The role of flow-independent exhaled nitric oxide parameters in the assessment of airway diseases

Nitric oxide (NO), the first gas known to act as a biological messenger, is one of the most widely studied free radical/gas in medicine, both for its biological function and therapeutic applications. The measurement of endogenous NO in exhaled air is widely used in the evaluation of lung disorders. Partitioning of exhaled nitric oxide (eNO) is of increasing interest because of the additional information about lung pathology and distal lung inflammation that can be obtained. Specifically, measuring exhaled NO at multiple flow rates allows assessment of the flow-independent NO parameters: alveolar NO concentration (CalvNO), bronchial NO flux (JNO), bronchial wall NO concentration (CWNO), and bronchial diffusing capacity of NO (DNO). Several studies have reported that there were different patterns of those parameters in different airway diseases and/or in different severities of the same disease, mostly in asthma. Specifically, while JNO seems to provide the same information as FeNO50, alveolar NO concentration appears to be an independent parameter that is putatively associated with increased distal lung inflammation and more severe disease. However, despite much research interest in partitioning exhaled NO, clinical usefulness has yet to be established

Immunoproteasome dysfunction augments alternative polarization of alveolar macrophages.

The proteasome is a central regulatory hub for intracellular signaling by degrading numerous signaling mediators. Immunoproteasomes are specialized types of proteasomes involved in shaping adaptive immune responses, but their role in innate immune signaling is still elusive. Here, we analyzed immunoproteasome function for polarization of alveolar macrophages, highly specialized tissue macrophages of the alveolar lung surface. Classical activation (M1 polarization) of primary alveolar macrophages by LPS/IFNγ transcriptionally induced all three immunoproteasome subunits, low molecular mass protein 2 (LMP2), LMP7 and multicatalytic endopeptidase complex-like 1, which was accompanied by increased immunoproteasome activity in M1 cells. Deficiency of LMP7 had no effect on the LPS/IFNγ-triggered M1 profile indicating that immunoproteasome function is dispensable for classical alveolar macrophage activation. In contrast, IL-4 triggered alternative (M2) activation of primary alveolar macrophages was accompanied by a transcriptionally independent amplified expression of LMP2 and LMP7 and an increase in immunoproteasome activity. Alveolar macrophages from LMP7 knockout mice disclosed a distorted M2 profile upon IL-4 stimulation as characterized by increased M2 marker gene expression and CCL17 cytokine release. Comparative transcriptome analysis revealed enrichment of IL-4-responsive genes and of genes involved in cellular response to defense, wounding and inflammation in LMP7-deficient alveolar macrophages indicating a distinct M2 inflammation resolving phenotype. Moreover, augmented M2 polarization was accompanied by amplified AKT/STAT6 activation and increased RNA and protein expression of the M2 master transcription factor interferon regulatory factor 4 in LMP7(-/-) alveolar macrophages. IL-13 stimulation of LMP7-deficient macrophages induced a similar M2-skewed profile indicative for augmented signaling via the IL-4 receptor α (IL4Rα). IL4Rα expression was generally elevated only on protein but not RNA level in LMP7(-/-) alveolar macrophages. Importantly, specific catalytic inhibition with an LMP7-specific proteasome inhibitor confirmed augmented IL-4-mediated M2 polarization of alveolar macrophages. Our results thus suggest a novel role of immunoproteasome function for regulating alternative activation of macrophages by limiting IL4Rα expression and signaling