IL‐13 signature in severe adult asthmatics with airway neutrophilia: A new endotype to treat!

To the editor, With great interest, we read the article by Azim and co-workers1 published in this issue of Allergy. To understand the inflammatory component of the peripheral airways of severe asthmatics uncontrolled by high dose inhaled corticosteroids (ICS), the authors examined the bronchoalveolar lavage (BAL) in two well-characterized asthma populations. In 78 severe asthma patients constituting the experimental cohort, an increase in IL-13 BAL levels associated with increased neutrophils, without eosinophils was reported. In the validation cohort (n = 18), the authors confirmed this ‘non-eosinophilic’ phenotype and further showed the presence of pathogenic bacteria, Moraxella catarrhalis, Haemophilus sp and Streptococcus sp. The significance of the study is threefold. First, the identification of a severe T2 asthma population with an IL-13 signature and airway neutrophils that may be misclassified due to absence of the ‘classic’ biomarker—eosinophils. Second, it highlights potential alternate cellular sources of IL-13 besides the T2 cells. Third, the association of high BAL IL-13 with microbial dysbiosis in a low-eosinophilic airway inflammation setting that indicates a novel pathogenetic role of IL-13, further contributing to asthma severity. Almost two decades back, Ward et al., reported a high variability in the BAL inflammatory cellular profile.2 Therefore, the confirmation of a neutrophilic airway cellularity in a follow-up validation cohort was essential and adds to the robustness of the study design. However, the presence of neutrophils in the BAL may simply be an innate immune response to the detected pathogenic bacteria or the ongoing effect of the corticosteroid therapy, rather than a direct IL-13 effect. Both confounders could have been easily addressed if the airway inflammation was investigated longitudinally after a course of antibiotics. In addition, the absence of eosinophils in the BAL could simply be a numerical anomaly where the increased number of neutrophils masks the eosinophil count. Again, this could have been checked by examining the cellularity in BAL post-infection (i.e. after a course of antibiotics) or by using a ratio between eosinophils and neutrophils instead of absolute values. Close inspection of the reported data reveal that an underlying eosinophilic/T2 component in the high tertile IL-13 severe asthma group cannot be ruled out altogether. First, the distribution on the scatter plot (Figure 2 in Azim et al1) shows half of the patients having >2% BAL eosinophils. In the replication cohort, the BAL eosinophil% were higher in the high IL-13 group vs low (1.30 ± 4.53 vs. 0.50 ± 1.65) but multiple comparison tests have not been reported to ratify a statistical difference. IL-5 levels were significantly higher in the high IL-13 compared to the low tertile group (0.61 ± 1.53 vs 0.32 ± 0.35 pg/ml, p = 0.001). IL-13 correlated with both BAL neutrophil% (r = 0.580, p < 0.001) and BAL eosinophil%, (r = 0.271, p = 0.017), even though the latter was a weaker association. Using routine sputum cell differential, ~10% of severe asthmatics consistently have a mixed granulocytic sputum, that is neutrophils >65% together with borderline eosinophils in the range of 2–3%. These patients usually have ongoing airway infection that masks their eosinophilia, as their eosinophilia re-emerges after the infection is resolved.3 A mixed granulocytic phenotype based on sputum cytology has been reported both by the Airways Disease Endotyping for Personalized Therapeutics (ADEPT) and Unbiased BIOmarkers in PREDiction of respiratory disease outcome (U-BIOPRED) cohorts. The patient population evaluated is similar to other observational studies where 7–10% of asthmatics have mixed phenotype,3 lowest lung function,4 and similar dysbiosis by 16s sequencing.5 Taken together, the high tertile IL-13 ‘neutrophilic’ group based on the BAL analysis may be representative of the mixed granulocytic phenotype. The debate continues on whether neutrophils contribute to the pathology of severe asthma in the high tertile IL-13 group or whether they are only present as a consequence of infection or the use of corticosteroids. It must be pointed out that, targeting different drivers of neutrophilic inflammation (eg IL-17, IL-23, CXCR2 [reviewed extensively in6]) had discouraging results in the clinical trials, although patient selection was not optimal. We next raise the question on whether increased IL-13 in the airway is contributory or collateral to the observed infections. The IL-4/IL-13 pathway has been implicated in dampening the neutrophil response to an infection by impairing IL-8 induced migration and neutrophil extracellular trap formation.7 LPS (infection mimic) induces airway hyperresponsiveness and corticosteroid resistance via IL-13 dependent signalling of pulmonary macrophages.8 Therefore, increased IL-13 due to an underlying T2 pathology may impair host defence. Of interest, increased sputum IL-13 levels predict the presence of airway autoantibodies in severe asthmatics with a mixed granulocytic phenotype. Moreover, autoantibody-induced macrophage dysfunction in the mixed phenotype can contribute to recurrent airway infections.9 That said, inherent immunodeficiencies or neutrophil dysfunction were not ruled out in this study as other underlying reasons for the observed airway infection in the replication cohort. In summary, elevated levels of IL-13 in the peripheral airways with increased neutrophils and low levels of eosinophils reveal an underlying T2 pathway for the mixed granulocytic asthma phenotype, together with alternate cellular sources of IL-13 besides eosinophils or the T2 cells, such as alveolar macrophages or mast cells (Figure 1). The study also adds a novel potential pathogenetic pathway (innate immune response impairment with microbial dysbiosis) to IL-13's ‘classic’ contribution to asthma severity, including but not limited to, airway smooth muscle phenotype switch, mucus hypersecretion, eosinophil recruitment and epithelial activation (Figure 1). Pending further validation through longitudinal studies, a therapeutic focus for this particular severe asthma endotype is therefore IL-13, a cytokine that is truly pleiotropic for its heterogeneity of cellular source and downstream functions. Targeting the IL-13/IL-4 pathway using an anti-IL-4Ra monoclonal antibody (dupilumab) was more successful than targeting IL-13 alone.10 This might be relevant for the population in question as IL-4 levels (though lowly detected) were correlated with IL-13 levels. We conclude by congratulating the authors1 for their distinct observation of a severe asthma endotype with raised IL-13, neutrophilia and dysbiosis in the peripheral airways. This population may benefit from an IL-4/IL-13 targeted therapy, although the ‘classic’ T2 selection biomarker—eosinophilia, is not immediately noticeable. The work also reinforces the value of using airway secretions, instead of blood, to investigate asthma endotypes. Dr. Mukherjee is supported by investigator award from Canadian Institutes of Health Research and Canadian Allergy, Asthma and Immunology Foundation. She has received grants from Methapharm Speciality Pharmaceuticals and honorarium from AstraZeneca and GlaxoSmithKline outside the submitted manuscript. Dr. Agache is Associate Editor of Allergy. MM and AI wrote the manuscript.

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