摘要Background:The maturation of the immune system is critical during early life,as it involves the differentiation,maturation,and establishment of immune tolerance of immune cells.This process is influenced not only by genetic factors but also by en-vironmental factors,particularly the symbiotic microbiota.Bifidobacterium animalis subsp.lactis(BB-12),originally found in dairy products,is widely used in infant for-mula and dietary supplements.However,its role and mechanisms in immune develop-ment during early life remain unclear.Methods:Using GF mice as the experimental model,B.animalis subsp.lactis BB-12 was administered via gavage during early life.In the juvenile stage,changes in T-cell subsets in the spleen,thymus,and gut intraepithelial lymphocytes(IEL)were assessed using spectral flow cytometry.Additionally,targeted metabolomics analysis of trypto-phan metabolism and short-chain fatty acid pathways in colonic tissue was conducted to explore how B.animalis subsp.lactis BB-12 influences the immune system through gut microbiota metabolism.Results:BB-12 effectively modulates the gut immune microenvironment,leading to beneficial changes in T-cell subsets in key immune tissues such as the spleen,thymus,and gut IELs.Metabolomics analysis further supports these findings by showing that BB-12 intervention greatly increased the production of tryptophan derivatives and acetic acid in the colon of GF mice.Conclusion:The findings provide theoretical evidence for the role of B.animalis subsp.lactis in immune system development and support its application in dietary supple-ments,suggesting potential as a component for infant immune health and in prevent-ing immune-related diseases.