Nitric oxide synthase 2 through an autocrine loop via respiratory epithelial cell-derived mediator

Understanding the Cellular Sources of the Fractional Exhaled Nitric Oxide (FeNO) and Its Role as a Biomarker of Type 2 Inflammation in Asthma

Fractional exhaled nitric oxide (FeNO) has gained great clinical importance as a biomarker of type 2 inflammation in chronic airway diseases such as asthma. FeNO originates primarily in the bronchial epithelium and is produced in large quantities by the enzyme inducible nitric oxide synthase (iNOS). It should be noted that nitric oxide (NO) produced at femtomolar to picomolar levels is fundamental for respiratory physiology. This basal production is induced in the bronchial epithelium by interferon gamma (IFNγ) via Janus kinases (JAK)/STAT-1 signaling. However, when there is an increase in the expression of type 2 inflammatory cytokines such as IL-4 and IL-13, the STAT-6 pathway is activated, leading to overexpression of iNOS and consequently to an overproduction of airway NO. Increased NO levels contributes to bronchial hyperreactivity and mucus hypersecretion, increases vascular permeability, reduces ciliary heartbeat, and promotes free radical production, airway inflammation, and tissue damage. In asthmatic patients, FeNO levels usually rise above 25 parts per billion (ppb) and its follow-up helps to define asthma phenotype and to monitor the effectiveness of corticosteroid treatment and adherence to treatment. FeNO is also very useful to identify those severe asthma patients that might benefit of personalized therapies with monoclonal antibodies. In this review, we revised the cellular and molecular mechanisms of NO production in the airway and its relevance as a biomarker of type 2 inflammation in asthma.

Tpl2 Ablation Leads to Hypercytokinemia and Excessive Cellular Infiltration to the Lungs During Late Stages of Influenza Infection

Tumor progression locus 2 (Tpl2) is a serine-threonine kinase known to promote inflammation in response to various pathogen-associated molecular patterns (PAMPs), inflammatory cytokines and G-protein-coupled receptors and consequently aids in host resistance to pathogens. We have recently shown that Tpl2-/- mice succumb to infection with a low-pathogenicity strain of influenza (x31, H3N2) by an unknown mechanism. In this study, we sought to characterize the cytokine and immune cell profile of influenza-infected Tpl2-/- mice to gain insight into its host protective effects. Although Tpl2-/- mice display modestly impaired viral control, no virus was observed in the lungs of Tpl2-/- mice on the day of peak morbidity and mortality suggesting that morbidity is not due to virus cytopathic effects but rather to an overactive antiviral immune response. Indeed, increased levels of interferon-β (IFN-β), the IFN-inducible monocyte chemoattractant protein-1 (MCP-1, CCL2), Macrophage inflammatory protein 1 alpha (MIP-1α; CCL3), MIP-1β (CCL4), RANTES (CCL5), IP-10 (CXCL10) and Interferon-γ (IFN-γ) was observed in the lungs of influenza-infected Tpl2-/- mice at 7 days post infection (dpi). Elevated cytokine and chemokines were accompanied by increased infiltration of the lungs with inflammatory monocytes and neutrophils. Additionally, we noted that increased IFN-β correlated with increased CCL2, CXCL1 and nitric oxide synthase (NOS2) expression in the lungs, which has been associated with severe influenza infections. Bone marrow chimeras with Tpl2 ablation localized to radioresistant cells confirmed that Tpl2 functions, at least in part, within radioresistant cells to limit pro-inflammatory response to viral infection. Collectively, this study suggests that Tpl2 tempers inflammation during influenza infection by constraining the production of interferons and chemokines which are known to promote the recruitment of detrimental inflammatory monocytes and neutrophils.

INTERCONNECTION BETWEEN NITRIC OXIDE FORMATION AND HYPERSENSITIVITY PARAMETERS UNDER GUINEA PIG MODEL OF ACUTE ASTHMA WITH MULTIPLE CHALLENGES

An immunoregulatory role of nitric oxide (NO) in the development of adaptive immune responses associated with allergic diseases is very important. The present study extended these observations by the examination of the reciprocal changes in characteristic immunologic parameters of the disease and NO level of bronchoalveolar lavage (BAL) cells under guinea pig model of acute asthma with multiple challenges. Development of guinea pig Th2 mediated asthma was accompanied by increasing the level of allergic markers: ovalbumin (OVA) specific IgG and IL-4. We demonstrated that the infiltrate of airway cells contributes to NO synthesis in the respiratory tract during allergic inflammation. The level of intracellular NO formation significantly correlated with plasma allergen specific IgG value in OVA-induced asthma. The presented data evidence that the elevated intracellular NO level in BAL fluid may reflect a nitrosative stress in respiratory tract in general, when allergic asthma exacerbation is present.

