Role of soluble factors secreted by MSCs in NK-cell degranulation and cytotoxicity. (A) NK cells were cultured in presence of K562 target cells for 20 minutes (ratio NK/K562, 2:1). pERK protein expression was determined by Western blot analysis and GAPDH was used as a loading control. CD107a degranulation (B) and NK-cell cytotoxicity was evaluated using the ⁵¹Cr-release assay (C) after IPS-MSC or H9-MSC culture. Results are expressed as the percentage of K562 lysis in the presence of inhibitors of HLA-G, IDO and PGE-2. One hundred percent indicates NK cells alone. Bars represent the median obtained from 6 independent experiments. The NK/K562 ratio was 15:1 for the ⁵¹Cr-release assay and 5:1 for the CD107a assay. Statistically significant differences between MSCs in presence or not with soluble inhibitors are indicated by asterisks, **P < .01.