The interleukin-6 (IL-6) partial antagonist (Q159E,T162P)IL-6 interacts with the IL-6 receptor and gp130 but fails to induce a stable hexameric receptor complex.

The extracellular "soluble" domains of the IL-6 receptor (sIL-6R) and gp130 (sgp130) form a hexameric ternary receptor complex together with IL-6, consisting of two molecules of each component. In this report we have investigated the interactions of the partial IL-6 antagonist (Q159E,T162P)IL-6 ((QT)IL-6), with the sIL-6R and sgp130. The kinetic rate constants of the binding of sIL-6R to immobilized monomeric (QT)IL-6 or IL-6 were obtained using an optical biosensor with analysis of the primary data by linear and nonlinear regression. Both methods of analysis showed that, due to a higher off-rate, sIL-6R has lower apparent affinity for (QT)IL-6 than IL-6. The lower affinity of (QT)IL-6 was further confirmed by equilibrium binding measurements at the sensor surface and in solution. Using the biosensor it was also shown that the (QT)IL-6 complex interacts with sgp130, supporting the notion that the biological activity of (QT)IL-6 is mediated via gp130. However, the IL-6 mutant, when incubated with sIL-6R and sgp130, failed to induce a stable hexameric receptor complex, as shown by narrowbore size exclusion chromatography.