The cell surface expression level of the human interleukin-5 receptor alpha subunit determines the agonistic/antagonistic balance of the human interleukin-5 E13Q mutein

The human interleukin-5 (IL-5) receptor consists of an a-chain that specifi cally binds the ligand with intermediate affinity, and a beta c-chain, that associates with the IL-5/IL-5R alpha complex, leading to a high-affinity, signal transducing receptor complex. Structure-function studies showed that modification of the putative beta c-chain binding site in IL-5 (E13Q mutei n) converted the molecule into an antagonist. However, analysis of the effe ct of this mutant IL-5 on COS-1 cells transfected with both receptor subuni ts, did not show reduced interaction with the beta c subunit [Tavernier, J. , Tuypens, T., Verhee, A., Plaetinck, G., Devos, R., Van der Heyden, J., Gu isez, Y. & Oefner, C. (1995) Proc. Natl Acad. Sci. USA 89, 7041-7045]. To g ain more insight into the mechanism of IL-5 antagonism by E13Q, we tested i ts biological activity on two FDC-P1 subclones that express clearly differe nt numbers of alpha-subunits yet an almost constant number of murine beta c -subunits. Here we show that E13Q has a biological activity comparable to w ild-type IL-5 only when a high number of alpha-chains is present on the cel ls. Confirming the critical role of the IL5R alpha cell-surface expression level, treatment with suboptimal doses of a neutralising anti-IL-5R alpha a ntibody results in reduced activity of the mutant but not of wild-type IL-5 .