A point mutation in interleukin-2 that alters ligand internalization

In previous studies, we have identified an interleukin-2 (IL-2) analog containing a point mutation at position 51 (T51P) that expresses nearly wild-type bioactivity, yet has similar to 10-fold lower receptor binding affinity, Since ligand-dependent receptor internalization may be the rate-limiting step controlling the duration of IL-2 receptor signaling, a reduction in the receptor internalization rate could contribute to the observed response enhancement for this analog, To evaluate this possibility, we compared the internalization of IL-2 and T51P in three separate assays, While the internalization rate for IL-2 agreed with values determined by others, the internalization of T51P was markedly reduced, The receptor binding rate constants for this analog were only slightly different; thus, altered binding kinetics could not explain the decreased internalization rate, The effects of reduced internalization were also observable in bioassays, where T51P maintained T-cell proliferation for a longer period compared with IL-2. These results indicate that the T51P point mutation reduces the receptor internalization rate compared with IL-2 in a fashion that is independent of the dissociation rate. This analog may represent a new approach to the preparation of cytokine analogs with potentiated agonist and antagonist properties.