Development of sIL-15Rα ELISA. (A) IL-15Rα ELISA curve using serial dilutions of sIL-15Rα–IL-2 standard. The absorption was measured at 405 nm. (B) The cell surface expression of IL-15Rα in 293T cells transfected with vector only, wild-type IL-15Rα, or extracellular domain of IL-15Rα (sIL-15Rα) as assessed by flow cytometry. (C) sIL-15Rα levels in the supernatants from 293T cells transfected with vector only, wild-type IL-15Rα, and sIL-15Rα measured with the assay. N.D. indicates nondetectable. (D) The cell surface expression of IL-15Rα in a murine mast cell line PT-18 and PT18-IL15Rα by flow cytometry. (E) sIL-15Rα levels in the supernatants from cell lines PT18 and PT18-IL15Rα. N.D. indicates nondetectable.