The MS-5–based culture system allows long-term proliferation of AML blasts and demonstration of effective AMG 330-mediated lysis, even at low E:T ratios. (A) AML blast viability over 24 days using the MS-5 feeder layer-based long-term culture system compared with a conventional suspension culture system. (B-E) Cytotoxicity assays based on flow cytometry analysis of AML blasts from primary diagnosis or relapse co-incubated with autologous T cells natively contained in the sample and AMG 330 or control antibody. Dot plots are gated on CD3⁻ cells. T-cell expansion is shown in the graph by columns. CD33⁺ AML blast expansion/lysis is demonstrated in the graph by lines. Murine (mu) CD29 represents MS-5 feeder cells within the culture. (B) Example of a cytotoxicity assay (PT8; E:T ratio = 1:11) with delayed lysis of AML blasts. (C) Example of a cytotoxicity assay (PT4; E:T ratio = 1:79) with demonstration of CD33⁺ AML blast regrowth on day 36 of culture after a decline of viable CD3⁺ T cells. (D) Example of a cytotoxicity assay (PT6; E:T ratio = 1:5) with complete lysis of primary AML blasts. Continuous blast viability and proliferation over 36 days is shown for the control antibody. (E) Summary of the cytotoxicity assays with 10 more AML samples. High AMG 330-mediated cytotoxicity is demonstrated compared with continuous blast viability and proliferation in control cultures. FSC, forward scatter; SSC, side scatter.