In the present study, we demonstrated the effect of RA on the severity of Con A-induced hepatitis and molecular changes of NKT

cells. First, we demonstrated that Con A-induced liver damage was ameliorated by RA. In correlation with cytokine levels in serum, RA regulated the production of IFN-γ and IL-4 but not TNF-α by NKT cells without influencing the NKT-cell activation status. However, RA did not alleviate α-GalCer-induced liver injury, even though it reduced IFN-γ and IL-4 but not TNF-α levels in serum. RAD001 in vitro This regulation was also detected when liver mononuclear cells (MNCs) or NKT hybridoma cells were treated with RA in vitro. The regulatory effect of RA on NKT cells was mediated by RAR-α, and RA reduced the phosphorylation of MAPK. These results suggest that RA differentially

modulates the production of effector cytokines by NKT cells in hepatitis, and the suppressive effect of RA on hepatitis varies with the pathogenic mechanism of liver injury. Liver damage induced by various agents, such as viral infection, results in serious problems accompanied by an excessive immune response [1]. Uncontrolled severe responses in the liver by immune cells are observed in diverse animal models, including Con A-induced hepatitis. Following the administration of Con A, T cells, granulocytes, and Kupffer cells infiltrate into the liver, resulting in the death of hepatocytes [2-4]. NKT cells are responsible SPTLC1 for liver injury in this model [5-10]. NKT cells are a distinct T-cell subset with an invariant T-cell receptor (TCR) that recognizes HDAC inhibitor glycolipids loaded on the cell-surface protein CD1d, and they rapidly secrete

cytokines upon stimulation [11-14]. In Con A-induced liver injury, inflammatory cytokines, such as IFN-γ, TNF-α, and IL-4, from NKT cells are pathogenic [5, 7, 9, 10]. In addition, a specific ligand of NKT cells, α-GalCer, can induce liver injury mediated by FasL and TNF-α rather than IFN-γ [15-17]. Although NKT cells are critical to induce both Con A- and α-GalCer-induced liver injury, their pathologic mechanisms are different from each other. Retinoic acid (RA), an active metabolite of vitamin A, regulates various diseases through anti-tumor and anti-inflammatory effects [18, 19]. RA is associated with anti-inflammatory effects in diverse diseases [20]. RA also enhances T-cell effector responses and is critical in vaccine responses [21-25]. These contradictory findings imply that the exact physiological function of RA remains to be discovered. RA promotes the proliferation and activation of NKT cells indirectly in vitro by increasing CD1d expression in APCs [26-28]. However, the direct effects of RA on NKT cells and NKT cell-dependent diseases in vivo have not been examined.