Structural and cellular mechanisms of mucus plugging in the larger airways

Abstract

Intractable mucus plugging drives mortality in cystic fibrosis (CF), asthma, and chronic obstructive pulmonary disease (COPD). While surface metaplasia narrows small airways, the massive volumetric reservoir capacity of hypertrophied submucosal glands (SMGs) dictates catastrophic mucus plugging in the larger airways. This review proposes a flush-load model to explain this structural failure: a biophysical imbalance in which SMG serous fluid secretion (the flush) is insufficient to hydrate and clear the burden of high-molecular-weight mucins (the load), leading to osmotic compression, ciliary collapse, and intractable luminal occlusion. The review delineates the disease-specific etiologies of this imbalance. In CF, dysfunction of ion channels, such as the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium channels (ENaC), causes mechanical uncoupling and mucin 5B (MUC5B) tethering to glandular ducts. In asthma, Type 2 inflammation disrupts the glandular stem cells, driving an explosive release of mucin. In COPD, epidermal growth factor receptor (EGFR)-driven remodeling and senescent inflammaging perpetuate chronic hypersecretion. Ultimately, anchored MUC5B strands from hypertrophic SMGs structurally integrate with surface-derived mucin 5AC (MUC5AC), forming an intractable adhesive mesh. Synthesizing these insights, we advocate shifting the therapeutic paradigm from symptomatic downstream clearance to upstream, disease-modifying interventions that target the glandular stem cell niche, restore ion channel homeostasis, and correct coordinated airway surface dysfunction.

Introduction

The conducting airways of the human respiratory system serve as the primary interface with the external environment and are continuously exposed to inhaled particles, pathogens, and irritants.¹ Yet in major obstructive airway diseases, this innate defense system fails.² Ultimately, pathologic mucus accumulates, forming widespread mucus plugs that obstruct airflow and become a nidus for chronic infection. Critically, these airway mucus plugs are not only a consequence of impaired mucociliary clearance but also a pervasive, fatal pathological entity that acts as an insidious airway occlusion in chronic obstructive pulmonary disease (COPD), asthma, and cystic fibrosis (CF).^3,4 This airway occlusion is further complicated by a significant diagnostic gap in routine clinical practice, which is often masked by the broader symptoms of airway obstruction. Conventional spirometry often fails to distinguish between dynamic airway collapse and fixed mucus plugging, leading to missed opportunities for intervention.⁵ Consequently, a significant mucus burden often goes undetected until visualized via high-resolution computed tomography (HRCT) scans or bronchoscopy.^3,6 Therefore, recognizing mucus plugging as a distinct, treatable target rather than a secondary feature is essential for addressing the high mortality associated with these obstructive disorders.

The prevalence of this pathology highlights its critical role in disease progression. In asthma, mucus plugging is a hallmark of severe phenotypes, present in up to 58% of patients with difficult-to-treat asthma and universally identified in autopsies of fatal status asthmaticus.^3,7 Similarly, in COPD, particularly the chronic bronchitis phenotype, widespread mucus plugging of the small airways drives the rapid decline in lung function.⁸ In CF, it is the primary cause of morbidity and mortality.⁹ The clinical concern lies in the insidious nature of these obstructions. Unlike bronchospasm, which responds rapidly to beta-agonists, mucus plugging creates a mechanical obstruction that is often refractory to standard pharmacologic interventions, leading to persistent regions of atelectasis and ventilation–perfusion mismatch that silently precipitate respiratory failure.⁵

It has long been recognized that submucosal glands (SMGs), complex tubuloacinar structures within the airway walls, which are composed of distinct cellular compartments that collaborate to sustain airway homeostasis, are central to mucus plugging because they hydrate the airway surface and produce the vast majority of mucus in large airways.¹⁰ This distinction underscores a paradigm shift: while surface goblet cells contribute to pathology, SMGs are the primary volume producers in cartilaginous airways and, therefore, the key drivers of recalcitrant mucus plugging.^2,10,11 While surface metaplasia fundamentally narrows the small airways, the massive volumetric mucin reservoir capacity of enlarged SMGs uniquely drives catastrophic plugging in the larger, cartilaginous airways.^2,12 However, SMGs are now understood to be far more than simple secretory units.¹⁰ Instead, emerging evidence has recast them as highly specialized micro-organs that perform a dual function: serving as the dominant source of protective secretions, while simultaneously acting as protected microenvironments, or niches, that harbor and regulate distinct populations of adult stem cells essential for airway maintenance and repair.13, 14, 15, 16, 17, 18

This review proposes a model that the recalcitrant mucus plugging driving severe obstructive disease arises not only from surface goblet cell hyperplasia but, critically, from remodeling of the airway SMG niche and its synergistic interaction with surface mucins. The fatal obstruction is a biophysical consequence of distinct mucin interplay: Mucin 5B (MUC5B)-rich strands from hypertrophic glands act as continuous tethers that intertwine with stiff, MUC5AC-rich domains from surface goblet cells, creating an immobile, adhesive mesh that osmotically compresses the underlying periciliary layer, neutralizing the protective gel-on-brush clearance mechanism in the airway.^12,19,20 By framing the gland as a continuous, maladaptive reservoir where the imbalance between mucin production and fluid clearance originates, there is an urgent need to bridge the diagnostic gap for patients with mucus hypersecretion phenotype before mucus plugs precipitate irreversible remodeling. Consequently, a therapeutic strategy shifts from symptomatic clearance to disease-modifying prevention, demonstrating that targeting upstream glandular stem cell plasticity and the MUC5B–MUC5AC axis can resolve this insidious airway occlusion at its source.

Glandular origins of obstruction: the flush–load imbalance

A comprehensive understanding of mucus plugging in large airways requires foundational knowledge of glandular anatomy and the physiological mechanisms that regulate mucus clearance. As noted, the massive volumetric mucin burden generated by SMG hypertrophy and hyperplasia frequently serves as the dominant driver of catastrophic luminal plugging in cartilaginous airways.^2,10,12 To integrate these anatomical features with observed clearance mechanics, we emphasize a functional paradigm of mucin–hydration balance. Efficient clearance relies on a dynamic interplay between the burden of secreted mucin (the load) and the volume of mobilizing serous fluid delivered atop the periciliary liquid (PCL) layer. At the surface epithelium, this interaction is best conceptualized as the “gel-on-brush” model, in which the PCL functions as a dense, protective brush that prevents the heavy, mobile mucus gel from adhering to the underlying ciliated cells.^12,21 In a healthy state, this ratio is maintained by the cellular composition of glands: distal serous acini lined by serous cells drain into proximal mucous tubules lined by mucous cells, which ultimately open onto the airway surface through the glandular duct. This specific spatial arrangement, with the fluid source placed behind the mucin source, is critical and ensures that low-viscosity serous fluid acts as a hydrostatic driving force to flush highly viscous mucin gel through the tortuous ductal network.¹⁰

The hydration component of this mechanism is primarily driven by serous cells, which actively secrete water, electrolytes, and antimicrobials. This process critically relies on the cystic fibrosis transmembrane conductance regulator (CFTR) channel, which establishes the osmotic gradient that draws water into the gland lumen.²² However, proper hydration requires a balance across multiple ion channels; in addition to CFTR, the dysfunction of the epithelial sodium channel (ENaC), which can hyper-absorb sodium, and calcium-activated chloride channels (e.g., transmembrane member 16A, TMEM16A), which fail to provide adequate compensatory chloride secretion, drastically impairs this hydration mechanism.^20,21,23 This hydraulic force not only hydrates but also provides the mechanical force required to detach mucus from the ductal walls. In the absence of continuous fluid secretion, the distinctive rheological properties of mucus cannot be maintained, leading to transport failure. Therefore, the integrity of coordinated ion transport within the serous acini constitutes the cellular basis for preventing ductal obstruction, while its dysfunction signifies the primary underlying defect associated with the formation of ductal plugs.^10,24

The mucin component is produced by mucous cells within the proximal glandular tubules, which synthesize and secrete high-molecular-weight, gel-forming mucins, primarily MUC5B. Structurally, glandular MUC5B granules are released as elongated, linear filaments (strands).²⁵ Functionally, this fibrous network serves as a conveyor mechanism that captures pathogens, distinctly differing from the surface clearance mechanism.²⁶ In contrast to MUC5B, the surface epithelium secretes MUC5AC, which emerges as diffuse, sheet-like structures rather than discrete strands. Pathologically, MUC5B dysfunction mainly involves quantitative and mechanical failure; excessive MUC5B output acts as pathological mucus tethers when the hydration volume is insufficient to detach them from gland ducts. Conversely, MUC5AC pathogenicity primarily represents a qualitative and biophysical failure; its overproduction forms a stiff, rubbery gel with altered viscoelastic properties that resists clearance regardless of hydration status.^27,28 Thus, the MUC5AC:MUC5B ratio serves as a critical determinant of plug biophysics. As evidenced by the SPIROMICS cohort data, an unfavorable change in this ratio is a main cause of pathology in COPD and asthma. The SPIROMICS analyses clearly demonstrate that the pathological adherence of airway surface MUC5AC, along with this altered ratio of MUC5AC to MUC5B mucins, fundamentally corrupts the gel architecture and precipitates plug formation.²⁸ However, this structural design carries an inherent risk: these glandular MUC5B strands necessitate rapid and substantial hydration to unfold and detach from the duct epithelium. The resulting vulnerability is explained by the mucus hyperconcentration (osmotic compression) hypothesis. When airway surface hydration fails, the physical polymers within the mucus tethers generate overwhelming osmotic forces.¹² As the mucus dehydrates, it osmotically compresses onto the airway surface, physically crushing the periciliary brush described in the gel-on-brush model.^12,21 Consequently, the cilia collapse beneath the heavy mucus tethers, halting mucociliary clearance entirely.¹⁹ If the viscoelastic properties of these MUC5B strands are not adequately moderated by serous fluid, they may fail to exit the duct, instead coalescing into dense, tangled aggregates that act as the nidus for a plug,²⁹ creating a hypoxic environment that drives further inflammation and invites bacterial infection (Fig. 1).²⁰

Fig. 1.

