Binding of interleukin-13 and interleukin-4 to the interleukin (IL)-4/IL-13 receptor of human synovial fibroblasts

Synovial fibroblasts expressed transcripts for IL-4R alpha, and IL-13R alph a 1 and IL-13R alpha 2. Using weighted nonlinear computer modeling of the d ata from equilibrium binding studies, a 2 bindings sites model fitted the d ata best. After occupation of the shared high affinity receptors by the non -signaling double mutant IL-4(121)R->D, Y-124->D (RY-IL-4) the high affinit y binding of IL-13 could be abolished. A 2 binding site model still could b e fitted, however the improvement in fit over a one-site model was not stat istically significant. Using affinity spectra, at least 2 binding sites are apparent. After treatment with RY-IL-4, some of the high affinity binding was abolished, however not completely. A correlation between the number of binding sites and the affinity is apparent, which seriously casts doubt on the classical evaluation of binding isotherms, where the parameters are ass umed to be independent. In a previous study we suggested that the large num ber of IL-13R alpha(2) monomers are silent receptors, likely representing a decoy target for IL-13. The high affinity binding therefore most likely re presents the binding to the heterodimer consisting of IL-4R alpha and IL-13 R alpha(1) or IL-13R alpha(2). The low affinity binding may represent the I L-13R alpha(2).