MOLECULAR MAPPING WITH FUNCTIONAL ANTIBODIES LOCALIZES CRITICAL SITESON THE HUMAN IL RECEPTOR COMMON GAMMA (GAMMA-C)CHAIN

The IL receptor common gamma (gamma c) chain is required for the forma tion of high affinity cytokine receptor complexes for IL-2, IL-4, IL-7 , IL-9, and IL-15, and for signals regulating cell survival, growth, a nd differentiation. Our current understanding of how gamma c chain ass ociates with multiple ligands and receptor subunits is drawn largely f rom its structural homology to the human growth hormone (hGH) receptor and known structure of the hGH/hGH receptor complex. These receptors share distinct features in their extracellular portions and are believ ed to function by a mechanism of ligand-induced association of recepto r subunits, Here, we report the first directed mutational analysis of the human gamma c chain by alanine scanning conducted across seven reg ions likely to contain residues required for intermolecular contact. F unctionally distinct, neutralizing anti-gamma c mAbs were employed to define critical residues. One particular mAb. CP.B8, unique in its abi lity to inhibit IL-2-, IL-4-, IL-7-, and IL-15-induced proliferation a nd high affinity cytokine binding of normal T cells as an intact mAb a nd as a Fab fragment, localized critical residues to four noncontinuou s stretches, namely residues in loops AB and EF of domain 1, in the in terdomain segment, and in loop FG of domain 2. Notably, these residues form a contiguous patch on the gamma c chain surface in a three-dimen sional structural model. These results provide functional evidence for the location of contact points on ye chain required for its associati on with multiple ligands.