By Sinéad Kinsella PhD
Interleukin-6 (IL-6) is a 21 kDa glycoprotein that was discovered in the 1980s. IL-6 was originally termed B cell stimulatory factor 2 and thought to be a member of the interferon (IFN) family; however, cloning of IL-6 led to the discovery that IL-6 does not have any IFN-inducing ability, and instead has since been identified to have many diverse functions including cell growth and robust pro-inflammatory responses (Hirano et al, 1988). IL-6 is primarily expressed by antigen presenting cells (APCs) such as macrophages and dendritic cells and B cells, although secretion is also described CD4+ T cells (Dienz et al, 2009; Hirano, 1998).
IL-6 binds to the transmembrane IL-6 receptor (IL-6R). The IL-6R interacts with a glycoprotein, termed gp130, at the membrane which facilitates its’ downstream signalling in the cytosol (Taga et al, 1989). The interaction between IL-6R and gp130 is not monogamous as gp130 has been shown to interact with several other cytokine receptors (Muller-Newman, 2003). The IL-6-mediated association of IL-6R and gp130 leads to the recruitment of several tyrosine kinases; Janus kinase 1 & 2 (JAK1 & JAK2) and tyrosine kinase 2 (Tyk2), which induces phosphorylation of signal transducer and activator of transcriptions 3 (STAT3) and translocation to the nucleus where multiple anti-apoptotic and cytokine-associated genes are transcribed (Akira, 1997) (see Figure 1). IL-6 signalling is negatively regulated by SOCS3 and STAT3, both of which are transcribed in and IL-6 dependent manner, demonstrating a negative feedback loop (Garbers et al, 2015; Croker et al, 2008). Crosstalk between cytokine pathways is common, and it has been described that both IL-6 and the potent anti-inflammatory cytokine IL-10 use a similar signalling mechanisms through STAT3 (Lai et al, 1996). IL-6 is produced in response to acute inflammation, primarily in response to simulation by TNFa, LPS, and IL-1 (Kamimura et al, 2003), and is a central regulator of maintaining chronic inflammation (Gabay, 2006). A proposed IL-6 – dependent shift from acute to chronic inflammation is mediated via the binding of IL-6 to the soluble IL-6 receptor, sIL-6Ra, leading to what has been termed as ‘trans-signalling’ and facilitates a sustained IL-6-induced inflammatory response, and the subsequent activation and infiltration of several mediators of sustained inflammation (Hurst et al, 2001).
The Role of IL-6 in Disease
High serum levels of IL-6 are associated with increased tumourigeneses (Lech-Maranda et al, 2006). IL-6 has a central role in inducing proliferation by inhibiting apoptosis, and studies inhibiting the IL-6/STAT3 pathway in vitro have reversed these anti-apoptotic functions in a multiple myeloma model (Chatterjee et al, 2004). IL-6 also has a detrimental role in the development of cancer, as it has been demonstrated that IL-6 induces the conversion of non-cancer cells into tumour stem cells (Kim et al, 2013). Furthermore, IL-6 is associated with the development of arthritis (Alonzi et al, 1998) and colitis (Atreya et al, 2000), with il-6-deficient mice susceptible to several bacterial pathogens, including Listeria monocytogenes and Candida albicans (Dalrymple et al, 1995; Romani et al, 1996).
IL-6 as an Inflammatory Target
As excess levels of IL-6 or the IL6R are associated with pathogeneses, blocking the IL-6R has revealed promising results in certain inflammatory diseases and cancer (Allocca et al, 2013; Coward et al, 2011). Targeting IL-6 signalling molecules is being explored as a therapeutic strategy (Aparicio-Siegmund et al, 2014). STAT3 inhibitors are currently being tested in clinical trials to treat various cancers, where the anti-apoptotic role of STAT3 is targeted (Page et al, 2011). Moreover, several efforts have been made to inhibit the activities of JAK1 and JAK2 in an effort to suppress the robust inflammatory response induced by the JAK/STAT pathway (Leonard and O’Shea, 1998), however an antibody specifically targeting the IL-6R, called tocilizumab, may prove to be more beneficial for treating inflammatory disease (Garbers et al, 2015). Additionally, another monoclonal antibody, targeting IL-6, called Siltuximab, has shown positive results in clinical trials treating prostate cancer and multiple myeloma (Dorff et al, 2010; Rossi et al, 2010).