Influenza A virus abrogates IFN-gamma response in respiratory epithelial cells by disruption of the Jak/Stat pathway

The innate immunity to viral infections induces a potent antiviral response mediated by interferons (IFN). Although IFN-gamma is detected during the acute stages of illness in the upper respiratory tract secretions and in the serum of influenza A virus-infected individuals, control of influenza A virus is not dependent upon IFN-gamma as evidenced by studies using anti-IFN-gamma Ab and IFN-gamma(-/-) mice. Thus, we hypothesized that IFN-gamma is not critical in host survival because influenza A virus has mechanisms to evade the antiviral activity of IFN-gamma. To test this, A549 cells, an epithelial cell line derived from lung adenocarcinoma, were infected with influenza virus strain A/Aichi/2/68 (H3N2) (Aichi) and/or stimulated with IFN-gamma to detect IFN-gamma-stimulated MHC class II expression. Influenza A virus infection inhibited IFN-gamma-induced up-regulation of HLA-DRalpha mRNA and the IFN-gamma induction of class II transactivator (CIITA), an obligate mediator of MHC class II expression. Nuclear translocation of Stat1alpha upon IFN-gamma stimulation was significantly inhibited in influenza A virus-infected cells and this was associated with a decrease in Tyr701 and Ser727 phosphorylation of Stat1alpha. Thus, influenza A virus subverts antiviral host defense mediated by IFN-gamma through effects on the intracellular signaling pathways.

Nitric oxide in health and disease of the respiratory system

During the past decade a plethora of studies have unravelled the multiple roles of nitric oxide (NO) in airway physiology and pathophysiology. In the respiratory tract, NO is produced by a wide variety of cell types and is generated via oxidation of l-arginine that is catalyzed by the enzyme NO synthase (NOS). NOS exists in three distinct isoforms: neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS). NO derived from the constitutive isoforms of NOS (nNOS and eNOS) and other NO-adduct molecules (nitrosothiols) have been shown to be modulators of bronchomotor tone. On the other hand, NO derived from iNOS seems to be a proinflammatory mediator with immunomodulatory effects. The concentration of this molecule in exhaled air is abnormal in activated states of different inflammatory airway diseases, and its monitoring is potentially a major advance in the management of, e.g., asthma. Finally, the production of NO under oxidative stress conditions secondarily generates strong oxidizing agents (reactive nitrogen species) that may modulate the development of chronic inflammatory airway diseases and/or amplify the inflammatory response. The fundamental mechanisms driving the altered NO bioactivity under pathological conditions still need to be fully clarified, because their regulation provides a novel target in the prevention and treatment of chronic inflammatory diseases of the airways.

Impaired nitric oxide synthase-2 signaling pathway in cystic fibrosis airway epithelium

Cystic fibrosis (CF) airway epithelial cells are more susceptible to viral infection due to impairment of the innate host defense pathway of nitric oxide (NO). NO synthase-2 (NOS2) expression is absent, and signal transducer and activator of transcription (STAT) 1 activation is reduced in CF. We hypothesized that the IFN-γ signaling pathway, which leads to NOS2 gene induction in CF airway epithelial cells, is defective. In contrast to a lack of NOS2 induction, the major histocompatibility complex class 2, an IFN-γ-regulated delayed-responsive gene, is similarly induced in CF and non-CF airway epithelial (NL) cells, suggesting an NOS2-specific defect in the IFN-γ signaling pathway. STAT1 and activator protein-1, both required for NOS2 gene expression, interact normally in CF cells. Protein inhibitor of activated STAT1 is not increased in CF cells. IFN-γ induces NOS2 expression in airway epithelial cells through an autocrine mechanism involving synthesis and secretion of IFN-γ-inducible mediator(s), which activates STAT1. Here, CF cells secrete IFN-γ-inducible factor(s), which stimulate NOS2 expression in NL cells, but not in CF cells. In contrast, IFN-γ-inducible factor(s) similarly inhibit virus in CF and NL cells. Thus autocrine activation of NOS2 is defective in CF cells, but IFN-γ induction of antiviral host defense is intact.

STAT-1 and c-Fos interaction in nitric oxide synthase-2 gene activation

Interferon-gamma (IFN-gamma) is required for induction of the human nitric oxide synthase-2 (NOS2) gene in lung epithelium. Although the human NOS2 promoter region contains many cytokine-responsive elements, the molecular basis of induction is only partially understood. Here, the major cis-regulatory elements that control IFN-gamma-inducible NOS2 gene transcription in human lung epithelial cells are identified as composite response elements that bind signal transducer and activator of transcription 1 (STAT-1) and activator protein 1 (AP-1), which is comprised of c-Fos, Fra-2, c-Jun, and JunD. Notably, IFN-gamma activation of the human NOS2 promoter is shown to require functional AP-1 regulatory region(s), suggesting a role for AP-1 activation/binding in the IFN-gamma induction of genes. We show that c-Fos interacts with STAT-1 after IFN-gamma activation and the c-Fos/STAT-1 complex binds to the gamma-activated site (GAS) element in close proximity to AP-1 sites located at 4.9 kb upstream of the transcription start site. Taken together, our findings support a model in which a physical interaction between c-Fos and STAT-1 participates in NOS2 gene transcriptional activation.