Structural Characterization and Immunomodulatory Activity of an Acidic Polysaccharide from Rhodomyrtus tomentosa (Aiton) Hassk. Fruits

A polysaccharide from Rhodomyrtus tomentosa (Aiton) Hassk. fruit (RTFP-2b) was isolated and purified. RTFP-2b has a molecular weight of 22.995 kDa and consists of nine monosaccharides, with arabinose (38.68%), galactose (21.86%), and galacturonic acid (14.83%) as its major components. Methylation and NMR analyses revealed dominant glycosidic linkages, including α-L-Araf-(1→, →4)-α-D-GalpA-(1→, →4)-α-D-Galp-(1→, →5)-α-L-Araf-(1→, →3)-α-L-Araf-(1→ , →2)-α-L-Rhap-(1→, and →3,4,6)-β-D-Galp-(1→. Bioactivity assays using lipopolysaccharide (LPS)-stimulated RAW264.7 cells showed that RTFP-2b exhibits dose-dependent immunomodulatory properties. When administered at lower concentrations (100–200 μg/mL), RTFP-2b enhanced phagocytosis and IL-1β production. At higher concentrations (300–400 μg/mL), it significantly suppressed nitric oxide and showed biphasic regulation of IL-1β, but unexpectedly increased IL-6 levels in LPS-stimulated RAW264.7 cells. These immunomodulatory effects of RTFP-2b at higher doses were accompanied by inhibition of NF-κB signaling. These findings indicate that RTFP-2b is a structurally distinct acidic polysaccharide with dose-dependent immunomodulatory properties, suggesting its potential application in functional foods or pharmaceuticals.