Does nitric oxide control the counter-inflammatory response in endotoxic shock?*

Interferon regulatory factor-1 regulates the autophagic response in LPS-stimulated macrophages through nitric oxide

The pathogenesis of sepsis is complex and, unfortunately, poorly understood. The cellular process of autophagy is believed to play a protective role in sepsis; however, the mechanisms responsible for its regulation in this setting are ill defined. In the present study, interferon regulatory factor 1 (IRF-1) was found to regulate the autophagic response in lipopolysaccharide (LPS)-stimulated macrophages. In vivo, tissue macrophages obtained from LPS-stimulated IRF-1 knockout (KO) mice demonstrated increased autophagy and decreased apoptosis compared to those isolated from IRF-1 wild-type (WT) mice. In vitro, LPS-stimulated peritoneal macrophages obtained from IRF-1 KO mice experienced increased autophagy and decreased apoptosis. IRF-1 mediates the inhibition of autophagy by modulating the activation of the mammalian target of rapamycin (mTOR). LPS induced the activation of mTOR in WT peritoneal macrophages, but not in IRF-1 KO macrophages. In contrast, overexpression of IRF-1 alone increased the activation of mTOR and consequently decreased autophagic flux. Furthermore, the inhibitory effects of IRF-1 mTOR activity were mediated by nitric oxide (NO). Therefore, we propose a novel role for IRF-1 and NO in the regulation of macrophage autophagy during LPS stimulation in which IRF-1/NO inhibits autophagy through mTOR activation.

Nitric oxide contributes to the development of a post-injury Th2 T-cell phenotype and immune dysfunction

Severe injury induces immune dysfunction resulting in increased susceptibility to opportunistic infections. Previous studies from our laboratory have demonstrated that post-burn immunosuppression is mediated by nitric oxide (NO) due to the increased expression of macrophage inducible nitric oxide synthase (iNOS). In contrast, others suggest that injury causes a phenotypic imbalance in the regulation of Th1- and Th2 immune responses. It is unclear whether or not these apparently divergent mediators of immunosuppression are interrelated. To study this, C57BL/6 mice were subjected to major burn injury and splenocytes were isolated 7 days later and stimulated with antiCD3. Burn injury induced NO-mediated suppression of proliferative responses that was reversed in the presence of the NOS inhibitor L-monomethyl-L-arginine and subsequently mimicked by the addition of the NO donor, S-nitroso-N-acetyl-penicillamine (SNAP). SNAP also dose-dependently suppressed IFN-gamma and IL-2 (Th1), but not IL-4 and IL-10 (Th2) production. Delaying the addition of SNAP to the cultures by 24 h prevented the suppression of IFN-gamma production. The Th2 shift in immune phenotype was independent of cGMP and apoptosis. The addition of SNAP to cell cultures also induced apoptosis, attenuated mitochondrial oxidative metabolism and induced mitochondrial membrane depolarization. However, these detrimental cellular effects of NO were observed only at supra-physiologic concentrations (>250 microM). In conclusion, these findings support the concept that NO induces suppression of cell-mediated immune responses by selective action on Th1 T cells, thereby promoting a Th2 response.