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Holothurian triterpene glycosides (cucumariosides) are known to possess multiple biological activities. Here we show that cucumariosides from the Far Eastern edible holothurian (sea cucumber), Cucumaria japonica, and their semisynthetic derivatives possess potent immunomodulatory properties. Intraperitoneal injection of cucumariosides (0.2-20 ng per mouse) induced macrophage lysosomal activity in a dose-dependent manner (up to 250% of control). The stimulatory effect was related to the chemical structure of cucumariosides and was especially influenced by the number and position of sulfate groups in the carbohydrate moiety of the molecules. In vitro, an inhibitory rather than a stimulatory effect of cucumariosides on phagocytosis and release of tumor necrosis factor-alpha (TNF-alpha) was found. Virtually all cucumariosides inhibited latex bead phagocytosis by human peripheral blood granulocytes in a dose-dependent manner. Also, the lipopolysaccharide-induced TNF-alpha production by immune cells in human blood diluted with RPMI-1640 was decreased, without a clear relation to the structure and dose of the compounds. The data are discussed in terms of possible mechanisms underlying immunomodulatory properties of triterpene glycosides from C. japonica.