Stimulation of turbot phagocytes by Ulva rigida C. Agardh polysaccharides

We previously reported that Ulva rigida polysaccharides enhance the respiratory burst activity of turbot kidney leucocytes. In the present study, water-soluble polysaccharides were separated into several components to determine the relationship between their structure and their effects on cell activity. U. rigida polysaccharides were found to be composed mainly of uronic acids, rhamnose and xylose with smaller amounts of other sugars. Anion exchange chromatography was used to separate the polysaccharides into acid (94%) and neutral sugars (6%). U. rigida polysaccharides induced respiratory burst activity of turbot kidney leucocytes and the effect was found to be related to the acid fraction. The acid fraction was separated into four fractions, according to the molecular size, and all had stimulatory capacity. Acid sugars contained mainly large homogeneous sulphated polysaccharides. The capacity to induce the respiratory burst was lost after desulphation of the polysaccharides, but was recovered again after non-specific resulphation. The acid polysaccharides induced also IL-1β expression in turbot peritoneal leucocytes. As above, this effect was lost after desulphation of the polysaccharides. The present results indicate that the sulphate groups are required for induction of turbot innate immune responses by U. rigida polysaccharides.