Discovery of small molecules against porcine reproductive and respiratory syndrome virus replication by targeting NendoU activity.

Porcine reproductive and respiratory syndrome (PRRS) remains a major threat to animal health and causes substantial economic losses worldwide. The nonstructural protein 11 (NSP11) of the causative agent, PRRS virus (PRRSV), contains a highly conserved nidoviral uridylate-specific endoribonuclease (NendoU) domain essential for viral replication and immune evasion. Targeting NSP11 offers a novel approach to antiviral intervention. Through in silico virtual screening followed by a fluorescence resonance energy transfer assay , we identified A8-A2 as a promising candidate that effectively inhibits NendoU activity. Molecular docking and mutational analysis revealed that A8-A2 and its analogs target the key catalytic residues His144 and Thr217 of NSP11, located within the NendoU enzyme activity loop and pocket region, respectively. A8-A2 demonstrated dose-dependent inhibition of PRRSV replication in porcine alveolar macrophages. Notably, the NendoU is conserved across PRRSV strains and other Nidoviruses, and A8-A2 exhibited antiviral activity against both type I and type II PRRSV strains, as well as the infectious bronchitis virus, a coronavirus in the order Nidovirales. Further investigations revealed that A8-A2 impedes viral replication early in infection and reverses NSP11-mediated suppression of Poly(I:C)-induced interferon production. However, this effect occurs independently of mRNA splicing inhibition. These findings indicate that A8-A2 could act as an effective antiviral agent against infections caused by diverse PRRSV strains and may serve as a broad-spectrum agent for other Nidoviruses. IMPORTANCE: Porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses in the pig industry, and vaccination is the principal method to prevent this viral infection currently. However, vaccination often fails to provide protection against heterologous strains, highlighting the need for alternative strategies for broad protection. The nidoviral uridylate-specific endoribonuclease (NendoU) domain plays a crucial role in viral replication and evasion of host immune responses. In this study, we identified a group of new compounds with similar chemical structures that could interfere with NendoU enzyme activity. Among these compounds, A8-A2 significantly inhibited PRRSV replication in host cells with minimal cytotoxicity. Our findings provide a new direction for developing potent antiviral compounds that can offer broad protection against different PRRSV strains, thereby mitigating their impact on pig health and benefiting the husbandry industry.