Phagocytes engulf pathogenic microbes, kill them and degrade their cellular 
macromolecules by hydrolytic enzymes in phagolysosomes. However, such enzymes 
are unable to degrade some microbial polysaccharides, and fate of such 
indigestible polysaccharides in phagocytes remains uncertain. Using the 
extracellular domain of Dectin-1 as β-glucan-specific probes, we succeeded in 
detection of soluble and Dectin-1-reactive β-glucan discharged from mouse RAW 
264.7 and human THP-1 macrophage cell lines as well as mouse peritoneal 
macrophages, which had phagocytized insoluble β-glucan particles. The RAW 264.7 
cell culture-supernatant containing the discharged β-glucan stimulated naïve RAW 
264.7 cells, resulting in the induction of cytokine expression. Such discharge 
of Dectin-1-reactive β-glucan from macrophage cells was inhibited by either 
NADPH oxidase inhibitors (apocynin and diphenylene iodonium) or radical 
scavengers (N-acetyl cysteine and MCI-186). Moreover, reactive oxygen species 
(ROS) produced by a Cu(2+)/ascorbic acid system solubilized insoluble β-glucan 
particles in vitro, and a part of the solubilized β-glucan was Dectin-1 reactive 
and biologically active in macrophage activation. The soluble and biologically 
active β-glucan was degraded further during prolonged exposure to ROS. These 
results suggest that degraded but Dectin-1-reactive β-glucan is discharged from 
macrophage cells phagocytizing insoluble β-glucan particles and stimulates not 
only themselves again but also the other naïve phagocytes, leading to the 
effective elimination of infecting microbes and the ultimate breakdown and 
inactivation of metabolically resistant β-glucan.