The biological macromolecule Nocardia rubra cell-wall skeleton (Nr-CWS) has well-established immune-stimulating and anti-tumor activities. However,the role of Nr-CWS on natural killer (NK) cells remains unclear. Here,we explore the function and related mechanisms of Nr-CWS on NK cells. Using a tumor-bearing model,we show that Nr-CWS has slightly effect on solid tumor. In addition,using a tumor metastasis model,we show that Nr-CWS suppresses the lung metastasis induced by B16F10 melanoma cells in mice,which indicates that Nr-CWS may up-regulate the function of NK cells. Further investigation demonstrated that Nr-CWS can increase the expression of TRAIL and FasL on spleen NK cells from Nr-CWS treated B16F10 tumor metastasis mice. The spleen index and serum levels of TNF-$\alpha$,IFN-$\gamma$,and IL-2 in B16F10 tumor metastasis mice treated with Nr-CWS were significantly increased. In vitro,the studies using purified or sorted NK cells revealed that Nr-CWS increases the expression of CD69,TRAIL,and FasL,decreases the expression of CD27,and enhances NK cell cytotoxicity. The intracellular expression of IFN-$\gamma$,TNF-$\alpha$,perforin (prf),granzyme-B (GrzB),and secreted TNF-$\alpha$,IFN-$\gamma$,IL-6 of the cultured NK cells were significantly increased after treatment with Nr-CWS. Overall,the findings indicate that Nr-CWS could suppress the lung metastasis induced by B16F10 melanoma cells,which may be exerted through its effect on NK cells by promoting NK cell terminal differentiation (CD27lowCD11bhigh),and up-regulating the production of cytokines and cytotoxic molecules.
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