Nitric Oxide Inhibits the Proliferation of T-Helper 1 and 2 Lymphocytes without Reduction in Cytokine Secretion

To study the effect of nitric oxide (NO) on the activity of Th subsets, cloned Th1 and Th2 lymphocytes were stimulated in the presence of an NO donor. NO, when present from the start of incubation, inhibited the proliferation of both Th subsets dose-dependently, achieving complete inhibition at a relatively low level. The addition of NO 24 h after the onset of T cell stimulation also resulted in reduced proliferation of both Th subsets, suggesting that NO affects a late process during T cell activation. Stimulation of T cells in the presence of NO did not induce apoptosis at the concentrations that completely inhibited proliferation, although apoptosis became evident at higher NO concentrations. The secretion of several cytokines (i.e., IFN-γ, IL-4, and IL-5) was slightly upregulated, while IL-2 production was modestly inhibited in the presence of NO. However, exogenous IL-2 did not reverse the NO-induced inhibition of T cell proliferation, nor did additional stimulation with phorbol esters. Finally, expression of IL-2R was modestly decreased in the presence of NO, although TCR expression was not affected. These studies demonstrate that relatively low concentrations of NO induce a strong and specific inhibition of T cell proliferation in both Th subsets, suggesting that local NO production may regulate Th-mediated tissue inflammation.