A potent dimeric peptide antagonist of interleukin-5 that binds two interleukin-5 receptor alpha chains

Two series of peptides that specifically bind to the extracellular domain o f the alpha chain of the human interleukin-5 receptor (IL-5R alpha). but sh are no primary sequence homology to IL-5, were identified from libraries of random recombinant peptides. Affinity maturation procedures generated a 19 -aa peptide that binds to the IL-5 receptor alpha/beta heterodimer complex with an affinity equal to that of IL-5 and is a potent and specific antagon ist of IL-5 activity in a human eosinophil adhesion assay. The active form of the peptide is a disulfide-crosslinked dimer that forms spontaneously in solution. Gel filtration analysis, receptor-binding studies, and analytica l ultracentrifugation reveal that the dimeric peptide binds simultaneously to two receptor alpha chains in solution. Furthermore, the dimer peptide, b ut not IL-5, can activate a chimeric receptor consisting of the IL-5R alpha extracellular domain fused to the intracellular domain of the epidermal gr owth factor receptor, thus demonstrating that the peptide also promotes rec eptor dimerization in a cellular context. The functional antagonism produce d by the bivalent interaction of the dimeric peptide with two IL-5R alpha c hains represents a distinctive mechanism for the antagonism of cytokines th at use heteromeric receptors.