Inhibition of the IFN-γ response of NK cells through cognate interaction between HLA class I and KIR. Donor 17 expresses KIR2DL3 but not the serologically cross-reactive KIR2DL2 and KIR2DS2 (Figure 3). Flow cytometric analysis with the GL-183 antibody (anti-KIR2DL2/3, 2DS2) was used to determine the frequency (10%) of NK cells of donor 17 that express KIR2DL3 (A) and are potentially inhibitable by HLA-Cw*0702. NK cells from donor 17 were cultured in medium alone, IL-2, or IL-12; challenged with 221 or 221-Cw*0702 cells; and assayed for IFN-γ by ELISPOT. (B) The proportion of NK cells that made IFN-γ in response to 221 but were inhibited by 221-Cw*0702. (C) The difference between the observed frequency of NK cells inhibited by HLA-Cw*0702 and that predicted from the frequency of NK cells expressing KIR2DL3. (D-E) Blocking of the 221-Cw*0702–mediated inhibition with anti-HLA class I (DX17; ▦) (D) or anti-KIR2DL3 antibody (GL-183; ▦) (E). Anti-CD56 (Leu19; □) was included as a control antibody, which binds to NK cells but does not affect their function. ▪ indicates no mAb. NK cells were cultured in IL-2 with target cells, and antibodies were added at 10 μg/mL. NK cells from donors 33 and 39 were used for the experiments shown in panels D and E, respectively. (KIR2DL3⁺ NK cells were 22% and 36% for donors 33 and 39, respectively.)