Interleukin

Interleukins (ILs) are a group of cytokines (secreted proteins and signal molecules) that were first seen to be expressed by white blood cells (leukocytes). ILs can be divided into four major groups based on distinguishing structural features. However, their amino acid sequence similarity is rather weak (typically 15–25% identity). The human genome encodes more than 50 interleukins and related proteins. Interleukins (ILs) are a group of cytokines (secreted proteins and signal molecules) that were first seen to be expressed by white blood cells (leukocytes). ILs can be divided into four major groups based on distinguishing structural features. However, their amino acid sequence similarity is rather weak (typically 15–25% identity). The human genome encodes more than 50 interleukins and related proteins. The function of the immune system depends in a large part on interleukins, and rare deficiencies of a number of them have been described, all featuring autoimmune diseases or immune deficiency. The majority of interleukins are synthesized by helper CD4 T lymphocytes, as well as through monocytes, macrophages, and endothelial cells. They promote the development and differentiation of T and B lymphocytes, and hematopoietic cells. Interleukin receptors on astrocytes in the hippocampus are also known to be involved in the development of spatial memories in mice. The name 'interleukin' was chosen in 1979, to replace the various different names used by different research groups to designate interleukin 1 (lymphocyte activating factor, mitogenic protein, T-cell replacing factor III, B-cell activating factor, B-cell differentiation factor, and 'Heidikine') and interleukin 2 (TSF, etc.). This decision was taken during the Second International Lymphokine Workshop in Switzerland (27-31 May 1979 in Ermatingen). The term interleukin derives from (inter-) 'as a means of communication', and (-leukin) 'deriving from the fact that many of these proteins are produced by leukocytes and act on leukocytes'. The name is something of a relic; it has since been found that interleukins are produced by a wide variety of body cells. The term was coined by Dr Vern Paetkau, University of Victoria. Some interleukins are classified as lymphokines, lymphocyte-produced cytokines that mediate immune responses. Interleukin 1 alpha and interleukin 1 beta (IL1 alpha and IL1 beta) are cytokines that participate in the regulation of immune responses, inflammatory reactions, and hematopoiesis. Two types of IL-1 receptor, each with three extracellular immunoglobulin (Ig)-like domains, limited sequence similarity (28%) and different pharmacological characteristics have been cloned from mouse and human cell lines: these have been termed type I and type II receptors. The receptors both exist in transmembrane (TM) and soluble forms: the soluble IL-1 receptor is thought to be post-translationally derived from cleavage of the extracellular portion of the membrane receptors. Both IL-1 receptors (CD121a/IL1R1, CD121b/IL1R2) appear to be well conserved in evolution, and map to the same chromosomal location. The receptors can both bind all three forms of IL-1 (IL-1 alpha, IL-1 beta and IL-1 receptor antagonist). The crystal structures of IL1A and IL1B have been solved, showing them to share the same 12-stranded beta-sheet structure as both the heparin binding growth factors and the Kunitz-type soybean trypsin inhibitors. The beta-sheets are arranged in 4 similar lobes around a central axis, 8 strands forming an anti-parallel beta-barrel. Several regions, especially the loop between strands 4 and 5, have been implicated in receptor binding.