Integrated Proteomics and Metabolomics Analysis Reveals Potential Pathways Underlying Onion-Mediated Regulation of Spleen Immune Function in Liangshan Black Sheep

Feed additives play a crucial role in boosting livestock immunity, but their underlying molecular mechanisms are often incompletely understood. This study used integrated proteomics and metabolomics to systematically investigate the immunomodulatory effects of dietary fermented onion (FO) on the spleen of Liangshan Black Sheep. Results showed that FO supplementation significantly improved systemic antioxidant capacity and immune function, indicated by a higher spleen index and increased serum concentrations of SOD, MCP-1, and IL-2 (<i>p</i> < 0.05). Multi-omics profiling of spleen tissues identified 169 differentially expressed proteins and 168 differential metabolites. Integrated pathway enrichment revealed calcium signaling and purine metabolism as potential core regulatory hubs for the observed immunomodulation. This molecular remodeling correlated with key molecules including protein F2R and metabolites adenosine and hypoxanthine, which may form a coordinated regulatory network. Overall, our findings suggest potential pathways linking dietary FO supplementation to enhanced splenic immune function in Liangshan Black Sheep, likely via synergistic regulation of specific signaling pathways. This work supports FO as a promising functional feed additive and provides a molecular framework for developing novel immunomodulatory strategies in livestock production.