Abstract: Highly pure polysaccharides from the dried stems of Dendrobium officinale were obtained by water extraction and alcohol precipitation combined with freeze-thawing, and they were further subjected to chemical modification by sulfation, deacetylation and carboxymethylation, respectively, with the aim of investigating the effects of these different modifications on the immunomodulatory activity of Dendrobium officinale polysaccharides in RAW264.7 macrophages. The results showed that Dendrobium officinale polysaccharides had high neutral sugar contents along with low contents of uronic acids and proteins, which were 87.37%, 3.19% and 0.51%, respectively. The neutral sugar content of Dendrobium officinale polysaccharides was significantly reduced by sulfation and carboxymethylation. The Fourier transform infrared spectroscopic (FTIR) analysis indicated that sulfation, deacetylation and carboxymethylation derivatives were successfully prepared. The molecular weight of Dendrobium officinale polysaccharides measured by high-performance gel permeation chromatography (GPGPC) was 960 kD, while it was significantly increased by sulfation and carboxymethylation. It was found that sulfation and deacetylation could enhance the immunomodulatory activity of Dendrobium officinale polysaccharides in vitro, while carboxymethylation had a significant opposite effect, indicating that grafted modification cannot necessarily improve the immunomodulatory activity of Dendrobium officinale polysaccharides.

Key words: Dendrobium officinale, polysaccharide, chemical modification, RAW264.7 macrophages, immunomodulatory activity