MSCs inhibit T-cell effector function without interfering with T-cell activation. Splenocytes from C6 mice transgenic for a K^k-restricted HY-specific T-cell receptor (TCR) (5 × 10⁶) were stimulated for 24 hours with irradiated female syngeneic (CBA, H2^k) spleen cells (5 × 10⁶) pulsed with the cognate HY K^kSmcy (TENSGKDI) peptide in the absence (A) or the presence (B) of MSCs (1 × 10⁵). After 24 hours, CD8⁺ cells were analyzed for intracellular IFN-γ and for the expression of activation molecules using anti-CD25 or anti-CD69 monoclonal antibodies. For IFN-γ staining, T cells were treated with Brefeldin A for the last 4 hours of cultures. Cells were permeabilized, and the proportion of CD8⁺/IFN-γ⁺ T cells was quantified. The fluorescence-activated cell sorter (FACS) plot is representative of 1 of 6 experiments of identical design (see “Results” for mean ± SD values).