The Role of IL-6 in T-Cell Differentiation
Although IL-6 alone is not described to induce T cell growth, its’ effects on T cell proliferation are mediated by the induction of anti-apoptotic molecules, demonstrated by the upregulation of Bcl-2 in isolated T cell populations (Teague et al, 1997). IL-6 may also mediate its effect on differentiating T cells subsets through downstream cytokine production (see Figure 2). Importantly, IL-6 is essential for fine tuning the CD4+ helper T cell response, by regulating the balance between the Th1 and Th2 responses (Diehl et al, 2000; Rincon et al, 1997). IL-6 downregulates IFNg production through the upregulation of SOCS1, which suppresses IFNg signalling, and therefore limits the differentiation of T-cells into the Th1 state (Diehl et al, 2000). IL-6 – induced transcription of NFATc leads to the upregulation of IL-4, which facilitates T-cell polarization towards the Th2 state (Heijink et al, 2002; Rincon et al, 1997). Additionally, IL-6 together with TGFb mediate increased levels of IL-17 through upregulating the receptor RORgt, which polarizes T cells towards the Th17 (Passos et al, 2010). Finally, IL-6 inhibits TGFb expression on CD4+ T cells, which prevents the differentiation into Tregs (Kuhn et al, 2017).
Figure 1: IL-6 signalling. IL-6 binds to the transmembrane IL-6 receptor (IL-6R), which induces the interaction between the glycoprotein gp130 and IL-6R, facilitating downstream interactions with the tyrosine kinases JAK1, JAK2 and Tyk2. These tyrosine kinases mediated signalling downstream and phosphorylates STAT3, inducing its’ nuclear translation and the transcription of multiple cytokine-related and anti-apoptotic genes, as well as the negative regulators of IL-6 signalling, SOC3 and STAT3.
Figure 2: Effects of IL-6 on T cell differentiation. IL-6 downregulates IFNg through the upregulation of the IFNg signalling suppressor, suppressor of cytokines 1 (SOCS1). Additionally, IL-6 mediates the transcription of IL-4, thus reducing T cell polarization to the Th1 subset and more towards the Th2 state. IL-6 in combination with TGFb leads to increased IL-17 production, facilitating the regulation of Th17 differentiation. IL-6 also inhibits CD4+ T cell expression of TGFb which limits the differentiation into the Treg state. (Adapted from Dienz and Rincon, 2009)
- Akira S. IL-6-regulated transcription factors. Int J Biochem Cell Biol. 1997. 29(12):1401-18.
- Allocca M, Jovani M, Fiorino G, Schreiber S, Danese S. Anti-IL-6 treatment for inflammatory bowel diseases: next cytokine, next target. Curr Drug Targets. 2013. 14(12):1508-21.
- Alonzi T, Fattori E, Lazzaro D, Costa P, Probert L, Kollias G, De Benedetti F, Poli V, Ciliberto G. Interleukin 6 is required for the development of collagen-induced arthritis. J Exp Med. 1998. 187:461–468.
- Aparicio-Siegmund S, Sommer J, Monhasery N, Schwanbeck R, Keil E, Finkenstädt D, Pfeffer K, Rose-John S, Scheller J, Garbers C. Inhibition of protein kinase II (CK2) prevents induced signal transducer and activator of transcription (STAT) 1/3 and constitutive STAT3 activation. Oncotarget. 2014. 5(8):2131-48.
