhibited interferon-stimulatory DNA-mediated IFN-β and IL-6 production during HSV-1 infection. Furthermore, UL24 selectively blocked nuclear factor κB (NF-κB) but not IFN-regulatory factor 3 promoter activation. Coimmunoprecipitation analysis demonstrated that UL24 bound to the endogenous NF-κB subunits p65 and p50 in HSV-1-infected cells, and UL24 was also found to bind the Rel homology domains (RHDs) of these subunits. Furthermore, UL24 reduced the tumor necrosis factor alpha (TNF-α)-mediated nuclear translocation of p65 and p50. Finally, mutational analysis revealed that the region spanning amino acids (aa) 74 to 134 of UL24 [UL24(74-134)] is responsible for inhibiting cGAS-STING-mediated NF-κB promoter activity. For the first time, UL24 was shown to play an important role in immune evasion during HSV-1 infection.IMPORTANCE NF-κB is a critical component of the innate immune response and is strongly induced downstream of most pattern recognition receptors (PRRs), leading to the production of IFN-β as well as a number of inflammatory chemokines and interleukins. To establish persistent infection, viruses have evolved various mechanisms to counteract the host NF-κB pathway. In the present study, for the first time, HSV-1 UL24 was demonstrated to inhibit the activation of NF-κB in the DNA sensing signal pathway via binding to the RHDs of the NF-κB subunits p65 and p50 and abolishing their nuclear translocation.

Xu H ，Su C ，Pearson A ，Mody CH ，Zheng C ... -  《-》

Marek's disease alphaherpesvirus (MDV) RLORF4 is not required for expression of glycoprotein C and interindividual spread.

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that causes Marek's disease (MD) in chickens. RLORF4 is a MDV-specific gene that is systematically deleted during attenuation of MDV in vitro. Concomitantly, the expression of glycoprotein C (gC) is diminished during attenuation, suggesting these two changes may be linked. Original studies in which RLORF4 was deleted utilized an infectious clone that lacked gC expression due to a frame-shift mutation within the gC open reading frame (UL44). Here, we utilized an infectious clone in which gC expression was restored to test our hypothesis that RLORF4 is important for expression of MDV gC, and subsequently, interindividual spread. Contrary to our hypothesis, gC expression was unaltered during both in vitro and in vivo replication of RLORF4-null MDV and was able to efficiently transmit from chicken to chicken, conclusively showing that RLORF4 does not regulate gC expression and is not required for horizontal transmission.

Vega-Rodriguez W ，Ponnuraj N ，Jarosinski KW 《-》