Spleen MSCs suppress NK cell cytotoxicity in a cell-cell contact–dependent manner. Splenic MSCs (CD11b⁺Gr-1⁺) from naive or tumor-bearing BALB/c mice were FACS sorted and cocultured with DX5⁺ NK cells at the ratios between 1:0 to 1:2 (NK/MSC) in a 24-well plate (A) or in a Transwell system (B). After 1 day (C) or 5 days (A-B) of NK-MSC coincubation, NK cell cytotoxicity against YAC-1–Luc cells (YAC-1–Luc/NK, 1:20) was assayed as described in “Cytotoxicity assay.” The data represent NK cell killing YAC-1–LUC activity at 4 hours after addition of YAC-1–Luc cells. Data are mean (± SEM) of triplicate wells of 3 independent experiments. *P < .05; **P < .001.