Requirement for direct cell-cell contact for the secretion of IL-7 from stromal cells. Cell contact between the IL-7–dependent pre-B cells and the stromal cells was prevented by use of a 0.45-μm membrane (no contact). (A) IL-7–dependent proliferation did not occur without direct cell contact with the stromal cells. (B) Proliferation of IL-7–dependent pre-B cells to rIL-7 was not affected by the presence or absence of cell contact with stromal cells. The results shown in (B) used rIL-7 at a concentration of 0.25 ng/mL; similar results were found with IL-7 at 1.0 ng/mL (data not shown). The results shown in (A) and (B) used the pre-B–cell line BC77; similar results were found with BC715. (□) No SC; (▪) 1-month-old SC; () 24-month-old SC.