Glucocorticoids are commonly used for treatment of chronic inflammatory diseases, while a number of patients show insensitivity to glucocorticoid treatment. The molecular basis of these individual differences in glucocorticoid pharmacodynamics has little been taken into account. Here we focus on the implication of Staphylococcus aureus-producing superantigen, such as toxic shock syndrome toxin-1 (TSST-1), in the glucocorticoid sensitivity of human peripheral blood mononuclear cells and cell-response to glucocorticoid to produce a transcript for FK506-binding protein (FKBP51). Peripheral blood mononuclear cell-sensitivity to glucocorticoid was assessed by a cell proliferation test. FKBP51mRNA expressions were determined by real-time reverse transcription-polymerase chain reaction (RT-PCR). We also compared concentrations of various cytokines produced in culture supernatant between concanavalin A- and TSST-1-stimulated peripheral blood mononuclear cells using a cytometric beads array. Mitogen-activated protein kinase (MAPK) phosphorylation activity in peripheral blood mononuclear cells stimulated with concanavalin A and TSST-1 was analyzed by a cell-based ELISA. Prednisolone markedly inhibited concanavalin A-induced peripheral blood mononuclear cell proliferation, but they scarcely inhibited TSST-1-induced peripheral blood mononuclear cell proliferation. The mean (S.D.) of immunosuppressant concentrations that would give 50% (IC(50)) values for prednisolone in concanavalin A-stimulated peripheral blood mononuclear cells was 52.6 (54.2) ng/ml, which was significantly lower than that in TSST-1-stimulated peripheral blood mononuclear cells, i.e., 574.2 (817.0) ng/ml (P<0.001). TSST-1-stimulated peripheral blood mononuclear cells for 48 h attenuated prednisolone-induced FKBP51mRNA expressions concomitantly with decreased sensitivity to the anti-proliferative effects of prednisolone. Concentrations of interleukin-2 (IL-2) produced from TSST-1-stimulated peripheral blood mononuclear cells were significantly higher than that from peripheral blood mononuclear cells stimulated with concanavalin A (P<0.0001). Both concanavalin A and TSST-1 enhanced the phosphorylation of extracellular signal-regulated kinase (ERK) and p38, whereas the level of c-jun terminal kinase (JNK) phosphorylation was only increased by TSST-1-stimulation in peripheral blood mononuclear cells. Furthermore, the decreased FKBP51mRNA by TSST-1was found to be recovered by JNK and mitogen-activated protein kinase (MEK)/ERK inhibitors. Our data suggest that TSST-1 reduces activity of glucocorticoid in peripheral blood mononuclear cells by JNK activation and subsequent production of IL-2. Therefore, JNK might be an attractive target for overcoming glucocorticoid insensitivity induced by TSST-1 in peripheral blood mononuclear cells.