Analysis of the interaction between human interleukin‐5 and the soluble domain of its receptor using a surface plasmon resonance biosensor

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Abstract
A surface plasmon resonance (SPR) biosensor was used to study the interaction of human interleukin‐5 (hIL5) with its receptor. IL5 is a major growth factor in the production and activation of eosinophilis. The receptor for IL5 is composed of two subunits, α and β. The α subunit provides the specificity for IL5 and consist of an extracellular soluble domain, a single transmembrane region and a cytoplasmic tail. We expressed the soluble domain of the human IL5 receptor α subunit (shIL5Rα) and human IL5 (hIL5) in Drosophila. Both hIL5 and shIL5Rα were immobilized separately through amine groups onto the carboxylated dextran layer of sensor chips of the BIAcore™ (Pharmacia) SPR biosensor after N‐hydroxysuccinimide/carbodiimide activation of the chip surface. Interactions were measured for the complementary macromolecule, either shIL5Rα or hIL5, in solution. Kinetics of binding of soluble analyst to immobilized ligand were measured and from this the association rate constant, dissociation rate constant and equilibrium dissociation constant (Kd) were derived. With immobilized shIL5Rα and soluble hIL5, the measured Kd was 2 nM. A similar value was obtained by titration calorimetry. The Kd for Drosophila expressed receptor and IL5 is higher than the values reported for proteins expressed in different systems, likely due to differences in the methods of interaction analysis used for differences in protein glycosylation. Receptor‐IL5 binding was relatively pH independent between pH 6.5 and 9.5. Outside this range the dissociation rate increased with compressibility little increased in association rate. The values obtained for the interaction of hIL5 and shIL5Rα were found to depend on which component was immobilized; the Kd was 5.5 nM with immobilized hIL5 and soluble shIL5Rα. The SPR biosensor provides a unified methodology to measure the interaction properties of shIL5Rα and hIL5 derivatives, mutants and mimetic as well as to evaluate potential antagonists of the receptor‐cytokine interaction.