Transforming growth factor 1a (TGF-,) has a growth-inhibitory effect on numerous different cell types of the immune system, including T lymphocytes. We show in this study that the inhibitory action of TGF-13 on T lymphocytes is accompanied by a block of interleukin 2 (IL-2) gene expression which is mediated, at least in part, by inhibition of IL-2 promoter/enhancer activity. The functional analysis of cis-regulatory (proto-enhancer) elements of the IL-2 enhancer/promoter region showed that the most TGF-13-responsive element maps to its so-called upstream promoter site. The proto-enhancer activity of the upstream promoter site element is also inhibited by cyclosporin A. (12,19,20,43). TGF-1 is a very efficient inhibitor of pre-B-cell maturation, preventing expression of the immunoglobulin kappa light-chain locus (26,29). Similarly, TGF-P was found to interfere with the appearance of membrane immunoglobulin heavy-chain (IgH) molecules and the secretion of immunoglobulin proteins in B lymphocytes (19). Moreover, TGF-P was found to suppress the generation of cytotoxic T cells (14), interleukin-2 (IL-2)-dependent T-cell proliferation, and the upregulation of IL-2 and transferrin receptors (20).A strong but quite different effect of TGF-3 on the synthesis and secretion of numerous cytokines has also been detected. In peripheral T lymphocytes, TGF-P was able to suppress the synthesis of IL-2 and gamma interferon (12, 39), whereas in similar cell types, the synthesis of tumor necrosis factor a and appeared to be resistant to or even enhanced by the drug (7,12). This quite selective effect of TGF-1 on a set of cytokines prompted us to compare the action of TGF-1 with that of the immunosuppressive agent cyclosporin A (CsA) at the molecular level. We and others have recently shown that CsA and its functional analog FK506 suppress early steps of T-cell activation through their interference with a few transcription factors that bind to the enhancer of the IL-2 gene (4, 10, 35). In particular, CsA and FK506 inhibited the activity of NFAT-1, a lymphoid-specific factor binding to the two purine boxes of the IL-2 enhancer, and the activity of factors that bind to the enhancer's upstream promoter site (UPS). We will show here that the latter site is specifically recognized by octamer and AP-1-like factors.In nonlymphoid cell types, TGF-1 has been reported to exert its negative effect on numerous genes through a variety of binding motifs for transcription factors. In several rat cell lines, an inhibitory activity of TGF-P on the transin/stromelysin promoter has been detected which appeared to be mediated through a noncanonical AP-1/Fos binding site (21). In other studies, an inhibitory effect of TGF-,B on the synthesis of jun mRNAs that results in suppression of AP-1-mediated gene activities has been reported (16,22,25,27,46). In other reports, an effect of TGF-,B on expression of the c-myc gene which seems to be mediated through the protein product of the retinoblastoma (Rb) gene has been reported (28, 30). A target site for the...