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Aims: Sepsis, a systemic inflammatory response to infection, represents the leading cause of death in critically ill patients. However, the pathogenesis of sepsis remains incompletely understood. Carbon monoxide (CO), when administered at low physiologic doses, can modulate cell proliferation, apoptosis, and inflammation in pre-clinical tissue injury models, though its mechanism of action in sepsis remains unclear. Results: CO (250 ppm) inhalation increased the survival of C57BL/6J mice injured by cecal ligation and puncture (CLP) through the induction of autophagy, the down-regulation of pro-inflammatory cytokines, and by decreasing the levels of bacteria in blood and vital organs, such as the lung and liver. Mice deficient in the autophagic protein, Beclin 1 (Becn1+/−) were more susceptible to CLP-induced sepsis, and unresponsive to CO therapy, relative to their corresponding wild-type (Becn1+/+) littermate mice. In contrast, mice deficient in autophagic protein microtubule-associated protein-1 light chain 3B (LC3B) (Map1lc3b−/−) and their corresponding wild-type (Map1lc3b+/+) mice showed no differences in survival or response to CO, during CLP-induced sepsis. CO enhanced bacterial phagocytosis in Becn1+/+ but not Becn1+/− mice in vivo and in corresponding cultured macrophages. CO also enhanced Beclin 1-dependent induction of macrophage protein signaling lymphocyte-activation molecule, a regulator of phagocytosis. Innovation: Our findings demonstrate a novel protective effect of CO in sepsis, dependent on autophagy protein Beclin 1, in a murine model of CLP-induced polymicrobial sepsis. Conclusion: CO increases the survival of mice injured by CLP through systemic enhancement of autophagy and phagocytosis. Taken together, we suggest that CO gas may represent a novel therapy for patients with sepsis. Antioxid. Redox Signal. 20, 432–442.

Sepsis is a systemic inflammatory response to a blood-borne infection that is associated with an extremely high rate of morbidity and mortality. The present article reviews our recent studies involving the role of cyclooxygenase (COX)-2 in host responses to bacterial endotoxemia and its role in the regulation of nitric oxide synthase (NOS)2 and heme oxygenase (HO)-1. COX-2-deficient (-/-) mice display a blunted and delayed induction of the cytokine-inducible genes NOS2 and HO-1 after administration of Escherichia coli lipopolysaccharide (LPS or endotoxin). Translocation and activation of transcription factors important for signaling events during an inflammatory response, such as nuclear factor-kappaB and activating protein-1, are also reduced. In addition, COX-2(-/-) mice have reduced leukocyte infiltration into critical organs (kidneys and lungs) after LPS administration. Interestingly, the absence of COX-2 does not alter the LPS induction of several proinflammatory cytokines in tissue macrophages, but induction of the antiinflammatory cytokine interleukin-10 is exaggerated. After LPS administration, 50% of wild-type (+/+) mice die; however, COX-2(-/-) mice display a dramatic improvement in survival during endotoxemia. Taken together, our findings suggest that COX-2(-/-) mice are resistant to many of the detrimental consequences of endotoxemia.

Heme oxygenase (HO) enzymes catalyze the initial reaction in heme catabolism. HO-1 is an inducible isoform that is up-regulated by diverse stimuli, including inflammatory cytokines and factors that promote oxidative stress. HO-1 is a cytoprotective enzyme that degrades heme, a potent oxidant, to generate carbon monoxide, biliverdin (subsequently reduced to bilirubin), and iron. Recently, we found that thioredoxin (TRX), a disulfide reductase enzyme known to be important for the binding of transcription factors to DNA, contributes to the induction of HO-1 by inflammatory mediators. In the present study, we extended this observation and determined that, similar to HO-1, TRX and TRX reductase (TR) are induced by bacterial lipopolysaccharide in macrophages at the level of mRNA and protein. However, maximal induction of TRX and TR precedes that of HO-1. Increased expression of HO-1 in the cytoplasm of inflammatory cells corresponds to a translocation of TRX into the nucleus of these cells. Finally, transfection of TRX into macrophages promoted an increase in HO-1 protein. Taken together, these data support the concept that the TRX system contributes to the up-regulation of HO-1 under conditions associated with increased oxidative stress.