MSC coculture protects primary BCP-ALL cells from cell death. (A) Isolated BCP-ALL blasts from ALL5 were cultured in the absence or presence of a confluent layer of iMSC #3. After 2 h, the blasts were briefly removed from the coculture and irradiated with 2 Gy of IR. The cells were then reintroduced to the coculture and harvested after 20 h for examination of cell death in the CD19+ cell fraction by CD19-FITC/PI co-staining as described in Materials and Methods. The histogram shows mean values of 10 independent experiments, with error bars indicating SEM values. *P < 0.0001 by paired samples t test. (B) Isolated BCP-ALL blasts from ALL5 (n = 3) and ALL16 (n = 3) cultured in the absence or presence of a confluent layer of primary BM-MSC were treated and analysed as described in the legend to Figure 1A. The left panel shows absolute cell death values for each experiment with bars representing median values. *P < 0.05 by Wilcoxon matched pairs signed rank test. The right panel shows relative reduction in cell death mediated by MSC coculturing for each BCP-ALL sample at 0 Gy and 2 Gy of irradiation, respectively. The horizontal bars represent median values. (C) Isolated BCP-ALL blasts from 9 different patient samples were treated and analysed as described in Figure 1A, and grouped according to cytogenetic sub-classification. The upper panel shows absolute cell death values after 0 and 2 Gy of irradiation (ALL5: n = 10, error bars represent SEM values; ALL16: n = 3, error bars represent SEM values; ALL18: n = 2, error bars represent range of values; ALL6/10/12/17/19/20: values represent results from single experiments). The lower panel shows the relative reduction of cell death imposed by iMSC#3 on irradiated BCP-ALL blasts.