Cyclosporin A suppresses the expression of the interleukin 2 gene by inhibiting the binding of lymphocyte-specific factors to the IL-2 enhancer.

Cyclosporin A (CsA), a powerful immunosuppressive drug, inhibits the synthesis of lymphokines in T lymphocytes at the level of gene transcription. Using protein extracts from El4 lymphoma cells we show that the binding of lymphocyte-specific factors interacting with the two so-called purine boxes (Pu-boxes) of the interleukin 2 (IL-2) enhancer are missing in CsA-treated cells. The CsA-sensitive factors are newly synthesized upon induction. The most prominent factor consists of 45 kd polypeptides and contacts both Pu-boxes at the two central G residues within the identical core sequence AAGAGGAAAA. The CsA-mediated suppression of factor binding to the Pu-boxes correlates well with functional studies in which the inducible, T cell-restricted proto-enhancer activity of Pu-boxes was selectively repressed by CsA. These observations support the conclusion that the suppression of factor binding to the Pu-boxes by CsA impairs the activity of IL-2 and of further lymphokine genes, thereby inhibiting the synthesis of lymphokines in T lymphocytes.