Soluble Interleukin-6 receptor (sIL-6R)
Scientific background
Interleukin-6 (IL-6) is a multifunctional cytokine that regulates pleiotropic roles in immune regulation, inflammation, hematopoiesis, and oncogenesis. Its biological activities are shared by IL-6-family of cytokines such as leukemia inhibitory factor and oncostatin M. IL-6 exerts its biological activities through interaction with specific receptors expressed on the surface of target cells. The receptor complex mediating the biological activities consists of two distinct membrane-bound glycoproteins:
- IL-6 Receptor (IL-6R, gp80) is 80 kDa alpha subunit, transmembrane glycoprotein that is 449 aa long. It binds IL-6 with low affinity (Kd= 5.5 nM)
- A signal-transducing non-ligand binding 130 kDa component (gp130) that is required for high-affinity binding complex (Kd = 50 pM)
Upon binding of IL-6 to the receptor (alpha subunit), gp130 is homodimerized and is subsequently involved in down-stream signal processes binding tyrosine kinases and activating STAT1 and STAT3 transcriptional factors. Both IL-6R and gp130 also occur in soluble form in biological fluids and has been purified from human serum and urine. The number of cell types expressing IL-6 R does not reflect the spectrum of cell types that can respond to IL-6. Cells known to express IL-6 R include CD4+ and CD8+ T cells, hepatocytes, CD34+ stem cells neurons, neutrophils, monocytes and osteoblasts.
Soluble IL-6 Receptor
Soluble IL-6 receptor (sIL-6R) is a 50-55 kDa ligand binding protein, derived from the extracellular part of the gp80 receptor by either proteolytic cleavage of their membrane moiety (PC-sIL-6R; isoform 1) or by alternative splicing (DS-sIL-6R). The soluble form of IL-6R can bind its ligand and induce cellular responses by association with gp130, thus acting as an IL-6 agonist. Interestingly, the association of IL-6 with the soluble form of IL-6R alpha is capable of eliciting a biological response in cells that express only the membrane gp130. This type of activation, called “trans-signalling”, renders virtually all cells capable of responding to IL-6/sIL-6R alpha complexes, making for a large new spectrum of IL-6 activities, ranging from the control of the immune response to involvement in pathological states.
Soluble interleukin-6 receptor as a prognostic factor in numerous clinical disorders:
In healthy individuals, blood levels of sIL-6R are in the range of 75-80 ng/mL, with elevated levels in myeloma (130-190 ng/mL) and HIV-infected patients (170 ng/mL). Elevated IL-6 concentrations have been reported in numerous clinical disorders (Jones S. A. et al, 2001, The FASEB J. 15:43-58), where they appear to orchestrate a variety of inflammatory responses. Given the agonistic properties of sIL-6R, it is evident that control of many of these IL-6-mediated events is regulated via sIL-6R. Consequently, when considering the role of IL-6 in disease progression, it is equally important to consider how sIL-6R affects its function. Although enhanced sIL-6R levels have been documented in a variety of disease states, we are only now beginning to appreciate the potential contribution sIL-6R may have in these pathologies. As a result, future studies must address this issue if the functional properties of IL-6 are to be fully understood. Initial clinical studies indicate the levels of sIL-6R, the alpha subunit, in the serum of patients with various diseases as summarized in table1.
