Sputum Interleukin 5 in eosinophilic and non-eosinophilic severe asthma

Abstract

<b>Background:</b> Interleukin 5, an archetypal type 2 cytokine, is fundamental to eosinophilic airways disease, considered to signal the bone marrow to promote eosinophil progenitor maturation and increase levels of eosinophils in the circulation. <b>Aim:</b> To investigate the relationship between airway IL-5 concentrations and inflammatory phenotypes of severe asthma. <b>Methods:</b> Healthy volunteers and biologic naïve severe asthma patients were clinically characterised and had blood and induced sputum samples collected. Sputum was analysed for differential cell count and supernatant (PBS processed) proteins, measured by single plex ELISA. Eosinophilic asthma was defined as sputum eosinophils &gt;2%. Between group comparisons were assessed by Kruskal Wallis and relationships between variables by spearman rank correlations. <b>Results:</b> 17 healthy controls, 53 non-eosinophilic and 26 eosinophilic patients with severe asthma participated in the study.&nbsp;In severe asthma, there were modest correlations between sputum IL-5 and sputum eosinophils&nbsp;(r = 0.519, p&lt;0.001) and&nbsp;blood eosinophils (r=0.673, p&lt;0.001).&nbsp;Those with severe eosinophilic asthma had significantly increased sputum eosinophils (8.2% [5.8,18.7]) and sputum IL-5 (11.6 ng/ml [2.1,18.9]), compared to those with non-eosinophilic asthma (both p&lt;0.001). Though sputum eosinophil percentages in non-eosinophilic severe asthma (0.2% [0.0,1.0]) did not differ from in health (0.4% [0.0,0.8]), sputum IL-5 (0.2 ng/ml [0.1,0.9] was higher in this non-eosinophilic phenotype compared to that in healthy volunteers (0.0 ng/ml [0.0,0.0], p=0.002). <b>Discussion:</b> Despite high levels of steroid treatment, there is persistent IL-5 generation within the airways of severe asthma.