Neutrophil Plasticity in Airway Disease: Balancing Damage and Repair

Abstract

Background

Neutrophils, previously viewed as short-lived microbial killers, are now recognized as highly adaptable regulators of innate immunity. Advances in transcriptomic, metabolic, and epigenetic profiling reveal their remarkable heterogeneity and ability to adopt microenvironment-specific phenotypes. In the lung, this plasticity gives neutrophils a double role: they fight infection but can also cause long-lasting inflammation, tissue damage, and scarring.

Summary

We review how neutrophils are activated, move, and act in lung disease, focusing on their release of proteases, production of reactive oxygen species, and formation of extracellular traps. We also describe repair-promoting neutrophil types and treatments that aim to reduce damage while keeping normal neutrophil defense intact.

Key Messages

Learning how neutrophils change within the lung microenvironment will help create better and more precise treatments for lung inflammation and tissue damage.

Introduction

Neutrophils, the most abundant leukocytes in the circulation of mice and men, comprising 50–70% of all white blood cells, express myeloid surface antigens such as CD13, CD15, CD16 (FcγRIII), and CD89 (FcαR) [1, 2]. Neutrophils carry out their immune functions through multiple mechanisms, including phagocytosis, degranulation, and the release of neutrophil extracellular traps (NETs) [3]. Neutrophils use oxidative and non-oxidative mechanisms to eliminate pathogens and regulate tissue responses, but the same processes can also damage host tissues. In addition, neutrophils release alarmins (such as S100A8/A9 and S100A12), cytokines, and chemokines that activate surrounding cells and coordinate immune responses [2, 4].

Neutrophils originate in the bone marrow, the primary site of granulopoiesis, and have a short lifespan; therefore, approximately 0.5–1 × 10¹¹ neutrophils are produced daily in humans to maintain steady-state circulating levels [5]. Hematopoietic stem cells first differentiate into granulocyte-monocyte progenitors, which then progress through a proliferative phase (myeloblasts, promyelocytes, myelocytes) followed by a maturation phase (metamyelocytes, band cells, and finally mature neutrophils). Throughout this process, neutrophils sequentially acquire distinct granule subsets – primary (azurophilic), secondary (specific), tertiary (gelatinase), and secretory vesicles – each enriched with specialized antimicrobial and immune-modulatory molecules. These include myeloperoxidase (MPO), defensins, and neutrophil elastase (NE) in primary granules; lactoferrin, lysozyme, and NADPH oxidase components in secondary granules; and gelatinase (MMP-9) and adhesion molecules in tertiary granules. Together, these granule components equip mature neutrophils with a broad repertoire of molecules that enable pathogen killing, tissue repair, and modulation of immune responses [5, 6].

Mature neutrophils are released into the bloodstream under strict chemokine regulation. The CXCR4-CXCL12 axis retains neutrophils within the bone marrow, whereas CXCR2 upregulation facilitates their mobilization into circulation. After a short lifespan in the blood, senescent neutrophils return to the bone marrow, where resident macrophages clear them to maintain homeostasis [3]. In humans, the bone marrow contains a large reserve of mature neutrophils, known as the postmitotic marrow pool, which contains approximately 5.6 × 10⁹ neutrophils per kilogram of body weight. Each day, about 0.9 × 10⁹ neutrophils per kilogram are released from this pool into the bloodstream to replace aged or expended cells [7]. This substantial reservoir allows for a rapid increase in circulating neutrophils during infection or inflammation.

Once considered short-lived and functionally limited, neutrophils are now recognized as highly plastic cells that dynamically adapt their migratory and effector functions in response to temporal and environmental cues, including circadian rhythms, chemokine gradients, and adrenergic signals [8, 9]. Moreover, growing evidence indicates that neutrophils exhibit remarkable phenotypic and functional diversity across tissues (Fig. 1). While some populations persist as long-term tissue residents, others are rapidly recruited during inflammation. Their phenotype and function are continuously shaped by the local microenvironment, underscoring the dynamic adaptability of neutrophils [7, 10, 11]. This spatial heterogeneity highlights the ability of neutrophils to perform tissue-specific roles that extend well beyond their traditional antimicrobial functions [12].

Fig. 1.

