CD3⁺IRS⁺ T cells lyse target cells bearing the appropriate HLA-I counter ligand. Polyclonal (A: PNH6; B: PNH7; C: PNH11) bulk populations homogeneously expressing CD3 and CLIR (CD94⁺) antigens were challenged in a 4-hour ⁵¹ Cr release assay with 721.221 target cells bearing HLA-E with G peptides as ligand for CD94/NKG2 complex of the activating type. Anti-HLA-I mAb or anti-CD94 mAb (5 μg/mL) was added to the cytolytic assay to determine whether the masking of CD94/NKG2-HLA-I interaction leads to inhibition of target lysis. (D-F) The CD3⁺KIR⁺CLIR^- T-cell clones KIR2DS2⁺ (D; from PNH7) or KIR2DS1⁺ (E; from PNH5) were used as effector cells with the 721.221 transfected with HLA-Cw3 or HLA-Cw4 (ligands of KIR2DS2 or KIR2DS1, respectively). E/T ratio indicates effector-target ratio. Anti-HLA-I mAb or anti-panKIR2D mAb (5 μg/mL) was added to the cytolytic assay to determine whether the masking of KIR2D-HLA-I interaction leads to inhibition of target lysis. The CD3⁺KIR⁺ T-cell clone expressing KIR2DL1, the inhibiting form of KIR2D recognizing HLA-Cw4, was challenged with 721.221 target cells HLA-Cw4 transfected. In this instance, the addition of anti-HLA-I mAb or anti-panKIR2D mAb (5 μg/mL) led to triggering of lysis as the impairment of KIR2D-HLA-I binding does not protect the target cells any more. Results are expressed as % ⁵¹ Cr-specific release, mean of triplicate samples plus or minus SD.