Secreted equine IL-1 receptor antagonist (IL-1ra) is a presumably 22 - 25 kDa glycoprotein produced by variety of cell types that antagonizes IL-1 activity (1 - 3). It is a member of the IL-1 family of proteins that includes IL-1 alpha and IL-1 beta. Although there is little amino acid (aa) identity (< 30%) among the three IL-1 family members, all molecules bind to the same receptors, all show a beta -trefoil structure, and all are believed to have evolved from a common ancestral gene (1 - 4). Equine IL-1ra is synthesized as a 177 aa precursor that contains a 25 aa signal sequence plus a 152 aa mature region. There is one intrachain disulfide bond and one potential N-linked glycosylation site (3, 5, 6). Mature equine sIL-1ra is 78%, 78%, 80%, 82%, and 76% aa identical to mature mouse, human, porcine, canine and bovine IL-1ra, respectively. In human, three non-secreted IL-1ra isoforms have also been identified. It is unknown if such an analogous situation exists in equine. Cells known to secrete IL-1ra include fibroblasts, vascular smooth muscle cells, intestinal columnar epithelium, chondrocytes, macrophages, mast cells, neutrophils and hepatocytes.
There are two type I transmembrane glycoprotein receptors for IL-1ra. The first is the bioactive 80 kDa type I IL-1 receptor (IL-1 RI), and the second is the inert (decoy) 65 kDa type II IL-1 receptor. IL-1ra binding to IL-1 RI competitively blocks IL-1 ( alpha or beta ) binding to the same receptor. This results in receptor ligation without activation (1, 7). The type II IL-1 receptor is inert, and any binding of IL-1ra not only fails to block co-existing IL-1 activity, but may actually potentiate it by removing an IL-1 antagonist. Functionally, all activities attributed to IL-1ra are explained by its role as a competitive inhibitor of IL-1 binding to IL-1 RI (1, 2, 8, 9).
