IL-1 is a name that designates two proteins, IL-1 alpha and IL-1 beta, that are the products of distinct genes, but recognize the same cell surface receptors. IL-1 alpha and IL-1 beta are structurally related polypeptides that show approximately 25% homology at the amino acid level. Both proteins are produced by a wide variety of cells in response to stimuli such as those produced by inflammatory agents, infections, or microbial endotoxins. The proteins are synthesized as 31 kDa precursors that are subsequently cleaved into proteins with molecular weights of approximately 17.5 kDa. The specific protease responsible for the processing of IL-beta, designated interleukin 1 beta -converting enzyme (ICE), has been described. Mature human and mouse IL-1 beta share approximately 75% amino acid sequence identity and human IL-1 beta has been found to be active on murine cell lines.
IL-1 alpha and IL-1 beta are potent pro-inflammatory cytokines that induce a wide variety of biological activities on different cell types. Two distinct types of IL-1 receptors have been identified and cloned from human and mouse cells. The IL-1 receptor type I is a 80 kDa transmembrane protein with demonstrated IL-1 signaling function. The IL-1 receptor type II is a 68 kDa membrane protein with a relatively short cytoplasmic tail and has no signaling function. The type II receptor acts as a decoy target for IL-1, inhibiting IL-1 activities by preventing the binding of IL-1 to the type I receptor. A soluble version of the type II receptor is induced by anti‑inflammatory agents such as glucocorticoids, IL-4 and IL-13.
