Nucleolin inhibits porcine circovirus type 3 replication by activating the JAK-STAT1 signaling pathway

Porcine circovirus type 3 (PCV3), a newly identified pathogen linked to multiple clinical diseases, threatens the global swine industry. Nucleolin, a multifunctional protein involved in various biological events, has been confirmed to suppresses PCV3 replication; however, the underlying molecular mechanism remains unclear. Herein, we demonstrate that nucleolin inhibits PCV3 replication by activating the type I interferon (IFN)/Janus kinase (JAK)-signal transducer and activator of transcription 1 (STAT1) pathway, a critical host antiviral defense axis. Mechanistically, nucleolin enhances STAT1/STAT2 phosphorylation, promotes their nuclear translocation, and interacts with IFN regulatory factor 9 (IRF9), thereby driving the formation of the IFN-stimulated gene factor 3 (ISGF3) complex and the subsequent production of interferon-stimulated genes (ISGs). Furthermore, PCV3 infection antagonizes JAK-STAT1 activation, an effect that is reversed by nucleolin overexpression. Notably, the JAK-STAT1 pathway agonist RO8191 not only inhibits PCV3 replication in cultured cells but also reduces PCV3 viral load and alleviates PCV3-induced pathological lesions in the lungs and spleens of mice. Collectively, these results indicate that PCV3 facilitates its own replication by suppressing this pathway, whereas nucleolin exerts its antiviral activity by counteracting this suppression, specifically by targeting the STAT1-STAT2-IRF9 signaling axis.