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-beta) 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-beta production and promotes viral replication, while knockdown of VP23 during MDV infection enhances the IFN-beta response and suppresses viral replication. VP23 selectively inhibits IFN regulatory factor 7 (IRF7) but not nuclear factor kappa B (NF-kappa B) 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-beta 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-beta 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-kappa B 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.