Feline Herpesvirus 1 US3 Blocks the Type I Interferon Signal Pathway by Targeting Interferon Regulatory Factor 3 Dimerization in a Kinase-Independent Manner

As a prevalent agent in cats, feline herpesvirus 1 (FHV-1) infection contributes to feline respiratory disease and acute and chronic conjunctivitis. FHV-1 can successfully evade the host innate immune response and persist for the lifetime of the cat. Several mechanisms of immune evasion by human herpesviruses have been elucidated, but the mechanism of immune evasion by FHV-1 remains unknown. In this study, we screened for FHV-1 open reading frames (ORFs) responsible for inhibiting the type I interferon (IFN) pathway with an IFN-beta promoter reporter and analysis of IFN-beta mRNA levels in HEK 293T cells and the Crandell-Reese feline kidney (CRFK) cell line, and we identified the Ser/Thr kinase US3 as the most powerful inhibitor. Furthermore, we found that the anti-IFN activity of US3 depended on its N terminus (amino acids 1 to 75) and was independent of its kinase activity. Mechanistically, the ectopic expression of US3 selectively inhibited IFN regulatory factor 3 (IRF3) promoter activation. Furthermore, US3 bound to the IRF association domain (IAD) of IRF3 and prevented IRF3 dimerization. Finally, US3-deleted recombinant FHV-1 and US3-repaired recombinant FHV-1 (rFHV-dUS3 and rFHV-rUS3, respectively) were constructed. Compared with wild-type FHV-1 and rFHV-rUS3, infection with rFHV-dUS3 induced large amounts of IFN-beta in vitro and in vivo. More importantly, US3 deletion significantly attenuated virulence, reduced virus shedding, and blocked the invasion of trigeminal ganglia. These results indicate that FHV-1 US3 efficiently inhibits IFN induction by using a novel immune evasion mechanism and that FHV-1 US3 is a potential regulator of neurovirulence. IMPORTANCE Despite widespread vaccination, the prevalence of FHV-1 remains high, suggesting that it can successfully evade the host innate immune response and infect cats. In this study, we screened viral proteins for inhibiting the IFN pathway and identified the Ser/Thr kinase US3 as the most powerful inhibitor. In contrast to other members of the alphaherpesviruses, FHV-1 US3 blocked the host type I IFN pathway in a kinase-independent manner and via binding to the IRF3 IAD and preventing IRF3 dimerization. More importantly, the depletion of US3 attenuated the anti-IFN activity of FHV-1 and prevented efficient viral replication in vitro and in vivo. Also, US3 deletion significantly attenuated virulence and blocked the invasion of trigeminal ganglia. We believe that these findings not only will help us to better understand the mechanism of how FHV-1 manipulates the host IFN response but also highlight the potential role of US3 in the establishment of latent infection in vivo.