MSC-induced inhibition of NK-cell proliferation. NK cells were cultured alone or with allogeneic irradiated MSCs in the presence of 100 U/mL IL-2. (A) Results of 1 of 8 representative experiments in which the proliferation of resting NK cells was evaluated using the CFSE dilution method. CFSE fluorescence of NK cells (identified as CD45⁺CD56⁺ lymphocytes) was analyzed after 7 days of culture alone or with MSCs (NK/MSC ratio, 8:1). (B) Proliferative response of IL-2-cultured proliferating NK cells analyzed at day 7 of culture in the absence or presence of MSCs. Data are presented as percentage of ³H-thymidine incorporation by NK cells cultured in the presence of MSCs (at different NK/MSC ratios) with respect to NK cells cultured alone (100%). Error bars represent mean ± SD of 6 independent experiments.