Targeting IL-6 trans-signalling: past, present and future prospects

Abstract

Interleukin-6 (IL-6) is a key immunomodulatory cytokine that affects the pathogenesis of diverse diseases, including autoimmune diseases, chronic inflammatory conditions and cancer. Classical IL-6 signalling involves the binding of IL-6 to the membrane-bound IL-6 receptor α-subunit (hereafter termed 'mIL-6R') and glycoprotein 130 (gp130) signal-transducing subunit. By contrast, in IL-6 trans-signalling, complexes of IL-6 and the soluble form of IL-6 receptor (sIL-6R) signal via membrane-bound gp130. A third mode of IL-6 signalling - known as cluster signalling - involves preformed complexes of membrane-bound IL-6-mIL-6R on one cell activating gp130 subunits on target cells. Antibodies and small molecules have been developed that block all three forms of IL-6 signalling, but in the past decade, IL-6 trans-signalling has emerged as the predominant pathway by which IL-6 promotes disease pathogenesis. The first selective inhibitor of IL-6 trans-signalling, sgp130, has shown therapeutic potential in various preclinical models of disease and olamkicept, a sgp130Fc variant, had promising results in phase II clinical studies for inflammatory bowel disease. Technological developments have already led to next-generation sgp130 variants with increased affinity and selectivity towards IL-6 trans-signalling, along with indirect strategies to block IL-6 trans-signalling. Here, we summarize our current understanding of the biological outcomes of IL-6-mediated signalling and the potential for targeting this pathway in the clinic.

Fig. 1. Principles of IL-6 and IL-11 classic and trans-signalling.

In classic signalling, interleukin-6 (IL-6) and IL-11 bind to membrane IL-6 receptor (mIL-6R) and mIL-11R, respectively, and immediately recruit the high-affinity complex with the signal-transducing glycoprotein 130 (gp130) receptor. Soluble IL-6 receptor (sIL-6R) and sIL-11R can be generated by the proteases ADAM17 ((a disintegrin and metalloproteinase 17) and ADAM10, respectively. In cluster signalling, a transmitter cell presents IL-6–mIL-6R complexes to cells expressing gp130. Cluster signalling has only been described for IL-6, but in principle it is also possible for IL-11. Low-affinity complexes of IL-6–sIL-6R and IL-11–sIL-11R can induce trans-signalling by the formation of high-affinity complexes with gp130. For the sake of simplicity, IL-6 complexes are shown as tetramers (IL-6–IL-6R–2×gp130), although experimental evidence suggest hexamers (2×IL-6–2×IL-6R–2×gp130). The same holds true for IL-11 signalling. Naturally occurring sgp130 might serve as a IL-6 trans-signalling and IL-6 cluster-signalling buffer system (see Box 1).

Fig. 2. Timeline of IL-6 trans-signalling — from discovery to targeted therapies.

The timeline shows the discovery milestones for interleukin-6 (IL-6) trans-signalling and compares the development of IL-6-targeted therapies for combined classic and trans-signalling inhibition. The key studies defining the IL-6 trans-signalling pathway took place between 1989 and 1994, and preclinical studies began in the early 2000s, with clinical trials for olamkicept starting in 2012. ADAM17, a disintegrin and metalloproteinase 17; CNS, central nervous system; cs130, chimeric soluble gp130; gp130, glycoprotein 130; IBD, inflammatory bowel disease; IL-6, interleukin-6; IL-6R, IL-6 receptor; RA, rheumatoid arthritis; sgp130Fc, soluble gp130–Fc fusion protein; sIL-6R, soluble IL-6R.

Fig. 3. Assembly of the IL-6–IL-6R–gp130 complex via site II and site III.

Hexameric assembly of glycoprotein 130 (gp130)–gp130* with interleukin-6 (IL-6)–soluble IL-6 receptor (sIL-6R) and IL-6*–IL-6R*complexes via site II and site III. The asterisks indicates the second gp130, IL-6 and IL-6R in the assembly of two trimeric complexes. A trimeric complex of gp130 and IL-6–sIL-6R is formed between site IIa of IL-6, site IIb of IL-6R and domain D2–D3 of gp130. A second trimeric complex of gp130* and IL-6–sIL-6R is formed between site IIIa of IL-6, site IIIb of IL-6R and domain D1 of gp130.

