Bioactivity of stromal cell–bound IL-7. Stromal cells (≈5 × 10⁴/well) derived from IL-7 knockout mice were treated in the presence or absence of heparitinase and susbequently incubated with rIL-7 (5 ng/well) for 30 minutes at 37°C. Cells were washed to remove unbound IL-7 and then 2 × 10⁴ BC7.12 lymphocytes were added to each well. Cultures were incubated for 3 days after which viable lymphocytes were enumerated. To assess maximal IL-7 bioactivity, the washing step was omitted in one treatment group (central bar). The results are presented as the average ± SD of four independent experiments.