The ligand‐binding α‐chain of the human interleukin 5 (IL‐5) receptor was expressed in its soluble form, lacking the transmembrane and cytoplasmic domains, from recombinant baculoviras. The soluble receptor was used in a scintillation proximity assay to identify two chemical compounds that inhibit binding of human IL‐5 to the soluble receptor α chain with IC50 of 8 μM and 11 μM. These compounds also inhibited the interaction of human IL‐5 with its membrane‐bound receptor, composed of the ligand‐binding a chain and signal‐transducing β chain, and prevented signaling through the receptor. Analysis by surface plasmon resonance and matrix‐assisted laser‐desorption/ionization mass spectrometry showed that the identified compounds bound irreversibly to the receptor at a 1:1 (mol/mol) ratio, suggesting a covalent interaction with the α chain of the human IL‐5 receptor. Both compounds also inhibited the interaction of the receptors for interleukin 3 (IL‐3) and granulocyte‐macrophage colony‐stimulating factor (GM‐CSF), which are involved in hematopoietic differentiation and activation of immune cells, thus eliminating them as potential therapeutic agents. The inhibition of the structurally closely related receptors for IL‐5, IL‐3 and GM‐CSF by both compounds, while binding of interleukin‐4 to its receptor was not affected, suggests that a similar reactive site exists in the ligand‐binding domains of the receptors for IL‐5, IL‐3 and GM‐CSF.

Wiekowski, M., Prosser, D., Taremi, S., Tsarbopoulos, A., Jenh, C., Chou, C., Lundell, D., Zavodny, P., & Narula, S. (1997). Characterization of Potential Antagonists of Human Interleukin 5 Demonstrates Their Cross‐Reactivity with Receptors for Interleukin 3 and Granulocyte‐Macrophage Colony‐Stimulating Factor. European Journal of Biochemistry, 246(3), 625–632. Portico.