Simple SummaryA previous study by our research group demonstrated that casein phosphopeptide-selenium (CPP-Se) chelate enhanced the immune system of dogs. In this study, after feeding CPP-Se to dogs for 30 days, the leukocytes and cytokines were quantified together with the analysis of blood gene expression and serum metabolites by RNA sequencing (RNA-Seq) and metabolomics, respectively. Our findings indicate that the differentially expressed genes (DEGs) and differentially expressed metabolites (DEMs) were notably enriched in immunomodulatory and amino acid metabolic pathways, respectively. These results showed that CPP-Se can enhance immunity by regulating those genes and metabolites which are involved in immune-related pathways and also provided a theoretical basis for the future use of CPP-Se in pet foods to enhance immunity.AbstractCasein phosphopeptide-selenium chelate (CPP-Se) is an organic compound produced by the chelation of casein phosphopeptide with selenium. This compound showed the ability to modulate canine immune response in our previous study; but its effect on the peripheral blood transcriptome and serum metabolome was unknown. This study aims to reveal the potential mechanism behind the immunomodulatory function of CPP-Se. We have identified 341 differentially expressed genes (DEGs) in CPP-Se groups as compared to the control group which comprised 110 up-regulated and 231 down-regulated genes. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis found that DEGs were mainly involved in immune-related signaling pathways. Moreover, the immune-related DEGs and hub genes were identified. Similarly, metabolomics identified 53 differentially expressed metabolites (DEMs) in the CPP-Se group, of which 17 were up-regulated and 36 were down-regulated. The pathways mainly enriched by DEMs were primary bile acid biosynthesis, tryptophan metabolism, and other amino acids metabolic pathways. Combined analysis of transcriptomic and metabolomic data showed that the DEGs and DEMs were commonly enriched in fatty acid biosynthesis, pyrimidine metabolism, glutathione metabolism, and glycerolipid metabolic pathways. Taken together, our findings provided a theoretical basis for further understanding of the immunomodulatory function of CPP-Se as well as a scientific reference for the future use of CPP-Se in pet foods as a dietary supplement to modulate the immunity.