Exploration of a bioactive oligosaccharide fragment from Saposhnikoviae Radix polysaccharides: preparation, structural characterization, and immunomodulatory mechanism.

Exploring bioactive oligosaccharide fragments (the direct mediators of polysaccharide bioactivity in vivo) and elucidating their structures represent a central challenge in polysaccharide research. A bioactive oligosaccharide fragment (named BOF III) was isolated and purified from Saposhnikoviae Radix polysaccharide (SRP) by an immunoactivity-guided screening method. Its structure was characterized by UHPLC-Q-TOF-MS/MS and NMR, and it was identified as an α-1,6-glycosidically linked heptaglucose, structurally defined as isomaltoheptaose. In vitro, BOF III enhanced the immune response by stimulating macrophage proliferation, phagocytosis, and the secretion of cytokines and NO. In cyclophosphamide-induced immunosuppressed mice, it exerted systemic immunomodulatory effects, including restored thymus and spleen indices, improved hematological parameters. Importantly, it demonstrated intestinal barrier-protective effects by repairing mucosal morphology, increasing goblet cells, upregulating tight junction proteins (ZO-1, Occludin), and reducing systemic lipopolysaccharide (LPS) and diamine oxidase (DAO) leakage. Mechanistic investigations indicated that BOF III may exert its immunomodulatory effects involving the intestinal barrier, spleen, and lung, with concurrent effects manifested as intestinal barrier repair, activation of the TLR4/MyD88 pathway in splenic and pulmonary tissues, and regulation of immune-related gene expression. This study not only elucidates the structural basis for the immunomodulatory activity of SRP but also lays the foundation for BOF III transformation into novel immunomodulatory drugs or functional products. Furthermore, this study provides a valuable strategy for investigating bioactive constituents from other polysaccharide-rich herbal medicines.