ACAR-mediated cytolysis seen at low target density and low E:T ratios. (A) SUPT1 targets were engineered to express a wide range of BCMA (i) and TACI (ii). Antigen densities of targets are indicated. Dot plots depict receptor levels found on primary MM tumor cells from 50 patients (population median is indicated, and dashed line represents threshold for positive expression) in comparison with engineered SUPT1 targets. (B) These SUPT1^BCMA (i) and SUPT1^TACI (ii) targets were then cocultured with ACAR-CD8 and H-spacer variants at a low E:T ratio (1:10), and target death determined at 48 hours by FACS and expressed as percentage cytolysis in comparison with media control. (C) Specific cytolysis at 48 hours of human myeloma cell lines MM.1s (i) and U266 (ii) when cocultured with ACAR-transduced T cells at reducing E:T ratios. Inset histograms show BCMA and TACI expression by FACS (blue shaded curves) in comparison with staining with isotype control (open curves). Mean ± SEM of number of experiments are indicated. *P < .05, **P < .01, ***P < .001 by t test, in comparison with PBMC NT.