Structural characterization and macrophage immunomodulatory activity of a polysaccharide isolated from Gracilaria lemaneiformis

•A sulfated polysaccharide named as GLP-2 was purified from Gracilaria lemaneiformis.•The composition and structural characteristics of GLP-2 were investigated.•GLP-2 promoted the secretion of ROS, NO, IL-6, and TNF-α in RAW264.7 cells.•GLP-2 upregulated mRNA expression of iNOS, IL-6 and TNF-α. Polysaccharides from Gracilaria lemaneiformis possess various beneficial bioactive functions, but the immunomodulatory activity remains incompletely known. In the present study, a new low molecular-weight sulfated polysaccharide named as GLP-2 was isolated from G. lemaneiformis. The chemical composition and structural characteristics of GLP-2 were determined by high-performance gel permeation chromatography (HPGPC), gas chromatography (GC), infrared (IR) spectrum, methylation and nuclear magnetic resonance (NMR) analyses. In addition, the immunomodulatory activity and underlying mechanism of GLP-2 were examined using murine macrophage RAW264.7 cells. The results indicated GLP-2 could significantly improve the proliferation and pinocytic capability of RAW264.7 cells, and promote the production of reactive oxygen species (ROS), nitric oxide (NO), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) by activating iNOS, IL-6 and TNF-α gene expressions. These results suggested that GLP-2 could be developed as a potent immunomodulatory agent for use in functional foods and dietary supplements.