LLT1 interaction with CD161 inhibits NK cell-mediated cytotoxicity. A, Monitoring of CD107a expression on NK cells unstimulated or stimulated with C1R and C1R-LLT1 for 5 h. The percentage of NK cells expressing CD107a is indicated. B, C1R and C1R-LLT1 were used as targets in a 4-h ⁵¹Cr release assay with polyclonal CD161⁺ NK cells. Blocking mAb and isotype control mAb were added at 10 μg/ml. C, Redirected killing assay using polyclonal NK cells and P815 cells transfected or not with LLT1. Suboptimal concentration of anti-CD16 mAb (0.5 μg/ml) was added to anti-CD161 (191B8) or isotype control (10 μg/ml) mAbs. Data are representative of four to eight experiments.