Anti-influenza A virus effects of fructan from Welsh onion (Allium fistulosum L.)

Abstract

A fructan that acts as an anti-influenza A virus substance was isolated from hot water extract of the green leafy part of a Welsh onion (Allium fistulosum L.). The structure of the fructan was characterised and elucidated by chemical and spectroscopic analyses. The fructan was composed of terminal (21.0%) and 2,1-linked β-D-Fruf residues (65.3%) with 1,6-linked β-D-Glcp residues (13.7%). The molecular weight of the polysaccharide and polydispersity was estimated to be 1.5×10(3) and 1.18, respectively. Although the fructan did not show anti-influenza A virus activity in vitro, it demonstrated an inhibitory effect on virus replication in vivo when it was orally administered to mice. In addition, the polysaccharide enhanced the production of neutralising antibodies against influenza A virus. Therefore, the antiviral mechanism of the polysaccharide seemed to be dependent on the host immune system, i.e., enhancement of the host immune function was achieved by the administration of the polysaccharide. From our observations, the fructan from Welsh onions is suggested to be one of the active principles which exert an anti-influenza virus effect.

Fig. 1

¹H- and ¹³C-NMR spectra of the fructan from A. fistulosum.

Fig. 2

Effects of oral administration of WH and fructan on body weight loss in mice infected with influenza A virus. BALB/c mice were treated with vehicle (closed circle), oseltamivir (open circle), WH (closed triangle; 0.5 mg/day, open triangle; 1.5 mg/day) or fructan (closed diamond; 0.5 mg/day, open diamond; 1.5 mg/day) from 3 days prior to viral infection to 7 days post-infection.

Fig. 3

Effects of WH and fructan on the virus replication in the mice infected with influenza A virus. Mice were treated as described in Fig. 2. Virus titres in the bronchoalveolar lavage fluid (BALF) (A) and lung (B) were determined at 3 days after virus infection. Data are shown as mean ± SD. Asterisks indicates statistically significant differences as compared with vehicle controls: ^∗p < 0.001.

Fig. 4

Effects of WH and fructan on the production of neutralising antibodies in BALFs and sera. Mice were treated as described in Fig. 1. Neutralising antibody titres in the BALF (A) and sera (B) were determined at 14 days after virus infection. Data are shown as mean ± SD. Asterisks indicates statistically significant differences as compared with oseltamivir-treated group: ^∗p < 0.05, ^∗∗p < 0.001.

Fig. 5

Effect of fructan on NO production in RAW 264.7 cells. No sample control (–) and lipopolysaccharide (LPS, 0.1 μg/ml) were also assayed. Data are shown as mean ± SD of triplicate cultures. Asterisks indicates statistically significant differences as compared with vehicle control: ^∗p < 0.05, ^∗∗p < 0.001.