ATTENUATION OF IL-5-MEDIATED SIGNAL-TRANSDUCTION, EOSINOPHIL SURVIVAL, AND INFLAMMATORY MEDIATOR RELEASE BY A SOLUBLE HUMAN IL-5 RECEPTOR

A soluble form of the human IL-5R alpha-chain (IL-5Ra) that contains t he extracellular IL-5 binding domain has been evaluated for its effect an IL-5 binding to and activation of human eosinophils and basophils. The truncated receptor was expressed in Escherichia coli and recovere d in biologically active form following renaturation and anion exchang e chromatography. The soluble receptor formed a 1/1 complex with IL-5 in solution and bound IL-5 with affinity comparable to that of cell-as sociated IL-5Ra, Soluble IL-5Ra also competed with IL-5 for binding to the native alpha beta IL-5R on human cells and inhibited IL-5-mediate d receptor activation and inflammatory mediator production. In this re gard, the soluble receptor prevented IL-5-induced tyrosine phosphoryla tion of JAK2 kinase and IL-5R beta-chain and inhibited IL-5 priming of leukotriene C4 release by human basophils. However, the E, coli-deriv ed receptor failed to inhibit IL-5 in longer term assays, including eo sinophil survival and TF-1 cell proliferation, possibly due to its pro pensity to aggregate in a time- and temperature-dependent manner, In c ontrast, we observed that a soluble IL-5Ra derived from baculovirus-in fected cells was less prone to aggregate and effectively antagonized I L-5-induced cell proliferation and survival, These findings indicate t hat the extracellular portion of the human IL-5Ra chain can prevent as sociation of IL-5 with cell surface receptors and can attenuate signal transduction, mediator release, and survival of inflammatory cells. A s such, soluble IL-5R may be useful in treating diseases such as human asthma, in which pulmonary injury is associated with the activity of IL-5R-bearing cells.