Evolutionary divergence and functions of the human interleukin (IL) gene family
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Evolutionary divergence and functions of the human interleukin (IL) gene family Chad Brocker,1 David Thompson,2 Akiko Matsumoto,1 Daniel W. Nebert3* and Vasilis Vasiliou1 1
Molecular Toxicology and Environmental Health Sciences Program, Department of Pharmaceutical Sciences, University of Colorado Denver, Aurora, CO 80045, USA 2 Department of Clinical Pharmacy, University of Colorado Denver, Aurora, CO 80045, USA 3 Department of Environmental Health and Center for Environmental Genetics (CEG), University of Cincinnati Medical Center, Cincinnati, OH 45267 – 0056, USA *Correspondence to: Tel: þ1 513 821 4664; Fax: þ1 513 558 0925; E-mail: [email protected]; [email protected] Date received (in revised form): 22nd September 2010
Abstract Cytokines play a very important role in nearly all aspects of inflammation and immunity. The term ‘interleukin’ (IL) has been used to describe a group of cytokines with complex immunomodulatory functions — including cell proliferation, maturation, migration and adhesion. These cytokines also play an important role in immune cell differentiation and activation. Determining the exact function of a particular cytokine is complicated by the influence of the producing cell type, the responding cell type and the phase of the immune response. ILs can also have pro- and anti-inflammatory effects, further complicating their characterisation. These molecules are under constant pressure to evolve due to continual competition between the host’s immune system and infecting organisms; as such, ILs have undergone significant evolution. This has resulted in little amino acid conservation between orthologous proteins, which further complicates the gene family organisation. Within the literature there are a number of overlapping nomenclature and classification systems derived from biological function, receptor-binding properties and originating cell type. Determining evolutionary relationships between ILs therefore can be confusing. More recently, crystallographic data and the identification of common structural motifs have led to a more accurate classification system. To date, the known ILs can be divided into four major groups based on distinguishing structural features. These groups include the genes encoding the IL1-like cytokines, the class I helical cytokines (IL4-like, g-chain and IL6/12-like), the class II helical cytokines (IL10-like and IL28-like) and the IL17-like cytokines. In addition, there are a number of ILs that do not fit into any of the above groups, due either to their unique structural features or lack of structural information. This suggests that the gene family organisation may be subject to further change in the near future. Keywords: IL, interleukin, cytokine, gene family, evolution, human genome
Introduction ILs are a large group of immunomodulatory proteins that elicit a wide variety of responses in cells and tissues. These cytokines comprise a large number of the known immunological ‘secondmessenger’ molecules within mammals.1 Interleukins initiate a response b
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