U5 snRNP200 antibodies induce death in vitro. (A) Addition of AML-specific U5 snRNP200 complex–binding antibodies sAT1331 (black triangles) and sAT1337 (black squares) to THP-1 cells at time point 0 blocked expansion of this cell line, whereas the flu antibodies AT1002 IgG1 (open circles) and AT1002 IgG3 (open squares) did not (left panel). (Right panel) Reculturing of THP-1 cells that survived antibody treatment (left panel), again in the presence of sAT1331, sAT1337, or control antibody. Antibodies were added at 24-hour intervals at a concentration of 7.5 μg/mL. Viable cells were 4′,6-diamidino-2-phenylindole⁻ (DAPI⁻). (B) THP-1 cells were incubated with AML-specific antibodies for 4 hours at 37°C, after which cell death was measured using a standard bead controlled assay. Numbers on the x-axis denote each specific AML-specific antibody; asterisks indicate antibodies that are specific for the U5 snRNP200 complex. Co, heat-inactivated medium control. (C) THP-1 cells were incubated with increasing concentrations of cytotoxic AML antibodies sAT1331 and sAT1337, resulting in a concentration-dependent death of target cells. (D) Primary AML blasts isolated from newly diagnosed patients with AML (BL-038: upper panels; BL-063: lower panels) cultured in the presence of sAT1331, sAT1337, or control antibody. (Left panels) Numbers of life cells; (right panels) percentage of DAPI⁺ cells.