Mechanism of airway glandular flush–load imbalance in mucus plugging. The schematic of airway mucus dynamics in healthy and diseased airways. The healthy panel shows serous acini producing fluid via ion channels located on the acinar serous cell membranes, supporting effective mucus clearance through hydrated mucins. The disease panel depicts dysfunctional ion channels (CFTR, ENaC, TMEM16A) on the membranes of acinar serous cells, leading to reduced fluid secretion, mucous cell hyperplasia, tangled mucin strands, and, ultimately, a large mucus plug obstructing the airway. ASL: Airway surface liquid; CFTR, Cystic fibrosis transmembrane conductance regulator; ENaC, Epithelial sodium channel; MUC5B, Mucin 5B; H₂O: Water; SMG, Submucosal gland; TMEM16A, Transmembrane member 16A.

This mechanism explains the susceptibility of airways to plugging in disease conditions characterized by CFTR dysfunction. Although this dysfunction constitutes the genetic hallmark of CF, analogous pathophysiological processes are increasingly acknowledged in COPD.³⁰ Exposure to cigarette smoke and chronic inflammation can induce acquired CFTR dysfunction and ENaC hyperactivity, effectively impairing serous cell function even in the absence of genetic mutations.30, 31, 32 This diminution in anion transport and fluid secretion recapitulates the defect observed in CF, resulting in a dehydrated and hyperconcentrated glandular environment in which MUC5B strands remain tethered to the ductal epithelium, thereby preventing their clearance from the airway surface.^30,33

Ultimately, the transition from physiological defense to pathological obstruction occurs when the glandular mucus tethers overwhelm the hydration. While surface goblet cell metaplasia and SMG overgrowth both increase stored micin in chronic bronchitis and asthma, it is the massive reservoir capacity of enlarged SMGs that uniquely drives severe, widespread airway obstruction.^2,10,34 When this increased mucin burden coincides with reduced serous output, whether due to genetic CFTR failure, dysregulation of complementary ion channels, or acquired dysfunction, a catastrophic imbalance occurs. The gland becomes mechanically obstructed by its own hyper-concentrated secretions, cementing the formation of the intractable, rubbery mucus plug that characterizes severe airway disease and drives mortality.^35,36

Physiological mechanisms of plugging and airway physicochemical properties

Physiological mechanisms of mucus plugging

The SMG-secreted mucus mixture works together to trap inhaled debris, inhibit microbial growth, and promote effective removal through ciliary action, thereby maintaining a sterile and healthy luminal environment.³⁷ Glandular secretion is a dynamic process tightly regulated by the autonomic nervous system to meet defensive needs.³⁸ Rather than being a static baseline, this regulation is a highly responsive system, primarily driven by the intrinsic nervous network of the airway. Under normal conditions, baseline function is largely mediated by parasympathetic control, in which the co-release of vasoactive intestinal peptide (VIP) and low concentrations of acetylcholine (ACh) acts synergistically.^39,40 In response to acute challenges, such as inhaled irritants, a centrally mediated vagal reflex amplifies cholinergic drive, coordinating the rapid, high-volume expulsion of mucus to flush out the insult agent.^38,41 This neural framework is further tuned by sympathetic control, in which norepinephrine acts on β-adrenergic receptors to elevate cyclic adenosine monophosphate (cAMP) and promote the secretion of a smaller volume of viscous, protein-rich mucus.¹⁰ Additionally, sensory or NANC (non-adrenergic, non-cholinergic) control provides a local defense mechanism; stimulation of sensory C-fibers by irritants triggers an axon reflex, releasing neuropeptides such as substance P and neurokinin A, which directly stimulate glandular secretion and vasodilation.⁴²

While complex neural networks determine the volume and mucin composition of these glandular secretions, the ultimate success of airway clearance depends entirely on the post-secretory luminal environment. When this environment is compromised, the transition from healthy mucus to a pathologic plug is fundamentally a physicochemical and biophysical failure. Proper mucus expansion depends on the alkaline microenvironment created by bicarbonate (HCO3^-) secretion. In a healthy airway, CFTR-mediated bicarbonate secretion sequesters calcium ions, allowing densely packed mucin granules to unpack and expand.^43,44 In obstructive diseases, this delicate ionic balance is lost, resulting in an acidic microenvironment that prevents normal mucin unpacking. An acidic airway surface liquid (as seen in CF and acute inflammation) prevents mucin expansion, producing condensed, intractable mucus that remains tethered to gland duct orifices. This leads to the tethering defect, in which mucus plaques are not only adherent to the surface epithelium but remain deeply anchored within the SMG ducts. These tethers physically bind the luminal plug to the ductal openings of glands, preventing cough clearance and serving as a scaffold for further accumulation of debris and inflammatory cells, forming the core of the obstructive plug.

Additionally, purinergic signaling functions as a vital injury-response pathway; extracellular ATP and uridine-5′-triphosphate (UTP), released from stressed or damaged epithelial cells, activate purinergic receptor P2Y, G-protein coupled, 2 (P2Y2) receptors on glandular cells, thereby stimulating significant fluid and mucin secretion independently of neural input.⁴⁵ Notably, the physical environment of the airway lumen provides feedback that regulates glandular function. The osmolarity and potential of hydrogen (pH) of the airway surface liquid (ASL) are crucial for modulating the viscosity of secretions, whereas mechanical shear stress from coughing can physically deform the gland, potentially aiding the expulsion of stored mucus from the ductal reservoir.⁴⁵ The key components and functions of the airway gland and surface goblet cells are listed in Table 1.^22,24,26, 27, 28^,36,41,46, 47, 48

Table 1.

Key components and functions of airway gland and surface goblet cells.

Cell type	Component(s)	Key function(s)	Reference(s)
Mucous cells	MUC5B (predominant glandular mucin)*	Gel-forming mucin, which traps particles/pathogens, and forms mucus strands for clearance. Pathogenicity primarily represents a quantitative and mechanical failure; excessive strands become tethered when the flush is insufficient to detach them	26,27,36
Surface goblet cells	MUC5AC (predominant surface mucin)*	Gel-forming mucin sheets. Pathogenicity primarily results from a qualitative and biophysical failure; excessive production forms a stiff, rubbery gel with altered viscoelastic properties that resist clearance regardless of hydration status	27,28
Serous cells	H2O, electrolytes (Cl-, HCO3-)	ASL hydration, mucus viscosity, mucociliary transport, pH balance. Maintains proper hydration for the periciliary "gel-on-brush" layer via coordinated ion transport (CFTR, ENaC, CaCCs)	22,41
	Antimicrobials (lysozyme, lactoferrin, LL-37, Muc7)	Kill bacteria, inhibit viruses	22,24
	Protective proteins (e.g., annexins, apolipoproteins, gelsolin, histones, keratins, lumican, major vault protein, prohibitin)	Anti-inflammatory, anti-proteolytic, anti-oxidant	22,46–48

ASL, Airway surface liquid; CaCCs, Calcium-activated chloride channels; CFTR, Cystic fibrosis transmembrane conductance regulator; Cl^-, Chloride ion; ENaC, Epithelial sodium channel; HCO₃^-, Bicarbonate ion; H₂O, Water; MUC5AC, Mucin 5AC; MUC5B, Mucin 5B; Muc7, Mucin 7; SMG, Submandibular gland.

^⁎MUC5AC is primarily secreted by surface epithelial goblet cells in healthy human, pig, and ferret airways. SMGs produce MUC5B strands that facilitate the transport of MUC5AC sheets produced by the surface epithelium.

