1-7F9 blocks interactions of inhibitory KIR2DL with HLA class I on B-EBV cells. (A) Binding of soluble KIR2DL1-hFc was blocked by anti-KIR mAbs GL183 or DF200 and binding of KIR2DL1-mFc blocked by 1-7F9, as measured by flow cytometry. Relative binding of KIR-Fc proteins to .221-Cw4 cells is shown as percentage of binding by KIR-Fc in the absence of mAbs. Similar data were obtained in a repeat experiment. (B) In a ⁵¹Cr release cytotoxicity assay, YTS cells efficiently killed LCL721.221-Cw4 cells (▲), whereas YTS-2DL1 cells did not (●; E:T ratio 12:1). Preincubation (30 minutes at 37°C) of the NK cells with increasing doses of 1-7F9 augmented the killing of LCL721.221-Cw4 targets by YTS-2DL1 cells, in a dose-dependent manner. Curve fitting using one-site receptor saturation equation gives an EC₅₀ of 0.71 μg/mL (95% CI, 0.2-1.2 μg/mL). Experiment shown is representative of multiple experiments giving equivalent results. (C) NK-cell clones were tested for cytolytic activity against B-EBV cell lines transfected with indicated HLA class I allotypes at 10:1 E:T ratio, with or without mAb at 10 μg/mL. NK clone MDC14 (top panels) is KIR2DL1⁺, KIR2DL2/3⁻, NKG2A⁻. NK clone MDC88 (bottom panels) is KIR2L1⁻, KIR2DL2⁺, KIR2DL3⁻, NKG2A⁻. Killing by these NK cells was tested against B-EBV cell lines transfected with HLA-Cw4 (left panels) and HAL-Cw1 (right panels). Results were analyzed by ANOVA, followed by Bonferroni posttest; only P < .001 are reported.