Neutrophil plasticity and function in homeostasis, inflammation, and tissue repair. Neutrophil lifecycle stages are indicated in red. Mature neutrophils are released from the bone marrow into circulation, characterized by low CXCR4 and high CXCR2 and CD62L expression. Upon activation, they follow chemoattractant gradients and extravasate into affected tissues. Exposure to IFNγ and/or LPS polarizes neutrophils toward a proinflammatory “N1” phenotype, exhibiting high degranulation, phagocytic activity, and NET formation. Hyperactivated or aged neutrophils also display elevated phagocytosis and NET formation. In contrast, exposure to TGFβ, IL-4, or glucocorticoids promotes the development of anti-inflammatory, pro-resolving “N2” neutrophils. Immature or tissue-resident neutrophils similarly contribute to inflammation resolution and tissue repair. This presentation was created in BioRender.

Advances in single-cell RNA sequencing, spatial transcriptomics, and intravital imaging now allow detailed characterization of neutrophils [13, 14]. Multiple subpopulations have been identified, including N1- and N2-like polarized types, low-density, immature, aged, tissue-resident, and circulating subsets (Fig. 1). Those in inflamed or diseased tissues differ from circulating or health neutrophils, reflecting epigenetic and metabolic adaptations to the local microenvironment [15–17]. Neutrophil plasticity allows them to adopt pro-inflammatory, immunosuppressive, or pro-repair phenotypes depending on their state. In the lung, hyperactivated or aged neutrophils can worsen tissue injury through proteases, reactive oxygen species, and NETs [18]. In tumors, N1-like neutrophils exhibit antitumor activity through cytotoxicity and pro-inflammatory signaling, whereas N2-like neutrophils promote tumor growth by supporting angiogenesis, suppressing immune responses, and facilitating immune evasion [19, 20]. Overall, these findings highlight how neutrophils integrate environmental signals to fine-tune their functions, acting as central regulators of inflammation and tissue homeostasis.

Lung Neutrophils

The human lung, with an extensive surface area of about 130 m² in adults, provides a vital interface for gas exchange, allowing oxygen to enter the blood and carbon dioxide to be removed. This surface is formed by roughly 480 million alveoli, each surrounded by a dense capillary network that ensures efficient gas transfer. Alveoli are lined by two types of epithelial cells, squamous alveolar epithelial type 1 (AT1) cells (involved primarily in gas and solute exchange), and cuboidal type 2 (AT2) cells (involved in surfactant synthesis and recycling). Additional cells in the alveolar septal wall during health include endothelial cells, fibroblasts, pericytes, macrophages, and mast cells [21, 22].

Inhaled air delivers oxygen but also exposes the lungs to pathogens and environmental irritants. Data from blood sampling methods in dogs, intravital imaging in mice, an indirect gamma camera method in humans, and stiffness analysis of human neutrophils revealed that the lungs have a major reservoir of neutrophils, which can be rapidly mobilized to sites of injury or infection to maintain homeostasis and defend against pathogens [23–25]. In patients requiring lung resection, Hogg et al. [26] found a 60–100-fold increased pulmonary capillary transit time for neutrophils compared to erythrocytes using ^99mTc-labeled macroaggregates and ⁵¹Cr-labeled erythrocytes to measure regional blood flow and volume in the resected lung.

Their proximity to the airway surface enables a fast and effective innate immune response [24, 25]. Moreover, lung neutrophils display distinct phenotypes compared to those in other tissues, likely to reflect the need for tight inflammatory control in the fragile alveolar environment [27]. Studies in mice have shown that most lung neutrophils are CD62L^lo and express high levels of CXCR4, similar to aged neutrophils, suggesting that CXCR4 helps their migration to and retention within the lungs [28, 29]. The CXCR4 antagonist plerixafor induces a massive neutrophil release from the lung into the circulation in mice and macaques indicating the role of CXCR4 for neutrophil margination [30].

Intravital microscopy in mice demonstrate that under homeostatic conditions, most neutrophils reside in the pulmonary vasculature, with only a few in the interstitium or subepithelial space and very few in the alveolar lumen (Fig. 2) [8, 31–34]. Within the vasculature, they interact with monocytes, macrophages, and lymphocytes, forming local networks that coordinate immune surveillance and inflammatory responses [35, 36]. The cytokine production of LPS-stimulated monocytes was reduced in patients with neutropenia compared to non-neutropenic individuals indicating the importance of neutrophils for monocyte/macrophage recruitment and function [37].

Fig. 2.

