Resto cells display a tumor B-cell supportive effect. (A) Growth of tumor B cell lines. BL2, VAL, and RL cells lines were cultured for 3 days in low serum concentration alone, or with confluent Resto cells, pretreated or not with TNF/LT for 7 days. Proliferation was assessed by tritiated thymidine (³H-TdR) incorporation determined in sixplicate culture wells. Stromal cells cultured alone always showed tritiated thymidine incorporation less than 500 cpm. Data are expressed as percentages of thymidine incorporation by B-cell lines cultured in the presence of Resto cells (pretreated or not with TNF/LT) with respect to B-cell lines alone (assigned to 100%). Bars represent mean values ± SD from 4 (BL2) or 5 (VAL, RL) independent experiments. *Mean value is statistically significantly different from that obtained without stromal cells (P<.05). **Mean value is statistically significantly different from that obtained with untreated Resto cells (P<.05). (B) Apoptosis of tumor B cell lines. BL2 cells were cultured under low serum concentration alone or on confluent Resto cells pretreated or not with TNF/LT. Cells were recovered after 3 days of coculture and CD45⁺ apoptotic cells were detected by staining with active caspase-3 staining. Percentage of caspase-3⁺ cells is indicated in each panel. Results are those of 1 experiment representative of 5.