First printed in R&D Systems' 1999 Catalog.
While a number of Interleukins such as Interleukin-1 (IL-1)1 and IL-102 are seemingly pleiotrophic in their effects, Interleukin 6 (IL-6) may be considered the prototypic pleiotrophic cytokine.3, 4 This is reflected in the variety of names originally assigned to IL-6 based on function, including Interferon beta 2, IL-1-inducible 26 kD Protein, Hepatocyte Stimulating Factor, Cytotoxic T-cell Differentiation Factor, B cell Differentiation Factor (BCDF) and/or B cell Stimulatory Factor 2 (BSF2).3 Once all the activities associated with the various names for IL-6 became connected with one common gene, the actual name IL-6 was proposed for this molecule.5 A number of cytokines make up an IL-6 cytokine family. Membership in this family is based on a helical cytokine structure and receptor subunit makeup.6, 7 For reviews on IL-6, see references 3, 4, 8-11.
|Figure 1. IL-6 and Acute Phase Proteins. SAA = Serum Amyloid Protein A. CRP = C-Reactive Protein.|
Human IL-6 is a variably glycosylated, 22-27 kDa secreted glycoprotein that serves as a prototype for a family of molecules that includes leukemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1) and IL-11. Although all molecules possess a similar helical structure, their association is due to their functional redundancy and receptor interactions.3, 6, 7, 11 IL-6 is translated as a 212 amino acid (aa) molecule, with a 28 aa signal sequence and a 184 aa mature segment.12-15 It contains four cysteines and two potential N-linked glycosylation sites with its primary structure showing limited homology to G-CSF.15 An alternate splice variant of IL-6 was identified in monocytes and lymphocytes.16 This form is 17 kDa and 148 aa long and appears to lack a binding site for the IL-6 signal transducing molecule gp130. A virally encoded form of IL-6 in human herpesvirus type 8 is 204 aa long and shows 25% aa identity to human IL-6.17, 18 While it is active on human cells, it is not clear if its binding properties are comparable to those of IL-6.18, 19 Mouse and rat IL-6 also have been cloned and are approximately 40% identical to human IL-6 at the aa level.20-22 Unlike human IL-6, mouse and rat IL-6 lack potential N-linked glycosylation sites, but may be O-glycosylated.20 The presence or absence of glycosylation, however, has no effect on bioactivity.
The functional receptor for IL-6 is a complex of two transmembrane glycoproteins (gp130 and IL-6 receptor) that are members of the Class I cytokine receptor superfamily. Members of this family are defined by their extracellular regions with at least two adjacent fibronectin type III domains that are often preceded by a C2-type Ig-like domain. Within the type III domains, one has four conserved cysteines, while a second shows a Trp-Ser-xxx-Trp-Ser (WSxWS) motif.6, 51 A number of cytokine receptor components belong to this superfamily, including LIF R, CNTF R alpha, IL-6 R alpha and gp130,6, 9 OSM R beta,52 IL-11 R alpha 53 and CT-1 R alpha.54
gp130 (or glycoprotein-130 [kDa]) is the signal transducing subunit of the functional IL-6 receptor (IL-6 R) complex. When membrane-bound, it can range anywhere from 130-145 kDa, with the 145 kDa form representing the glycosylated “gp130”.55, 56 In humans, gp130 is 896 aa long, with a 597 aa extracellular region, a 22 aa transmembrane domain and a 277 aa cytoplasmic segment.56 In the extracellular region, there are six type III fibronectin domains plus the expected WSxWS motif and four conserved cysteines. The cytoplasmic domain lacks an intrinsic tyrosine kinase domain. Nevertheless, it contains at least three “boxes” that can associate with cytoplasmic tyrosine kinases following gp130 homodimerization.51, 57 Three pathways are activated, including the JAK/STAT, Ras/Raf, and Src-family of kinases51, 58 Although IL-6 binding to gp130 has been described,59 it is generally reported that the 80 kDa IL-6 R is the actual IL-6 binding protein.6 Mouse gp130 has been cloned and found to be 77% identical to human gp130 at the aa level (72% identity exists in the extracellular region).60 It possesses the same features as human gp130, and it will associate with human IL-6 and human IL-6 R to form a functional signaling complex.60 In the blood of normal individuals, soluble gp130 (sgp130) exists at concentrations approaching 400 ng/mL.61 Circumstances surrounding the generation of the soluble molecule are not clear. Two alternate splice forms lacking the transmembrane segment have been reported, one 624 aa62 and one 658 aa long.63 sgp130 functions as a down-modulator of IL-6 activity,61 something that is consistent with the observation that the transmembrane region of gp130 is necessary for full IL-6 signaling.64 Soluble gp130 will not bind to sIL-6 R unless IL-6 is present.61 Numerous cell types are responsive to IL-6 suggesting that gp130 is widely expressed.8, 65, 66
The human ligand-binding IL-6 receptor (IL-6 R) is an 80 kDa, transmembrane glycoprotein that is 449 aa long.67 It contains a 358 aa extracellular region, a 28 aa transmembrane domain and an 82 aa cytoplasmic segment. As with other cytokine receptor family members, its extracellular region shows four conserved cysteine residues plus a WSxWS motif. In addition, there is also an Ig-like domain at its N-terminus.51, 67 IL-6 binds to IL-6 R with a Kd = 5.5 nM, and this low affinity complex subsequently recruits a gp130 molecule to form a high-affinity complex with a Kd = 50 pM.56 Signaling occurs when two trimeric complexes associate to form a hexamer where homodimerization of gp130 can occur.57, 68-70 Mouse and rat IL-6 R are 54% identical to human IL-6 R in overall aa sequence, with 50% identity in the extracellular region.71, 72 Soluble 55 kDa forms of IL-6 R (sIL-6 R) are suggested to arise via both proteolytic processing73, 74 and alternative splicing.75, 76 In healthy individuals, blood levels of sIL-6 R are in the range of 75-80 ng/mL,77, 78 with elevated levels in myeloma (130-190 ng/mL)78 and HIV-infected patients (170 ng/mL).77 Soluble IL-6 R binds circulating IL-6 and extends its half-life, and, on the surface of cells expressing gp130, it forms a signal transducing complex.66, 79, 80 The number of cell types expressing IL-6 R does not reflect the spectrum of cell types that can respond to IL-6.81 Cells known to express IL-6 R include CD4+ and CD8+ T cells,82 hepatocytes,83 CD34+ stem cells84 neurons,85 neutrophils,86 monocytes74, 79 and osteoblasts.87
Human IL-6 is active on both mouse and rat cells,21, 22 while mouse IL-6 has no activity on human cells.21, 23 Cells known to express IL-6 include CD8+ T cells,24 fibroblasts,12, 25 synoviocytes,26 adipocytes,27 osteoblasts,28 megakaryocytes,29 endothelial cells (under the influence of endothelins),30 sympathetic neurons,31 cerebral cortex neurons,32 chromaffin cells of the adrenal medulla,33 retinal pigment cells,34 mast cells,35 keratinocytes,36, 37 Langerhans cells,36 fetal and adult astrocytes,38, 39 neutrophils,40, 41 monocytes,16 eosinophils,41, 42 colonic epithelial cells,43 B1 B cells,44 and most likely, pancreatic islet beta cells.45 IL-6 production is generally correlated with cell activation. Circulating IL-6 can be found in the blood of normal individuals in the 1 pg/mL range,46, 47 with slight elevations during the menstrual cycle,48 modest elevations in certain cancers (melanoma) (10 pg/mL),49 and large elevations after surgery (30-430 pg/mL).50
IL-6 has been described as both a pro-inflammatory and anti-inflammatory molecule, a modulator of bone resorption, a promoter of hematopoiesis, and an inducer of plasma cell development.4, 9, 88 These activities are described in more detail in references 3, 4, 7, 9, and 89. IL-6 also has been shown to influence IL-4 production. It has been suggested that antigen-driven, APC-derived IL-6 may influence naive CD4+ T cells to produce IL-4 and express the IL-4 receptor. Thus, given the close approximation of APC and T cell, transient APC-derived IL-4 could initiate a T cell autocrine loop whereby naive T cells direct their own phenotype commitment.