Schematic representation of marginated neutrophils in the lung. The lung contains a large population of marginated neutrophils characterized by high CXCR4^hi and low CXCR2^lo differing from circulating neutrophils expressing CD62L^lo. These cells are primarily located in alveolar capillaries and pulmonary vessels supplying the airways. Marginated neutrophils adhere to the endothelium without obstructing blood flow, forming a dynamic reservoir that can be rapidly mobilized when needed. Approximately one-third of these cells crawl along endothelial surfaces to patrol for circulating pathogens. In healthy lungs, few extravascular neutrophils are present, and alveolar macrophages as well as epithelial AT1 and AT2 cells are not affected by marginated neutrophils. This presentation was created in BioRender.

Upon activation, pulmonary neutrophils act as frontline defenders, crawling along the capillary endothelium, capturing pathogens, and initiating phagocytose within minutes [32]. Although lung capillaries primarily support gas exchange, they also serve as a key site for neutrophil margination, allowing large numbers to accumulate. This localization enables neutrophils to patrol for circulating pathogens, unlike the mononuclear cell-dominated defenses of the spleen and liver, and provides an efficient environment for cell-cell interactions that promote apoptosis of aged neutrophils [32, 33, 38].

In mice, high-resolution imaging has shown that most pulmonary neutrophils remain intravascular under steady-state conditions, migrating along narrow (<10 µm) capillaries without rolling, while only a small proportion (∼14%) reside the interstitium or subepithelial regions, and very few enter the alveolar space [8, 32]. Neutrophil retention in the lung was thought to be a passive consequence of narrow capillaries, branched vasculature, and low pulmonary blood pressure. Recent studies, however, demonstrate that this process is actively regulated: aged neutrophils are preferentially recruited during inflammation, whereas adrenergic signaling, particularly via β₂-adrenergic receptors, can rapidly mobilize them back into circulation [8, 30, 39]. As demonstrated with ¹¹¹In-labeled neutrophils in sheep, the pulmonary neutrophil reservoir includes both, the capillaries, which support gas exchange, and the bronchial vessels supplying airways, lymph nodes, and nerves, forming a specialized niche that enables rapid immune responses while limiting tissue damage [40]. These latter highlight the lung as a dynamic interface that regulates neutrophil trafficking and retention and contributes to systemic immune homeostasis.

Pulmonary Neutrophil Recruitment

Pulmonary neutrophils exhibit highly dynamic, context-dependent migration in response to infectious and sterile inflammatory stimuli [41, 42]. The degree of infiltration into the lung parenchyma correlates with pathogen burden, virulence, and disease severity. During viral infections such as influenza, Middle East respiratory syndrome coronavirus, and respiratory syncytial virus, in human and non-human primates neutrophils rapidly accumulate in the airways and alveolar spaces [43, 44]. Cytokines like interleukin-6 (IL-6) and interleukin-1β (IL-1β) enhance neutrophil recruitment by airway epithelial cells and alveolar macrophages to express CXC chemokines, including CXCL1, CXCL2, and CXCL8 (IL-8). Experiments using neutrophils from healthy donors, primary human lung cells, and mouse models revealed key regulators of neutrophil migration. Neutrophils respond via chemokine receptors CXCR1 and CXCR2, which recognize CXC chemokines (CXCL1, CXCL2, CXCL5, CXCL6, CXCL7, and CXCL8/IL-8) containing ELR motif (Glu-Leu-Arg) ELR+ CXC as well as complement receptors like C5aR1, guiding them to sites of infection or tissue damage [45, 46].

CXCR2 signaling plays a central role in neutrophil mobilization and transmigration across the alveolar-capillary barrier, and CXCR2 antagonists are currently being evaluated clinically for acute lung injury and viral infections [47]. As neutrophils migrate through intrapulmonary compartments (endovascular, interstitial, and intra-alveolar), they adjust adhesion molecule expressions, indicating that specific adhesive interactions regulate compartmental checkpoints. CXCL5 (epithelium-derived neutrophil-activating peptide-78, ENA-78) is expressed in human alveolar type 2 epithelial cells (A549), human lung fibroblast and pulmonary endothelial cells, macrophages, monocytes, and, in contrast to CXCL1 and CXCL8, CXCL5 is stored in cytoplasmic membranous structures [48]. Upon activation, CXCL5 can be released immediately from intracellular stores. In rodents, lung alveolar type 2 cells (AT2) were identified as the primary source of CXCL5. Neutrophil accumulation in the LPS-challenged lung was strongly reduced in mice pretreated with anti-CXCL5 antibodies demonstrating the importance of this chemokine [49].

