Cell surface IL-15 is not associated with its own receptor. (A) MONO-MAC-6 were treated with IFN-γ (500 U/mL) for 24 hours. Cells were then washed and incubated in PBS, acetate buffer (pH 4.4), or trypsin as described in Materials and Methods. All groups were stained with M112 MoAb and analyzed on FACSort. Fluorescence intensity is represented by open histograms; solid histograms refer to the background staining of isotype-matched control MoAb. (B) MONO-MAC-6 treated with IFN-γ as described above were incubated with PBS (left panel) or trypsin (right panel). Cells were then incubated with biotin-conjugated IL-15 IgG2b fusion protein (open histogram) or with equal amount of isotype-matched biotinylated IgG (solid histogram). In the central panel, cells were incubated with the IL-15 IgG2b fusion protein and then with acetate buffer, pH 4.4. The x axis represents the intensity of green fluorescence expressed in a log scale as mean channel, and the y axis represents the number of cells per channel.