Soluble β-glucan from Grifola frondosa induces proliferation and Dectin-1/Syk signaling in resident macrophages via the GM-CSF autocrine pathway.

MD-Fraction, a highly purified, soluble β-(1,3) (1,6)-glucan obtained from Grifola frondosa (an oriental edible mushroom), has been reported to inhibit tumor growth by modulating host immunity. β-Glucan, a major component of the fungal cell wall, is generally recognized by PRRs expressed on macrophages and DCs, such as Dectin-1, and the ability of β-glucans to modulate host immunity is influenced by their structure and purity. Most cellular studies have used particulate β-glucans, such as yeast zymosan (crude β-glucan) and curdlan (purified β-glucan). However, little is known about the cellular mechanism of soluble β-glucans, including MD-Fraction, despite significant therapeutic implications. In this study, we investigated the cellular mechanism of MD-Fraction in murine resident macrophages and compared it with two well-known β-glucan particles. MD-Fraction induced GM-CSF production rapidly through Dectin-1-independent ERK and p38 MAPK activation. Subsequently, MD-Fraction-induced GM-CSF enhanced proliferation and Dectin-1 expression, which permitted Dectin-1-mediated TNF-α induction through the Syk pathway. Curdlan induced not only the proliferation and activation of Dectin-1/Syk signaling in a manner similar to MD-Fraction but also the uncontrolled, proinflammatory cytokine response. Contrastingly, zymosan reduced proliferation and Dectin-1 expression significantly, indicating that the mechanism of macrophage activation by MD-Fraction differs from that of zymosan. This is the first study to demonstrate that purified β-glucans, such as MD-Fraction and curdlan, induce GM-CSF production directly, resulting in Dectin-1/Syk activation in resident macrophages. In conclusion, we demonstrated that MD-Fraction induces cell proliferation and cytokine production without excessive inflammation in resident macrophages, supporting its immunotherapeutic potential.