Cell-cell contact is required for stroma-induced sensitization of KG1a cells to apoptotic signals. (A) KG1a cells physically separated from stroma remain insensitive to proapoptotic ligands. HS5 (bottom chamber) and KG1a cells (top chamber) were cultured in noncontact transwell plates and exposed to TNF-α (100 ng/mL). After 18 hours, apoptosis was assessed as described for Figure 1A. (B) Sustained cell-cell contact was protective against apoptotic signals. Cocultures of HS5 and KG1a cells were incubated in the presence or absence of TNF-α (100 ng/mL). After 18 hours, the supernatant containing nonadherent KG1a cells was collected. The remaining cells, stromal and attached KG1a cells, were collected after a short exposure to trypsin. Apoptosis in KG1a cells was assessed in both groups (attached and nonattached) as described for Figure 1A. KG1a cells that had remained attached to stroma showed no apoptosis, whereas those detached from stroma showed apoptosis (Ω indicates P ≤ .001).