Characterization of the soluble murine IL-2R and estimation of its affinity for IL-2

IL-2 induces cells of the cytotoxic T cell line C30.1 to express large numbers of membrane IL-2R (mIL-2R). At the height of activation, these cells also release a soluble form of IL-2R (sIL-2R). Using either crude supernatant or a semi-purified preparation of sIL-2R obtained by affinity chromatography, studies were performed to characterize murine sIL-2R. Its m.w. was determined by both gel filtration and SDS-PAGE. The affinity of sIL-2R for a panel of mAb known to recognize different epitopes of mIL-2R (p55 subunit) was assessed by saturation and competition experiments. The relationship between the various epitopes was studied by cross-inhibition experiments. The data suggest that sIL-2R and mIL-2R (p55 subunit) are structurally similar. The ability of sIL-2R to bind IL-2 was assessed by measuring the dissociation and the inhibition constant of the molecule for IL-2. Both values coincide and indicate that the affinity of sIL-2R for IL-2 is at least 10-fold lower than the that of low affinity mIL-2R. The biologic implications of these findings are discussed.