Tumor necrosis factor-α (TNFα)-induced maturation of dendritic cells (DC), with down-regulation of their endocytic ability, has been reported to be mediated by the accumulation of the lipid messenger ceramide. We have now studied the effects and mechanisms of action of NO on endocytosis, investigated with fluorescein isothiocyanate-labeled dextran using human monocyte-derived DC, both immature and after treatment with TNFα. Exposure of DC to NO, released by either bystander phagocytes or NO donors, reversed the inhibition of endocytosis induced by TNFα. The intracellular accumulation of ceramide induced by TNFα was also inhibited by NO. In addition, NO was found to exert an inhibitory effect downstream of the TNFα- triggered ceramide accumulation, because NO donors reversed the inhibition of endocytosis induced by the cell-permeant C2-ceramide. These effects of NO were mimicked by the membrane-permeant cyclic GMP analogue, 8-Br cyclic GMP, and prevented by inhibition of the soluble guanylyl cyclase. At variance with rodents, the inducible isoform of the NO synthase was expressed neither in immature human DC nor after cell treatment with TNFα, interferon-γ, and lipopolysaccharide, suggesting that regulation of these cells depends on exogenous NO. NO, working through cyclic GMP, might therefore prolong the ability of human DC to internalize antigens at the site of inflammation and thus modulate the initial steps leading to antigen-specific immune responses.
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