Differential susceptibility of cycling and quiescent CD34⁺ CML cells to HLA-identical sibling donor NK-cell cytotoxicity. (A) Quiescent CD34⁺ CML cells (QSC) are less susceptible to NK-cell cytotoxicity than cycling CD34⁺ (CD34+DIV) and CD34⁻ (CD34NEG) progeny from the same bulk CML CD34⁺ culture. (Unique patient identifier [UPI] 360, accelerated-phase CML; 4-hour cytotoxicity using NK cells cultured for 3 days with IL-2 and IL-15; AUTO = autologous control). Data representative of cytotoxicity assays from 4 separate patient-donor pairs. (B) Donor NK cells expanded after 11 to 13 days are more cytotoxic after 4-hour incubation with control K562 cells, but QSCs are still less susceptible than other cycling cell populations (UPI 400, accelerated-phase CML). Data representative of cytotoxicity assays from 4 separate patient-donor pairs.