IL-10 is required for human CD4(+)CD25(+) regulatory T cell-mediated suppression of xenogeneic proliferation

Cellular rejection of xenografts is predominantly mediated by CD4(+) T cells. Human CD4(+)CD25(+) regulatory T cells (Tregs) are capable of suppressing the CD4(+) T cell-mediated xenogeneic response in vitro. However, the precise mechanisms that are involved remain to be identified. In this study, we analyzed whether interleukin-10 (IL-10) is required for Tregs to suppress xenogeneic responses in vitro by small interfering RNA (siRNA)-mediated IL-10 knockdown. After siRNA transfection, Tregs were analyzed for IL-10 gene and protein expression and their phenotype. Mixed lymphocyte reactions (MLRs) were performed by stimulating human CD4(+)CD25(-) T cells with allogeneic or pig peripheral blood mononuclear cells (PBMCs) in the presence or absence of Tregs in a coculture or transwell system. The production of effector cytokines by xeno- or alloreactive CD4(+)CD25(-) T cells, or suppressive cytokines by Tregs, was examined using enzyme-linked immunosorbant assay (ELISA). We showed that IL-10 knockdown resulted in a substantially reduced IL-10 production by Tregs, leading to impaired Treg-mediated suppression of xeno- but not alloreactive CD4(+) CD25(-) T-cell proliferation. However, IL-10 knockdown had no effect on Treg phenotype, their suppression of effector cytokine production by xeno- or alloreactive T cells and the production of the Treg-suppressive cytokine, transforming growth factor-beta (TGF-beta). This study shows that IL-10 is required for human Tregs to suppress xenogeneic but not allogeneic proliferation in vitro.