PHGDH restricts porcine reproductive and respiratory syndrome virus replication by sustaining cellular antioxidant capacity and redox homeostasis.

Porcine reproductive and respiratory syndrome virus (PRRSV) remains a major threat to the swine industry. Although redox imbalance and oxidative stress are known to favor PRRSV infection, the host determinants that preserve redox homeostasis during PRRSV replication remain insufficiently defined. Here, we identify phosphoglycerate dehydrogenase (PHGDH), the rate-limiting enzyme of the de novo serine synthesis pathway, as a novel restriction factor for PRRSV. We found that PRRSV infection downregulated PHGDH at both mRNA and protein levels. PHGDH knockdown significantly enhanced PRRSV replication in MARC-145 cells and primary porcine alveolar macrophages, whereas ectopic PHGDH expression suppressed viral replication. Specifically, PHGDH depletion promoted PRRSV replication and release, but not attachment or internalization. Catalytically inactive PHGDH mutants retained PRRSV-restriction activity comparable to wild type, indicating a non-canonical, catalysis-independent function. Mechanistically, PHGDH depletion coordinately reduced antioxidant defense genes, oxidized cellular reducing pools, and increased intracellular ROS and mitochondrial superoxide. Importantly, antioxidant treatment with N-acetylcysteine, reduced glutathione, or Mito-TEMPO abolished the proviral effect of PHGDH knockdown, establishing oxidative stress as an essential mediator of enhanced PRRSV replication. PHGDH depletion also caused mitochondrial structural and functional abnormalities and promoted glycolytic features; however, antioxidants failed to restore mitochondrial integrity, and glycolysis inhibition did not reverse the elevated viral replication in PHGDH-depleted cells, indicating that oxidative stress-rather than glycolysis or secondary mitochondrial injury-drives the proviral phenotype. Collectively, our data uncover a PHGDH-redox axis that limits PRRSV replication and highlight redox homeostasis as a tractable host process for antiviral intervention.IMPORTANCEPorcine reproductive and respiratory syndrome virus is a leading cause of respiratory disease and reproductive failure in pigs, creating major economic losses worldwide. Effective control remains challenging, so identifying host processes that naturally limit infection can reveal new intervention opportunities. Our study shows that a host metabolic enzyme, phosphoglycerate dehydrogenase, helps cells maintain antioxidant capacity and redox balance, and this cellular "buffer" restricts porcine reproductive and respiratory syndrome virus replication. When this protection is weakened, oxidative stress rises and the virus benefits; restoring redox balance counters this effect. These findings highlight redox homeostasis as an important, druggable host pathway and suggest that strengthening the cell's own stress defenses may complement existing strategies to reduce virus burden and improve swine health.