Direct cell-to-cell contact is essential for cell adhesion–mediated drug resistance. CLL cells were treated with 10 μM F-ara-A and incubated with KUSA H1 (A) and StromaNKTert (B) in the presence or absence of micropore membrane insert for 24, 48, and 72 hours. Bars represent the mean viability of CLL cells compared with untreated control (100%). Data shown are the mean (± SEM) of 6 independent experiments. *Significant protection of CLL cells from F-ara-A–induced apoptosis in direct CLL–MSC contact compared with control sample (P < .05). (C) Presented are contour plots of CLL cells from a representative patient after 48 hours of coculture with StromaNKTert in the conditions indicated above and on the side of the plots. The relative percentages of viable cells are displayed above each of the gates. Direct cell-to-cell contact is essential not only for protection from fludarabine-induced apoptosis but also from spontaneous apoptosis.