Heterodimerization of the IL-2 receptor β- and γ-chain cytoplasmic domains is required for signalling

THE interaction of interleukin-2 (IL-2) and IL-2 receptors criti-cally regulates the T-cell immune response following antigen activation 1,2 . IL-2 can signal through high or intermediate affinity receptors which contain IL-2Rα (refs 3, 4) + β (refs5–8) + γ (ref. 9) or β + γ chains, respectively. IL-2Rγ is a common γ chain, γ c , also shared by the IL-7 (ref. 10) and IL-4 (refs 11,12) receptors, which when mutated results in X-linked severe combined immunodeficiency 13 . Using chimaeric receptor constructs together with monoclonal or bispecific antibodies we demonstrate here that IL-2 signalling requires ligand-induced extracellular-domain-mediated heterodimerization of the β- and γ c -chain cytoplasmic domains. Anti-I L-2Rα monoclonal antibodies trigger proliferation of cells transfected with chimaeric constructs in which the extracel-lular domains of IL-2Rβ and γ c are replaced by that of IL-2Rα. Other experiments using chimaeric constructs indicated that IL-2 binds monomerically and monovalently to IL-2Rα and that the β-transmembrane domain is not required for receptor chain inter-actions. Finally, we provide a method for mapping residues in the γ c cytoplasmic domain even in cells that constitutively express γ c .