| Summary: | <i>Pleurotus ostreatus</i> mushroom preparations have been investigated because of their ability to modulate the immune function. However, there is still no consensus regarding the activation and polarizing effect on macrophages by <i>Pleurotus</i>-derived bioproducts. This study examined the immune-activating effect of a mycelium-derived <i>P. ostreatus</i> aqueous extract (HW-Pm) on macrophage functions, by means of the determination of nitric oxide (NO) production, the mRNA expression of inducible nitric oxide synthase (iNOS), Arginase-1 and FIZZ and the cytokine levels. The phagocytic activity and the activation of NF-κB in U937 reporter cells were also investigated. No cytotoxicity was observed in macrophages treated with HW-Pm (IC<sub>50</sub> > 1024 μg/mL) by the resazurin test. HW-Pm induced high levels of NO production and iNOS expression in macrophages. In contrast, HW-Pm did not induce Arginase-1 and FIZZ mRNA expressions. The mushroom extract increased TNF-α and IL-6 production and the phagocytic function in murine macrophages. It also stimulated the activation of the NF-κB promoter. The <i>P. ostreatus</i> mycelium extract has a potential application as a natural immune-enhancing agent, by targeting macrophage activation towards the classically activated subset and stimulating macrophage-mediated innate immune responses.
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