Immunostimulatory effect of dried bonito extract on mouse macrophage cell lines and mouse primary peritoneal macrophages

Dried bonito is a preserved food used in Japan, which contains abundant flavor ingredients and functional substances. We focused on the immunostimulatory effect of dried bonito extract (DBE) on mouse macrophage-like J774.1 cells, RAW264.7 cells, and mouse primary peritoneal macrophages. DBE significantly stimulated the production of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) by both J774.1 cells and peritoneal macrophages by enhancing the cytokine gene expression levels. In addition, DBE stimulated nitric oxide production by enhancing the expression of inducible nitric oxide synthase in RAW264.7 cells. DBE also increased the phagocytosis activity of J774.1 cells. Immunoblot analysis revealed that DBE has an immunostimulatory effect on macrophages through activation of mitogen-activated protein kinase and nuclear factor-κB cascades. TNF-α production enhanced by DBE was partially inhibited by treatment with TLR4 Inhibitor TAK-242, whereas IL-6 production enhanced by DBE was almost inhibited. These results suggested that DBE is thought to strongly stimulate the TLR4 signaling pathway for macrophage activation, and its activation is also involved in other signaling. Finally, the phagocytosis activity of peritoneal macrophages from DBE-administered BALB/c mice increased significantly, suggesting that DBE has the potential to stimulate macrophage activity in vivo. In conclusion, these data indicated that DBE contributes to activating host defense against pathogens by activating innate immunity.