Purification of soluble β-glucan with immune-enhancing activity from the cell wall of yeast

β-Glucan, one of the major cell wall components of Saccharomyces cerevisiae, has been found to enhance immune functions, especially by activating macrophages. However, a major obstacle to the clinical application of β-(1→3)-glucan is its low solubility in aqueous media. In this study, soluble β-glucan, free of mannoprotein, was prepared, and its effects on TNF-α secretion and phagocytosis by macrophages were evaluated. β-Glucan was first rendered soluble from the yeast cell wall by alkaline extraction (glucan-p1). The extract contained 2.8% of protein which was subsequently removed by successive DEAE-cellulose and ConA chromatography. β-Glucan thus prepared was completely free of mannoprotein and was soluble at neutral pH (glucan-p3). The effects of β-glucan on phagocytosis and TNF-α release activity were investigated. While glucan-p1 moderately induced TNF-α secretion at 200 μg/ml (550 pg of TNF-α/5 × 10⁵ cells), glucan-p3 markedly stimulated macrophages at 200 μg/ml (2,860 pg of TNF-α/5 × 10⁵ cells). Furthermore, glucan-p3 stimulated phagocytosis about 20% more than glucan-p1 did. In conclusion, we purified water-soluble β-glucan which was completely devoid of mannoprotein and effectively stimulated the macrophage function, enabling it to be used as an intravenous injection for sepsis.