Primary CLL cells cocultured for 24 hours with mouse or human stroma are resistant to the BH3-mimetic ABT-737. (A) Representative primary FACS data of PB-derived CLL cells cocultured with either CD154 ⁺ or CD154⁻ stroma shows that the CD154⁺ cells significantly maintain CLL cell viability (annexin V and propidium iodide negative) in response to 100nM ABT-737. (B) Aggregate data for CLL cells cocultured ± 100nM ABT-737 with CD154 stroma confirms stromal protection (n = 4), with means depicted as horizontal bars ± SEM; P ≤ .05 between each group. (C) Aggregate data for CLL cells cocultured ± 100nM ABT-737 with or without the human stromal cell line StromaNKTert confirms the strongly protective effect of human stroma (n = 7). (D) Aggregate data from dose-response curves on 5 CLL patient samples cocultured ± ABT-737 with or without StromaNKTert. Both drugs kill consistently in a dose-dependent manner in the absence of stroma; induction of apoptosis by both drugs is significantly inhibited in the presence of stroma.