Immunosuppressive effect of silymarin on mitogen-activated protein kinase signalling pathway: The impact on T cell proliferation and cytokine production

Silymarin, a polyphenolic flavonoid derived from milk thistle (Silybum marianum), is known to have anti-inflammatory, hepatoprotective and anticarcinogenic effects. In this study, the in vitro immunomodulatory effect of silymarin was investigated using human CD4+ T cells. Peripheral blood mononuclear cells (PBMC) from healthy individuals were activated with anti-CD3 (5 μg/ml) plus anti-CD28 (2 μg/ml) and treated with 10, 50 and 100 μM silymarin. Cells were incubated 72 hr for proliferation assay using MTT and for viability analysis using PI staining and flow cytometry. Naive CD4+ T cell was also isolated from PBMC, activated with PHA/anti-CD28 and treated with 100 μM silymarin for 72 hr. MAPKs' activity of cell lysate from activated naive CD4+ T cells was assessed using an ELISA-based MAPKinase activity kit, and Th1/Th2/Th17-related cytokine expression was determined by Multi-analyte ELISA array kit. Results indicated a significant inhibition in proliferation of activated PBMC after 48-hr incubation with 100 μM silymarin without causing cell death. Moreover, MAPKs' activity (ERK1/2 and P38) and Th1-related cytokines (IL-2, TNF-α, IFN-γ) were significantly reduced in silymarin-treated cells compared with control after 72 hr. This study shows that silymarin has the ability to inhibit T cell proliferation and pro-inflammatory cytokine secretion in vitro. Furthermore, silymarin is able to inhibit ERK1/2 and P38 pathway activation in T cells stimulated through TCR engagement, a property that is likely associated with its ability to inhibit T cell proliferation and cytokine secretion. Therefore, silymarin, as an immune-response modifier, might be a valuable drug in therapeutic situations in which immunosuppression is required.