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

Abstract

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.

Figure 1

Schematic representation of the enzymatic production of nitric oxide (NO) by nitric oxide synthases (NOS) indicating substrate and cofactors.

Figure 2

Functions of NO in the airway depending on its concentration from physiological conditions to pathological effects. iNOS: inducible nitric oxide synthase. iNANC: noncholinergic inhibitory noncholinergic.

Figure 3

Schematic representation of the homeostatic mechanism of iNOS expression in bronchial epithelial cells under physiological conditions via INFγ receptor and the STAT-1 pathway. Once INFγ binds its receptor, STAT-1 is phosphorylated and translocated to the nucleus inducing the expression of interferon response factor 1 (IRF1) which binds to its response element in the promoter of the NOS2 gene that encodes for iNOS. Together with the STAT-1 binding elements, they maintain iNOS expression in low concentrations for the physiological production of NO in exhaled air. iNOS promoter also has binding sites for NFκB and AP-1 which increase its expression in response to microbial or environmental signals detected by pattern receptors.

Figure 4

Airway diseases that occur with high and low FeNO levels. ppb: parts per billion. AERD: aspirin-exacerbated respiratory disease.

Figure 5

Schematic representation of excessive activation of the IL-4/IL-13 receptor and potentiation of STAT-6 signals (red lines) that alter the balance of STAT-1-mediated signals and lead to overexpression of iNOS in patients with type 2 inflammation.

Figure 6

Effects of cytokines characteristic of type 2 inflammation on the production of FeNO and other markers of the high T2 asthma phenotype.