Stephen Schworer, MD, PhD, is an assistant professor of medicine at the University of North Carolina at Chapel Hill School of Medicine.

Stephen Schworer, MD, PhD

Insights Into the Role of Mucus Plugging in Asthma

In recent research, mucus plugging has been shown to play a key role in severe cases of 

 and to be the most common driver of fatal events related to asthma. This role, enhanced by thoracic CT imaging studies of large airways, was explored in new research on the molecular characteristics of the airway epithelium in the peripheral lung.

This study is being led by Stephen Schworer, MD, PhD, an assistant professor of medicine at the University of North Carolina at Chapel Hill School of Medicine. Schworer spoke in the second issue of 

 that is powered by the American Lung Association Research Institute, on his team’s current research in this area.

The team hypothesized that asthmatic small airways would exhibit mucus plugging with high expression of MUC5AC, the secreted mucin which is known to be upregulated in the large airway among those with asthma.

To test this hypothesis, we characterized small airways in tissues from fatal asthma cases and biopsies from steroid-resistant severe asthmatics. Using AB-PAS staining, we showed that there is a high mucus plug burden in bronchioles in both cohorts.

“We expand on previous fatal asthma morphology studies by adding the study of biopsies of severe, steroid-resistant asthmatics, and using innovative technologies including spatial transcriptomics and multiplex immunostaining to define the small airway molecular asthmatic phenotype,” Schworer said. “We hypothesized that asthmatic small airways would have mucus plugging with high expression of the MUC5AC, the secreted mucin known to be upregulated in the large airway and asthma. To test this hypothesis, we characterize small airways and tissues from fatal asthma cases and biopsies from steroid resistant severe asthmatics.”

To learn more about Schworer’s research, view the interview above or read his contribution to the second issue of The Respiratory Report here:

Small Airway Mucus Plugging in Severe Asthma: Insights and Implications

Videos

This interview in the latest issue of The Respiratory Report features Stephen Schworer, Md, PhD, discussing the role of mucus plugging in severe asthma.

Stephen Schworer, MD, PhD
Credit: American Lung Association

Asthma is a common, chronic obstructive airway disease characterized by inflammation, airway hyper-reactivity, and mucus plugging.

 Recent studies have demonstrated that mucus plugging is a key feature in severe asthma and the most common driver of fatal asthma events.

 The understanding of the role of mucus plugging in asthma has been enhanced by thoracic CT imaging studies of large airways. 

These cross-sectional imaging studies have demonstrated that: 1) over half of severe asthmatics in the Severe Asthma Research Program (SARP) cohort have persistent CT-defined large airway mucus plugging despite inhaled corticosteroid therapy; 2) the amount of mucus plugging correlates with airflow limitation; and 3) the mucus plugs are long-lived within a given pulmonary segment.

However, in asthma, along with most muco-obstructive diseases, mucus obstruction in small airways are sites of early and severe disease.

 These small airways are not effectively visualized by CT. Clinical studies, including the ATLANTIS study, have reported an association between small airways dysfunction and both asthma control and exacerbations.

 Importantly, the molecular pathophysiology and the characteristics of mucus plugging in small airways disease in asthma are not well-characterized.

Our study aims to better understand molecular characteristics of the airway epithelium in the peripheral lung. We expand on previous fatal asthma morphology studies by adding VATS biopsies of severe, steroid-resistant asthmatics and using innovative technologies, including spatial transcriptomics and multiplex immunostaining, to define the small airway molecular asthmatic phenotype.

We hypothesized that asthmatic small airways would exhibit mucus plugging with high expression of MUC5AC, the secreted mucin known to be upregulated in the large airway in asthma.

 To test this hypothesis, we characterized small airways in tissues from fatal asthma cases and biopsies from steroid-resistant severe asthmatics. Using AB-PAS staining, we showed that there is a high mucus plug burden in bronchioles in both cohorts. 

