FL-MSCs and macrophages cooperate to sustain FL B-cell growth. (A-C) GC-derived B-cell lines were cultured in low serum concentration alone (CTRL), in the presence of FL-MSCs, in vitro–differentiated macrophages (Macro), or with a combination of Macro and FL-MSCs that have previously established a bidirectional crosstalk during a 4-day coculture. (A) Cell growth was evaluated at day 3 on VAL (left) and RL (right) by the incorporation of tritiated thymidine (³H-TdR). Results represent the mean ± SD from 6 experiments. MSCs and macrophages cultured without B cells always showed a ³H-TdR incorporation ≤ 500 cpm. (B) Apoptosis was evaluated at day 1 on VAL by the use of active caspase-3 staining gated on CD19/CD20^pos B cells. Results represent the mean ± SD from 6 experiments. (C) Proliferation was evaluated at day 3 on VAL using BrdU staining gated on CD19/CD20^pos B cells. Results represent the mean ± SD from 4 experiments. *P < .05; **P < .01; ***P < .001; ns: not significant. (D) Purified B cells obtained from 4 patients with FL were cultured alone (CTRL), in the presence of FL-MSCs, in vitro–differentiated macrophages (Macro), or with a combination of Macro and FL-MSCs that have previously established a bidirectional crosstalk during a 4-day coculture. The absolute number of B cells was assessed using TOPRO-3 staining and calibrated beads. **P < .01; ***P < .001.