Purified polysaccharides from Plantago asiatica L.: Preparation, characterization and immune-activating effects

Many plant-derived polysaccharides exhibit potent immunomodulatory activities. Here, two polysaccharide fractions (PLP-I: Mw = 1424.82 Da, and PLP-II: Mw = 12,726.85 Da) were obtained through water extraction and alcohol precipitation method and DEAE-52 cellulose column chromatography from Plantago asiatica L. to investigate their structure and immune-activating capacity. PLP-I was a neutral polysaccharide and PLP-II was an acid polysaccharide. Monosaccharide composition analysis shows that PLP-I contains fucose, arabinose, rhamnose, galactose, glucose, xylose, mannose, galacturonic acid, glucuronic acid, and guluronic acid with a molar ratio of 0.46:5.33:3.10:26.29:49.41:1.80:5.06:6.01:2.40:0.13. PLP-II contains fucose, arabinose, rhamnose, galactose, glucose, xylose, mannose, galacturonic acid and glucuronic acid with a molar ratio of 1.11:15.89:17.89:24.00:5.67:7.12:3.79:22.68:1.86. Methylation analysis confirmed the structural details of both fractions. FT-IR, methylation, and 1D/2D NMR analyses show that PLP-I primarily consists of glucan and galactose polymer. PLP-II was a heteropolysaccharide composed mainly of HG-type domains, RG-I pectin domains, and a few AGX-type domains. The side chains of PLP-II contain type I and II arabinogalactans (AG-I and AG-II). In vitro, PLP-I and PLP-II significantly increased maturation markers of dendritic cells (DCs), including MHC-II, CD80, and CD86. Additionally, they reduced DC endocytosis and promoted the secretion of the inflammatory cytokine IL-12 p70. In vivo, PLP, PLP-I and PLP-II increased the percentage of CD4⁺ and CD8⁺ T cells in inguinal lymph nodes, as well as the secretion of IFN-γ in serum. They also improved CD11c⁺ cell subpopulations in both the spleen and lymph nodes and significantly decreased the tumor weights of the prophylactic 4 T1 tumor-bearing mice model. Overall, PLP and its purified polysaccharides highlight their potential as immunotherapeutic agents in the development of functional foods aimed at boosting immune function and enhancing antitumor immune responses exhibit broad application potential and were shown to enhance immunomodulatory effects.