Physicochemical properties and the airway microenvironment

Indeed, the functionality of SMG secretions is strictly governed by the physicochemical properties of ASL, particularly its ionic composition, pH, and oxygen tension. Proper mucus expansion is dependent on a specific ionic milieu that regulates the transition of mucins from a condensed intracellular state to an expanded extracellular gel. Upon secretion, mucin granules are densely packed and electrostatically shielded by high concentrations of calcium ions (Ca²⁺), which mask the repulsive negative charges of the mucin polymers. The rapid, explosive expansion of these granules upon exocytosis requires the immediate sequestration of the shielding calcium, a biophysical process driven primarily by bicarbonate (HCO3^-) that acts not only as a pH buffer but as a crucial calcium chelator. In a healthy airway, CFTR-mediated bicarbonate secretion creates an alkaline microenvironment that facilitates Ca²⁺ removal, and the subsequent electrostatic repulsion drives the unpacking of mucin polymers.^43,44 This coordinated ion transport, which requires the precise balance of CFTR, the epithelial sodium channel ENaC, and calcium-activated chloride channels, maintains the periciliary liquid (PCL) layer, a dense gel-on-brush system that prevents the mobile mucus gel from adhering to the ciliated cells.^{12,19,20,49,50} However, in diseases like CF, the specific failure of anion channels to secrete HCO3^- (often coupled with ENaC sodium hyperabsorption) leads to an acidic, calcium-rich ASL. In this environment, mucins fail to expand and remain condensed and tethered to the gland duct, forming viscous, intractable plaques that characterize the earliest stages of obstructive pathology.^51,52 Driven by osmotic compression, this highly concentrated mucus load physically damages the periciliary brush, entirely inhibiting mechanical clearance and anchoring the plaque to the airway surface.^12,21

In addition, the respiratory microenvironment is affected by local oxygen metabolism, which plays a reciprocal, detrimental role in disease progression. The thick, static mucus plugs create steep oxygen gradients, leading to localized zones of profound hypoxia at the epithelial surface.⁵³ This hypoxic microenvironment has profound pathological consequences, primarily through the stabilization of hypoxia-inducible factor 1-alpha (HIF-1α), a transcription factor that directly binds to the promoter regions of mucin genes, significantly upregulating MUC5AC expression while simultaneously driving epidermal growth factor receptor (EGFR) signaling pathways that promote goblet cell hyperplasia.⁵⁴

Crucially, this hypoxic microenvironment is a primary driver of the imbalance between flush and load, rather than an isolated symptom. Because active ion transport for serous fluid secretion, especially via CFTR, is highly ATP-dependent, serous cells are particularly susceptible to hypoxia-induced mitochondrial dysfunction.⁵⁵ This loss quickly reduces the CFTR-mediated fluid flush. Furthermore, evidence suggests that hypoxia-induced stabilization of HIF-1α can directly reduce CFTR expression and function at the apical membrane, substantially impairing anion and fluid secretion.^56,57 This mechanism establishes a detrimental positive feedback loop within the flush-load model: initial plug formation induces local hypoxia, which concurrently impairs serous cell function (reducing the flush), elevates mucin production (increasing the load), and promotes glandular hypertrophy, ultimately solidifying the plug and exacerbating the obstruction.⁵³

Age-related SMG structural changes

Progressing age is correlated with both the incidence and mortality rates of various respiratory diseases.⁵⁸ While SMG development is largely completed during early life, aging induces distinct structural modifications.⁵⁸ In murine models, aging correlates with the emergence of age-related gland-like structures (ARGLS) in the trachea.⁵⁹ These structures, which are presumably formed through de novo budding of the surface epithelium rather than originating from pre-existing glands, differ from normal SMGs by lacking myoepithelial cells (MECs) and appropriate ductal connections. In humans, aging is generally associated with a decline in mucociliary function, driven by reduced ciliary beat frequency and changes in mucus rheology, which may be further exacerbated by alterations in glandular structure.⁶⁰ Such age-dependent modifications impair the innate defenses of the airway, thereby increasing susceptibility to infections and chronic obstructive conditions.

Glandular stem cell niche: the origin of remodeling

The chronic mucus hypersecretion observed in obstructive airway diseases is not a transient functional change but a permanent structural remodeling driven by dysregulated stem cells. To date, beyond stem cells in airway and alveolar epithelia,⁶¹ the SMG has been defined as a protected niche for two distinct stem cell populations, glandular ductal stem cells (GDSCs) and MECs. These glandular progenitor cells remain quiescent for long periods but are activated following injury to proliferate and differentiate, replenishing both the glandular and surface epithelia.¹⁷ This regenerative capacity is fundamental for restoring barrier integrity after damage from infection, inflammation, toxic exposures, or mucus plugging (Table 2).13, 14, 15, 16^,62, 63, 64, 65, 66, 67, 68

Table 2.

Comparative profile of airway glandular stem/progenitor cells.

Profile of features	Glandular duct stem cells (GDSCs)	Myoepithelial cells (MECs)
Primary anatomical location	Gland ducts (ciliated and collecting)14	Basal side of secretory acini and tubules15,16,62
Key molecular markers	K5+, K14+, CD166+, ALDHhi, NGFRLow/ITGA2Low14	K5+, K14+, αSMA/ACTA2+, SOX9+, NGFRhi/ITGA2hi15,16,62,63
Role in quiescent states	Glandular maintenance and turnover14	Quiescent: Structural and contractile function15,16,64
Role in injured states	Moderate to severe injury: Routine glandular and epithelial repair13,65	Severe injury only (reserve population)15,16,64
Multipotency	High: Generate all surface and glandular cell types14	High (upon activation): Generate all surface and glandular cell types15,16,64
Lineage bias	Balanced: Differentiates into a normal mix of basal, ciliated, and secretory cells14	Secretory-biased: Preferentially differentiates into secretory cells15,64
Key regulatory pathways	Wnt/β-catenin/Lef-166	Wnt/β-catenin/Lef-1, SOX916,66, 67, 68

ACTA2, Actin alpha 2, smooth muscle; ALDH, Aldehyde dehydrogenase; αSMA, Alpha-smooth muscle actin; ITGA2, Integrin subunit alpha 2; K14, Keratin 14; K5, Keratin 5; Lef-1, Lymphoid enhancer binding factor 1; NGFR, Nerve growth factor receptor; SOX9, SRY-box transcription factor 9.

Airway GDSCs

The GDSCs are a population of multipotent progenitors located within the epithelial lining of the airway glandular ducts. They are the main drivers of homeostatic turnover within the gland and serve as the first responders in repairing the surface epithelium after mild or moderate injury.^13,14 Their strategic location in the ducts enables them to rapidly sense and respond to injury signals on the surface.⁶⁵ GDSCs are characterized by specific molecular signatures, including basal cell markers such as cytokeratin 5 (KRT5) and cytokeratin 14 (KRT14), the epithelial marker cytokeratin 19 (KRT19), and cell-surface antigens CD166 and CD44.⁶⁹ They can be isolated by combining their specific epithelial markers with a surface marker profile of low nerve growth factor receptor and low integrin alpha 2 (NGFR^low/ITGA2^low).^69,70 In chronic diseases, they are stimulated to proliferate, thereby contributing to gland enlargement (hypertrophy) and expansion of the secretory territory.⁶⁵ Functionally, they exhibit high aldehyde dehydrogenase (ALDH) activity, which is essential for their self-renewal capacity in vitro.^69,71 Compelling lineage tracing and transplantation studies also confirmed their multipotency to differentiate into multiple epithelial cell types, as well as their ability to regenerate a fully differentiated surface epithelium and, uniquely, the SMG tubules and ducts, indicating that they are stem cells for the glandular unit.^13,14

Airway glandular MECs

Beyond the GDSCs, the airway SMG niche harbors a second, notable progenitor, MECs, which were originally recognized only for their contractile ability, expressing alpha-smooth muscle actin (α-SMA) to contract the gland, before they were identified as a potent, deeply quiescent reserve stem cell population that activates only during severe airway trauma, such as catastrophic injury and chronic inflammation (as in COPD and severe asthma).^15,16 MECs possess a hybrid molecular signature, co-expressing basal epithelial markers such as KRT5 and KRT14, along with myoid markers including α-SMA. Their surface marker profile (NGFR^hi/ITGA2^hi) differs from that of GDSCs, allowing for their separate isolation.¹⁴ Lineage tracing revealed their remarkable plasticity, and a single MEC can regenerate at least seven mature cell types in both the gland and surface epithelium.^15,68 However, activated MECs possess a strong lineage bias toward secretory differentiation. When they proliferate to repair the airway, they preferentially generate mucus cells, thereby directly fueling the mucus metaplasia that underpins chronic hypersecretion.¹⁵ Mechanistically, this activation is tightly controlled by signaling pathways, including the Wnt/β-catenin/lymphoid enhancer binding factor 1 (Lef-1) pathway, positioning MECs as a contingency population that ensures the airway can recover from severe injuries.^66,67,72

