Cytotoxicity of DKK1 peptide-specific T-cell clones. (A) P20-specific or (B) P66v-specific T-cell clones against HMCLs U266, IM-9, XG1, and ARP-1. K562 was used as control for NK-cell activity. (C) Cytotoxicity of the T-cell clones (P20 and P66v) against primary myeloma cells from 4 patients with MM and normal cells including MSCs, DCs, B cells, and PBMCs from HLA-A*0201⁺ healthy donors. Patients 1 and 2 were HLA-A*0201⁺, and patients 3 and 4 were HLA-A*0201⁻. All primary myeloma cells expressed DKK1 protein. U266 was used as a positive control. (D) Inhibition of the T-cell clone–mediated cytotoxicity against peptide-pulsed T2 cells by mAbs against MHC class I (aMHC-I), MHC class II (aMHC-II), or HLA-A*0201 (aHLA-A2). Isotypic IgG was used as control. An effector-target (E/T) ratio of 10:1 was used in panels C and D. Results of 4 independent experiments are shown. Error bars show SD. Similar results were obtained with other T-cell clones from blood donors and patients with myeloma.