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Mechanisms regulating the balance of T-helper 1 (Th1) and T-helper 2 (Th2) immune responses are of great interest as they may determine the outcome of allergic and infectious diseases. Recently, in mice, nitric oxide (NO), a powerful modulator of inflammation, has been reported to preferentially down-regulate Th1-mediated immune responses. In the present study, we investigated the effect of NO on the production of Th1- and Th2-associated cytokines by activated human T cells and human T-cell clones. Cytokine secretion was measured in the presence of the NO-donating agents 3-morpholinosydnonimine (SIN-1) and S-nitroso-N-acetylpenicillamine (SNAP). Both NO-donors markedly inhibited the release of interferon-γ (IFN-γ), interleukin-2 (IL-2), IL-5, IL-10 and IL-4 by anti-CD3 activated T cells. A preferential inhibition of Th1-associated cytokines was not observed. Neither was nitrite found in the supernatants of activated T cells, nor was specific mRNA for inducible and constitutive NO synthase detectable, indicating that T cells themselves did not contribute to the observed effect of the NO donors. Costimulation with anti-CD28 monoclonal antibodies (mAb) prevented SIN-1/SNAP-mediated down-regulation of cytokine production only in part. In contrast, when T cells were stimulated by phorbol-ester and ionomycin, they were refractory to SIN-1-induced inhibition of cytokine production. When SIN-1 was added after the onset of anti-CD3 stimulation, the inhibitory effect was found to be less pronounced, indicating that SIN-1 may interfere with early signal transduction events. The addition of superoxide dismutase (SOD) and catalase did not restore the effects of SIN-1, demonstrating that the inhibition of cytokines was due to NO and not to oxygen intermediates. Furthermore, 8-Br-cGMP-mediated increase of intracellular cGMP caused the same pattern of cytokine inhibition as observed with SIN-1 and SNAP. Using a single cell assay, these agents were shown to reduce the frequency of IFN-γ-producing T cells, suggesting that not all T cells are susceptible to SIN-1/SNAP. However, cytokine production by purified T-cell subpopulations (CD4+, CD8+, CD45RA+, and CD45RO+) was equally impaired by NO donors. In conclusion, in contrast to the murine system, our results do not provide evidence that NO preferentially inhibits Th1-cytokine secretion of activated human T cells in vitro.