Cytoprotection by bone marrow stromal cells. (A) Cells from patient CLL1 were incubated for 18 hours with or without contact with marrow stromal cells (MSCs), as indicated. PF-956980 was added at the concentrations shown. Proteins were extracted and analyzed by Western blotting. (B) Cells from CLL12 were left untreated or incubated with 100μM chlorambucil for 3 hours, washed, and resuspended in fresh medium. Cells were placed in wells of a 24-well culture plate in the absence or presence of MSCs, as indicated. PF-956980 was added at the concentrations shown. After 18 hours of further incubation, proteins were extracted and PARP cleavage was quantified by Western blotting. Data are representative of 5 experiments.