NK cells that reconstitute at an early stage in vivo from CD34⁺ HSCs have impaired functional activity against leukemic targets. (A) NK cells from 7 patients at 30 days after haploidentical HSCT (■) or from 5 healthy blood donors (▴) were tested in a standard 4-hour ⁵¹Cr release assay against the 721.221 BLCL (left panel) or against fresh allogeneic AML blasts (right panel) after overnight activation with 100 IU/mL IL-2. Patient NK cells were also tested against AML blasts in the presence of 20 μg/mL anti-NKG2A mAb (□). Shown are the average values with SE. Asterisks indicate effector-target ratios where differences between donors and patients after haploidentical HSCT were statistically significant. (B) NK cells from 6 patients at 30 days after haploidentical HSCT () or from 5 healthy blood donors (□) were tested by standard ELISA for IFN-γ release after 24-hour incubation in response to medium containing 100 IU/mL rhIL-2 alone, the 721.221 BLCL, or fresh allogeneic AML blasts. Patient NK cells were also tested against AML blasts in the presence of 20 μg/mL anti-NKG2A mAb () or isotype control anti-NGFR mAb (■). Shown are the average values with SE. *P value from a Mann-Whitney U test; **P value from a Wilcoxon test for paired data.