Immunological characterization of antigenic domains on human IL–2 receptor β subunit: epitope-function relationships

Five mAb directed at the IL-2R β chain were analyzed for their binding and functional properties. They define three epitopes on a recombinant soluble β chain or on the β chain expressed at the surface of YT-2C2 cells. Epitope 1 (A41 and 6E8 mAb) is part of the IL-2 binding domain, whereas epitope 2 (CF1 and 6E10 mAb) is not involved in IL-2 binding. Epitope 3 (6B5 mAb) also partly overlaps the IL-2 binding domain but does not overlap epitopes 1 and 2. None of the mAb can by themselves inhibit IL-2 induced proliferation of a human activated T cell clone. Only epitope 1 mAb can synerglze with an anti-ct chain mAb to inhibit this proliferation. Using epitope 1 and 2 mAb as well as a purified, recombinant form of the IL-2R p chain extracellular domain, an ELISA-based immunoassay was set up which allows the quantitative determination of soluble and detergent solubillzed IL-2R β chains. Epitopes 1 and 2 are in non-competitive interaction: the binding of a mAb to one epitope decreases the affinity of a mAb for the second epitope. Epitope 2 mAb have binding stoichiometries (˜16,000 sites/cell) which are ˜80% higher than that of epitope 1 mAb and IL-2 itself (˜9000 sites/cell). Upon binding of epitope 2 mAb, the stoichiometries of epitope 1 mAb and IL-2 are increased to reach the stoichiometry of epitope 2 mAb. A molecular model is proposed in which the IL-2R β chain exists in two different states on YT-2C2 cells: one associated with the intermediate-affinity IL-2R ;β/γ complex, the other being part of a receptor structure which is not accessible to epitope 1 mAb and IL-2.