Bacillus subtilis exopolysaccharide enhances Lactobacillus johnsonii-kynurenic acid to restore intestinal T helper 17/regulatory T cell balance via aryl hydrocarbon receptor.

Weaning-induced intestinal dysfunction remains a significant challenge, characterized by microbial dysbiosis and immune suppression. However, the precise molecular mechanisms by which specific structural features of levan-fructans influence host homeostasis through microbial metabolic pathways are not fully understood. Here, a branched levan-type exopolysaccharide from Bacillus subtilis BS21 (BS21EPS), characterized by a β-(2 → 6)-D-fructofuranosyl backbone and β-(2 → 1) linkages, reduces colonic damage and systemic inflammation. Metagenomic sequencing shows that BS21EPS supplementation selectively increases Lactobacillus johnsonii 428 in the colon, which contains specialized GH32 enzymes for levan degradation. Integrative metabolomics identifies kynurenic acid (KYNA) as the primary microbial metabolite consistently increased both in vivo and in vitro, which mechanistically serves as a potent endogenous ligand for the aryl hydrocarbon receptor (AhR). In a mouse colitis model, supplementation with L. johnsonii 428 or KYNA enhances intestinal barrier function and reduces inflammation by influencing T-cell lineage decisions, especially by promoting Treg expansion while inhibiting Th17 differentiation. Notably, AhR antagonism with CH223191 abrogates these immunomodulatory effects, confirming the crucial role of the KYNA-AhR axis in maintaining immune balance. Collectively, these findings reveal a structural-functional link between dietary levans, providing a targeted nutritional strategy for managing weaning-related dysfunction and inflammatory bowel diseases.