Herpes simplex virus 1 serine/threonine kinase US3 hyperphosphorylates IRF3 and inhibits beta interferon production

Viral infection initiates a series of signaling cascades that lead to the transcription of interferons (IFNs), finally inducing inter-feron-stimulated genes (ISGs) to eliminate viruses. Viruses have evolved a variety of strategies to modulate host IFN-mediated immune responses. Herpes simplex virus 1 (HSV-1) US3, a Ser/Thr kinase conserved in alphaherpesviruses, was previously re-ported to counteract host innate immunity; however, the molecular mechanism is elusive. In this study, we report that US3 blocks IFN- production by hyperphosphorylating IFN regulatory factor 3 (IRF3). Ectopic expression of US3 protein signifi-cantly inhibited Sendai virus (SeV)-mediated activation of IFN- and IFN-stimulated response element (ISRE) promoters and the transcription of IFN-, ISG54, and ISG56. US3 was also shown to block SeV-induced dimerization and nuclear translocation of IRF3. The kinase activity was indispensable for its inhibitory function, as kinase-dead (KD) US3mutants K220M and D305A could not inhibit IFN- production. Furthermore, US3 interacted with and hyperphosphorylated IRF3 at Ser175 to prevent IRF3 activation. Finally, the US3 KDmutant viruses were constructed and denoted K220M or D305AHSV-1, respectively. Cells and mice infected with both mutant viruses produced remarkably larger amounts of IFN- than those infected with wild-type HSV-1. For the first time, these findings provide convincing evidence that US3 hyperphosphorylates IRF3, blocks the production of IFN-, and subverts host innate immunity. Innate immunity is a conserved, rapid defense mechanismagainst pathogen invasion. The type I interferon (IFN-I) path-way plays a crucial role in mediating the antiviral respons