Human MSCs protect CLL cells from spontaneous and drug-induced apoptosis. Displayed are the mean relative viabilities of CLL cells in the absence or presence of murine MSCs and/or 10 μM F-ara-A at the time points shown on the horizontal axis. Results represent data for 20:1 CLL/MSC ratios and are the mean (± SEM) relative viabilities, compared with untreated controls (100%) from 3 to 5 different patients with CLL for each MSC line. All human MSC lines (except for the primary hMSC pt#2; see supplemental Table 3) provided significant levels of protection from F-ara-A–induced apoptosis, although the levels of protection generally were lower than those provided by murine MSCs. The names of the human MSCs are displayed above each diagram.