Impact of aging on the airway glandular niche

The regenerative capacity of the airway declines with age, a process closely linked to the depletion and dysfunction of stem cell reservoirs.⁷³ Aging results in the accumulation of senescent cells within the airway epithelium, which display a senescence-associated secretory phenotype (SASP) that secretes pro-inflammatory cytokines and proteases.⁷⁴ This inflammatory environment can disrupt the quiescent state of glandular progenitors, potentially impairing their efficient response to injury.⁷⁵ Moreover, oxidative stress and telomere shortening in aging airways may bias stem cell differentiation towards maladaptive phenotypes, thereby contributing to delayed repair and persistent inflammation observed in elderly patients with COPD and other airway diseases.⁷⁶

Mechanisms driving mucus plugging in obstructive airway diseases

The central role of the SMG is clearly demonstrated by its widespread dysfunction across a range of obstructive airway diseases.⁷⁷ In these conditions, SMGs undergo aberrant remodeling, typically marked by hypertrophy and hyperplasia (Fig. 2). This glandular expansion is not a balanced scaling of all cell types. Instead, it is often accompanied by mucous metaplasia, in which the mucous-to-serous cell ratio increases,² leading to chronic mucus hypersecretion. This key clinical feature causes airway obstruction and impairs mucociliary clearance.⁷⁸ Of note, although the clinical presentation of mucus plugging is similar across these diseases, the underlying structural and cellular drivers differ significantly. Understanding these specific mechanisms is key to targeted therapy. In CF airways, SMG pathology constitutes a primary etiological factor, directly attributable to the genetic deficiency of the CFTR anion channel. This fundamental defect compromises the glandular capacity to secrete fluid and bicarbonate from birth, making SMG dysfunction a core aspect of the disease.²⁴ Conversely, in asthma and COPD, glandular dysfunction is predominantly secondary to persistent inflammation, in which a continuous influx of inflammatory mediators induces pathological changes within the normally functioning stem cell niche (Table 3).^2,24,27,42,78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89

Fig. 2.

Representative Alcian Blue-stained images of human bronchi. The images show the bronchus of a healthy airway (left panel) and the pathological changes of SMG hypertrophy and hyperplasia in asthma (middle-left panel), COPD (middle-right panel), and CF (right panel) bronchi. Scale bars in all images equal 100 µm. CF, Cystic fibrosis; COPD, Chronic obstructive pulmonary disease; GD, Glandular duct; SMG, Submucosal gland.

Table 3.

Pathological changes in airway submucosal glands across major airway obstructive diseases.

Disease	Key pathological changes	Primary mechanisms	Clinical consequence(s)
Cystic fibrosis (CF)	CFTR and coordinated ion channel dysfunction in serous cells, hyperplasia and mucin occlusion of gland ducts, thickened/elastic/acidic mucus, impaired fluid/anion secretion, MUC5B hyperconcentration, causing osmotic compression24,80	Genetic defect (CFTR), loss of anion/fluid secretion81	Chronic infections, impaired mucociliary transport, declining lung function82
Asthma	Hypertrophy/hyperplasia of SMG, excessive mucus production (massive volumetric increase compared to surface goblet cells), mucus plugging, mucus tethering, exudation27,83,84	Chronic inflammation (allergic), airway remodeling, biophysical and ion transport defects, neural mechanisms85	Airflow obstruction, airway hyperresponsiveness, increased morbidity/mortality due to poor asthma control42
COPD/chronic bronchitis	Hypertrophy/hyperplasia of SMG, increased mucous:serous cell ratio, excessive mucus production (dominating over surface metaplasia in larger airways), dilated gland ducts, increased MUC5B, compromised SIgA distribution, inflammation-driven2,86	Chronic irritant exposure (smoking), inflammation, oxidative stress, neutrophil recruitment, EGFR activation87,88	Persistent productive cough, rapid FEV1 decline, poor quality of life, increased exacerbations/hospitalizations78,79,89

CFTR, Cystic fibrosis transmembrane conductance regulator; COPD, Chronic obstructive pulmonary disease; EGFR, Epidermal growth factor receptor; FEV_1, Forced expiratory volume in 1 second; MUC5AC, Mucin 5AC; MUC5B, Mucin 5B; SIgA, Secretory immunoglobulin A; SMG, Submucosal gland.

Dysregulation of the glandular niche in the main airway obstructive diseases

In chronic airway diseases, the finely tuned regenerative system often becomes dysregulated. This dysregulation differs fundamentally between conditions such as CF, where the stem cell niche is inherently defective, and asthma and COPD, in which extrinsic factors progressively damage the niche. In CF, the airway microenvironment is congenitally dysfunctional; this acidic and dehydrated state triggers a self-perpetuating pathological feedback loop.⁹⁰ Here, the stagnant, nutrient-rich mucus facilitates bacterial colonization and biofilm formation, provoking a robust neutrophil influx. These cells secrete proteases, oxidants, and cytokines that induce further epithelial damage and activate the glandular compartment to proliferate, thereby perpetuating the airway in a state of persistent obstruction, infection, and inflammation.^91,92 Chronic obstruction and low-grade infection continually send injury signals, likely driving the proliferation of progenitor populations in a misguided effort at repair, leading to SMG hyperplasia.⁹³ The inherent secretory lineage bias of MECs is particularly detrimental, as their differentiation generates more secretory cells that produce aberrant mucus, thereby exacerbating the cycle of obstruction.^15,92 In asthma and COPD, chronic inflammation mediated by Type 2 cytokines (e.g., IL-13) or irritant-driven neutrophilic cascades compromises the niche. Mechanistically, these signals, along with EGFR ligands, bias the differentiation of glandular progenitors (and possibly transdifferentiate serous cells) toward a mucous phenotype.^42,94 Consequently, the massive increase in mucin production creates a highly viscous gel that forms static mucus plugs. Furthermore, the total output of antimicrobials (lysozyme and lactoferrin) is insufficient to protect this increased mucus volume, leaving the airway highly vulnerable to infection. The distinct pathogenic mechanisms in SMGs across diverse airway diseases thus reflect unique disruptions in the cellular biology and regulatory pathways of glandular stem cell niches (Table 4).13, 14, 15^{,24,27,46,83,86,87,}92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106

Table 4.

Features of SMG dysfunction in the pathogenesis of major obstructive airway diseases.

Feature of SMG dysfunction	Airway diseases
	Cystic fibrosis (CF)	Asthma/COPD
Primary etiology	CFTR genetic defect; intrinsically faulty niche93	Secondary consequence of chronic inflammation/irritation; corrupted niche95,96
Timing of involvement	Primary and early pathogenetic event93	Secondary, develops over time due to persistent injury95,96
Pathological role of GDSCs	Proliferate in response to chronic obstruction/infection13,14	Contribute to hyperplasia and remodeling under inflammatory stimuli95, 96, 97
Pathological role of MECs	Proliferation exacerbates mucus plugging due to secretory bias in a dehydrated environment15,24,46,98,99	Chronic and persistent severe injury drives mucous cell hyperplasia and gland hypertrophy83,86,94,100, 101, 102
Glandular pathology	Hyperplasia, ductal plugging with thick, dehydrated, acidic mucus, driving severe osmotic compression and ciliary collapse92,103	Hypertrophy and hyperplasia with excessive mucus production; airway epithelial remodeling27,86,101
Stem cell-related therapeutic goal	Correct the CFTR defect in ductal/serous cells and balance ion channels (e.g., gene therapy in GDSCs)104	Inhibit inflammatory signals driving pathological proliferation and differentiation87,105,106

CFTR, Cystic fibrosis transmembrane conductance regulator; COPD, Chronic obstructive pulmonary disease; GDSCs, Glandular duct stem cells; MECs, Myoepithelial cells; SMG, Submucosal gland.

CF: a proposed model of pathological tethering and surface adhesion

To elucidate the complex biophysical interactions that drive CF airway obstruction, we outline a unifying framework that combines the tethering strand mechanism with surface adhesion. Consistent with this volumetric mucin burden, SMG dysfunction is the primary mechanical driver of massive plugging in the larger airways in CF, where it is the primary defect (the tether). In healthy cartilaginous airways, SMGs secrete mucus, primarily the gel-forming mucin MUC5B, as long, continuous strands. Beating cilia pull on these strands until they break free from the gland ducts and are swept toward the larynx. In CF, this mechanism fails at the source.^29,51,107 The pathology is fundamentally driven by a mechanical uncoupling of mucin exocytosis and fluid secretion due to the absence of functional CFTR. However, this mechanical uncoupling is exacerbated by concurrent dysfunction of complementary ion channels, such as hyperabsorption of sodium via ENaC, which further depletes the ASL. This ion transport failure prevents Cl^- and HCO3^- secretion, leading to airway surface dehydration and PCL depletion.

