Airway monocyte modulation relates to TNF dysregulation in neutrophilic asthma
BackgroundDysregulation of TNFα signalling is implicated in neutrophilic asthma. TNFα signalling involves membrane-bound and soluble ligand (TNFα) and receptors (TNFRs); however, little is known about how these factors are altered in asthma. We hypothesized that intercompartment-, immune cell- and/or asthma inflammatory phenotype-dependent regulation could relate to TNF factor dysregulation in neutrophilic asthma.MethodsMeasures were made in 45 adults with asthma (36 non-neutrophilic, 9 neutrophilic) and 8 non-asthma controls. Soluble TNFα, TNFR1 and TNFR2 were quantified in plasma and sputum supernatant by ELISA, and membrane-bound TNFα/TNFR1/TNFR2 measured on eosinophils, neutrophils, monocytes, and macrophages in blood and sputum by flow cytometry. Marker expression was compared between inflammatory phenotypes and compartments, and relationship of membrane-bound and soluble TNF markers and immune cell numbers tested by correlation.ResultsSoluble sputum TNFR1 and TNFR2 were increased in neutrophilic versus non-neutrophilic asthma (p=0.010 and p=0.029). Membrane-bound TNFα expression was upregulated on sputum versus blood monocytes, while TNFR1 and TNFR2 levels were reduced on airway versus blood monocytes and neutrophils. Soluble TNFR1 and TNFR2 in sputum significantly correlated with the number of airway monocytes (p=0.016, r=0.358 and p=0.029, r=0.327).ConclusionOur results imply that increased sputum soluble TNF receptor levels observed in neutrophilic asthma relate to the increased recruitment of monocytes and neutrophils into the airways and their subsequent receptor shedding. Monocytes also increase TNFα ligand expression in the airways. These results suggest an important contribution of airway monocytes to the altered inflammatory milieu in neutrophilic asthma.