- Atreya R, Mudter J, Finotto S, Müllberg J, Jostock T, Wirtz S, Schütz M, Bartsch B, Holtmann M, Becker C, Strand D, Czaja J, Schlaak JF, Lehr HA, Autschbach F, Schürmann G, Nishimoto N, Yoshizaki K, Ito H, Kishimoto T, Galle PR, Rose-John S, Neurath MF. Blockade of interleukin 6 trans signaling suppresses T-cell resistance against apoptosis in chronicintestinal inflammation: evidence in crohn disease and experimental colitis in vivo. Nat Med. 2000. 6(5):583-8.
- Chatterjee M, Stühmer T, Herrmann P, Bommert K, Dörken B, Bargou RC. Combined disruption of both the MEK/ERK and the IL-6R/STAT3 pathways is required to induce apoptosis of multiple myeloma cells in the presence of bone marrow stromal cells. Blood. 2004. 104(12):3712-21.
- Coward J, Kulbe H, Chakravarty P, Leader D, Vassileva V, Leinster DA, Thompson R, Schioppa T, Nemeth J, Vermeulen J, Singh N, Avril N, Cummings J, Rexhepaj E, Jirström K, Gallagher WM, Brennan DJ, McNeish IA, Balkwill FR. Interleukin-6 as a therapeutic target in human ovarian cancer. Clin Cancer Res. 2011. 17(18):6083-96.
- Croker BA, Kiu H, Nicholson SE. SOCS regulation of the JAK/STAT signalling pathway. Semin Cell Dev Biol. 2008. 19(4):414-22.
- Croker BA, Kiu H, Nicholson SE. SOCS regulation of the JAK/STAT signalling pathway. Semin Cell Dev Biol. 2008 Aug;19(4):414-22.
- Dalrymple SA, Lucian LA, Slattery R, McNeil T, Aud DM, Fuchino S, Lee F, Murray R. Interleukin-6-deficient mice are highly susceptible to Listeria monocytogenes infection: correlation with inefficient neutrophilia. Infect Immun. 1995. 63:2262–2268.
- Diehl S, Anguita J, Hoffmeyer A, Zapton T, Ihle JN, Fikrig E, Rincón M. Inhibition of Th1 differentiation by IL-6 is mediated by SOCS1. Immunity. 2000. 13 (6):805-15.
- Dienz O, Rincon M. The effects of IL-6 on CD4 T cell responses. Clin Immunol. 2009. 130(1):27-33.
- Dorff TB, Goldman B, Pinski JK, Mack PC, Lara PN Jr, Van Veldhuizen PJ Jr, Quinn DI, Vogelzang NJ, Thompson IM Jr, Hussain MH. Clinical and correlative results of SWOG S0354: a phase II trial of CNTO328 (siltuximab), a monoclonal antibody against interleukin-6, in chemotherapy-pretreated patients with castration-resistant prostate cancer. Clin Cancer Res. 2010. 16(11):3028-34.
- Elevated IL-10 plasma levels correlate with poor prognosis in diffuse large B-cell lymphoma. Eur Cytokine Netw. 2006. 17(1):60-6.
- Gabay C. Interleukin-6 and chronic inflammation. Arthritis Res Ther. 2006. 8 Suppl 2:S3.
- Garbers C, Aparicio-Siegmund S, Rose-John S. The IL 6/gp130/STAT3 signaling axis: recent advances towards specific inhibition. Curr Opin Immunol. 2015. 34:75-82.
- Heijink IH, Vellenga E, Borger P, Postma DS, de Monchy JG, Kauffman HF. Interleukin-6 promotes the production of interleukin-4 and interleukin-5 by interleukin-2-dependentand -independent mechanisms in freshly isolated human T cells. Immunology. 2002. 107(3):316-24.
- Hirano T, Matsuda T, Hosoi K, Okano A, Matsui H, Kishimoto T. Absence of antiviral activity in recombinant B cell stimulatory factor 2 (BSF-2). Immunol Lett. 1988.17:41-45.
