IL-15Rα is a subunit of the IL-15 receptor together with IL-2Rβ (CD122) and IL-2Rγ (CD132), two subunits shared with IL-2 receptor. A soluble form of IL-15Rα, including the extracellular sushi domain, is naturally released by antigen-presenting cells in vivo. Soluble IL-15Rα binds with high affinity IL-15 and acts as IL-15 antagonist through the prevention of IL-15 binding on its α/β/γ receptor. However, on lymphocytes that express only the heterodimeric IL-2Rβ/γ receptor, IL-15Rα presents IL-15 in trans to the IL-2Rβ/γ receptor, and enhances the effects of IL-15, increasing lymphocyte proliferation and survival. Recombinant Human IL-15Rα sushi is composed of the extracellular sushi domain of IL-15Rα plus a natural hinge domain, and it is a high affinity agonist of IL-15 in culture, maintaining the natural biological effect of soluble IL-15Rα.