BACKGROUND:Recent studies show that mesenchymal stem cells(MSCs)have immunomodulatory properties. They suppress the immune response to alloantigen and modify the proliferation of T cells.CD4 + CD25 + regulatory T cells have strong immunomodulatory potential.However, little is known about the effects of rat MSCs(rMSCs)on the development of regulatory T cells. METHODS:MSCs were obtained from bone marrow of male Sprague-Dawley rats,and co-cultured with CD3 + T cells from allogeneic spleen cells.The proportion of CD4 + CD25 + regulatory T cells was analyzed by flow cytometry.To further confirm the immunosuppressive activity of rMSCs, we used MTT assay and flow cytometry of CD3 + T cells to investigate the proliferative responses of CD3 + T cells to mitogenic stimuli.Enzyme-linked immunosorbent assay was performed to detect alterations of the cytokines TNF-α, TGF-βand IL-10. RESULTS:The proliferation of CD3 + T cells decreased when co-cultured with rMSCs,and the degree of inhibition was concentration-dependent.The percentage of CD4 + CD25 + regulatory T cells increased when CD3 + T cells were co- cultured with different concentrations of rMSCs.The levels of pro-inflammatory cytokine(TNF-α)decreased while anti- inflammatory(TGF-β,IL-10)cytokines increased in mixed lymphocyte reaction. CONCLUSIONS:rMSCs inhibit allogeneic T cell proliferation in mixed cell cultures.This immunosuppressive effect seems to be mediated by inducing the generation of CD4 + CD25 + regulatory T cells and soluble factors. BACKGROUND: Recent studies show that mesenchymal stem cells (MSCs) have immunomodulatory properties. They suppress the immune response to alloantigen and modify the proliferation of T cells. CD4 + CD25 + regulatory T cells have strong immunomodulatory potential. METHODS OF MSCs were obtained from bone marrow of male Sprague-Dawley rats, and co-cultured with CD3 + T cells from allogeneic spleen cells. The proportions of CD4 + CD25 + regulatory T cells was analyzed by flow cytometry. To further confirm the immunosuppressive activity of rMSCs, we used MTT assay and flow cytometry of CD3 + T cells to investigate the proliferative responses of CD3 + T cells to mitogenic stimuli. Enzyme-linked immunosorbent assay was performed to detect alterations of the cytokines TNF-α, TGF-β and IL-10. RESULTS: The proliferation of CD3 + T cells decreased when co-cultured with rMSCs, and the degree of inhibition was c oncentration-dependent. The percentage of CD4 + CD25 + regulatory T cells increased when CD3 + T cells were co-cultured with different concentrations of rMSCs. The levels of pro-inflammatory cytokine (TNF- β, IL-10) cytokines increased in mixed lymphocyte reaction. CONCLUSIONS: rMSCs inhibit allogeneic T cell proliferation in mixed cell cultures. This immunosuppressive effect seems to be mediated by inducing the generation of CD4 + CD25 + regulatory T cells and soluble factors.