Mutant CFTR impairs fluid and bicarbonate secretion, leading to a failure in calcium sequestration that prevents MUC5B from expanding normally. As a result, the inadequate serous hydration renders these excessive, condensed MUC5B strands into highly elastic tethers. Because they cannot mechanically detach, they remain firmly secured to the SMG ducts, directly causing airway obstruction.^41,51,108 This tethering effectively ties the mucus layer to the airway wall, stalling mucociliary clearance at the site of secretion,¹⁰⁷ leading to interaction between the surface epithelium and the SMG (the adhesion).

Beyond the SMGs, mucus bundles also suffer from surface adhesion along the airway epithelium. This involves a second mucin type, MUC5AC, secreted by surface goblet cells. These tethered MUC5B strands act as a sticky scaffold that traps MUC5AC secreted by surface goblet cells.^12,29,109 Recent models show that the thick MUC5B strands from the SMGs interact with MUC5AC sheet tethered to these goblet cells. In a healthy airway, this interaction serves as a pivot point, helping reorient mucus bundles for efficient clearance.¹¹⁰ However, in the dehydrated environment of a CF airway, the number of MUC5AC-mediated adhesion sites is vastly increased. The mucus bundles become heavily coated and permanently adherent to surface goblet cells, completely preventing their clearance and leading to massive mucus plugs.¹¹¹ These tethered mucin strands cannot be cleared by ciliary action or coughing. As a result of this progression, the combined MUC5B/MUC5AC mass forms a dehydrated, static plaque that rapidly becomes colonized by bacteria, leading to characteristic infected, dehydrated mucus plugs that destroy the CF lung (Fig. 3).⁵¹ Indeed, mucin blockage of gland ducts is among the earliest pathological changes observed in the CF lung.¹¹² Furthermore, calcitonin gene-related peptide (CGRP), which is elevated in CF airways, may induce secretory cell hyperplasia, resulting in an expanded population of cells that overproduce pathological mucus.^99,113

Fig. 3.

Potential mechanism of mucus plug formation in CF airways. The diagram illustrates the “tethered strand” mechanism of biophysical failure in CF. The left panel shows a normal airway in which functional CFTR secretes Cl^- and HCO₃^- to hydrate and “unpack” MUC5B granules, thereby facilitating normal mucus transport. The right panel depicts the consequence of non-functional CFTR: The lack of ion and HCO₃^- secretion prevents proper MUC5B expansion, resulting in tethered MUC5B strands that remain attached to the gland duct. This tethered sticky mesh traps surface MUC5AC, inhaled debris, and bacteria, creating a dehydrated mucus plug that obstructs the airway over non-functional cilia. CF, Cystic fibrosis; CFTR, Cystic fibrosis transmembrane conductance regulator; Cl^-, Chloride ion; HCO₃^-, Bicarbonate ion; H₂O, Water; MUC5AC, Mucin 5AC; MUC5B, Mucin 5B; SMG, Submucosal gland.

Asthma: SMG remodeling and explosive mucin secretion

To elucidate the distinct biophysical mechanics of asthmatic plugging, we propose a mechanistic framework driven by localized microenvironmental SMG remodeling and explosive mucin secretion. In asthma, the profound volumetric mucin burden from SMG hypertrophy serves as the primary mechanical driver of massive plugging in the larger airways; this pathology is rooted in chronic Type-2 inflammation (driven by cytokines like IL-13) that aberrantly programs the healthy basal progenitor compartment⁴² and disrupts the normal balance of ciliated and secretory cells via persistent epigenetic and transcriptional reprogramming.^114,115 Chronic exposure to cytokines such as IL-13 and IL-4 can directly reprogram glandular stem cells; these Type 2 cytokines are significant drivers of MUC5AC and MUC5B production.¹¹⁶ Mechanistically, IL-13 signaling via signal transducer and activator of transcription 6 (STAT6) and forkhead box A3 (FOXA3) drives transcriptional changes that repress the ciliated cell transcription factors forkhead box J1 (FOXJ1) and upregulate SAM pointed domain containing ETS transcription factor (SPDEF), promoting differentiation into MUC5AC-secreting goblet cells (mucous metaplasia) on the airway surface.^117,118 Instead of differentiating into homeostatic epithelial lineages, the aberrant progenitor compartment enters a state of goblet cell metaplasia. This creates a localized, heterogeneously distributed MUC5AC-high environment throughout the bronchioles. The failure of these regions to produce mature ciliated cells, coupled with mucin overproduction, abrogates local airway clearance and sets the stage for permanent structural plugging.^33,119 Ultimately, this pathological cascade leads to extensive airway remodeling, including SMG hypertrophy and hyperplasia driven by a distinct shift in the gland stem cell niche.¹⁰

Of note, it is instructive to contrast it with the aforementioned tethered strand and surface-adhesion mechanisms observed in CF. While CF plugging is initiated by a primary failure of hydration and detachment at the SMG duct, the pathology in asthma is instead driven by the explosive mucin degranulation.^{27,51,107,110,111} The pathology in asthma is further exacerbated hypersecretion of surface epithelial cells, which contributes to the formation of a luminal mucus cap over the airway surface. This pathology is driven by a critical mucin imbalance: a disproportionate elevation of surface MUC5AC relative to MUC5B creates a gel that is significantly stiffer and more adhesive. In this context, the obstructive defect primarily reflects a qualitative and biophysical failure; its excessive production results in the formation of a stiff, highly cohesive gel with altered viscoelastic properties that persistently resists clearance, irrespective of the airway hydration level.^27,28 Concurrently, oxidative stress catalyzes disulfide bond cross-linking, transforming the gel into a highly viscoelastic solid. The resulting plug is a complex matrix of rapidly secreted glandular MUC5B interlaced with surface-derived MUC5AC sheets (Fig. 4).²⁷ Furthermore, the neural environment is influenced: neuropeptides such as neuropeptide Y (NPY) can inhibit VIP-stimulated fluid secretion, thereby disrupting gland function.⁴⁰

Fig. 4.

Potential mechanism of mucus plug formation in asthmatic airways. The diagram delineates the SMG remodeling mechanism driven by the Type 2 inflammatory circuit. The left panel illustrates a healthy airway with an organized glandular structure, including intact ciliated ducts and serous acini. The middle panel shows an asthmatic airway with SMG remodeling, and the right panel depicts the molecular pathway. This cellular shift results in mucous cell hypertrophy and excessive mucin secretion, contributing to the formation of tenacious, eosinophil-rich plugs in the airway lumen. IL-13, Interleukin-13; IL-13R, Interleukin-13 receptor; MUC5AC, Mucin 5AC; SMG, Submucosal gland; STAT6, Signal transducer and activator of transcription 6.

While pathological remodeling creates the pathophysiological substrate by driving massive intracellular mucin accumulation, actual acute luminal occlusion is triggered by the explosive degranulation of these mucin reservoirs from both surface goblet cells (MUC5AC) and SMGs (MUC5B).^83,120 Airway mucin secretion operates on two distinct mechanisms: a constitutive basal pathway for homeostatic clearance, and an agonist-stimulated pathway. During an asthma exacerbation, local secretagogues, such as ATP, histamine, and massive parasympathetic acetylcholine efflux, activate cognate receptors on metaplastic goblet cells and hypertrophied SMGs. This triggers a massive intracellular calcium transient that engages the secretory machinery (specifically synaptotagmin 2 [Syt2] and mammalian Unc-18 homolog b [Munc18b]), driving the explosive, near-total degranulation of intracellular mucin stores.121, 122, 123 Upon cholinergic stimulation, SMGs contract violently, squeezing high-viscosity mucus out of glands. This explosive release ejects highly concentrated, dehydrated mucin polymers into the airway lumen. The rapid volume expansion of the mucus gel immediately overwhelms the local ciliary beat capacity, causing acute asphyxiation and forming the dense plugs characteristic of fatal asthma.¹²⁴

COPD: EGFR-driven remodeling and inflammaging

To contextualize the distinct pathobiology of chronic bronchitis in COPD, we postulate a mechanistic framework centered on EGFR-driven remodeling and inflammaging. In a healthy airway, EGFR activation is a tightly regulated, transient process essential for epithelial repair following acute injury.¹²⁵ However, in COPD, chronic exposure to noxious stimuli (such as cigarette smoke) induces continuous shedding of EGFR ligands (such as amphiregulin) by ADAM metallopeptidase domain 17 (ADAM17, also known as tumor necrosis factor-alpha [TNF-α]-converting enzyme [TACE]). Ultimately, sustained signaling drives profound SMG hyperplasia and secretory dysfunction in chronic bronchitis.^126,127

