Drug resistance of CD40-stimulated CLL cells is reversed by c-Abl kinase inhibitors. (A) CLL samples (n = 4) were cultured on 3T3 (control) or CD40L-expressing cells in the presence of the indicated inhibitors for 48 hours, and after detachment and washing cultured for 24 hours in medium or with the cytotoxic drugs. Average results for apoptosis measured via Mitotracker staining are shown. (B) The same as in panel A for an experiment with p53 dysfunctional cells. Data are representative for 3 similar experiments performed; the variation among samples in particular for background apoptosis in the absence of external stimuli precluded averaging. (C) A similar experiment as in panel A was performed with decreasing concentrations of dasatinib as indicated. Drug susceptibility was assessed by incubation with 5 μM GSI-1 for 24 hours. Results represent averages of 4 experiments or 2 where indicated. At 3 nM there was no effect of dasatinib detectable (not shown). (D) Sequential CD40 stimulation followed by incubation with c-Abl kinase inhibitors. CLL cells were cocultured with 3T3 cells expressing CD40L for 48 hours, detached and washed before addition of dasatinib (300 nM) for an additional 48 hours, and were then tested for drug susceptibility. Results represent average data of 3 experiments.