MSCs are viable but anergic. MSCs from C57BL/6J mice inhibit proliferation of effector T cells from C57BL/6J mice in response to irradiated MNCs from CD1 mice (A), of effector cells from CD1 mice to irradiated C57BL/6J feeders (B), and of effector human PBMCs to allogeneic irradiated human feeder cells (C). (A-C) ▦ indicates T cells stimulated in the absence of MSCs; ▪, MSC-treated T cells. ^*P < .05 by Mann-Whitney U test. Error bars indicate SD values. Proliferative responses are expressed as mean CPM of at least 3 independent experiments. hMLR indicates human MLR. (D) MSCs inhibit production of IFN-gamma (▪) and TNF-alpha (▦) by ConA-stimulated T cells in a dose-dependent manner. ^*P < .05 by Mann-Whitney U test compared with cytokine levels induced by ConA stimulation. Columns represent the mean ± SD from at least 3 separate experiments. MSCs do not significantly increase (P < .2 by Mann-Whitney U test) the percentage of apoptotic T cells upon anti-CD3 or ConA stimulation (E). (E) □ indicates T cells stimulated in the absence of MSCs; ▦, with 0.25 × 10⁵ MSC-treated T cells; and ▪, with 2 × 10⁵ MSC-treated T cells. Bars represent the mean ± SD percent variation (n = 3) of annexin V-positive cells. The inhibition of proliferation induced by ConA stimulation is partially reverted by the administration of 100 IU/mL IL-2. Such an effect is more pronounced at lower MSC concentrations but still enough to increase by about 2-fold CPM at each concentration (F). (F) □ indicates conA-stimulated T cells; ▪, MSC-treated T cells; and ▦, IL-2-stimulated MSC-treated T cells. Results are expressed as mean CPM ± SD of at least 3 independent experiments.