Mechanistically, environmental insults, typically smoke (oxidative stress), activate TACE/TNF-α, which transactivates EGFR on glandular progenitors, drives structural hypertrophy, increases the Reid Index (gland thickness), and skews the epithelium toward a hypersecretory phenotype.^88,127,128 This sustained signaling fundamentally alters the trajectory of epithelial cell differentiation. It suppresses ciliated cell development (by downregulating transcription factors such as FOXJ1) and forces the epithelium into a state of severe goblet cell hyperplasia, while concomitantly driving profound SMG hypertrophy.^96,127 This physical remodeling replaces the natural clearance machinery of the airway with a massive, overactive mucus-producing apparatus, creating a permanent structural burden that causes chronic airway occlusion and a functional imbalance: the gland loses its capacity to secrete antimicrobial-rich serous fluid while simultaneously inducing mucus hypersecretion.¹²⁹

Of note, COPD is now widely understood not just as an inflammatory disease, but as a condition of accelerated cellular aging characterized by inflammaging, a chronic, low-grade, sterile inflammation driven primarily by cellular senescence.⁷⁶ Chronic oxidative stress from smoking induces premature senescence in airway epithelial cells and local fibroblasts.^130,131 Rather than facilitating homeostatic repair, chronic insults drive progenitors into a state of senescence and induce SASP. The subsequent release of inflammatory cytokines (such as IL-6 and IL-8) drives inflammaging,^76,132 establishing a pro-inflammatory microenvironment that provides relentless paracrine stimulation for hypertrophied SMGs to hypersecrete mucus.^127,132,133 Specifically, the continuous paracrine signaling from these SASP factors directly corrupts the glandular stem cell niche. These pro-inflammatory cytokines disrupt the normal homeostatic signals that regulate GDSCs, heavily skewing their differentiation toward secretory lineages rather than maintaining a healthy balance of ciliated and serous cells.^134,135 Furthermore, this persistent inflammatory environment threatens the deep quiescence of reserve MECs, triggering their aberrant activation and directly contributing to pathological mucous metaplasia.^15,16 Evidence suggests that chronic oxidative stress from cigarette smoke may selectively induce senescence in specific progenitor populations, particularly the more actively dividing GDSCs, leaving the glandular niche fundamentally compromised.^131,136 The accumulation of clearance-resistant senescent cells, together with ongoing inflammaging and selective progenitor vulnerability, provides a mechanistic explanation for the persistence of structural remodeling and mucus hypersecretion in COPD airways decades after smoking cessation.^76,131 This transforms the gland itself into a persistent source of inflammation that recruits neutrophils and perpetuates tissue damage even after smoking cessation (Fig. 5). Therefore, this pathophysiological paradigm provides a strong mechanistic rationale for targeted senolytic therapies. By selectively inducing apoptosis in these therapy-resistant, SASP-producing senescent cells, senolytics could effectively eliminate the compromised elements from the stem cell niche. This targeted clearance would abrogate the chronic inflammatory feedback loop, thereby restoring a permissive microenvironment essential for glandular regeneration and function.

Fig. 5.

Potential mechanism of mucus plug formation in COPD airways. The diagram delineates the EGFR-driven remodeling and inflammaging mechanisms in COPD. The left panel shows well-organized glandular structures, including ciliated and collecting ducts, mucous tubules, and serous acini of a healthy airway. The middle panel shows a COPD airway; environmental insults, such as cigarette smoke, induce oxidative stress that activates TACE-mediated cleavage of TNF-α, thereby stimulating EGFR activity. The right panel depicts the molecular pathway inducing the inflammaging. COPD, Chronic obstructive pulmonary disease; EGFR, Epidermal growth factor receptor; GDSCs, Glandular ductal stem cells; MECs, Myoepithelial cells; MUC5AC, Mucin 5AC; SASP, Senescence-associated secretory phenotype; TACE, Tumor necrosis factor-alpha-converting enzyme; TNF-α, Tumor necrosis factor alpha.

Furthermore, cigarette smoke exposure induces an acquired CFTR dysfunction via oxidative stress, which mimics the dehydration defects seen in CF, further compounding the clearance issue.³⁰ Importantly, this acquired ion transport failure extends beyond CFTR; smoke-induced inflammation simultaneously hyperactivates the epithelial ENaC and impairs calcium-activated chloride channels, such as TMEM16A.²³ This concurrent dysfunction of multiple ion channels results in profound airway surface dehydration and hyperconcentration of the mucin load, which directly mediates the mechanical intractability of the resulting plugs.^21,23,81,137 Additionally, secretory immunoglobulin A (SIgA) produced by glands can become entrapped in viscous mucus, thereby impairing the local immune barrier and contributing to the high incidence of infectious exacerbations.¹³⁸

Clinically, anatomical evidence demonstrating that intractable plugs remain physically attached to the SMG ducts elucidates the frequent failure of traditional, surface-acting mucolytics. Thus, an effective therapeutic intervention necessitates targeting disease-specific upstream drivers: restoring the ionic homeostasis to disrupt calcium-dependent cross-links in CF, abrogating Type-2 cytokine signaling pathways (e.g., IL-13) to rescue the progenitor niche in asthma, or interrupting aberrant EGFR cascades and deploying senolytics to eliminate senescent cells in COPD.

Therapeutic strategies targeting mucus plugging

Given the critical role of SMG dysfunction in the pathophysiology of chronic airway diseases, therapeutic strategies are increasingly shifting toward modulating glandular function and abrogating luminal mucus hypersecretion. Achieving true disease modification requires a strategy pivot toward mechanism-based therapies and progenitor-targeting intervention. These advancements reflect a deeper understanding of SMG biology, transitioning from broad-spectrum symptomatic management of mucous occlusion to highly targeted therapies designed to correct fundamental pathophysiological drivers. This shift includes next-generation mucoactive agents, targeted biologics, and emerging stem cell therapies (Table 5).^{13,15,16,18,47,86,88,98,101,}139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151 Crucially, a clear mechanistic distinction exists among these modalities: while some agents (e.g., anticholinergics, EGFR inhibitors) directly target receptors on glandular cells to arrest hypersecretion or hyperplasia, others (e.g., corticosteroids, biologics) act indirectly by suppressing the upstream inflammatory mediators that drive secondary glandular remodeling.

Table 5.

Therapeutic strategies targeting mucus burden.

Category/agents/approaches	Mechanism of action and primary target (direct/indirect)	Primary target regarding SMG dysfunction (direct vs. indirect)	Primary disease(s) targeted/relevant	Reference(s)
Mucoactive agents
Mucolytics
N-acetylcysteine (NAC)	Breaks disulfide bonds linking mucin polymers; antioxidant, anti-inflammatory	Direct (mucus physicochemical structure)	CF, COPD, bronchiectasis, asthma	139
Dornase alfa	Depolymerizes DNA polymers in purulent mucus	Direct (mucus structure/DNA burden)	CF	140–142
Carbocysteine	Modulates mucus production, restores viscoelasticity; antioxidant, anti-inflammatory; increases Cl- transport	Mixed (direct on mucin metabolism; indirect via anti-inflammatory effects)	COPD, chronic bronchitis	143
Erdosteine, gelsolin, thymosin β4, dextran, heparin	Various: Modulate mucus production, sever actin cross-links, increase hydration	Direct (mucus viscoelasticity)	General airway diseases	141
Mucoregulators
Anticholinergic agents (ipratropium, tiotropium)	Block parasympathetic nerve activity, reduce mucus secretion	Direct (reduction of mucus secretion)	COPD, asthma	144,145
CFTR modulators	Restore ionic environment to facilitate proper mucin detachment	Direct (restoration of serous cell ion transport)	CF	146
Glucocorticoids   (corticosteroids)	Reduce airway inflammation and mucin secretion; modulate remodeling	Direct (glandular receptors and secretion)	Asthma, COPD, chronic rhinosinusitis	147
Macrolide antibiotics	Reduce airway inflammation and mucin secretion; inhibit Ca2+ entry in gland cells; partial muscarinic agonists	Primarily indirect (upstream inflammatory mediators)	Chronic bronchitis, bronchiectasis	105,148
Anti-inflammatory therapies
Corticosteroids	Suppress inflammation, modulate mucosal remodeling, reduce eosinophilia	Mixed (direct on glandular Ca2+signaling; indirect on neutrophil inflammation)	Asthma, COPD	147
Biologics (e.g., anti-IgE, anti-IL-5, anti-IL-4Rα, anti-TSLP, anti-IL-25, anti-IL-33)	Target specific inflammatory pathways to reduce inflammation, mucus plugging, and remodeling	Direct (targeting of epithelial signaling pathways that regulate glandular differentiation)	Severe asthma, potentially COPD	149
Novel approaches
Gene therapy (CFTR)	Delivers functional CFTR transgene to correct genetic defect	Indirect (upstream inflammatory signaling pathways)	CF	150
Inhibitors of nerve activity/mucin exocytosis	Modulate neural signals, prevent mucin release	Indirect (upstream neural signals)	Hypersecretory diseases	86,101
EGFR inhibitors	Target growth factor pathways driving gland hypertrophy/hypersecretion	Direct (glandular stem cells)	COPD, chronic bronchitis	88,151
SMG organoids for drug testing	In vitro modeling for drug screening and pathophysiology studies	Direct (glandular remodeling/hyperplasia pathways)	All airway diseases	18
Stem cell modulation	Promotes epithelial repair and modulates remodeling	Direct (glandular stem cell niche behavior)	All airway diseases	13,15,16, 98

Ca²⁺, Calcium ion; CF, Cystic fibrosis; CFTR, Cystic fibrosis transmembrane conductance regulator; Cl^-, Chloride ion; COPD, Chronic obstructive pulmonary disease; EGFR, Epidermal growth factor receptor; IgE, Immunoglobulin E; IL, Interleukin; IL-4Rα, Interleukin-4 receptor alpha chain; SMG, Submucosal gland; TSLP, Thymic stromal lymphopoietin.

