Role of α chain - IL-2 complex in the formation of the ternary complex of IL-2 and high-affinity IL-2 receptor

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Abstract
Using anti-Tac (antl-α chain) and 2R-B (anti-β chain) antibodies, we studied the roles of IL-2 receptor subunits (α and β chains) in the formation of IL-2 and high-affinity IL-2 receptor complex, which is the Initial event of IL-2 induced T cell growth. Hlgh-afflnity IL-2 binding which was undetectable in the presence of 2R-B antibody at 4°C became fully detectable when examined at 37°C, which explained the lack of inhibition by 2R-B antibody of IL-2-lnduced proliferation of the cells expressing high-affinity IL-2 receptor. We further studied the mechanism of the ‘reappearance’ of high-affinity IL-2 binding in the presence of 2R-B antibody. The addition of IL-2 to the cells preincubated with radiolabeled or fluorescence-labeled 2R-B antibody resulted in a marked decrease In the antibody bound to the cells expressing high-affinity IL-2 receptor at 37°C. This decrease was blocked by the presence of anti-Tac antibody, which inhibited IL-2 binding to α chain, but not by 7G7/B6 antibody, which recognized a non-IL-2 binding site of its chain. Furthermore, the decrease in cell-bound 2R-B antibody was not due to the internalization of β chain-2R-B antibody complex, because the amount of cell-bound Mlk-β3 antibody recognizing a non-IL-2 binding epltope of β chain remained unchanged, nor to the inhibition by simple competitive binding of IL-2 molecules to β chain as judged from comparative studies of competitive binding Inhibition. Taking these data together, the reappearance of high-affinity IL-2 binding was considered to be caused by the replacement of 2R-B antibody at the IL-2 binding site of β chain by α chain-mediated IL-2, and it was strongly suggested that α chain -IL-2 complex has a key role in the formation of the ternary complex of IL-2 and high-afflnlty IL-2 receptor, α chain may function as a dimension converter of IL-2 to effectively deliver IL-2 molecules to a relatively small number of β chains in the dynamics of the formation of hlgh-afflnity IL-2 binding in T cells.