Interleukin-6 (IL-6) antagonism by soluble IL-6 receptor alpha mutated in the predicted gp130-binding interface

Interleukin-6 (IL-6) triggers the formation of a high affinity receptor complex constituted by the ligand-binding subunit IL-6 receptor alpha (IL-6R alpha) and the signal-transducing beta chain gp130. Since the cytoplasmic region of IL-6R alpha is not required for signal transduction, soluble forms of IL-6R alpha (sIL-6R alpha) show agonistic properties because they are still able to originate IL-6.sIL-6R alpha complexes, which in turn associate with gp130. A three-dimensional model of the human IL-6.IL-6R alpha.gp130 complex has been constructed and verified by site-directed mutagenesis of regions in shIL-6R alpha (where "h" is human) anticipated to contact hgp130, with the final goal of generating receptor variants with antagonistic properties. In good agreement with our structural model, substitutions at Asn-230, His-280, and Asp-281 selectively impaired the capability of shIL-6R alpha to associate with hgp130 both in vitro and on the cell surface, without affecting its affinity for hIL-6. Moreover, the multiple substitution mutant A228D/N230D/H280S/D281V expressed as a soluble protein partially antagonized hIL-6 bioactivity on hepatoma cells.