The sushi domain of soluble IL-15 receptor alpha is essential for binding IL-15 and inhibiting inflammatory and allogenic responses in vitro and in vivo

IL-15 is a pleiotropic cytokine that plays important roles in both innate a nd adaptive immunity. It is associated with a range of immunopathology, inc luding rheumatoid arthritis and allograft rejection. IL-15 functions throug h the trimeric IL-15R complex, which consists of a high affinity binding a- chain and the common IL-2R beta- and gamma -chains. Characterization of IL- 15/IL-15R interactions may facilitate the development of improved IL-15 ant agonists for therapeutic interventions. We previously constructed soluble m urine IL-15R alpha (sIL-15R alpha) by deleting the cytoplasmic and transmem brane domains. To localize the functional domain of IL-15R alpha, we have n ow constructed various truncated versions of sIL-15R alpha. The shortest re gion retaining IL-15 binding activity is a 65-aa sequence spanning the Sush i domain of IL-15R alpha. Sushi domains, common motifs in protein-protein i nteractions, contain four cysteines forming two disulfide bonds in a 1-3 an d 2-4 pattern. Amino acid substitution of the first or fourth cysteine in s IL-15R alpha completely abolished its IL-15 binding activity. This also abr ogated the ability of sIL-15R alpha to neutralize IL-15-induced proinflamma tory cytokine production and anti-apoptotic response in vitro. Furthermore, the mutant sIL-15R alpha lost its ability to inhibit carrageenan-induced l ocal inflammation and allogenic cell-induced T cell proliferation and cytok ine production in vivo. Thus, the Sushi domain is critical for the function al activity of sIL-15R alpha.