An IgG3-IL-2 fusion protein has higher affinity than hrIL-2 for the IL-2R alpha subunit: real time measurement of ligand binding.

The alpha subunit of the interleukin-2 (IL-2) receptor (IL-2R alpha)3 has the highest individual affinity for IL-2 and is the only subunit not known to bind other cytokines. The interactions between IL-2 and IL-2R alpha studied in cell binding assays have revealed a number of factors which may vary significantly in different cell lines used for these assays in different laboratories. In order to avoid the problems associated with cellular assays we used an optical biosensor to examine the interaction between IL-2R alpha and hrIL-2. Real-time measurement of association and dissociation resulted in a calculated KD of 1.9 x 10(-7) M for this interaction. We then examined the IL-2R alpha binding of a potentially bivalent IgG3-IL2 fusion protein previously shown to have a higher affinity than hrIL-2 for the high affinity IL-2R but not the intermediate affinity IL-2R. Biosensor measurements of association and dissociation of IgG3-IL2 to IL-2R alpha yielded a similar association rate but a decreased dissociation rate compared to hrIL-2, resulting in a KD of 5.3 x 10(-8) M. This system is applicable to the numerous IL-2 mutants with different affinities and activities and is generalizable to other cytokine/receptor interactions.