Modulation of natural killer cell activity by water soluble polysaccharides from the roots of Asparagus racemosus Linn

Asparagus racemosus L. (Fam. Liliaceae) is an ethno-pharmacologically acclaimed Ayurvedic medicinal plant [1]. The plant belongs to the category of \u27Rasayana\u27 i.e., plants possessing immunomodulatory and adaptogenic activity [2]. Although there have been claims on immune system potentiation by aqueous extracts of A. racemosus, there is a lack of information specifying the exact molecular mechanism and phytochemical components responsible for the immuno-stimulatory activity. In the present study, an aqueous extract of A. racemosus (ARC) was further fractionated to isolate the polysaccharide fraction (ARP). The isolated polysaccharides (ARP) were characterized by enzymatic, size exclusion, GC-FID, and thin layer chromatographic analyses. Phytochemical evaluation confirmed the presence of 26.7% of 2β1 fructo-oligosaccharides with a degree of polymerization (DP) of nearly 9-10. Cytotoxicity evaluation was carried out on P388 cell lines and the IC50 values were found to be 293.4, and 526.5µg/mL for ARC and ARP, respectively. These high IC50 values are consistent with low cytotoxicity of the extracts. In-vitro natural killer (NK) cell activity was evaluated using human peripheral blood mononuclear cells (PBMC) isolated from whole blood on a ficoll-hypaque density gradient [3]. K562 tumor cells, a myeloid leukemia cell line, were used as target cells. ARC, tested over the range 0.2 to 50µg/mL, showed a dose-related stimulation of NK cell activity with a peak increase of 16.9% at 5.6µg/mL. However, ARP demonstrated a higher stimulatory activity of 51.8% at 25µg/mL. The results clearly indicate that the polysaccharides of A. racemosus have the ability to potentiate NK cell activity, and the modulation of natural killer cell activity could be an important mechanism underpinning the \u27Rasayana\u27 properties of this plant