NKC^KD NK cells exhibit reduced cytotoxicity toward MHC-I–deficient tumor cells in vitro and in vivo. (A-B) The ability of WT versus NKC^KD lymphokine-activated killer (LAK) cells to kill tumor target cells was tested by ⁵¹Cr-release assay. LAK cells generated from WT and NKC^KD mice were used as effectors cells against untreated (A) RMA, (B) RMA-S target cells, or targets precoated with anti-Thy1.2 mAb to test antibody-dependent cellular cytotoxicity (ADCC) function. The data are displayed as the mean ± SD percentage of chromium release from triplicate wells. (C) In vivo rejection of RMA-S relative to RMA cells was assessed. The mean rejection is indicated by a horizontal line. Each symbol represents the data from a single mouse. (D) Production of a MHC-I–negative C1498 subline. Flow cytometry for β₂m (dark histogram) is shown for the indicated C1498 lines. The gray histogram indicates isotype control mAb staining. The percentage of cells positively staining for β₂m is indicated. (E) In vivo rejection of C1498-MHC-I–negative cells versus C1498-MHC-I–positive tumor cells. Note that both cell lines received EMS treatment. Data are representative of ≥ 3 similar experiments. Groups differed significantly as shown (**P < .01; ***P < .001). These experiments were performed with mice on the B6 background.