Primary stromal cells render KG1a cells sensitive to apoptotic signals. (A) Stroma cells were isolated from marrow aspirates. Shown are the results of flow cytometric phenotyping as described in “Methods.” (B) When the cultures reached confluence, the stromal cells were seeded in 96-well plates and KG1a cells were added. After overnight incubation in the presence or absence of TNF-α (100 ng/mL), apoptosis in the KG1a cells was assessed as described for Figure 1A. Primary stromal cells were obtained from healthy volunteers (NBM) or patients with various stages of MDS (RCMD, refractory cytopenia with multilineage dysplasia; RARS, refractory anemia with ring sideroblasts). Ω indicates P < .01 compared with vehicle-treated cultures.