Structural Characteristics of Polysaccharide GP2a in Gardenia jasminoides and Its Immunomodulatory Effect on Macrophages

Here, we elucidated the structural characteristics of a polysaccharide isolated from Gardenia jasminoides Ellis (labeled as GP2a) and its immunomodulatory activity. GP2a is an acidic polysaccharide with a molecular weight of 44.8 kDa, mostly comprising galacturonic acid. Methylation analysis revealed 4-GalpA (74.8%) to be the major sugar residue in GP2a. Nuclear magnetic resonance analysis indicated that its main chain comprised →4)-α-D-GalpA-6-OMe-(1→4)-α-D-GalpA-(1→ and →4)-α-D-GalpA-6-OMe-(1→2)-α-L-Rhap-(1→, with galactan and arabinans linked to the C-4 position of →2)-α-L-Rhap-(1→ residue as branched chains. Furthermore, GP2a showed no obvious toxicity to macrophages (RAW 264.7) while enhancing cell viability in a dose- and time-dependent manner. Compared with untreated cells, nitric oxide production and secretion of cytokines, such as tumor necrosis factor-α, interferon-γ, interleukin (IL)-1β, IL-6, and granulocyte macrophage colony stimulating factor, in GP2a-treated cells significantly increased after 48 h. At 300 µg/mL GP2a concentration, there was no significant difference in the cytokine levels in GP2a- and lipopolysaccharide-treated cells (the positive control). In summary, GP2a is a pectic polysaccharide with homogalacturonan and rhamnogalacturonan-I structural regions in the main chain. Based on its immunomodulatory effects in vitro, GP2a may have potential uses in functional food and medicine.