Recent studies show that circadian fluctuations in club cell-derived CXCL5, controlled by clock gene-dependent glucocorticoid responses, drive diurnal variations in alveolar neutrophilia in response to inhaled LPS in mice [50]. Both CXCL5 and CXCL15, another murine ELR+ CXC chemokine with an as-yet poorly defined receptor, are produced by respiratory epithelial cells, including alveolar type 1 (AT1), type 2 (AT2), and club cells (CXCL5), as well as airway epithelial cells (CXCL15), highlighting the key role of epithelial-immune cross-talk in orchestrating neutrophil recruitment [8].

Beyond chemotaxis, neutrophil recruitment is regulated by metabolic and transcriptional reprogramming. For example, activation of HIF-1α-dependent glycolysis by viral RNA or damage-associated molecular patterns (DAMPs) enhances neutrophil longevity and functional activity within hypoxic alveolar regions [51]. In a zebrafish inflammatory model, HIF-1α stimulation reduces neutrophil death, delaying the resolution of inflammation [52]. Neutrophils detect pathogens via pattern recognition receptors (PRRs), including Toll-like receptors, NOD-like receptors, and RIG-I-like receptors, which sense viral proteins, double-stranded RNA, and other pathogen-associated molecular patterns [53]. Stimulation of TLR7, TLR8, or TLR9 triggers NF-κB-dependent IL-8 production and NET formation in mammals, mechanisms that can restrict viral dissemination while contributing to lung tissue injury, as seen in diseases such as COVID-19 [54]. Human neutrophils can also directly interact with influenza A virus via surface glycoproteins such as CD43, Siglec-9, and DC-SIGN, facilitating viral recognition and internalization [55, 56]. This interaction triggers ROS generation and NET release but may also support viral persistence through neutrophil-mediated immune evasion.

Pulmonary neutrophil recruitment is regulated by chemokine gradients, complement activation, PRR signaling, and local metabolic factors. Chemokines from epithelial cells and macrophages guide neutrophils via CXCR1 and CXCR2 into the alveolar space, while complement C5a amplifies their migration and responsiveness. PRR signaling through Toll-like receptors and NOD-like receptors senses pathogen-associated molecular patterns and DAMPs, triggering cytokines like TNF, IL-1β, and G-CSF to sustain neutrophil production and recruitment. Local metabolic conditions – such as hypoxia, lactate buildup, and changes in glucose and lipid metabolism – further shape neutrophil activation and survival. High-resolution single-cell analyses reveal that neutrophils recruited to the inflamed lung are highly heterogeneous. Identified subsets include pro-inflammatory neutrophils with high CXCL8 and S100A8/A9, interferon-responsive populations expressing ISGs, and tissue-repairing neutrophils producing VEGFA and MMP9. Single-cell spatial analysis of COVID-19 patient lungs and single-cell transcriptome analysis of a bacterial lung infection mice model show that neutrophil recruitment is not just a quantitative process but a functionally diverse response balancing antimicrobial defense and tissue injury [13, 57, 58].

Pulmonary Neutrophil Plasticity and Functional Roles

Neutrophils show remarkable plasticity, rapidly adapting their migration, phagocytosis, and signaling in response to local and systemic signals [8, 9] (Fig. 3). Upon activation, they initiate diverse effector responses, including ROS generation, degranulation, and the formation of NETs, web-like DNA-histone structures decorated with granule proteins such as NE and MPO, which immobilize pathogens and concentrate antimicrobial activity [59, 60]. During NETosis, human neutrophils decondense chromatin and mix it with granule enzymes before it is released as a three-dimensional network of DNA fibers (∼15–17 nm) coated with protein domains (∼25 nm) [61, 62]. While essential for host defense, excessive NETosis can damage tissue and exacerbate inflammation, contributing to chronic obstructive pulmonary disease (COPD) [63]. Clearance of apoptotic neutrophils and NETs by alveolar macrophages limits secondary injury and promotes repair.

Fig. 3.

Schematic presentation of neutrophil subtypes and functions in alveolar damage, inflammation, and tissue repair. Upon stimulation, large numbers of neutrophils emigrate from the circulation, migrate through the interstitial layer, and transmigrate across the alveolar epithelium to invade alveolar airspaces. Therefore, they degranulate, phagocytose pathogens, and form NETs to combat infections. Granule-derived products can disrupt endothelial and epithelial barrier functions, contributing to edema formation and recruitment of adaptive immune cells. Activated alveolar type 2 (AT2) cells help protect the epithelial lining and support pathogen clearance through enhanced mucus production. Neutrophils also contribute to tissue repair and resolution of inflammation by NET-mediated pathogen trapping, phagocytosis of debris, and secretion of pro-angiogenic and pro-resolving factors. This presentation was created in BioRender.

