Hydrogen peroxide signals E. coli phagocytosis by human polymorphonuclear cells; up-stream and down-stream pathway

Redox processes are major regulators of leukotriene synthesis in neutrophils exposed to bacteria Salmonella typhimurium; the way to manipulate neutrophil swarming

Neutrophils play a primary role in protecting our body from pathogens. When confronted with invading bacteria, neutrophils begin to produce leukotriene B4, a potent chemoattractant that, in cooperation with the primary bacterial chemoattractant fMLP, stimulates the formation of swarms of neutrophils surrounding pathogens. Here we describe a complex redox regulation that either stimulates or inhibits fMLP-induced leukotriene synthesis in an experimental model of neutrophils interacting with Salmonella typhimurium. The scavenging of mitochondrial reactive oxygen species by mitochondria-targeted antioxidants MitoQ and SkQ1, as well as inhibition of their production by mitochondrial inhibitors, inhibit the synthesis of leukotrienes regardless of the cessation of oxidative phosphorylation. On the contrary, antioxidants N-acetylcysteine and sodium hydrosulfide promoting reductive shift in the reversible thiol-disulfide system stimulate the synthesis of leukotrienes. Diamide that oxidizes glutathione at high concentrations inhibits leukotriene synthesis, and the glutathione precursor S-adenosyl-L-methionine prevents this inhibition. Diamide-dependent inhibition is also prevented by diphenyleneiodonium, presumably through inhibition of NADPH oxidase and NADPH accumulation. Thus, during bacterial infection, maintaining the reduced state of glutathione in neutrophils plays a decisive role in the synthesis of leukotriene B4. Suppression of excess leukotriene synthesis is an effective strategy for treating various inflammatory pathologies. Our data suggest that the use of mitochondria-targeted antioxidants may be promising for this purpose, whereas known thiol-based antioxidants, such as N-acetylcysteine, may dangerously stimulate leukotriene synthesis by neutrophils during severe pathogenic infection.

MK2 Is Required for Neutrophil-Derived ROS Production and Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic disease that is commonly accompanied by increased inflammatory responses and elevated reactive oxygen species (ROS) of the gastrointestinal tract. Here, we found that MAPK-activated protein kinase 2 (MK2) modulates ROS production and is required for dextran sulfate sodium (DSS)-induced IBD in the mouse model. Genetic ablation of MK2 in the myeloid lineage cells (MK2^Lyz2−KO) protected against DSS-induced colitis injury. In response to DSS challenge, compared to MK2^lyz2−WT mice, MK2^Lyz2−KO mice exhibited less damage of epithelial and goblet cells, decreased generation of interleukin (IL)-6, tumor necrosis factor (TNF)-α, and ROS, as well as reduced Ki67-positive cells and concentrations of myeloperoxidase (MPO) in the intestinal epithelium. Furthermore, upon treatment with formylated peptide N-formyl-methionyl-leucyl-phenylalanine (fMLF), the generation of ROS was attenuated in MK2-deficient neutrophils, in which the phosphorylation of Akt and p38 MAPK was also reduced. Collectively, these findings indicated that MK2 is required for neutrophil-derived ROS production and IBD, and MK2 and ROS are promising therapeutic targets for IBD.

LETMD1 Regulates Phagocytosis and Inflammatory Responses to Lipopolysaccharide via Reactive Oxygen Species Generation and NF-κB Activation in Macrophages

LETM1 domain-containing protein 1 (LETMD1), also known as HCCR-1, is a mitochondrial protein and is known to regulate p53 and STAT3 activities in cancer cells. In this study, we present, for the first time (to our knowledge), data indicating that LETMD1 suppresses multiple immune responses in monocyte/macrophage lineage cells and mouse primary macrophages. Attenuation of LETMD1 expression with specific small interfering RNA and short hairpin RNA constructs enhanced LPS-induced expressions of inflammatory mediators in macrophages. In addition, LETMD1 attenuation caused potentiation of phagocytosis as well as migration in a macrophage-like cell line, U937. These enhancing effects were associated with altered activation of signaling adaptors (such as NF-κB, MAPKs, p53, and JAK-STAT) involved in TLR4 signaling. Especially, LETMD1 selectively regulated TLR4-induced NF-κB activation via MyD88 but not via TIR-domain-containing adapter-inducing IFN-β (TRIF). Attenuation of LETMD1 expression caused mitochondrial hyperpolarization and subsequent decrease in ATP production and increase in mitochondrial/cellular reactive oxygen species (ROS) and intracellular calcium levels. LETMD1 attenuation also enhanced LPS-induced expression of NADPH oxidase (NOX) 2, the main producer of cellular ROS in phagocytes, through augmenting IFN regulatory factor 1. Accordingly, treatment with ROS scavenger, NOX2 suppressing agents, or calcium chelators resulted in suppression of LPS-induced cytokine production as well as NF-κB activation in cells with LETMD1 attenuation. These findings reveal a previously unknown function of LETMD1 and provide evidences showing LETMD1 negatively regulates macrophage functions by modulating mitochondrial function, subsequent ROS generation, and NF-κB activation.

