CHARACTERIZATION OF POTENTIAL ANTAGONISTS OF HUMAN INTERLEUKIN-5 DEMONSTRATES THEIR CROSS-REACTIVITY WITH RECEPTORS FOR INTERLEUKIN-3 AND GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR

The ligand-binding alpha-chain of the human interleukin 5 (IL-5) recep tor was expressed in its soluble form, lacking the transmembrane and c ytoplasmic domains, from recombinant baculovirus. 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 a chain with IC50 of 8 mu M and 11 mu M. These compounds also inhibited the interaction of human IL-5 with its membrane-bound receptor, compo sed of the ligand-binding alpha chain and signal-transducing beta chai n, and prevented signaling through the receptor Analysis by surface pl asmon resonance and matrix-assisted laser-desorption/ionization mass s pectrometry showed that the identified compounds bound irreversibly to the receptor at a 1:1 (mol/mol) ratio, suggesting a covalent interact ion with the alpha chain of the human IL-5 receptor. Both compounds al so 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 inh ibition of the strucuturally closely related receptors for IL-5, IL-3 and GM-CSF by both compounds, while binding of interleukin-4 to its re ceptor 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-C SF.