Structure characterization and immune-enhancing properties of a neutral polysaccharide from Sarcopyramis nepalensis wall.

A neutral polysaccharide, SNP-1a, was extracted from Sarcopyramis nepalensis Wall via hot water extraction and subsequent chromatographic purification. Structural analysis revealed a 19.32 kDa polysaccharide composed of glucose and galactose in a molar ratio of 0.59:0.41 with 92.7% purity. SNP-1a is characterized by a predominantly linear backbone of alternating glucose and galactose residues linked via →4)-α-D-Glcp-(1→ and →4)-β-D-Galp-(1→ bonds, with low-frequency branching possibly involving a terminal glucose side chain. In cyclophosphamide-induced immunosuppressed mice, SNP-1a restored spleen and thymus indices, increased immune cell counts, and elevated IL-2, IL-6, TNF-α, and IFN-γ levels by activating Toll-like receptor 4-mediated MAPK/NF-κB pathways. 16S rRNA sequencing showed that SNP-1a modulated the gut microbiota, reducing the Firmicutes/Bacteroidetes ratio, enriching beneficial bacteria, and inhibiting pathogens. Functional prediction further indicated that SNP-1a intervention supported specific metabolic functions and restored key host-microbe signaling pathways, such as secondary bile acid biosynthesis. These findings highlight the structure-activity relationship of SNP-1a, supporting its potential as an immunomodulatory agent.