Glutathione depletion reveals impairment of antigen processing and inhibition of cathepsin activity by nitric oxide in antigen-presenting cells

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Nitric oxide has been shown to induce immunosuppression by inhibiting class II MHC molecule expression and T-lymphocyte proliferation. However, much less is known about the ability of NO to interfere with antigen processing and presentation. So we questioned whether B lymphoma cells exposed to NO could be impaired in their ability to process lysozyme and to stimulate proliferation of a syngeneic T-cell hybridoma. As immunosuppressive pathological conditions are often associated with a pro-oxidative milieu, we also examined the influence of intracellular GSH levels on NO responsiveness. Exposure of GSH-depleted B cells to NO-releasing compounds lowered their capacity to present a reduced and alkylated lysozyme (TAP–HEL), although presentation of a lysozyme-derived peptide was unaffected. Cells with a normal GSH content were protected from this inhibition. Fluid phase endocytosis, protein synthesis and expression of class II molecules remained normal in GSH-depleted cells. However, proteolysis of a dye conjugate of ovalbumin was strongly inhibited, suggesting that protease inhibition might be involved. Cathepsin B activity, which was necessary to TAP–HEL processing, was inhibited by the NO-donors. The inhibition was higher in GSH-depleted cells and reproduced by treatment of A20 B cells by two cathepsin inhibitors. These results show that, in addition to cytostasis and reduction in class II expression, NO-induced immunosuppression could also implicate inhibition of antigen processing under oxidative stress conditions.