N-terminal domain of classical swine fever virus N pro induces proteasomal degradation of specificity protein 1 with reduced HDAC1 expression to evade from innate immune responses

Abstract

ABSTRACT Classical swine fever virus (CSFV) poses a major threat to the pig industry. The mechanisms that CSFV uses to evade host innate immunity are not fully understood. Acetylation of histones and non-histone proteins is involved in modulating innate immune responses. Histone deacetylase 1 (HDAC1) could be proviral or antiviral by modulating the acetylation status of histones, viral proteins or non-histone host proteins, depending on the type of viruses involved. First, we found that CSFV infection in IPEC-J2 cells resulted in reduced expression of HDAC1. By chemical inhibition, gene silencing, and overexpression, we revealed that HDAC1 acts as a negative regulator of CSFV replication in IPEC-J2 cells probably through activation of poly(I:C) and IFN-λ3-induced IFN-I/III innate immunity. Mechanistically, CSFV N pro downregulated HDAC1 and its transcriptional regulator specificity protein 1 (Sp1). N pro interacted with Sp1 to facilitate its degradation through the ubiquitin-proteasome pathway via its N-terminal domain, a region that does not have significant effect on IRF3 stability. Thus, it is clear that CSFV deploys the two domains of its N pro to counteract the innate immune responses, the C-terminal one targeting the IRF3 pathway as previously reported, and the N-terminal one targeting the Sp1-HDAC1 axis. IMPORTANCE Of the flaviviruses, only CSFV and bovine viral diarrhea virus express N pro as the non-structural protein which is not essential for viral replication but functions to dampen host innate immunity. We have deciphered a novel mechanism with which CSFV uses to evade the host antiviral immunity by the N-terminal domain of its N pro to facilitate proteasomal degradation of Sp1 with subsequent reduction of HDAC1 and ISG15 expression. This is distinct from earlier findings involving N pro -mediated IRF3 degradation via the C-terminal domain. This study provides insights for further studies on how HDAC1 plays its role in antiviral immunity, and if and how other viral proteins, such as the core protein of CSFV, the nucleocapsid protein of porcine epidemic diarrhea virus, or even other coronaviruses, exert antiviral immune responses via the Sp1-HDAC1 axis. Such research may lead to a deeper understanding of viral immune evasion strategies as part of their pathogenetic mechanisms.