Structural Analysis of an Inulin-Type Fructan from Ophiopogon japonicus and Its Immunomodulatory Properties

A water-soluble, low-molecular-weight polysaccharide fraction, designated OJP-2, was extracted from the roots of Ophiopogon japonicus; its structural characteristics and immunomodulatory activity in macrophages were subsequently investigated. OJP-2 exhibited a narrow apparent molecular weight distribution with an average molecular weight (Mw) of 3568 Da and was primarily composed of fructose (0.784) and glucose (0.208). Structural analysis—integrating UV spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, and 1D/2D nuclear magnetic resonance (NMR) spectroscopy—revealed that OJP-2 is an inulin-type fructan. Its structure is characterized predominantly by β-(2→1)-linked Fruf chains and terminal α-D-Glcp residues, with potential signals indicating C-6-substituted Fruf units. In vitro immunological studies demonstrated that OJP-2 promoted nitric oxide (NO) production and enhanced the secretion of IL-6, IL-1β, and TNF-α in RAW264.7 macrophages. Under LPS/IFN-γ stimulation, OJP-2 induced non-monotonic changes in the CD86/CD206 macrophage phenotype, with the most pronounced effects observed at a concentration of 50 μg/mL. Furthermore, Western blot analysis showed that, compared to the Model group, treatment with OJP-2 resulted in a downward trend in the relative levels of p-p65/p65 and p-IκBα/IκBα across the tested concentration range. Collectively, these findings indicate that OJP-2 modulates macrophage activation phenotypes and influences NF-κB-related signaling pathways. Thus, OJP-2 is a candidate fructan for further investigation of immunomodulatory activity.