PSP Exerts Immunoregulatory Effect Via Myeloid Differentiation Factor 88-Dependent Signaling Pathway

Objective:The present study is to investigate the anti-tumor mechanism of polysaccharopeptide (PSP) in mice bearing Ehrlich’s ascites carcinoma (EAC).Methods:Chromogenic end-point tachypleus amebocyte lysate assay was used to exclude possible contamination of PSP. Twelve female wild-type C57 mice were used to extract peritoneal macrophages and were randomly divided into three groups. Another twelve female myeloid differentiation factor 88 (MyD88)-deficient mice were randomly assigned to three groups. Cell survival and peritoneal macrophage phagocytosis were measured using WST8 assay. The concentration of nitric oxide was determined by Griess reaction. Enzyme-linked immunosorbent assay was used to measure the contents of tumor necrosis factor-a and interferon-γ. Quantitative real-time polymerase chain reaction was employed to measure the expression of mRNAs, while Western blotting was performed to determine protein expression.Results:PSP significantly inhibited the proliferation of EAC cells via the activation of macrophages. PSP-primed macrophages exhibited a higher tumoricidal activity than untreated macrophages. Administration of PSP markedly inhibited the growth of the tumor and increased macrophage phagocytosis, nitric oxide release and cytokine secretion. Expression of MyD88 was markedly increased in PSP-treated groups, while ST2825 inhibited MyD88 signaling and interfered with nitric oxide release and the secretion of cytokines such as tumor necrosis factor-a and interferon-y. Moreover, the mRNA and protein levels associated with MyD88-dependent signaling pathway in MyD88-deficient (MyD88-/-) mice group were significantly down-regulated compared with wild-type mice group under the regulation by PSP.Conclusions:PSP has an immunoregulatory effect through the regulation of MyD88-dependent signaling pathway.