Porcine ISG15 fused IFN-λ3 as a novel antiviral agent for treating porcine reproductive and respiratory syndrome virus infection in vivo.

IFN-λs hold promise as therapeutic candidates against mutable respiratory viruses, but their efficacy against porcine reproductive and respiratory syndrome virus (PRRSV) remains unclear. In this study, we expressed a recombinant fusion protein consisting of porcine ISG15 linked porcine IFN-λ3 (ISG15-IFN-λ3) via a rigid protein linker in Escherichia coli (E. coli). In vitro experiments demonstrated that treatment of porcine alveolar macrophage (PAM)-derived CRL-2843 cells with ISG15-IFN-λ3 induced upregulation of several Interferon-stimulated Genes (ISGs) proteins, including ISG15, ISG56, and HERC5. CRL-2843 cells pretreated with ISG15-IFN-λ3 exhibited heightened resistance to Newcastle disease virus infection, while PRRSV-permissive cells treated with ISG15-IFN-λ3 before and during PRRSV exposure showed significantly inhibited PRRSV replication as well. In animal experiments, at 21 days post-infection, ISG15-IFN-λ3-treated piglets displayed milder lung tissue pathology and significantly reduced serum PRRSV-RNA, indicating enhanced viral clearance and faster recovery. Additionally, PAMs collected from ISG15-IFN-λ3-treated piglets showed significantly reduced mRNA expression of representative cytokines, chemokines, suggesting that ISG15-IFN-λ3 treatment may mitigate pneumonia severity by reducing the levels of these inflammatory mediators. These findings indicate that recombinant ISG15-IFN-λ3 expressed in E. coli may serve as a novel, effective, and affordable agent for treating severe PRRSV infection in piglets, potentially benefiting the pork industry.