LAUSR

Abstract Alpha‐MMC is a type I ribosome‐inactivating protein purified from bitter gourd that has strong anti‐tumour and antiviral activity. Alpha‐MMC also has immunosuppressive effects, but the mechanism of these immunosuppressive effects remains unclear. It is reported that the binding of &agr;‐MMC to its specific cell membrane LRP1 receptor is key to its biological effects. In this study, we investigated the effect of &agr;‐MMC on cytotoxicity and cytokine release regulation in three immune cells, human monocyte THP‐1 cells, B‐lymphocyte WIL2 cells and T‐lymphocyte H9 cells, and explored the correlation between this effect and LRP1 receptor distribution on these three cell types. We demonstrate that &agr;‐MMC has a significant effect of apoptosis induction and cytokine release in THP‐1 cells but has no effect on WIL2‐S and H9 cells. Specifically, at a non‐cytotoxic dose (80 &mgr;g/ml), &agr;‐MMC regulates THP‐1 cells by inhibiting IL‐1&bgr;, IL‐2, IL‐8, IL‐9, IL‐12, MIP‐1&agr;/&bgr;, MCP‐1 and TNF‐&agr; expression and enhancing IL‐1ra and RANTES expression, resulting in the inhibition of cellular immune function. Subsequent experiments showed that the cytokine expression regulated by &agr;‐MMC can be blocked by silencing the LRP1 receptor of &agr;‐MMC. Further research indicated that phosphorylation of 9 signalling proteins of the MAPK pathway was significantly regulated by &agr;‐MMC and was blocked by LRP1 silencing. We conclude that the regulation of cytokine expression induced by &agr;‐MMC in monocyte THP‐1 cells is mediated by the LRP1 receptor, likely via the MAPK signalling pathway. Our results suggest that the inhibition effect on monocytes/macrophages mediates the immunosuppressive function of &agr;‐MMC. Due to the selective cytotoxicity and cytokine release regulation of &agr;‐MMC in monocytes/macrophages, &agr;‐MMC may be used for killing Tumour‐Associated Macrophages (M2 subtypes) or inhibiting their cytokine release in the tumour microenvironment. HighlightsApoptosis and regulation of cytokine release by &agr;‐MMC occurred mainly in monocyte THP‐1 cells.The regulation by &agr;‐MMC manifested as cell function inhibition of monocyte THP‐1 cells.The regulation is mediated by the LRP1 receptor, possibly via the MAPK signalling pathway.