Neutrophil behavior is highly adaptable and shaped by both external signals and intracellular checkpoints [27, 64, 65]. Activation of the V-domain Ig suppressor of T-cell activation (VISTA) receptor shifts neutrophils toward degranulation instead of phagocytosis, and in mice, Streptococcus pneumoniae infection promotes degranulating neutrophil phenotype, while Escherichia coli infection favors a pro-phagocytic phenotype [66]. Intracellular regulators such as Src homology 2 domain-containing phosphatase-1 (SHP1) limit excessive activation. Loss of SHP1 in inflamed lungs leads to uncontrolled SYK signaling, hyper-inflammation, enhanced degranulation and NET formation, pulmonary hemorrhage, and lethality in mice models of acute lung injury [27, 67].

The local microenvironment further fine-tunes neutrophil responses. Data from healthy donor neutrophils, ARDS patients, and mice models show that neutrophils exhibit increased degranulation and protease release, thereby exacerbating injury in hypoxic regions of inflamed lungs. Hypoxia-inducible factors integrate metabolic and inflammatory signals, promoting degranulation and NETosis while linking oxygen sensing to immune activity [68–70]. Collectively, these findings illustrate that pulmonary neutrophils are functionally diverse, with distinct subtypes contributing variably to inflammation, or tissue repair, depending on their transcriptional program and local environmental signals [17, 51, 71, 72].

Phenotypic and Functional Heterogeneity of Neutrophils across Airway Inflammatory Diseases

Airway inflammation involves the infiltration of immune cells and the release of cytokines, chemokines, proteases, and ROS, disrupting epithelial integrity and tissue balance [18, 73]. Across acute and chronic lung diseases like ARDS, COPD, alpha1-antitrypsin deficiency (AATD)-related emphysema, asthma, cystic fibrosis (CF), and idiopathic pulmonary fibrosis, neutrophils play a central, often disease-defining role. Data from human cohorts and different animal models show that in ARDS, excessive degranulation and NET formation cause acute alveolar injury; in COPD and AATD-related emphysema, persistent neutrophilic inflammation and protease-antiprotease imbalance drive alveolar destruction; in CF, neutrophil dysfunction contributes to mucus obstruction and chronic infection, and in fibrotic lung diseases, dysregulated neutrophil activity amplifies fibro-proliferative signaling [74–76]. During acute respiratory infections such as influenza, SARS-CoV-2, and bacterial pneumonia, neutrophils are rapidly recruited to the airways, where they phagocytose pathogens, release ROS, and form NETs. While these responses aid pathogen clearance, excessive neutrophil activation can damage the alveolar-capillary barrier, causing pulmonary edema, and hypoxemia [77–79]. In chronic conditions like COPD, sustained neutrophilic inflammation, driven by cigarette smoke, environmental exposures, or persistent infection, leads to continuous protease release, degrading extracellular matrix and impairing tissue repair [80, 81]. These examples demonstrate the plasticity of neutrophils, whose tightly regulated recruitment, activation, and clearance maintain lung homeostasis. When dysregulated, they shift from protective defenders to drivers of chronic tissue injury and remodeling, reflecting their dual roles in inflammation and repair (Table 1).

Table 1.

Key neutrophil mechanisms and therapeutic targets by disease

Disease	Key neutrophil mechanisms (pathogenic)	Translational targets/therapies (examples)
COPD	CXCR1/2-driven recruitment, NE/MMP release, NETs, PGP generation	CXCR2 antagonists; NE inhibitors; macrolides; anti-NET strategies (PAD4 inhibitors, DNase) [82–85]
AATD emphysema	Unopposed NE/PR3/cathepsin G → elastin degradation; neutrophil priming and NETs	AAT augmentation (IV/inhaled); selective NE inhibitors [86–89]
Pulmonary fibrosis	NETs + proteases → TGF-β activation, fibroblast → myofibroblast transition	NET inhibitors (PAD4), DNase, anti-TGF-β adjuncts (preclinical + translational) [90]
PH	NETs → endothelial dysfunction, in situ thrombosis, PASMC proliferation	Antithrombotic/anti-NET strategies; protease inhibition; prostacyclin analogues; anti-inflammatory modulators [91, 92]
Asthma (neutrophilic)	IL-17/CXCR2 axis, steroid resistance, NETs, mucus hypersecretion	CXCR2 antagonists; macrolides; IL-17/IL-23 pathway modulators; NET-limiting therapies [93, 94]
Bronchiectasis	Chronic infection → persistent NE/NET release, mucus stasis, tissue destruction	Long-term macrolides; inhaled/systemic NE, cathepsin C inhibitors; airway clearance; NET clearance strategies [95, 96]
Lung cancer	TAN plasticity (N1 vs. N2), NE/NETs promote invasion, immunosuppression and metastasis	TAN reprogramming (TGF-β blockade), CXCR2 inhibition, NET/NE neutralization + combination immunotherapy [97–99]

