The chromatin structure of the interleukin-2 (IL-2) gene was probed by DNase I treatment of isolated nuclei. The 5' region of the IL-2 gene contains three regions of hypersensitivity to DNase I. When peripheral blood T cells or Jurkat T cells are stimulated with mitogens, IL-2 message is induced, and the promoter region of the IL-2 gene develops an additional hypersensitive site. This suggests that a DNA sequence close to the transcriptional start site is involved in the transduction of the extracellular signal. Such a conclusion is further supported by DNA transfection experiments. A short segment of DNA, which includes the region of induced hypersensitivity, confers inducibility on the linked chloramphenicol acetyltransferase gene in transiently transfected Jurkat cells. In addition, cells of nonhematopoietic origins exhibit a strikingly different chromatin pattern of IL-2, suggesting a role during differentiation for some of the hypersensitive sites.Activation of T lymphocytes by mitogens or antigens leads to secretion of the growth factor interleukin-2 (IL-2) (14,27,30,32,34). This factor interacts with the IL-2 receptor on the surface of activated T cells, and such an interaction is required for progression of these cells through the cell cycle (3). T cells thus proliferate in part via an autocrine mechanism. Subsequent to the induction phase, IL-2 expression appears to be specifically down modulated (6). Consequently, the magnitude of the initial pulse of IL-2 synthesis determines to a large degree the magnitude of the proliferative stimulus. Several agents which interfere with IL-2 production, such as cyclosporin A (CsA) (7, 22) and glucocorticoids (12), have powerful immunosuppressive actions. These agents most likely prevent the expansion of clones of antigen-specific T lymphocytes. The effects of glucocorticoids are broad, whereas the dramatic immunosuppressive effects of CsA are based largely on the inhibition of IL-2 production and give some insight into the critical role of IL-2 regulation in governing the immune response.The IL-2 gene product has been purified (28, 32), both cDNA and genomic clones for IL-2 have been identified, and the complete structure of the gene is known (11,18,19,37 Normal peripheral blood T cells also require two signals for activation; namely, antigens (or mitogens) and macrophages (3,26,29,38). Macrophages probably exert their effect through the IL-1 factor (5, 29). The mitogen PHA is thought to substitute for the signal normally provided by antigen, and PMA is probably a substitute for IL-1 or macrophages or both.Despite such extensive knowledge about the structure and function of the IL-2 gene and the signals leading to its activation in both normal T cells as well as in model cell lines, very little is known about the intracellular events that result in transcriptional activation or the determinants for its tissue-specific expression. Therefore, we set out to study the regulation of the IL-2 gene at the DNA level. We approached this in two ways: first by probing the chr...