Fowl adenovirus serotype 4 (FAdV-4), belonging to the family Adenoviridae, genus Aviadenovirus, is a double-stranded DNA virus [1]. Pathogenic FAdV-4 is the predominant etiological agent of hepatitis-hydropericardium syndrome (HHS) [2]. Broilers, especially 3- to 6-week-old ones, are considered susceptible to FAdV-4 infection, with up to 80 % mortality [3,4]. Clinical signs presented by infected animals are lethargy, diarrhea, and prostration. Post-mortem examination exhibits typical HHS gross lesions, including the accumulation of straw-colored fluid in the pericardial sac and the discolored liver with necrotic foci [5]. HHS, sporadically outbreaking in China before 2015, becomes prevalent since then and causes great economic losses to poultry industry [6].

Innate immunity, acting as the first line of defense, is initiated by pathogen-associated molecular patterns (PAMPs) binding to pattern-recognition receptors (PRRs) and activate the production of Type I Interferon (IFN-I) and multiple IFN-stimulated genes (ISGs) to promote viral clearance [7,8]. Cyclic GMP-AMP synthase (cGAS) is generally considered to be the principal sensor of cytosolic DNA. After detecting viral DNA, cGAS synthesizes the second messenger cGAMP to activate stimulator of interferon genes (STING), which recruits TANK-binding kinase 1 (TBK1) to phosphorylate and activate IFN regulatory factor 3 (IRF3) and IRF7, eventually resulting in the production of IFN-I [9,10]. Currently, avian DNA viruses, such as FAdV-4 and Marek's disease virus (MDV), have been reported to trigger IFN-β expression via cGAS/STING/TBK1/IRF7 signal axis [11,12]. Moreover, many viruses could inhibit IFN-I production through targeting the DNA-sensing pathway to evade innate immune response [[13], [14], [15]].

Viruses have evolved multiple strategies to antagonize host innate immune response, and FAdV-4 is no exception. Studies have reported that FAdV-4 infection caused depletion of B and T cells in lymphoid organs in specific pathogen-free (SPF) chickens [16]. Afterwards, data found that FAdV-4 infection induced lymphocytes apoptosis and subsequent growth impairment of thymus and bursa, therefore inhibiting antibody responses to inactivated vaccines against Newcastle disease virus (NDV) and H9 subtype avian influenza virus (AIV-H9) [17,18]. Apart from directly causing pathological injury of immune organs, blocking IFN-I response is another effective way for FAdV-4 to evade antiviral response. Since cGAS/STING signal pathway plays critical roles in inducing IFN-β production to antagonize FAdV-4 infection, viral proteins that inhibit cGAS/STING-mediated antiviral response and the underlying molecular mechanism will be deeply discussed in this study.

The genomic dsDNA of FAdV-4, approximately 45 kb in length, encodes some well-studied proteins and others with unknown functions. As structural proteins, fiber1, fiber2, hexon, and penton not only constitute the viral capsid, but determine the viral invasion [19], pathogenicity [[20], [21], [22]], and immunogenicity [23]. For other open reading frames (ORFs), however, their encoding potential and biological function during virus infection remain to be elucidated. In this study, we found that nonstructural protein ORF1B, a product of early transcript, acted as the strongest inhibitor of cGAS/STING DNA sensing pathway. Mechanically, ORF1B targeted IRF7 to impair its nuclear translocation, leading to the blockage of IFN-β production. These findings reveal a novel strategy employed by FAdV-4 to evade host innate immunity.