The interferon regulatory factors IRF3 and IRF7 are the master transcription factors to sequentially induce the early and late phase of IFN-stimulated genes (ISG) upon TBK1-mediated phosphorylation, which limits viral replication through multiple mechanisms. In addition, the NF-?B transcription factor is a critical regulator of the pathogen-induced proinflammatory response and its activity is primarily regulated by the SCF-?TrCP [SKP1-CUL1-F-box-?TrCP (?-transducin repeat-containing protein)] E3 ligase complex-mediated ubiquitination and degradation. Various intracellular restriction factors, including the tripartite motif (TRIM) protein family, cooperate with TBK1, IRF3/7, and NF-?B transcription factors to develop effective anti-viral immune responses. From our functional screening of 75 TRIM proteins, we have identified a novel TRIM9 E3 ligase as a major innate yin-yang effector to develop balanced immune responses against viral infection. TRIM9 is primarily expressed in the brain, but its expression is rapidly induced in various other tissues upon viral infection. Our study discovered that TRIM9 interacts with TBK1 and IRF3/7, leading to the robust activation of the type I IFN pathway. On the other hand, TRIM9 binds the WD repeat-containing F-box protein ?TrCP of SCF E3 ligase complex in a phosphorylation dependent manner, resulting in the marked suppression of NF-?B pathway. Based on these results, we hypothesize that TRIM9 shapes innate immune homeostasis by integrating positive and negative regulation of type I IFN anti-viral and NF-?B proinflammatory responses, respectively: as a novel anti-viral factor, the TRIM9 E3 ligase differentially regulates type I IFN and NF-?B signaling pathways in a genetically and functionally separable manner, developing well-balanced immune responses against viral infection.
While a robust antiviral response is essential for controlling viral infection, this response must be metered and ultimately stopped when it reaches a late stage to avoid excessive production of inflammatory cytokines, which could otherwise lead to deleterious effects on the host. This suggests that halting anti-viral signaling is as important as initiating it. Thus, understanding the mechanisms that ensure the beneficial production of proinflammatory cytokines and type I IFNs are of particular importance.
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