Recombinant FDC-SM-8D6 supports L3055 cell growth. 8D6-3T3 cells were prepared as described in “Materials and methods.” (A) Transfected 3T3 cells express Flag tag-FDC-SM-D6. 8D6-3T3 cells were trypsinized and subjected to FACS analysis by using mAb 8D6 and anti-Flag Ab followed by FITC-conjugated goat anti–mouse immunoglobulin (solid histograms). 3T3 cells were used as a negative control. (B and insert) 8D6-3T3 cells stimulate L3055 cell growth. Different numbers of irradiated 8D6-3T3 and 3T3 cells were seeded in 24-multiwell plates 1 day before the addition of L3055 cells. Monoclonal Ab 8D6 (20 μg/mL) was used to block stimulatory effect of 8D6-3T3 cells, and mAb 3C8 was used as an isotype control (empty histograms). Viable L3055 cell number at day 4 from 1 of 3 reproducible experiments is shown. (C) Soluble FDC-SM-8D6 further enhanced HK cell–supported L3055 cell growth. L3055 cells (2 × 10⁴ cells/well) were cocultured with HK cells (1 × 10⁴ cells/well) in the presence of different doses of GST-8D6 or GST as indicated. Viable L3055 cells were counted at day 4. Data from 1 of 3 experiments are shown.