# Advancements in Research on Duck Tembusu Virus Infections
## 5 Innate Immune Responses  
Numerous studies have investigated the innate immune response to DTMUV in avian and mammalian hosts both in vivo and in vitro. Female ducks and ducklings were experimentally infected with DTMUV, and it was found that the virus replicated rapidly in various organs (brain, spleen, kidney, heart, pancreas, thymus, and bursa of Fabricius). Infection led to the activation of the RLR and TLR signaling pathways and overexpression of PRRs such as RIG-I and MDA-5. This activation resulted in the upregulation of interferon-stimulated genes (ISGs) from the Mx and OAS families in the brain and spleen [123]. Injection of \(10^{4}\) TCID50 of TMUV-TC2B into 5-day-old ducklings via a vein resulted in a significant upregulation of ISGs in the immune organs, causing dynamic damage to the immune system and providing insights into pathogenicity [124]. On the first day after DTMUV infection, the spleen exhibited the highest virus titers, with upregulation of Rig-I, Mda5, and Tr\(\mathrm{\SI}\) expression, as well as the upregulation of pro-inflammatory cytokines (such as II-I, II-2, II-6, and Cxcl8) and antiviral proteins (such as Mx and Oas), with II-6 showing the most significant increase in expression across all tested tissues [125]. In ducks infected with DTMUV, there was a significant negative correlation between the levels of CD4+ and CD8+ T, B, and non-T and -B lymphocytes and the viral load in the blood and target organs (spleen). Additionally, significant neutralizing antibody responses were observed [126]. DTMUV infection upregulates suppressor of cytokine signaling 1 (SOCS1), leading to ubiquitination and proteasomal degradation of IRF7, ultimately inhibiting the production of type I IFNs and promoting viral replication [127]. [PERSON] et al. elucidated the importance of TBK1 in an experimental duck model, as TBK1 is involved in multiple type I IFN signaling pathways. Through overexpression and knockdown experiments, they proposed a crucial role for TBK1 in mediating IFN-\(\beta\) production in the antiviral innate immune response in DEF [128]. Avian IRF1 and IRF7 may regulate the expression of IFN-\(\beta\) and VIPERIN, as well as many other ISGs, to inhibit the replication of DTMUV through IFN-dependent and/or -independent mechanisms [129]. DEF cell-derived exosomal miR-148a-5p promotes DTMUV replication by negatively regulating TLR3 expression [130]. In ducks infected with DTMUV, the duck-DDX3 (duDDX3) protein modulates the innate immune response, and overexpression of duDDX3 suppresses TMUV. However, despite the potential impact of duDDX3 on DTMUV replication, the virus can inhibit the expression of duDDX3, suggesting the existence of potential mechanismsfor immune evasion [131]. A series of experiments involving infection of CEF and 293T cells also demonstrated the significance of RLR and TLR pathways in the innate immune response to DTMUV infection. Infection with DTMUV strongly increased the expression of a set of type I IFN genes and key ISGs (Mx1, OAS1, IFITM3, and OASL) through activation of the molecular adaptors MDA5 and TLR-3 involved in the RLR and TLR pathways, respectively, leading to decreased viral replication [132]. Based on Stable Isotope Labeling by Amino Acids in Cell Culture (SILAC), protein profiling analysis of duck embryo fibroblast (DEF) cells infected with DTMUV revealed an upregulation of duck interferon-induced protein 35 (duIFI35). Moreover, duIFI35 was found to inhibit the activation of the IFN-\(\beta\) promoter induced by duck retinoic acid-inducible gene I (duRIG-I), thereby disrupting the duRIG-I-mediated host antiviral response and aiding DTMUV evasion of the host innate immune response [133]. The aforementioned studies highlight that DTMUV infection can elicit a robust immune response, but the virus must first overcome the host's innate antiviral immune barriers to establish successful infection [134]. To replicate effectively within the host, flaviviruses must develop complex strategies to evade or subvert the host's innate immune response.