Table 1. Soluble IL-6R level in disease
| Clinical condition | sIL-6R | Soluble IL-6R levels (Alpha subunit) | Reference |
| Cancers | |||
| Breast cancer |  | Serum sIL-6R levels were lower than in normal individuals | Jablonska, E. Cytokine 10,540-543, 1998 |
| Pancreatic cancer | | Soluble IL-6R levels unaltered from normal individuals | Barber, M. D. et al, Clin. Sci. 96,83-87, 1999 |
| Multiple myeloma |  | The concentrations of sIL-6R were significantly higher in the patients who died within 3 years compared with those who survived. | Pulkki, K. et al, Cytokine. 16(3): 79-86, 2001 |
| Inflammatory disorders | |||
| Rheumatoid arthritis | | Elevated sIL-6R levels in the sera of patients were higher than those of the control group. | Polgar A et al, Med Sci Monit. 6(1): 13-8, 2000 |
| Juvenile chronic arthritis | | Significant increases in both IL-6 and sIL-6R, which correlated with fever | Keul R et al Cytokine.10 (9): 729-34, 1998 |
| Osteoarthritis | | Elevated levels, but not to the extent associated with rheumatoid arthritis | Kotake S et al, J Bone Miner Res. 11(1): 88-95,1996 |
| Asthma | | Asthmatic patients have high serum sIL-6R levels | Yokoyama A et al, Am. J. Respir. Crit. Care Med., 156 (5), 1688-1691, 1997 |
| Crohn’s disease | | Increased levels of IL-6 and sIL-6R have been demonstrated in both serum and intestinal tissues of the patients | Ito H. Curr Drug Targets Inflamm Allergy. 2(2): 125-30, 2003 |
| Interstitial lung disease | | Raised sIL-6R levels contribute to systemic and local responses in pneumonia and sarcoidosis patients | Yokoyama A et al, Clin Exp Immunol.100 (2): 325-9, 1995 |
| Inflammatory bowel disease | | Elevated IL-6 and sIL-6R levels contribute to the pathogenesis of chronic intestinal inflammation | Atreya R et al, Nat Med. 6(5): 583-8, 2000 |
| Systemic sclerosis | | sIL-6R levels correlate with the severity of pulmonary fibrosis associated with systemic sclerosis | Hasegawa M et al, Rheumatology 38,612-617, 1999 |
| Intraocular inflammation | | Elevated sIL-6R and IL-6 levels in the aqueous humor of uveitis patients | Petrinovic-Doresic, J. et al, Ocular Immunol. Inflamm. 7,75-84, 1999 |
| Graves’ disease | | An increased was observed in the serum concentration of sIL-6R in the course of Graves’ disease. | Bossowski A et al, J Pediatr Endocrinol Metab. 14,741-7, 2001 |
| Endometriosis | | Peritoneal and serum sIL-6R levels significantly higher than in other benign gynecologic conditions | Schroder, W. et al, Clin. Exp. Obstet. Gynecol. 23,10-14, 1996 |
| Neurological conditions | |||
| Depression | | Significantly decreased sIL-6R and IL-6 concentrations in CSF compared with healthy subjects | Stubner, S et al, Neurosci. Lett. 259,145-148, 1999 |
| Multiple sclerosis | | Elevated serum sIL-6R concentrations correlate with disease severity | Padberg, F. et al, J. Neuroimmunol. 99,218-223, 1999 |
| Schizophrenia and mania | | Higher sIL-6R levels in psychotic patients than healthy volunteers | Maes, M. et al, J. Psych. Res. 29,141-151, 1995 |
| Psychological stress | | Elevated in post-traumatic stress disorder, especially in patients with major depression | Maes, M et al, Biol. Psych. 45,833-839, 1999 |
| Pathogen infections | |||
| HIV | | Viral infection promotes sIL-6R release | Honda, M. et al J. Immunol. 148,2175-2180, 1992 |
| Urinary tract infection | | Increased serum sIL-6R concentrations correlate with a loss of glomerular filtration rate | Jacobsen, S. H et al Nephron 80,401-407, 1998 |
| Cerebral malaria | | Elevated IL-6 and sIL-6R levels after P. falciparum infection | Jakobsen, P et al Infect. Immun. 62,4374-4379, 1994 |
SBH Sciences construct of sIL-6R is expressed as the isoform 1 that doesn’t have the COOH-terminal tail (GSRRRGSCGL).
SBH Sciences recombinant human soluble IL-6 receptor alpha, purified from mammalian 293 cells, is fully biologically active. Its activity is measured by its ability to increase
IL-6 activity (growth inhibition of murine M1 cells).
Collaboration Opportunity
SBH Sciences is looking for partners to investigate SBH Sciences fully functional soluble IL-6 receptor as therapeutic agent as well as diagnostic tool. We are open for suggestions and would be pleased to hear from you.
