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BACKGROUND: Nitric oxide (NO) has frequently been shown to display immunosuppressive activities. We describe here a molecular mechanism contributing to this effect. MATERIALS AND METHODS: Murine T cell lymphoma EL4-6.1 cells were activated with the physiological stimulus interleukin (IL)-1beta to express IL-2 mRNA in the presence or absence of subtoxic concentrations of the physiological spontaneous NO donor S-nitrosocysteine (SNOC). Subsequently, semiquantitative RT-PCR and gel shift assays with nuclear extracts were performed to analyze the effects of NO on IL-2 mRNA expression and on the activity of the dominant regulating transcription factors Sp1, EGR-1, and NFATc. RESULTS: NO inhibits IL-1beta-induced IL-2 mRNA expression in EL4-6.1 cells. The suppressive activity of NO was concentration dependent and found to be completely reversible. Importantly, NO at the concentrations used induced neither apoptosis nor necrosis. Dominant regulation of IL-2 gene expression is known to reside in the zinc finger transcription factors Sp1 or EGR-1 and in the non-zinc finger protein NFAT. NO abrogates the DNA binding activities of recombinant Sp1 and EGR-1. More importantly, gel shift assays also showed a lack of DNA binding of native Sp1 derived from NO-treated nuclear extracts and that from NO-treated viable lymphocytes. This effect is selective, as the DNA binding activity of recombinant NFATc was not affected by NO. CONCLUSION: Inactivation of zinc finger transcription factors by NO appears to be a molecular mechanism in the immunosuppressive activity of NO in mammals, thus contributing to NO-mediated inhibition of IL-2 gene expression after physiological stimuli. The exact understanding of the molecular mechanism leading to NO-mediated, fully reversible suppression of immune reactions may lead to use of this naturally occurring tool as an aid in inflammatory diseases.