The translocator protein (TSPO) ligands affected inflammatory and immune responses. However, the exact effects of TSPO ligands on Th1 responses in vitro and in vivo are still unclear. In the present study, we found that TSPO ligands, FGIN1-27 and Ro5-4864, suppressed the cytokine production in a dose-dependent manner by purified human CD4+ T-cells from peripheral blood mononuclear cells (PBMCs) after stimulation. TSPO ligands inhibited the production of interferon γ (IFN-γ) by memory CD4+ T-cells and the differentiation of naïve CD4+ T-cells into Th1 cells via suppressing the activity of the corresponding transcription factors as indicated by reduced expression of T-bet and down-regulation of STAT1, STAT4 and STAT5 phosphorylation. TSPO ligands suppressed cell proliferation and activation of CD4+ T-cells by the inhibition of TCR signal transduction including membrane proteins: Zap, Lck, Src; cytoplasm proteins: Plcγ1, Slp-76, ERK, JNK and the nucleoproteins: c-Jun and c-Fos. In addition, FGIN1-27 inhibited mixed lymphocyte reactions by human or murine cells. After the transplantation of allogeneic murine skin, injection of FGIN1-27 into mice prevented graft rejection by inhibition of cell infiltration and IFN-γ production. Taken together, our data suggest that TSPO ligands inhibit Th1 cell responses and might be novel therapeutic medicine for the treatment of autoimmune diseases and prevention of transplant rejection.

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