BTK^Cys481Serexpressing WM and ABC DLBCL cells confer a protective effect to BTK^WTmalignant cells through paracrine mediated prosurvival signaling. For coculture experiments, BTK^WT or BTK^Cys481Ser expressing GFP⁺ BCWM.1 or TMD8 cells were cultured with ibrutinib (0.5-1.0 µM) for 6 hours. Nontransduced BTK^WT GFP⁻ BCWM.1 or TMD8 cells were then added to wells, and cocultured for 24 to 48 hours in the absence or presence of ibrutinib (0.5-1.0 µM) Annexin V–APC staining was used to assess apoptotic changes on nontransduced GFP⁻ cells. Experiments were done in triplicate, and the representative plots with the percentage of apoptotic cells are shown in panel A. The statistics for all 3 experiments are displayed in panel B. For Transwell coculture experiments, BTK^WT or BTK^Cys481Ser expressing BCWM.1 WM or TMD8 ABC DLBCL cells were pretreated with vehicle control or ibrutinib at indicated concentrations in the upper chambers of 0.4-μm filtered plates for 6 hours. Nontransduced counterparts were then cultured for 72 hours with added ibrutinib at the indicated concentrations in the lower compartments and assessed for viability using the CellTiter-Glo assay. The single (DMSO) line shows viability of nontransduced cells with drug vehicle control alone. The x-axis shows ibrutinib log concentration (C). ***P < .001.