Interleukin signaling

Interleukins are low molecular weight proteins produced by leukocytes that bind to cell surface receptors and act in an autocrine and/or paracrine fashion. These secreted factors modify the cell differentiation program (survival, proliferation, maturation, induction of or protection from apoptosis) of the target cells to which they bind. As such, interleukins impact on diverse biological processes: tissue growth and repair, hematopoietic homeostasis, and at multiple levels of the host defense against pathogens. It is therefore not surprising that the rather generic designation ‘interleukin’ does not refer to a homogeneous family of proteins, but instead to groups of proteins that adopt different threedimensional structures, bind to distinct classes of receptors, and initiate unique signal transduction pathways. Our discussion of interleukins will be structured according to the two main classes of receptors to which they bind: the type I and type II cytokine receptor families and the IL1R/TLR (interleukin-1 receptor/ Toll-like receptor) superfamily.