NETs, neutrophil extracellular traps; PAD4, protein arginine deiminase 4; PASMC, pulmonary artery smooth muscle cells; PGP, proline-glycine-proline; PR3, proteinase-3; TAN, tumor-associated neutrophil.

Chronic Obstructive Pulmonary Disease

In COPD, regardless of clinical phenotype, neutrophils are key drivers of persistent airway inflammation, ECM degradation, and impaired tissue repair [100]. The degree of neutrophilic inflammation correlates with airflow limitation and accelerated decline in lung function, as measured by forced expiratory volume in 1 s (FEV₁) [80]. Cigarette smoke and air pollutants activate epithelial cells and alveolar macrophages to release neutrophil chemoattractants, including CXCL8/IL-8, CXCL1/2, and GM-CSF, thereby sustaining neutrophil recruitment, activation, and delayed apoptosis [100]. Activated neutrophils release proteolytic enzymes such as NE, MPO, and matrix metalloproteinases (notably MMP-9), along with ROS, which collectively degrade elastin, collagen, and other structural components of the lung. This process not only impairs mucociliary clearance but also amplifies inflammation through collagen-derived fragments like proline-glycine-proline (PGP) [81].

In COPD, neutrophils show enhanced sensitivity of danger signals. During acute exacerbations, key innate immune receptors, TLR2, TLR4, and the NLRP3 inflammasome, are upregulated compared with stable disease [101]. This makes neutrophils more responsive to DAMPs such as S100 proteins, defensins, and HMGB1, which are increased in COPD lungs, leading to the release of IL-1β and IL-18 and worsening inflammation [102]. Interestingly, changes in neutrophil precursors can be seen even in peripheral blood from early-stage COPD patients, suggesting that the immune system is primed before symptoms appear [103].

Recent single-cell and functional studies have identified distinct neutrophil subsets in COPD. One subset exhibits hyperinflammatory or proteolytic features, with enhanced degranulation and NETosis, while another shows immunoregulatory or low-activation characteristics [51, 103]. The abundance of these subsets correlates with exacerbation frequency and lung function decline. Both human and mouse studies have shown that senescent neutrophils, characterized by high CXCR4, low CD62L, smaller size, hyper-segmented nuclei, and increased NETosis, accumulate during chronic inflammation and contribute to tissue damage [81, 104]. These CXCR4^hi neutrophils are elevated in COPD patients [105]. At the same time, chronic inflammation can trigger emergency granulopoiesis, releasing immature neutrophils with low CD16, CD10, and CXCR2, band-shaped nuclei, and reduced granule content [81]. While these immature populations are observed in COPD, their functional roles remain unclear.

Beyond their antimicrobial and tissue-destructive roles, neutrophils also contribute to tissue repair and resolution of inflammation [51, 106]. During the later stages of inflammation, they help clearing cellular debris and apoptotic cells, remodeling the extracellular matrix, and releasing growth- and repair-promoting mediators. These reparative functions are regulated by lipid mediators, such as resolvins, protectins, and lipoxins, and pro-resolving signaling pathways that help to restore tissue homeostasis and limit chronic inflammation [107]. In COPD, however, these processes are often impaired. Neutrophil-derived NETs and proteases like NE can drive fibroblast-to-myofibroblast differentiation, promoting aberrant remodeling and fibrosis, while excessive NE suppresses epithelial regeneration and compromises host defense [108, 109]. Elevated NET levels in sputum correlate with worse lung function, and reduced neutrophil phagocytic capacity, suggesting that excessive NETosis may impair bacterial clearance [110].

In summary, neutrophils in COPD drive proteolysis, oxidative injury, NET-mediated inflammation, and abnormal tissue remodeling, while their capacity for repair and resolution is limited [100]. Understanding the molecular mechanisms that control these divergent neutrophil phenotypes is essential for developing therapies that lower harmful activities while preserving or restoring beneficial functions.

