β-Glucans inhibit intracellular growth of Mycobacterium bovis BCG but not virulent Mycobacterium tuberculosis in human macrophages

The yeast polysaccharide, β-glucan, has been shown to promote both anti-microbial and anti-tumor activities through its interaction with macrophages. Here we analyzed the effects of an insoluble whole glucan particle (WGP), a 1,3/1,6-β-glucan from Saccharomyces cerevisiae, and a soluble poly-1-6-β-d-glucopyranosyl-1-3-β-d-glucopyranose (PGG), a hydrolytic product of WGP, on the anti-microbial response of human macrophages against mycobacterial infection. Treatment of macrophages with WGP and PGG significantly decreased cell association and intracellular growth of Mycobacterium bovis BCG, but not Mycobacterium tuberculosis (M.tb) when compared to untreated controls. We characterized the influence of β-glucans on the generation of macrophage oxidative products and pro-inflammatory cytokines, two important anti-microbial defense mechanisms. WGP but not PGG treatment enhanced the oxidative response of macrophages as determined by the 2′,7′-dichlorofluorescin (DCF) assay. WGP treatment also induced macrophages to produce pro-inflammatory cytokines. The β-glucan receptor, Dectin-1, was found to be involved in the WGP-induced macrophage oxidative burst and intracellular growth inhibition of M. bovis BCG. This report indicates that although some forms of β-glucan are able to stimulate the respiratory burst and cytokine production in human macrophages, and exhibit anti-microbial properties against M. bovis BCG, the β-glucans tested here did not inhibit growth of M.tb within human macrophages.

► We tested particulate and soluble beta-glucans for anti-mycobacterial activity. ► Both forms of beta-glucan inhibited Mycobacterium bovis BCG growth in macrophages. ► Beta-glucans did not inhibit Mycobacterium tuberculosis growth in macrophages. ► Dectin-1 mediates immune response and growth inhibition by particulate beta-glucan. ► Beta-glucans appear to have limited therapeutic potential for tuberculosis.