Fig. 4. Inhibitory principles of IL-6 and IL-11 classic and trans-signalling.

Classic and trans-signalling of interleukin-6 (IL-6) and IL-11 can be inhibited by antagonistic substances (for example, antibodies and small molecules) directed against the cytokine, the α-receptors, or glycoprotein 130 (gp130)-associated Janus kinases (JAKs). Antibodies to gp130 are currently not in clinical development. Trans-signalling is inhibited by variants of soluble gp130. Sgp130Fc (soluble gp130–Fc fusion protein) inhibits IL-6 and IL-11 trans-signalling, chimeric soluble gp130 (cs130) inhibits IL-6 trans-signalling, and the autoinhibitory N-terminal ADAM17 (a disintegrin and metalloproteinase 17) pro-domain (A17pro) inhibits the release of soluble IL-6 receptor (sIL-6R), prevents IL-6 trans-signalling and might increase classic signalling because mIL-6R level might increase. A selective IL-11 trans-signalling inhibitor has not been described. Selective classic-signalling inhibitors have also not been developed thus far.

Fig. 5. Endogenous and therapeutic buffer system to limit IL-6 signalling.

The buffer for interleukin-6 (IL-6) in the blood is formed by soluble IL-6 receptor (sIL-6R) and sgp130 (Box 1). a, Under healthy, noninflammatory conditions, the buffer system allows local homeostatic signalling mainly in immune cells and hepatocytes but not in typical tissue cells, smooth muscle cells and endothelial cells. A low level of IL-6 and the buffer system prevents systemic IL-6 signalling. b, Under acute and chronic inflammatory conditions, the buffer system is overloaded and allows systemic IL-6 trans-signalling. c, Therapeutic application of sgp130Fc (soluble gp130–Fc fusion protein) under acute and chronic inflammatory conditions reinforces the buffer system and prevents systemic IL-6 trans-signalling. Coloured shapes have the same meanings as in Fig. 4.

Fig. 6. Next-generation IL-6 trans-signalling inhibitors.

a, First-generation sgp130Fc (soluble gp130–Fc fusion protein) is composed of domains D1–D6 of glycoprotein 130 (gp130; blue) fused to a human IgG1 Fc (grey). The first-generation trans-signalling inhibitor efficiently inhibits interleukin-6 (IL-6) and IL-11 trans-signalling and, with lower efficiency, oncostatin M (OSM) and leukaemia inhibitory factor (LIF) signalling. The second-generation sgp130 variants have improved IL-6 trans-signalling activity and/or reduced effects on the signalling of IL-11, LIF and OSM. Point mutations in domains D1 and D3 that improve the activity and specificity of sgp130Fc are indicated by yellow lines. Third-generation inhibitors such as chimeric soluble gp130 (cs130) are composed of the domains D1–D3 of gp130 (blue) fused to the single-domain antibody VHH6 (green), an antibody to the complex of IL-6 and soluble IL-6 receptor (IL-6–sIL-6R) (yellow and red). Bispecific cs130⁺ variants are based on cs130 fused to VHH6 and a second nanobody (purple) enabling additional functions including SARS-CoV-2-neutralization, inhibition of other inflammatory cytokines or cell-specific targeting. b, Proposed tetrameric and hexameric assembly mode of sgp130 and cs130 with IL-6–sIL-6R complexes. According to Boulanger et al., the initial trimeric complex of gp130 and IL-6–sIL-6R is formed between site IIa of IL-6, site IIb of IL-6R and domain D2–D3 of gp130. Subsequently, hexameric complexes are formed by binding of two trimeric IL-6–sIL-6R–sgp130 complexes via site IIIa of IL-6, site IIIb of IL-6R and domain D1 of gp130.