Bidirectional interaction between tumor B cells and BM-MSCs. (A) Meshwork induction. BM-MSCs were cultured alone or in the presence of TNF/LT, purified CD19⁺ peripheral blood B cells, BL2 cell line, or purified CD19⁺ primary FL B cells for 7 days. Expression of transglutaminase (TG) was revealed by fluorescence microscopy. Bar represents 20 μm. (B) Growth of tumor B cell lines. BL2 was cocultured for 3 days in low serum concentration alone or with confluent BM-MSCs, pretreated or not with TNF/LT for 7 days. Cell growth 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 the thymidine incorporation by BL2 cells cultured in the presence of BM-MSCs (pretreated or not with TNF/LT) with respect to BL2 cells alone (assigned to 100%). Results represent mean values ± SD from 4 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 BM-MSCs (P<.05). (C) Apoptosis of BL2 cell line. Confluent BM-MSCs pretreated or not with TNF/LT for 7 days were thereafter cocultured with BL2 cell line under limiting serum concentration. Apoptotic CD45⁺ tumor cells were detected by active caspase-3 staining after 3 days of coculture. Percentage of caspase-3⁺ cells is indicated in each panel. Results are those of 1 experiment representative of 5.