Hu-Mikβ1 inhibits IL-15–induced proliferation of wild-type Kit 225 cells but not Kit 225 7.10 cells that had been transfected with IL-15Rα. (A) Hu-Mikβ1 inhibited IL-15– but not IL-2–mediated proliferation of wild-type Kit 225 cells that express IL-2Rα, IL-2/IL-15Rβ, and γc but not IL-15Rα. (B) Anti-Tac (anti–IL-2Rα) inhibited IL-2– but not IL-15–mediated proliferation of wild-type Kit 225 cells. (C) In contrast neither anti-Tac (♦) nor Hu-Mikβ1 (■) inhibited the IL-15 (1 ng/mL)–induced proliferation of Kit 225 7.10 cells that had been transfected with IL-15Rα and that expressed IL-15Rα, IL-2/IL-15Rβ, and γc chains in cis. Anti–IL-15 (○) did inhibit IL-15–mediated proliferation in these cells.