Increased susceptibility of leukemic blasts to “single-KIR” NK-cell lines after treatment with valproic acid (VA). (A) FACS analysis of ULBP1 and MICA/B expression levels by leukemic blasts from 6 AML patients untreated (black area), and after 2 days treatment with medium alone (black line) or VA (gray area); broken line, isotype-specific mAb staining. (B) Specific lysis of cells treated with medium alone (open symbols) and VA (filled symbols) by KIR-HLA class I–mismatched (squares) and matched NK-cell lines (circles). (*P < .05, **P < .01, significant difference between cytolysis of VA-treated and untreated AML blasts by NK cells.) (C) Reduction of specific lysis of AML cells by α-NKG2D blocking mAbs. KIR-HLA class I–mismatched NK cells were preincubated with blocking mAbs and used as effectors against AML cells cultured in medium without VA (□) and with VA (▒). The effect of α-NKG2D mAbs is the average of results obtained at 3 NK cell/target cell ratios of 10:1 (or 5:1), 2.5:1, and 1.2:1. Lysis after use of α-NKG2D mAbs is shown as . No blocking was observed with control IgG1 Abs (not shown). Error bars represent SD. (*P < .05, **P < .01, significant difference between lysis of VA-untreated and VA-treated AML blasts in the presence of α-NKG2D mAbs.)