HLA-A2.1–restricted lysis of the breast-carcinoma cell lines. MDA-MB-157-HHD and MDA-MB-231 by CTL against BA46-derived peptides and patient-derived tumor extract (T.E). Mice were immunized as described in Methods with patient-derived tumor extract or with BA46-6, BA46-7, and BA46-9 peptides loaded on RMA-S-HHD-B7.1 transfectants. Lysis of MDA-MB-157(HLA-A2.1–negative), MDA-MB-157-HHD, and MDA-MB-231 (HLA-A2.1–positive) breast carcinoma cells was monitored by CTL assays. Specific lysis of HHD-transfected MDA-MB-157-HHD was significantly higher (P < 0.0006) than that of wild-type MDA-157 target cells. The E/T ratio of 100:1 is shown. Results are representative of three similar experiments for MDA-MB-157/HHD and two experiments for MDA-MB-231.