Cytofluorometric analysis of the binding of IL-15 on ATL cell lines. After incubation of the cell lines with IL-15 in the absence (a) or presence (b) of 100-fold excess amount of IL-2, the cell-surface–bound IL-15 was detected by an anti–IL-15 MoAb (closed histogram). All cell lines had almost the equivalent amounts of the IL-15 binding sites, which were not inhibited by IL-2. Open hitograms indicate the cells incubated with a control MoAb.