The study was purposed to investigate the immune regulatory effects of human bone marrow mesenchymal stem cells (hMSCs) on Foxp3 expressing CD4(+)CD25(+) regulatory T cells and to explore the mechanism of immune modulation by hMSCs. Human MSCs were isolated and expanded from bone marrow cells, and identified with cell morphology, and the phenotypes were assessed by immunohistochemistry. Human peripheral blood mononuclear cells (hPBMNCs) were prepared by centrifugation on a Ficoll Hypaque density gradient. The hMSCs (1 x 10(3), 1 x 10(4), 1 x 10(5)) were added into wells containing hPBMNCs (1 x 10(6)) from an unrelated donor in the presence of rhIL-2. After 5 days of co-culture, the percentage of CD4(+)CD25(+) T cells was detected by flow cytometry. T cell proliferation was assessed by [(3)H] thymidine incorporation using a liquid scintillation counter. The expression of Foxp3 in CD4(+)CD25(+) T cells was detected by reverse transcription polymerase chain reaction (RT-PCR). Cytokines (TGF-beta, IL-12, IFN-gamma, IL-10) concertrations of cultured supernatants were measured with ELISA. The results indicated that in all the experiments, the presence of hMSCs with hPBMNCs resulted in a statistically significant decrease in T cell proliferation, in dose-dependent manner. The increase of percentage of CD4(+)CD25(+) T cells in the peripheral CD4(+) T cell was observed after coculturing lymphocytes with hMSCs (p < 0.01). The expression of Foxp3-mRNA (Foxp3/beta-actin) in hMSCs groups was significantly higher than that in the control and was negatively associated with the value of CPM representing T proliferation. The levels of TGF-beta and IL-10 were higher in hMSCs groups than that in the control, and the levels of TGF-beta and IL-10 correlated positively with Foxp3-mRNA expression and the percentage of CD4(+)CD25(+) T cells. However, the secretion of IL-12 and IFN-gamma was significantly attenuated by hMSCs coculture, and there was no correlation with Foxp3-mRNA expression and the percentage of CD4(+)CD25(+) T cells. It is concluded that the Foxp3 expressing regulatory T cells may play an important role in the immune regulatory by hMSCs. Its mechanism is related to that the hMSCs-mediated TGF-beta and IL-10 convert CD4(+)CD25(-) T cells into CD4(+)CD25(+) T regulatory T cells, which specifically inhibits the proliferation of T cells.