Alpha-1 Antitrypsin Deficiency-Related Emphysema

Alpha-1 antitrypsin deficiency (AATD) results from mutations in the SERPINA1 gene, which encodes alpha-1 antitrypsin (AAT), the main circulating inhibitor of neutrophil proteases [111]. The Z allele (Glu342Lys) is the most clinically studied; homozygous PiZZ individuals have severely reduced functional AAT levels (about 10% of normal, which is in blood 1–2 g/L) and can develop early-onset emphysema, even without smoking [112, 113]. Inadequate AAT allows unopposed proteolysis, driving ECM degradation and tissue injury, a key feature of the pathogenesis of AATD-related lung disease [114]. Excessive NE and proteinase-3 (PR3) degrade elastin, collagen, and the basement membrane, impair mucociliary clearance, and generate collagen-derived chemoattractants such as PGP, which sustain neutrophil recruitment via CXCR1/CXCR2 [115–117]. This self-amplifying cycle drives alveolar destruction, loss of elastic recoil, and emphysema [112]. Elevated NE, PR3, and MPO activity in sputum or plasma correlate with lung function decline and CT-quantified emphysema in patients with AATD [118]. Several studies have shown that neutrophils from PiZZ AATD patients exhibit a “primed” phenotype with increased ROS generation, enhanced degranulation, and exaggerated NETosis, contributing to alveolar injury even in the absence of infection [112, 119–121]. Heterozygous PiMZ carriers also show increased neutrophil infiltration and worse airflow limitation compared to PiMM (having normal AAT) individuals [122]. A key but often overlooked mechanism in AATD is impaired clearance of apoptotic neutrophils and NET remnants [123].

Proteomic analyses of neutrophils from AATD patients reveal upregulation of integrins (αL, αM, αX, β2) and cytoskeletal regulators such as talin-1, indicating enhanced adhesion and migratory capacity [124]. Hypoxic conditions associated with advanced AATD-related emphysema further activate neutrophils, increasing production of hydrogen peroxide, peroxynitrite, and nitric oxide, while also stimulating the release of IL-1β and TNF-α, which amplify inflammation [125, 126]. Hypoxia has been shown to potentiate degranulation and NETosis in AATD neutrophils, highlighting a synergistic effect of inflammatory and hypoxic microenvironments on neutrophil [127, 128]. NETs from AATD neutrophils are abnormally persistent and resistant to DNase [129], likely due to citrullinated histones and oxidative DNA modifications [130]. Components of these NETs, including histones, proteases, and mitochondrial DNA, can activate alveolar macrophages and fibroblasts, promoting airway remodeling and fibrosis [131]. Normally, macrophages remove dying neutrophils through efferocytosis to resolve inflammation. In AATD lungs, persistent protease activity, oxidative stress, and low AAT impair this clearance, leading to secondary necrosis and continuous DAMP release. Accumulated NET debris further activates the NLRP3 inflammasome and sustains IL-1β-driven inflammation, linking neutrophil persistence to ongoing tissue damage [132, 133]. NET-related markers, such as cell-free DNA, citrullinated histone H3, and extracellular vesicle (EV) proteases, are emerging as potential biomarkers for disease progression and exacerbation risk [79].

Neutrophil-derived EVs, enriched in proteases like NE and microRNAs, are detected in plasma and BAL fluid of AATD patients. NE is a major driver of tissue destruction in both COPD and AATD-related emphysema. Endogenous inhibitors such as AAT, secretory leukocyte proteinase inhibitor (SLPI) [134], and α2-macroglobulin (α2M) [135] tightly regulate NE activity. However, NE is stored at extremely high concentrations within azurophil granules (≈5.33 mm; ∼67,000 molecules per granule). Upon quantal release, this creates a transient pericellular burst in which local NE levels exceed those of available antiproteases – such as α₁-antitrypsin (∼30 µm) – by nearly three orders of magnitude, allowing a brief period of essentially unopposed proteolytic activity [117, 136, 137]. Moreover, COPD patient-derived exosomes bound NE resists AAT inhibition, preserving its proteolytic activity and promoting lung matrix degradation, while sustaining systemic inflammation and barrier dysfunction [138–140].

Crosstalk between neutrophils, epithelial cells, and macrophages is central to AATD pathology [141]. NE and PR3 activate protease-activated receptor 2 (PAR-2) in human bronchial epithelial cells, causing mucus hypersecretion [142], while ROS and proteases stimulate epithelial CXCL8 and GM-CSF release, further enhancing neutrophil recruitment and survival. Activated macrophages release TNF-α and IL-1β, priming neutrophils, while fibroblasts exposed to neutrophil proteases and EVs acquire a pro-fibrotic phenotype, contributing to airway remodeling and tissue injury [143, 144].