- Hirano T. Interleukin 6 and its receptor: ten years later. Int Rev Immunol. 1998. 16(3-4):249-84.
- Hurst SM, Wilkinson TS, McLoughlin RM, Jones S, Horiuchi S, Yamamoto N, Rose-John S, Fuller GM, Topley N, Jones SA. Il-6 and its soluble receptor orchestrate a temporal switch in the pattern of leukocyte recruitment seen during acute inflammation. Immunity. 2001. 14(6):705-14.
- Kamimura D, Ishihara K, Hirano T. IL-6 signal transduction and its physiological roles: the signal orchestration model. Rev Physiol Biochem Pharmacol. 2003;149:1–38.
- Kim SY, Kang JW, Song X, Kim BK, Yoo YD, Kwon YT, Lee YJ. Role of the IL-6-JAK1-STAT3-Oct-4 pathway in the conversion of non-stem cancer cells into cancer stem-like cells. Cell Signal. 2013. 25(4):961-9.
- Kuhn C, Rezende RM, M’Hamdi H, da Cunha AP, Weiner HL. IL-6 Inhibits Upregulation of Membrane-Bound TGF-β 1 on CD4+ T Cells and Blocking IL-6Enhances Oral Tolerance. J Immunol. 2017. 198(3):1202-1209.
- Lai CF, Ripperger J, Morella KK, Jurlander J, Hawley TS, Carson WE, Kordula T, Caligiuri MA, Hawley RG, Fey GH, Baumann H. Receptors for interleukin (IL)-10 and IL-6-type cytokines use similar signaling mechanisms for inducing transcription through IL-6 response elements. J Biol Chem. 1996. 271(24):13968-75.
- Lech-Maranda E, Bienvenu J, Michallet AS, Houot R, Robak T, Coiffier B, Salles G.
- Leonard WJ, O’Shea JJ. Jaks and STATs: biological implications. Annu Rev Immunol. 1998;16:293-322.
- Müller-Newen G. The cytokine receptor gp130: faithfully promiscuous. Sci STKE. 2003 Sep 23;2003(201):PE40.
- Page BD, Ball DP, Gunning PT. Signal transducer and activator of transcription 3 inhibitors: a patent review. Expert Opin Ther Pat. 2011. 21(1):65-83.
- Passos ST, Silver JS, O’Hara AC, Sehy D, Stumhofer JS, Hunter CA. IL-6 promotes NK cell production of IL-17 during toxoplasmosis. J Immunol. 2010.184(4):1776-83.
- Rincón M, Anguita J, Nakamura T, Fikrig E, Flavell RA. Interleukin (IL)-6 directs the differentiation of IL-4-producing CD4+ T cells. J Exp Med. 1997. 185(3):461-9.
- Romani L, Mencacci A, Cenci E, Spaccapelo R, Toniatti C, Puccetti P, Bistoni F, Poli V. Impaired neutrophil response and CD4+ T helper cell 1 development in interleukin 6-deficient mice infected with Candida albicans. J Exp Med. 1996. 183:1345–1355.
- Rossi JF, Négrier S, James ND, Kocak I, Hawkins R, Davis H, Prabhakar U, Qin X, Mulders P, Berns B. A phase I/II study of siltuximab (CNTO 328), an anti-interleukin-6 monoclonal antibody, in metastatic renal cell cancer. Br J Cancer. 2010. 103(8):1154-62.
- Taga T, Hibi M, Hirata Y, Yamasaki K, Yasukawa K, Matsuda T, Hirano T, Kishimoto T: Interleukin-6 triggers the association of its receptor with a possible signal transducer, gp130. Cell. 1989. 58:573-581.
- Teague TK, Marrack P, Kappler JW, Vella AT. IL-6 rescues resting mouse T cells from apoptosis. J Immunol. 1997. 158:5791–579