The receptor binding to interleukin (IL)-13 is composed of the IL-13 receptor alpha 1 chain (IL-13R alpha 1) and the IL-4 receptor alpha chain (IL-4R alpha). In order to investigate the interaction of IL-13 with IL-13R alpha 1 and ILARcx, the DNA fragments coding the extracellular regions of human IL-13[Rotl and the IL-4R alpha (containing a cytokine receptor homologous region) were fused with mouse Fc and expressed by a silkworm-baculovirus system. The expressed receptors were successfully purified by affinity chromatography using protein A, and the Fc region was removed by thrombin digestion. After further purification with anion-exchange chromatography, these receptors were used to investigate the ligand-receptor interaction. Size exclusion chromatography and SPR analysis revealed that mixture of IL-13 and IL-13R alpha 1 showed predominant affinity to IL-4R alpha, although neither detectable affinity of IL-13 nor IL-13R alpha 1 was observed against IL4Ra. Combining these data with the moderate affinity of IL-13 to IL-13R alpha 1, this indicates that IL-13 first binds to IL-13R alpha 1 and recruits consequently to IL-4R. (c) 2008 Elsevier Inc. All rights reserved.