Interaction affinity between cytokine receptor components on the cell surface

The anti-common gamma chain (γ(c)) mAb CP.B8 is shown to inhibit interleukin 4 (IL-4)-dependent proliferation of phytohemagglutinin (PHA) activated T cells noncompetitively with respect to cytokine by blocking the IL-4-induced heterodimerization of IL-4Rα and γ(c) receptor chains. Affinities for the binding of IL-4 to Cos-7 cells transfected with huIL-4Rα, and to PHA blasts expressing both IL-4Rα and γ(c), were used to estimate the affinity of the key interaction between γ(c) and the binary IL-4Rα⋅IL-4 complex on the cell surface. This affinity was defined in terms of the dimensionless ratio [IL-4Rα⋅IL-4⋅γ(c)]/[IL-4Rα⋅IL-4], which we designate K(R). The results show that on PHA blasts this interaction is relatively weak; K(R) ≈ 9, implying that ≈10% of the limiting IL-4Rα chain remains free of γ(c) even at saturating concentrations of IL-4. This quantitative treatment establishes K(R) as a key measure of the coupling between ligand binding and receptor activation, providing a basis for functional distinctions between different receptors that are activated by ligand-induced receptor dimerization.