Cytotoxicity of in vitro–generated and immunomagnetically sorted NY-ESO-1–specific CD8⁺ T cells from BM of HLA-A*0201–positive MGUS patients (n = 3) and MM patients (n = 6) in whom we also detected NY-ESO-1–expressing plasma cells. Shown are representative flow cytometric plots of CD38-, CD138-, and NY-ESO-1–immunostained BMMC from 1 MGUS (A) and 1 MM (B) subject, as well as representative flow cytometry plots before and after selection of NY-ESO-1_157-165 pentamer-binding CD8⁺ T cells (expanded in vitro with NY-ESO-1_157-165 peptide) from 1 MGUS (C) and 1 MM (D) subject. Percentage-specific lysis of autologous plasma cells (E) and U266 myeloma cells (F) at different effector:target ratios by MGUS () and MM () NY-ESO-1_157-165 pentamer-binding CD8⁺ T cells (*P < .001, Mann-Whitney test). (G,H) Inhibition of this lysis by a mAb to HLA class I antigen. (I) Percentage-specific lysis of autologous plasma cells by MGUS () and MM () NY-ESO-1_157-165 pentamer-binding CD8⁺ T cells in the absence or presence of IL-6. *P < .001, paired t test.