We measured mucin expression and mucus plugging by MUC5AC and MUC5B using RNA in situ hybridization (RNA-ISH) and immunohistochemistry (IHC), which showed increased MUC5AC expression into small airways of asthmatics. We found that small airways disease is strikingly heterogeneous in terms of the expression of MUC5AC, both between different airways in a single lung segment and even within a single airway.

To better characterize the epithelial heterogeneity of MUC5AC expression in asthmatic bronchioles, we performed spatial transcriptomics on fatal and severe asthma bronchiolar epithelia. We utilized this technology to measure gene expression differences between MUC5AC expressing epithelia (MUC5AC-High) and epithelia not expressing MUC5AC (MUC5AC-Low). 

We compared each of these epithelial regions with control epithelia from organ donors not accepted for transplant and lobectomy specimens. We identified that, in addition to an increase in goblet cell genes and a loss of normal small airway secretory cell genes, MUC5AC-High epithelia expressed increased early ciliated cells and basal cells.

We further investigated the basal cells in the MUC5AC-High epithelia, by performing spatial transcriptomics segmented on the basal cell marker, cytokeratin 5. (KRT5). In MUC5AC-High regions, basal cells expressed high levels of genes associated with a type 2 inflammatory signature, namely IL-33, ALOX15, and POSTN.

 These findings suggest that the basal cells in MUC5AC-high epithelium are primed to differentiate into MUC5AC-expressing goblet cells.

Finally, we used multiplex immunostaining to identify whether epithelial heterogeneity of MUC5AC expression was associated with accumulation of immune cells. These studies revealed that specific populations of immune cells did not co-localize with MUC5AC niches, suggesting the importance of epithelial-intrinsic dysfunction in the mucus plugging characteristic of bronchiolar disease in asthma.

Our ongoing work supported by the ALA/AAAAI Allergic Respiratory Diseases Award focuses on better understanding the specific cell type alterations in asthma using single cell sequencing and chromatin accessibility in freshly isolated cells and cell culture from asthmatic individuals. These studies aim to identify important changes in gene expression and epigenetics of asthmatic goblet cells and basal progenitor cells. Understanding the mechanisms driving goblet cell metaplasia and subsequent MUC5AC-dominant mucus plugging in small airways will reveal potential targets for modulating small airway dysfunction in asthma.

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Schworer S, Okuda K, Dang H, et al. Mucus plugging and regional heterogeneity are features of asthmatic small airways. 

Journal of Allergy and Clinical Immunology

. 2023;151(2):AB223. doi:10.1016/j.jaci.2022.12.694

Schworer S, Okuda K, Dang H, et al. Transcriptional pathways in small airway basal cells define niches of goblet cell metaplasia in asthma. In: 

C19. CHARTING THE LUNG: INNOVATION IN MULTIOMICS

. American Thoracic Society; 2024:A5015-A5015. doi:10.1164/ajrccm-conference.2024.209.1_MeetingAbstracts.A5015

Schworer S, Okuda K, Dang H, et al. Steroid-resistant Severe Asthma and Fatal Asthma Small Airways Are Defined by Mucus Plugging and Heterogeneous MUC5AC Expression. In: 

C19. EMERGING MECHANISMS IN COPD AND ASTHMA

. American Thoracic Society; 2023:A4535-A4535. doi:10.1164/ajrccm-conference.2023.207.1_MeetingAbstracts.A4535

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Articles

Asthma is a chronic obstructive airway disease marked by inflammation and mucus plugging, with recent studies highlighting mucus plugging as a key factor in severe asthma and fatal events. Thoracic CT imaging has shown persistent mucus plugging in severe asthmatics, correlating with airflow limitation. Small airways, often sites of early disease, are not well-visualized by CT. Research using VATS biopsies and advanced technologies aims to understand the molecular characteristics of small airway disease, focusing on MUC5AC expression and its role in mucus plugging, revealing potential therapeutic targets.

Exploring mucus plugging in severe asthma, focusing on small airway dysfunction, MUC5AC expression, and epithelial heterogeneity using advanced methods.