DIFFERENTIAL BINDING OF HUMAN INTERLEUKIN-1 (IL-1) RECEPTOR ANTAGONIST TO NATURAL AND RECOMBINANT SOLUBLE AND CELLULAR IL-1 TYPE-I RECEPTORS

A recently described factor, interleukin-1 receptor antagonist binding factor (IL-1raBF), in serum of normal individuals is immunologically related to the interleukin-1 receptor type I (IL-1RI). It is presumabl y a soluble form of the receptor that binds exclusively to interleukin -1 receptor antagonist (IL-1ra). Recombinant soluble human IL-1RI expr essed in COS cells (sIL-1RI) consists of the extracellular part of the receptor and binds all three known IL-1 species but preferentially to IL-1ra. We further characterized the sizes and binding of IL-1raBF an d sIL-1RI to IL-1ra by polyacrylamide gel electrophoresis in the prese nce of sodium dodecylsulfate, ligand binding interference analyses, N- glycosidase treatment, concanavalin A affinity chromatography, and wit h the use of monoclonal antibodies (mAb) to human recombinant IL-1ra. We also evaluated the binding of IL-1ra to cellular IL-1RI on MRC5 fib roblasts and the interference afforded by the soluble receptors. The r esults show that the protein backbones of IL-1raBF and sIL-1RI are of similar size (approximate to 35-40 kDa) and that there are differences in the glycosylation of the two molecules. These carbohydrates were n ecessary for optimal binding of both molecules to IL-1ra. Both factors blocked binding of IL-1ra to cellular IL-1RI, as did mAb to IL-1ra, b ut the sites on IL-1ra which bound to the mAb, and to IL-1raBF and sIL -1RI, differed. We conclude that there are important differences betwe en the natural and recombinant forms of soluble IL-1RI and that IL-1ra binds differently to these molecules and to cellular IL-1RI.