The balance of interleukin (IL)-6, IL-6·soluble IL-6 receptor (sIL-6R), and IL-6·sIL-6R·sgp130 complexes allows simultaneous classic and trans-signaling

J Biol Chem. 2018 May 4;293(18):6762-6775. doi: 10.1074/jbc.RA117.001163. Epub 2018 Mar 20.


Interleukin (IL-)6 is the major pro-inflammatory cytokine within the IL-6 family. IL-6 signals via glycoprotein 130 (gp130) and the membrane-bound or soluble IL-6 receptor (IL-6R), referred to as classic or trans-signaling, respectively. Whereas inflammation triggers IL-6 expression, eventually rising to nanogram/ml serum levels, soluble IL-6R (sIL-6R) and soluble gp130 (sgp130) are constitutively present in the upper nanogram/ml range. Calculations based on intermolecular affinities have suggested that systemic IL-6 is immediately trapped in IL-6·sIL-6R and IL-6·sIL-6R·sgp130 complexes, indicating that sIL-6R and sgp130 constitute a buffer system that increases the serum half-life of IL-6 or restricts systemic IL-6 signaling. However, this scenario has not been experimentally validated. Here, we quantified IL-6·sIL-6R and IL-6·sIL-6R·sgp130 complexes over a wide concentration range. The amounts of IL-6 used in this study reflect concentrations found during active inflammatory events. Our results indicated that most IL-6 is free and not complexed with sIL-6R or sgp130, indicating that the level of endogenous sgp130 in the bloodstream is not sufficient to block IL-6 trans-signaling via sIL-6R. Importantly, addition of the single-domain antibody VHH6, which specifically stabilizes IL-6·sIL-6R complexes but did not bind to IL-6 or sIL-6R alone, drove free IL-6 into IL-6·sIL-6R complexes and boosted trans-signaling but not classic signaling, demonstrating that endogenous sIL-6R has at least the potential to form complexes with IL-6. Our findings indicate that even though high concentrations of sIL-6R and sgp130 are present in human serum, the relative ratio of free IL-6 to IL-6·sIL-6R allows for simultaneous classic and trans-signaling.

Keywords: Trans-signaling; classic signaling; cytokine; glycoprotein; interleukin 6 (IL-6); interleukin 6 receptor (IL6R); signal transduction.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cell Line
  • Cytokine Receptor gp130 / metabolism*
  • Enzyme-Linked Immunosorbent Assay
  • Epitopes / immunology
  • Humans
  • Interleukin-6 / blood
  • Interleukin-6 / immunology
  • Interleukin-6 / metabolism*
  • Receptors, Interleukin-6 / blood
  • Receptors, Interleukin-6 / immunology
  • Receptors, Interleukin-6 / metabolism*
  • Recombinant Proteins / blood
  • Recombinant Proteins / immunology
  • Recombinant Proteins / metabolism
  • Signal Transduction*
  • Single-Domain Antibodies / immunology


  • Epitopes
  • IL6 protein, human
  • Interleukin-6
  • Receptors, Interleukin-6
  • Recombinant Proteins
  • Single-Domain Antibodies
  • Cytokine Receptor gp130