The Characterization of a New AG-II-like Glycoprotein from Cynanchum thesioides (Freyn) K. Schum and Its Immunostimulatory Activity Through Activation of TLR4/9-Mediated MAPK/NF-κB Signaling Pathways

The structural and immunomodulatory properties of arabinogalactan proteins (AGPs) from edible medicinal plants remain largely unexplored. Here, A homogenous AG-II-like arabinogalactan protein (CTSP-W2, 9862 Da) was isolated from Cynanchum thesioides via hot-water extraction, ethanol precipitation, and column chromatography. Its structure was thoroughly characterized by high-performance gel permeation chromatography (HPGPC), Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), Congo-red, scanning electron microscopy (SEM), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), methylation analysis. The mechanism of immune activity was examined using specific inhibitors, Western blotting, and molecular docking. It comprises galactose, arabinose, glucose, galacturonic acid, xylose, and 18 amino acids (asparagine-rich), with a backbone of →3,6)-Galp-(1→ and →6)-Galp-(1→. CTSP-W2 significantly enhanced macrophage proliferation, phagocytosis, and secretion of Nitric oxide (NO), Tumor necrosis factor-alpha (TNF-α), and Interleukin-6 (IL-6). Inhibitor assays showed that Toll-like receptor 4 (TLR4, TAK-242) and Toll-like receptor 9 (TLR9, E6446) antagonists markedly reduced CTSP-W2-induced TNF-α, IL-6, and NO in a dose-dependent manner, whereas Toll-like receptor 2 (TLR2) inhibition (C29) unexpectedly upregulated these mediators. Western blot revealed that CTSP-W2 upregulated TLR4 and TLR9 protein expression and increased phosphorylation of Inhibitor of nuclear factor kappa-B alpha (IκBα), nuclear factor kappa B (NF-κB p65), and p38, indicating activation of the TLR4/9–NF-κB–p38 mitogen-activated protein kinase (MAPK) signaling axis. Furthermore, Molecular docking analysis further indicated that CTSP-W2 forms extremely strong hydrogen-bonding and hydrophobic interactions with TLR4 through its galactose chains. This study elucidates the immunoregulatory mechanism of CTSP-W2 and establishes a molecular basis for arabinogalactan proteins as potential natural immunomodulators.