New insights into the airway microbiome and severe asthma

Published Online: July 25, 2015

Severe asthma is clinically heterogeneous for which the underlying mechanisms are incompletely understood. In milder asthma, the composition of microbiota in the lower airways differs from that seen in healthy individuals, and attributes of the microbiome have been associated with clinical outcomes. Whether severe asthma and its features are also associated with particular patterns of airway microbiota composition is an important question, for chronic infection has been implicated as one possible mechanism of asthma.

To address this knowledge gap, in a study published in The Journal of Allergy and Clinical Immunology (JACI), Y.J. Huang and colleagues examined whether bronchial bacterial community composition is associated with clinical and inflammatory features in severe asthma. The subjects – participants in the BOBCAT study [Bronchoscopic Exploratory Research Study of Biomarkers in Corticosteroid-Refractory Asthma; previously reported in JACI (Jia et al. 2011)] – were extensively characterized clinically as well as in assessments of blood and airway inflammatory markers. The authors additionally compared the bacterial profiles with bronchial epithelial gene expression patterns, and also bacterial profiles of milder asthmatic subjects previously analyzed by the same lead investigators.

Patterns of bronchial bacterial composition were significantly associated with several phenotypic features of severe asthma. Certain bacterial groups (mainly members of the Bacteroidetes and Firmicutes phyla) were significantly more abundant in obese subjects with severe asthma compared to non-obese counterparts. The investigators also found that indicators of unstable or worse asthma control correlated significantly with greater abundance of many Proteobacteria phylum members, which include known potential respiratory pathogens. In contrast, indicators of stable asthma control correlated with greater abundance of Actinobacteria, a phylum that encompasses species prolific at producing anti-inflammatory/anti-microbial compounds.  Finally, the investigators did not observe positive relationships between bacterial microbiota and markers of type 2-driven or eosinophil-mediated inflammation which, coupled with other findings of the study, suggests that bacterial microbiota may be more contributory to non-type 2 mechanisms of asthma.

In summary, extensive analyses of the bronchial bacterial microbiome in severe asthma identified relationships with a number of clinical and inflammatory features, including non-type-2 related mechanisms. Moreover, patterns of bacterial dysbiosis in severe asthma contrasted with that observed in mild-moderate asthmatics taking lower doses of corticosteroids, suggesting further avenues of investigation to be pursued.

The Journal of Allergy and Clinical Immunology (JACI) is an official scientific journal of the AAAAI, and is the most-cited journal in the field of allergy and clinical immunology.

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