Self-association of interleukin 2 bound to its receptor

Although radioiodinated interleukin 2 (IL-2) has been used to define the binding characteristics of the cytokine to the alpha chain of the receptor complex, we have found that unsubstituted IL-2 behaves differently. Whereas previous investigations with radioiodinated IL-2 have shown binding to the alpha chain with a Kd of 10 nM, we show that unsubstituted IL-2 binds to the alpha chain but does not reach saturation between 100 and 1000 nM. The explanation for the discrepancy between the analysis of radioiodinated and unsubstituted cytokine involves the propensity of unsubstituted IL-2 for self-association, a property that is abrogated by radioiodination. The functional relevance of our findings is indicated by the different capacities of unsubstituted and iodinated cytokine to induce prolonged proliferation of human T lymphocytes.