Characterization of an exopolysaccharide synthesized by Lacticaseibacillus rhamnosus B6 and its immunomodulatory activity in vitro

An exopolysaccharide, designated F1, was purified from the fermented milk by Lacticaseibacillus rhamnosus strain B6 (CGMCC No. 13310). F1, with the weight average molecular weight of 1.577 × 106 Da, is consisted of rhamnose, glucose and galactose in a molar ratio of 3.7:1.5: 1. The backbone included 1,3-linked Rha, 1,2,3-linked Rha, 1,2-linked Glc and 1,3-linked Glc residues, with the branching point located at O2 position of 1,2,3-linked Rha residue, and the branch chain composed of terminal linked galactose residue with a pyruvate substituent. F1 could significantly stimulate the phagocytic activity and TNF-α expression in RAW 264.7 macrophages in a dose-dependent manner, and the release of NO at 200 μg/mL as well. F1 at 200 μg/mL could stimulate the expression of the pro-inflammatory cytokine encoding genes including TNF-α and iNOS, but with a negligible upregulating effect on the mRNA expression of IL-10. F1 could up-regulate the expression of NF-κBp65 and skew macrophage polarization towards M1 phenotype. These results suggest F1 elicit an immunomodulatory effect through the NF-κB signaling pathway.