Mucoactive agents: managing airway mucus burden

Mucoactive agents are essential for symptom management because they modify the physical properties of airway mucus, facilitating its clearance. By disrupting the cycle of obstruction and infection, these medications provide immediate relief and are classified by mechanism of action, including mucolytic agents and mucoregulators (Table 5). Therapies such as CFTR modulators directly restore ion transport in serous cells, thereby directly enhancing the hydration and facilitating proper mucin detachment. Their anti-inflammatory effects are secondary to this improved clearance.147, 148, 149, 150, 151, 152 Furthermore, targeting alternative ion channels, such as using ENaC inhibitors to prevent sodium hyperabsorption or TMEM16A activators to stimulate compensatory chloride secretion, represents a critical strategy to rehydrate the airway surface, restore the hydration, and alleviate osmotic compression.^23,81,153 Inhaled bicarbonate or agents that chelate calcium could, in theory, help dissolve tethered plugs by mimicking the natural unpacking mechanism.¹⁵⁴ Mucolytic agents work by directly degrading the complex polymer structure of mucus, thereby reducing its viscosity and improving its flow properties. For example, N-acetylcysteine (NAC) contains a free thiol group that cleaves the disulfide bonds that cross-link mucin polymers. In addition to its mucolytic function, NAC acts as a precursor to the antioxidant glutathione, enabling it to scavenge reactive oxygen species (ROS) and protect the airway epithelium from oxidative damage.¹³⁹ Dornase alfa is a recombinant human DNase I that cleaves the high molecular-weight extracellular DNA networks released by necrotic neutrophils in purulent secretions. The DNA significantly increases mucus viscosity, particularly in diseases with prominent neutrophilic inflammation, such as CF and bronchiectasis. By breaking down this DNA mesh, dornase alfa improves lung function and reduces the frequency of exacerbations in this patient population.140, 141, 142 Carbocysteine modulates mucus production, helping restore the normal balance between sialomucins and fucomucins. This change in composition affects the viscoelastic properties of mucus, partly by increasing chloride transport across the epithelium.¹⁴³

Mucoregulators are a class of medications designed to attenuate mucus hypersecretion by targeting regulatory pathways governing SMG activity. Anticholinergic agents, such as tiotropium bromide, serve as cornerstone therapeutics for COPD and critical adjunctive treatments for asthma.^155,156 They act as competitive antagonists at muscarinic acetylcholine receptors (mainly the M1 and M3 subtypes on SMG cells). This action blocks parasympathetic (cholinergic) nerve signals, the main triggers of glandular secretion, thereby decreasing mucus production.¹⁴⁴ Furthermore, specific macrolides exhibit profound mucoregulatory and immunomodulatory properties. While they act indirectly to suppress the neutrophil-driven inflammation that perpetuates the hypersecretory state, evidence suggests that they may also directly inhibit intracellular calcium influx in glandular cells, effectively blunting stimulated mucin release.^105,148

Anti-inflammatory therapies: indirect amelioration of mucus production and glandular remodeling

Unlike anticholinergics, which directly block glandular receptors, broad anti-inflammatory agents act indirectly by rescuing progenitor microenvironment from pathological signaling. In this regard, corticosteroids represent the foundational example of this class, serving as first-line therapy for attenuating chronic airway inflammation. By binding to intracellular glucocorticoid receptors, they initiate broad genomic effects that profoundly suppress pro-inflammatory gene expression. This widespread immunomodulation indirectly reduces local inflammatory mediators that drive mucin gene overexpression, secretory metaplasia, and glandular hyperplasia. However, their clinical efficacy is frequently limited in steroid-resistant asthma phenotypes and the predominantly neutrophilic inflammation characteristic of COPD,¹⁴⁷ highlighting the need for more precision-targeted interventions.

Biologics, on the other hand, represent a paradigm shift toward precision medicine; they target specific inflammatory pathways with high specificity. In severe Type 2 asthma, biologic agents that neutralize key cytokines have revolutionized management and significantly altered disease progression. For instance, dupilumab blocks the IL-4Rα subunit (inhibiting both IL-4 and IL-13 signaling), while tezepelumab blocks the upstream alarmin thymic stromal lymphopoietin (TSLP).¹⁵⁷ Crucially, these interventions reduce mucus plugging not only as a secondary benefit of resolving inflammation, but also by directly intercepting the signaling cascades that drive glandular dysfunction.¹⁵⁸ The specific blockade of IL-4/IL-13 signaling addresses the structural basis of plugging by inhibiting the differentiation of glandular progenitors; this blockade also prevents the IL-13-driven overproduction of MUC5AC, restoring the physiological MUC5B:MUC5AC ratio and reducing the viscoelasticity that tethers mucus to the ductal opening.^28,159 Simultaneously, anti-TSLP therapy restores homeostasis to the glandular microenvironment by halting the influx of eosinophils and mast cells. This reduction in inflammatory burden prevents the release of potent secretagogues, thereby averting the massive mucous surges that lead to catastrophic airway occlusion.^160,161 In clinical practice, this translates to resolution of the plugging phenotype, reducing the persistent, adherent mucus associated with severe asthma, although established hypertrophy regresses more slowly.^161,162 Fundamentally, these therapies directly modulate epithelial signaling pathways responsible for glandular differentiation, and preserve the integrity of the glandular stem cell niche by interrupting the cycle of chronic inflammation.^162,163 Protecting this microenvironment both dampens acute hypersecretory responses and inhibits the drivers of tissue remodeling, thereby securing the long-term regenerative viability of the airway.

Emerging novel approaches

A more thorough understanding of airway epithelial and SMG cell biology, particularly the role of SMG as a stem cell niche, is supporting the development of innovative therapeutic strategies. These strategies focus on targeting the underlying causes of the disease, with the potential to truly modify the disease and, in the long run, develop cures. These strategies at least include approaches to targeting progenitor cells and modulating signaling pathways.

The progenitor-targeting strategies focus on directly modulating the behavior of resident stem cell populations. In CF, targeting long-lived progenitors, such as GDSCs and/or MECs with gene-editing tools (e.g., clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 [CRISPR/Cas9]) could permanently correct the anion transport defect throughout the gland lineage, thereby preventing the formation of tethered plugs at the source.¹⁶⁴ The goal is to deliver a functional copy of the CFTR gene, or to correct the mutated CFTR gene, in this self-renewing stem cell pool. A successful intervention could lead to the gradual repopulation of the entire gland with corrected, fully functional cells, thereby enabling a one-time, permanent cure. However, significant challenges remain, including the need to develop vectors that can efficiently penetrate the mucus barrier and transduce this quiescent cell population.¹⁰⁴

However, in complex diseases like asthma and COPD, therapeutic efforts focus on modulating the stem cell niche to interrupt the pathological signaling that drives airway remodeling. A primary target is the EGFR pathway, which is frequently activated by irritants such as cigarette smoke and serves as a pivotal regulator of mucin gene transcription and glandular hyperplasia.⁸⁸ Alongside EGFR, the Wnt/Lef-1 pathway is similarly implicated in aberrant remodeling through its role in the post-injury activation of MEC progenitors. While its controlled activation offers a potential regenerative strategy for acute lung injury,^15,16,66 the primary goal in chronic airway disease is pharmacological inhibition of this pathway to attenuate hyperproliferative glandular remodeling. Consistent with the broader strategy of suppressing these hyperactive pathways, preclinical investigations using human SMG organoids have demonstrated that EGFR/ErbB pathway inhibitors effectively prevent the glandular proliferation and secretory differentiation.¹⁸ This provides a compelling rationale for repurposing oncological EGFR inhibitors to attenuate SMG hypertrophy and mucus plugging. Similarly, biologics (anti-IL-13/TSLP) act by shielding the stem cell niche from the pathological inflammatory signals. By depriving glandular progenitors of the stimuli that induce mucous metaplasia,^158,165 these therapies allow the gland to revert to a normal secretory phenotype, ultimately reducing the formation of rubbery asthmatic plugs.¹⁶⁶

In addition, SMG organoids provide a personalized platform for high-throughput drug screening. For CF, organoids derived from rectal or nasal biopsies are already used to predict patient responses to CFTR modulators (e.g., swelling assays).^18,167,168 Moreover, they enable testing of novel mucolytics or anti-inflammatory agents directly on human glandular tissue, assessing endpoints such as mucus viscosity, ciliary beat frequency, and stem cell regenerative capacity.^167,169 This patient-derived ex vivo model facilitates the translation of fundamental biological insights into clinical practice.

