Senescent hMSCs have a reduced capacity to inhibit lymphocyte proliferation. Human PBMCs (105) were cultured in triplicate with complete RPMI medium in the presence of PHA (10 μg/ml), and with or without various numbers of bone marrow-derived hMSCs (0:1 to 1:5 MSC/PBMC ratios) from different donors (hMSC19, 33, 44, and 45) in flat-bottomed 96-well plates. After 72 hours of culture, proliferation was evaluated by colorimetric measurement of BrdU incorporation. White bars, PBMC without hMSC; gray bars, PBMC cocultured with wild-type hMSC; black bars, PBMC cocultured with senescent hMSC; NS, non-stimulated PBMC. All data correspond to experiments performed using PBMCs from the same donor. *, p < .05; **, p < .01; ***, p < .005, versus cocultures with nonsenescent hMSCs. Error bars represent SEM. Abbreviations: hMSC, human mesenchymal stem cell; PHA, phytohemagglutinin.