Alpha-herpesvirus UL55 synergizes with ICP27 to suppress type I interferon production through conserved and host-adapted mechanisms.

Herpesviruses employ sophisticated immune evasion strategies to establish lifelong infections, subverting type I interferon (IFN-I) responses critical for antiviral defense. However, their adaptive mechanisms across species remain poorly characterized. Using duck plague virus (DPV)-an avian alphaherpesvirus model-we identify a cooperative immune evasion axis wherein ICP27 orchestrates UL55-mediated immunosuppression through dual regulatory mechanisms: its RNA-binding domain (RGG) facilitates UL55 mRNA nuclear export, while its C-terminal domain (CTD) stabilizes UL55 protein via direct interaction. This partnership enables synergistic suppression of IFN-I signaling-co-expression of ICP27 and UL55 inhibits Poly(I:C)-induced immune genes (IFN-β, Mx, OASL, IL-6) more potently than either protein alone. UL55 functions as a precision-targeted IFN-I antagonist, selectively degrading RIG-I and IRF7 through proteasomal pathways-confirmed by proteasome inhibitor rescue (MG132), structural modeling (AlphaFold), and binding assays (Co-IP). Evolutionarily, UL55 homologs (DPV, Herpes simplex virus type 1 [HSV-1], Varicella zoster virus [VZV]) conserve RIG-I targeting but diverge in IRF3/IRF7 regulation-adaptations shaped by UL55 sequence divergence (38.68% identity) and host biology (e.g., waterfowl IRF3 deficiency). This work establishes ICP27-UL55 as a key regulatory axis in herpesviral immune evasion and redefines UL55 as a conserved yet adaptable immunosuppressor in Alphaherpesvirinae.IMPORTANCEThis study fundamentally advances herpesvirology by defining a novel immune evasion paradigm in duck plague virus. We reveal ICP27 as a master regulator that coordinates UL55 immunosuppression through a two-tiered mechanism: RGG domain-mediated mRNA nuclear export and CTD-dependent protein stabilization-an unreported strategy in herpesviruses. UL55 selectively degrades RIG-I and IRF7 via proteasomal pathways, enabling precise IFN-I suppression with minimal immune activation. Crucially, ICP27-UL55 synergy inhibits Poly(I:C)-induced immune genes (IFN-β, Mx, OASL, IL-6) more effectively than individual proteins. Evolutionary analyses demonstrate conserved targeting of RIG-I across alphaherpesvirus UL55 homologs (DPV, HSV-1, VZV) but host-adapted divergence in IRF3/IRF7 regulation, shaped by UL55 sequence variation (38.68% identity) and host biology (e.g., avian IRF3 deficiency). These findings provide the first evidence of effector coordination through integrated transcriptional/post-translational regulation in herpesviruses. Disrupting ICP27-UL55 interaction offers new antiviral targets, while UL55-deficient strains serve as vaccine candidates for poultry disease control.