Interleukins

Abstract

The interleukins (IL) belong to a category of biologically active proteins, termed cytokines. These proteins are derived mainly from white blood cells and act on white cells, although these criteria are not necessarily exclusive of other cell sources and activities. Interleukins, among other proteins, maintain the haematopoietic and immune systems and control their functions. They have been classified specifically by a designated nomenclature committee of the International Union of Immunological Societies/World Health Organization. In general, numbering of interleukins has followed the order of their discovery and this has led to some anomalies. It has become evident that interleukins can be classified into subgroups or families based on shared structural characteristics, and/ or shared receptor subunits and/or shared chromosomal locations. For example, IL‐1α, IL‐1β, IL‐18 and IL‐33 belong to the ‘IL‐1 family’, IL‐2, IL‐15 and IL‐21 to the ‘IL‐2 family’ (IL‐4, IL‐7, IL‐9 and IL‐13 might also be included as they share at least one receptor subunit with IL‐2 family members), IL‐10, IL‐19, IL‐20, IL‐22, IL‐24, IL‐26, IL‐28 and IL‐29 to the ‘IL‐10 family’, IL‐12, IL‐23 and IL‐27 to the ‘IL‐12 family’. IL‐3 and IL‐5 together with GM‐CSF and IL‐6 and IL‐11 together with several other noninterleukin named cytokines – LIF, OSM, CNTF, CT‐1 – can also be grouped as families of ‘like’ cytokines. The IL‐17 family is comprised of six related proteins (IL‐17A–F), one of which, the murine homologue of IL‐17E, was anomalously designated IL‐25. With hindsight, the remaining interleukins might have been better classified with other biological modifiers. For instance, IL‐8 is clearly a chemokine; IL‐14 and IL‐16 bear scant resemblance to any other interleukins, or other cytokines for that matter. In the following paper, the molecular characteristics of interleukin proteins, their cognate receptors and intracellular signalling pathways and their biological activities are briefly described. The interactions of particular interleukins in the regulation of immune cell growth, differentiation and function will be illustrated by short sections at the end of the paper covering (1) T‐lymphocyte development and activation and (2) chronic inflammation and autoimmunity.