MOLECULAR AND BIOLOGICAL PROPERTIES OF AN INTERLEUKIN-1 RECEPTOR IMMUNOADHESIN

Overproduction of the cytokine interleukin 1 (IL-1) is an important fa ctor in the pathogenesis of several autoimmune and inflammatory diseas es. To develop a recombinant inhibitor of IL-1 with an extended pharma cologic half-life, we constructed an IL-1 receptor immunoadhesin (IL-1 R-IgC), by fusing the extracellular domain of the type IL-1 receptor w ith the hinge and Fc regions of human IgG1 heavy chain. Transfected hu man 293 cells express IL-1R-IgG as a secreted, disulfide-bonded homodi mer. The secreted protein contains an intact antibody Fc region, as in dicated by immunoblotting, and a functional IL-1 receptor region, as i ndicated by ligand-blotting. Saturation binding analysis indicates an equilibrium dissociation constant (K-D) of 350 pM for the binding of I L-IR-IgG to its ligand, IL-1(beta). Kinetic analysis of the binding re veals an off rate of 0.1 min(-1) and an on rate of 1.5 x 10(8) min(-1) M(-1), yielding a calculated K-D of 770 pM. These binding properties are similar to those of cell-surface type I IL-1 receptor. IL-1R-IgG i s capable of inhibiting the biological activity ofIL-1(beta) in vitro, as evidenced in a thymocyte proliferation assay. Pharmacokinetic anal ysis in mice indicates that IL-1R-IgG has a terminal half-life of 91 h r in the blood circulation. This half-life is markedly longer than the values reported for other recombinant inhibitors of IL-1 such as the IL-1 receptor antagonist or soluble IL-1 receptor. Thus, IL-1R-IgG may be useful for investigating the interaction of IL-1 with its receptor and the role of IL-1 in disease, as well as for potential interventio n in pathological situations involving overproduction of IL-1.