Behçet's Syndrome as a Model of Thrombo-Inflammation: The Role of Neutrophils

Behçet's syndrome (BS) is a systemic vasculitis, clinically characterized by different organ involvement and often complicated by thrombosis which occurs in vessels of all sizes. Thrombosis is more frequent in male patients with active disease and represents an important cause of morbidity and mortality. Neutrophil involvement in BS has been repeatedly suggested in the last few years. Indeed, neutrophils have been shown to be hyperactivated in BS patients, probably with a HLAB51 related contribution, and represent the main cells infiltrating not only oral and genital ulcers or erythema nodosum, but also other sites. Besides being deputed to host defense against micro-organisms, neutrophils display fundamental roles both in inflammation and tissue damage becoming inappropriately activated by cytokines, chemokines and autoantibodies and subsequently producing large amounts of superoxide anion (O2.) via NADPH oxidase (NOX2). The strict relationship between inflammation and hemostasis has been already demonstrated. Indeed, inflammation and immune-mediated disorders increase the risk of thrombosis, but the pathways that link these processes have not been completely elucidated. In this regard, we recently demonstrated, in a large population of BS patients, a new neutrophil-dependent pathogenetic mechanism of thrombosis. In particular, it was shown that neutrophils, mainly through NADPH oxidase, produce excessive amounts of reactive oxygen species (ROS), which are able to markedly modify the secondary structure of fibrinogen and hence the overall architecture of the fibrin clot that becomes less susceptible to plasmin-induced lysis. These data point out that BS represents “per se” a model of inflammation-induced thrombosis and suggest that neutrophils specifically contribute to thrombo-inflammation in this rare disease. In particular, it is suggested that an alteration in fibrinogen structure and function are associated with enhanced ROS production via neutrophil NADPH oxidase. Altogether, these findings improve our understanding of the intricate pathogenetic mechanisms of thrombo-inflammation and may indicate potential new therapeutic targets.

Neutrophils and redox stress in the pathogenesis of autoimmune disease

Polymorphonuclear leukocytes, or neutrophils, are specialist phagocytic cells of the innate immune system. Their primary role is host defence against micro-organisms, which they kill via phagocytosis, followed by release of reactive oxygen species (ROS) and proteolytic enzymes within the phagosome. ROS are generated via the action of the NADPH oxidase (also known as NOX2), in a process termed the 'Respiratory Burst'. This process consumes large amounts of oxygen, which is converted into the highly-reactive superoxide radical O₂^- and H₂O₂. Subsequent activation of myeloperoxidase (MPO) generates secondary oxidants and chloroamines that are highly microbiocidal in nature, which together with proteases such as elastase and gelatinase provide a toxic intra-phagosomal environment able to kill a broad range of micro-organisms. However, under certain circumstances such as during an auto-immune response, neutrophils can be triggered to release ROS and proteases extracellularly causing damage to host tissues, modification of host proteins, lipids and DNA and dysregulation of oxidative homeostasis. This review describes the range of ROS species produced by human neutrophils with a focus on the implications of neutrophil redox products in autoimmune inflammation.

Oxidative stress induced by Se-deficient high-energy diet implicates neutrophil dysfunction via Nrf2 pathway suppression in swine

The mechanism of the interaction between Se deficiency and high energy remains limited. The aim of the current study was to identify whether Se-deficient, high-energy diet can induce oxidative stress, and downregulate the Nrf2 pathway and phagocytic dysfunction of neutrophils. We detected the phagocytic activity, ROS production, protein levels of Nrf2 and Nrf2 downstream target genes, and the mRNA levels of 25 selenoproteins, heat shock proteins, and cytokines in neutrophils. Cytokine ELISA kits were used to measure the serum cytokines. The concentration of ROS was elevated (P < 0.05) in obese swine fed on a low Se diet (less than 0.03 mg/kg Se) compared to control swine. The protein levels of Nrf2 and its downstream target genes were depressed during Se deficiency and high-energy intake. The mRNA levels of 16 selenoproteins were significantly decreased (P < 0.05) in the Se-deficient group and Se-deficient, high-energy group compared to the control group. However, the mRNA levels of 13 selenoproteins in peripheral blood neutrophils were upregulated in high energy group, except TrxR1, SelI and SepW. In summary, these data indicated that a Se-deficient, high-energy diet inhibits the Nrf2 pathway and its regulation of oxidative stress, and prompted a pleiotropic mechanism that suppresses phagocytosis.