Unsung Heroes? Decoding the Protective Effects of Airway Microbiota in Chronic Obstructive Pulmonary Disease

Traditionally defined by broad-spectrum methods of bronchodilation and glucocorticoids, chronic obstructive pulmonary disease (COPD) research stands at a crossroads of innovation and is now transitioning toward more nuanced strategies (1). The advent of emerging biologics and precision therapeutics is set to redefine treatment options after years of ambiguous results (2). The efficacy and potential of these pioneering approaches may hinge on our understanding of host immune response, a factor that is intricately linked to host–microbe interaction reflected by airway microbiome composition (3).

Key pathogens of the COPD microbiome such as Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae are firmly established (4). What is less clear is what constitutes a healthy microbiome or which microbes are protective or beneficial. Furthermore, the line between commensal and pathogen has become blurred as more holistic studies of microbiome–host interaction unfold (5). This may reflect an overemphasis on discrete microbes and biomarkers that cannot, individually, account for the full breadth of clinical and biological variation inherent to complex disease (6). Of interest are key tipping points in overall microbiome community structure whereby shifts in composition arise in correlation with clinical phenotypes. Keystone taxa in the healthy airway may serve as sentinel markers of health. Instead of awaiting the emergence of pathogenic microbes and worsening symptoms, early shifts in phylogenetic diversity affecting keystone eubiotic taxa might indicate early warning signals disease. Could these “unsung heroes” of the health airway microbiome represent an overlooked facet of COPD pathobiology?

In contrast to the gut microbiome, the association between airway microbial diversity and health status is not as cut and dry and points to the existence of more subtle effects (7). An early lesson of microbiome research has been the breath of microbial diversity that defines intersubject variability (8). Although this opens opportunities for refined stratification and more targeted intervention, the influence of lurking confounders such as environmental and dietary factors or host genetic diversity demands large and well-phenotyped cohorts (9).

In this issue of the Journal, Opron and colleagues (pp. 186–200) present a well-designed and impressively large study of the COPD airway microbiome, with an emphasis on patients with milder disease phenotypes (10). The study leverages SPIROMICS (Subpopulations and Intermediate Outcome Measures in COPD Study), including patients without COPD and those with mild to moderate disease. This work convincingly demonstrates a clear correlation between α diversity and clinical phenotypes including lung function, symptoms, and quantitative imaging assessments of small airway disease or emphysema. Intriguingly, the study uncovered shifts in diversity associated with early stages of disease (Global Initiative for Chronic Obstructive Lung Disease stages 0–2), highlighting potential tipping points of entry into established pathological states. Microbiome profiles, extensive baseline clinical data, and mucin measurements were obtained from 877 patients, while longitudinal lung function data from 365 patients allowed comparison of rapid decliners with those who exhibited stable or improved lung function. Through combined analysis of the entire cohort as well as a subset of patients with available longitudinal lung function data, several taxa corelated with reduced phylogenetic diversity, and distinct aspects of pathobiology were identified (Figure 1). Strikingly, observed associations with pathobiological markers strongly implicated commensal taxa, including Alloprevotella, Oribacterium, and Veillonella, as beneficial and protective. Analysis of subjects with longitudinally stable or improved lung function further highlighted robust associations with additional taxa of the core airway microbiome such as Prevotella, Leptotrichia, Neisseria, Mogibacterium, Aggregatibacter, and Corynebacterium, consistent with a protective effect of phylogenetic diversity. The associations of Streptococcus, Actinobacteria, Actinomyces, and Moraxella with reduced phylogenetic diversity and elevated pathobiological markers suggested a dysbiotic state favoring disease progression. This sharply contrasts with the positive taxonomic correlations seen in milder forms of disease, underscoring the potential but perhaps underappreciated role of airway taxa as sentinels of respiratory health. Bacteria have the ability to degrade mucins for nutrients, and overgrowth of mucin-degrading bacteria can compromise the mucus barrier, leading to inflammation or infection (11). A further strength of this study is its investigation of airway mucin–microbiome correlates - a pathological signature of increasing clinical relevance in this setting (12). Here, the authors report the largest such investigation to date, identifying clear associations between total mucin concentration and several bacterial species, while delineating further association between higher and lower MUC5AC:MUCB ratios linked to Streptococcus and Leptotrichia, respectively.

Figure 1.

Microbiome correlates of disease status in SPIROMICS. Analysis of baseline microbiome profiles, clinical parameters, and mucin concentrations was performed in 877 participants, revealing significant microbial correlates. Longitudinal assessment of pulmonary function decline was conducted in 365 individuals, further highlighting microbial signatures associated with this clinical phenotype. Taxa associated with milder disease are illustrated as green nodes in the “eubiosis” network on the left side of the figure. These taxa potentially serve beneficial functions as integral nodes of the phylogenetically diverse eubiotic network. In contrast, microbial biomarkers indicative of disease progression and imbalance are depicted as red nodes in the “dysbiosis” network on the right, highlighting their potential role in disease pathogenesis and progression. GOLD = Global Initiative for Chronic Obstructive Lung Disease; SPIROMICS = Subpopulations and Intermediate Outcome Measures in COPD Study.

Although the application of microbiome analysis to such a large and well-characterized cohort is both impressive and highly informative, the use of 16S amplicon sequencing rather than whole-genome shotgun metagenomics is a limitation. The decision to go wide rather than deep is understandable and appropriate but comes at the cost of the taxonomic and functional resolution that metagenomic analysis would have provided. Given the increasingly recognized role of the fungi in COPD, analysis of the mycobiome might have yielded further insight, particularly if incorporated into stratification analysis or interkingdom network inference approaches (13). Although the use of sputum over lower airway sampling is oft debated and constitutes a further limitation, deployment at comparable scales is likely unfeasible. Furthermore, it is noteworthy and commendable that the authors previously investigated lower airway sampling of the microbiome, in the same cohort, highlighting the distinct features of such analysis that further contribute to our understanding of pathobiology (14). Finally, the importance of independent validation is underscored by the notable failure of certain SPIROMICS results to replicate in independent cohorts, such as COPDGene (Genetic Epidemiology of COPD) (15). Such apparent discrepancies highlight the complexity of COPD, calling for cautious interpretation and extensive cross-validation of research findings.

Initiated with the aim of dissecting complex phenotypes and identifying disease subgroups, SPIROMICS has meticulously assembled a large and diverse cohort ranging from smokers without airflow obstruction to those with severe COPD. This inclusive spectrum has allowed an unparalleled exploration of disease heterogeneity, including the assessment of early COPD. Given the scope for early intervention, decoding early signals of disease is of significant interest. The dissolution of beneficial microbial associations and loss of phylogenetic diversity may represent such a signal, preceding the emergence of more readily identified dysbiosis and the dominance of pathogenic taxa observed in the later stages of disease. Indeed, recognizing the importance of keystone taxa to maintaining a healthy airway may prove not only preventive but potentially restorative if therapeutic approaches that seek to modulate the microbiome are realized (16). Large-scale collaborative research efforts such as SPIROMICS are essential and continue to pay dividends not alone in understanding COPD but as an essential platform to understand the microbiome and its fundamental importance in respiratory health as well as disease.