Beyond endogenous progenitors, exogenous stem cells show great promise for treating chronic lung diseases.¹⁷⁰ Notably, the therapeutic efficacy of mesenchymal stem cells (MSCs) and their derived extracellular vesicles (EVs) in airway disease is likely driven by a synergistic dual mechanism.¹⁷¹ Primarily, MSCs function through robust paracrine mechanisms, exerting immunomodulation that restores homeostasis within the inflammatory microenvironment. By inhibiting key upstream mediators, specifically Th2 cytokines and neutrophil elastase, they abrogate the signal cascades that propagate secondary glandular remodeling. In addition to this broad immunosuppression, MSC-EVs engage in direct molecular crosstalk with the glandular epithelium; they function as biological shuttles, transferring mitochondria to metabolically exhausted cells and delivering bioactive miRNA cargo (e.g., miR-146a) to downregulate MUC5AC expression and inhibit STAT6 signaling.¹⁷² Consequently, these biologic interventions extend beyond basic anti-inflammatory action, actively recalibrating the aberrant glandular microenvironment toward a regenerative state. These therapeutic advances, ranging from the clearance of mucus plugs and attenuation of inflammation to the reprogramming of glandular stem cells, underscore our growing understanding of the pivotal role of the SMG in both airway health and disease pathology.

Conclusion and prospective

Therapeutic implications of airway gland pathobiology

Airway mucus plugs are complex structures anchored in SMGs and solidified by dehydration and inflammation. These plugs arise directly from SMG remodeling, specifically hypertrophy and hyperplasia driven by disruption within stem cell niches. The flush–load framework highlights that when the massive mucin load from hyperplastic glands overwhelms the coordinated ion-channel flush, the resulting hyperconcentrated mucus osmotically compresses the periciliary flush, leading to catastrophic ciliary collapse. To date, the study of airway SMGs has undergone a significant shift, moving from a specialized area within respiratory physiology to a primary focus on understanding the development of major obstructive airway diseases. Through this review, we have synthesized these biophysical failures into disease-specific frameworks: the pathological tethering and surface adhesion in CF; SMG remodeling and explosive mucin secretion in asthma; and EGFR-driven remodeling and inflammaging in COPD. Our understanding of these structures has evolved from perceiving them as simple secretory units to recognizing them as highly complex, multifunctional micro-organs. As this review states, SMGs are not only responsible for producing the protective mucus layer but also serve as crucial, protected niches for a hierarchy of adult stem cells, which are vital for airway maintenance and repair. This dual role, as both secretory entities and regenerative reservoirs, is fundamental to understanding their contributions to airway health and disease.

The clinical significance of SMGs is underscored by their widespread dysfunction across major airway obstructive diseases. A unifying hallmark of CF, asthma, and COPD is pathologically altered glandular tissue, driven by hypertrophy, hyperplasia, and aberrant mucus production. Nonetheless, the distinct pathogenic mechanism underlying these structural changes dictates different therapeutic approaches. Recognizing these mechanistic differences is essential for moving beyond symptomatic mucolytics toward interventions that target the root cause of glandular remodeling.

Future directions and unanswered questions

Although distinct stem cell populations (such as GDSCs and MECs) have been identified, the dynamic cooperative mechanisms that coordinate their activity during injury repair remain largely unclear. Present evidence indicates a functional hierarchy wherein GDSCs sustain homeostasis and MECs function as a reserve pool for severe injury; however, the exact molecular interactions, including specific paracrine signals and direct cell–cell contacts, that enable these populations to assess injury severity and orchestrate a graded regenerative response have not yet been fully characterized. Understanding this crosstalk is essential for developing therapeutic strategies that selectively mobilize or suppress specific progenitor subsets in pathological conditions.

Despite significant advances in our understanding of SMG biology in muco-obstructive diseases, several critical knowledge gaps remain. Moving forward, the field can transition from descriptive pathological observations to focused mechanistic research to target the underlying flush–load imbalance and glandular remodeling. First, studies should delineate the specific paracrine signals (e.g., Wnt and notch ligands) that regulate communication between GDSCs and MECs during injury repair, and identify how chronic inflammation disrupts these pathways, so that the stem cell niche can be therapeutically restored. Second, researchers should overcome the main physical barriers, namely mucous penetration and epithelial basement membrane transit, that restrict therapeutic access to the basolateral SMG surface. This will require testing advanced delivery platforms, such as mucolytic lipid nanoparticle (LNP) formulations, designed to transport payloads directly to glandular progenitors. Finally, the field should advance beyond basic SMG organoid models to sophisticated microfluidic co-culture systems. By incorporating functional neural elements and circulating immune cells, these models can more accurately replicate the complex in vivo neuro–immune–glandular axis driving hypersecretion. Addressing these fundamental objectives will be crucial for translating structural and cellular insights into visible disease-modifying therapies.

Furthermore, biological sex is a critical but historically overlooked factor in the development of muco-obstructive diseases. Emerging epidemiological evidence highlights notable sex-based differences in mucus hypersecretion and overall disease severity. For instance, females experience more severe asthma phenotypes, exhibit greater susceptibility to early-onset COPD despite shorter smoking histories, and face worse clinical outcomes with faster decline in lung function in CF (often called the "CF gender gap").¹⁷³ Mechanistically, these differences may be driven by hormonal regulation of airway ion channels. Fluctuations in sex hormones, especially estrogen, can directly impact CFTR expression and function.¹⁷⁴ In the context of the flush–load model, this hormone-induced downregulation of CFTR impairs serous fluid secretion. When compounded by other risk factors for acute COPD exacerbations,¹⁷⁵ this mechanism ultimately exacerbates mucus dehydration and plugging in females. Additionally, intrinsic sex-based differences in immune responses, such as heightened or prolonged Type 2 inflammatory cascades in females, may further disrupt the glandular stem cell niche and accelerate mucous metaplasia. Consequently, future research should explicitly consider sex as a biological variable (SABV). Understanding how sex hormones and sex-specific immune cascades independently modulate SMG progenitor biology, organoid secretory dynamics, and drug efficacy will be essential for developing truly personalized therapies for airway mucus plugging.

Notably, the SMG organoid system provides a robust platform for elucidating human-specific disease mechanisms and accelerating high-throughput drug discovery.¹⁸ While this ex vivo culture model and the growing focus on airway stem cell dysregulation offer promising new therapeutic avenues, the anatomical sequestration of the SMG remains a formidable barrier to treatment. Unlike the surface epithelium, SMGs reside deep within the submucosa and become heavily obstructed by viscous, dehydrated mucus during disease progression. Consequently, developing targeted delivery strategies capable of accessing the SMG niche is critical. Future investigations must prioritize engineering mucus-penetrating nanoparticle carriers or identifying small-molecule modulators that can reach the gland basolaterally via systemic circulation. Overcoming these biophysical barriers is an absolute prerequisite for translating gene-editing or niche-modulatory interventions into successful clinical therapies.

In summary, airway mucus plugging is an active consequence of coordinated dysfunction between deep SMG remodeling and epithelial goblet cell hyperplasia. Rather than focusing exclusively on the airway lumen, where goblet cells are the primary source of MUC5AC, future respiratory medicine should adopt a dual approach that equally prioritizes the SMG. By targeting the fundamental stem cell dysregulation and signaling pathways that drive glandular remodeling and goblet cell hyperplasia, we can shift the clinical paradigm from managing existing mucus plugs to preventing their formation, offering new hope for patients with chronic obstructive airway diseases.

Funding

This work was funded by National Natural Science Foundation of China (No. 82460200), Special Project of Central Government Guiding Local Science and Technology Development of Ningxia (No. 2024FRD05046), Key Research and Development Project of Ningxia (No. 2023BEG02021 and No. 2025BEG02002), National Science and Technology Major Project of China (No. 2024ZD0528905), Ningxia Science and Technology Benefits for the People Project (No. 2025CMG03002), and Ningxia Hui Autonomous Region Scientific and Technological Infrastructure Construction Project (No. 2025DPC05027).

Artificial intelligence disclaimer

During the preparation of this work, the authors used the artificial intelligence tool Gemini Pro to assist data collection and generation of tables. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the published article.

CRediT authorship contribution statement

Zheqing Hu: Writing – original draft, Validation, Software, Investigation, Data curation. Liyuan Yang: Validation, Investigation, Data curation. Yabo Ma: Investigation, Data curation. Juan Chen: Writing – review & editing, Supervision, Investigation, Funding acquisition, Conceptualization. Xiaoming Liu: Writing – review & editing, Visualization, Validation, Supervision, Investigation, Conceptualization.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Edited by: Peifang Wei