Interleukin-1 receptor antagonist: discovery, structure and properties.

Interleukin-1 receptor antagonist (IL-1ra) is a recently-described member of the IL-1 family. This unique human protein has 30% amino acid sequence homology to IL-1 beta and binds to human types I and II IL-1 receptors without apparent cellular activation. IL-1ra blocks the in vitro stimulatory effects of IL-1 on thymocytes, fibroblasts, endothelial cells and bone cells. In addition, IL-1ra is a potent inhibitor of the inflammatory effects of IL-1 in vivo. IL-1ra represents the first naturally-occurring cytokine inhibitor and may be important in modulating IL-1 effects in both normal and abnormal physiology.