Currently used AAT augmentation therapy restores antiprotease capacity, reduces systemic inflammation, modifies neutrophil membrane proteome, and limits degranulation [145, 146]. Confocal microscopy and mass spectrometry show that PMA-induced NETs of PiZZ AATD patient neutrophils isolated directly after AAT infusion differ in size and shape from NETs of those isolated before AAT infusion [147]. Experimental strategies targeting CXCR2 signaling, NE activity or NET formation have shown promise in preclinical AATD and emphysema models [148–150]. Given the heterogeneity of neutrophils, future therapies may aim not only to suppress excessive activation but also to promote resolution and repair-oriented neutrophil functions.

Neutrophils in Other Pulmonary Diseases

Asthma

Neutrophilic asthma accounts for approximately 20%–30% of all asthma cases and represents a severe subtype characterized by excessive neutrophil accumulation and activation in the airways [151, 152]. It is primarily associated with non-type 2 (non-Th2) inflammation and steroid-resistant endotypes, although neutrophils can coexist with eosinophils in mixed granulocytic forms. Unlike eosinophilic (type 2) asthma, which is driven by IL-4, IL-5, and IL-13 signaling and typically responds to corticosteroids, neutrophilic asthma is dominated by a Th1/Th17 imbalance and innate immune-driven inflammation, making it less responsive to standard anti-inflammatory therapies [153–156].

The role of neutrophils in asthma is complex and not fully understood [157]. Airway neutrophilia is driven by epithelial-derived CXCL8, IL-17 (from Th17 and γδ T cells), and IL-6 and is sustained by microbial colonization or repeated pollutant exposure. Neutrophils in asthmatic airways show increased NETosis, protease release (e.g., NE, MMP-9), and prolonged survival due to GM-CSF and delayed apoptosis, which impair mucociliary clearance, damage epithelium, and enhance airway hyperresponsiveness [158, 159]. NETs and proteases can activate sensory nerves and stimulate mucus production via EGFR signaling, contributing to airflow obstruction [160, 161]. Neutrophil-dominant asthma is often steroid-resistant but may respond to targeted therapies such as CXCR2 antagonists, IL-17/IL-23 inhibitors, macrolides, or NET-limiting strategies (e.g., protein arginine deiminase 4 [PAD4] inhibition, DNase) while preserving host defense [162–164].

Importantly, NETs can also be beneficial: they trap and kill bacteria [61], viruses [165], and fungi [166, 167]. NETs can promote the resolution of inflammation by degrading cytokines and chemokines [168] and inhibit GM-CSF/IL-4-induced dendritic cell differentiation from human peripheral blood monocytes [169]. In a mouse model for colitis-associated cancer, MMP-9 was found to reduce ROS accumulation and DNA damage [170]. Finally, through the release of antimicrobial peptides, growth factors, and pro-resolving mediators, neutrophils can contribute to mucosal barrier integrity and tissue repair. Understanding how epithelial injury, PRR activation, and neutrophil responses interact is essential for developing therapies that target innate immune pathways in severe treatment-resistant asthma.

Cystic Fibrosis

CF is an autosomal recessive disorder that impairs mucus clearance, promoting bacterial colonization and chronic lung infection [171]. From CF airways, neutrophil populations were isolated, which can suppress T-cell function by arginase I upregulation [172, 173], and under certain conditions can acquire features of antigen-presenting cells, including expression of CD80, CD86, and MHC II, potentially influencing adaptive immune responses [174]. Immature neutrophils can also upregulate CXCR4 upon activation, enabling them to migrate from inflamed tissues into lymphatic vessels [105, 174, 175]. This trafficking to lymph nodes has been linked to T-cell proliferation, suggesting that neutrophils in CF may contribute to adaptive immunity [176].

Most CF patients are currently treated with CFTR modulators, such as elexacaftor/tezacaftor/ivacaftor (ETI), which effectively increase CFTR function in multiple organs including the lungs. However, single-cell RNAseq studies of CF children’s nasal cells have shown that ETI only partially restored the expression of inflammatory genes in CF immune cells (neutrophils and macrophages) [177]. Consistent with this, ETI reduces the activities of NE, PR3, and cathepsin G and decreases levels of IL-1β and IL-8 but does not fully normalize the inflammatory profile in CF patients [178]. Importantly, CF neutrophils continue to exhibit functional defects, including poorly resolving inflammatory responses, impaired bacterial clearance, excessive degranulation, and elevated store-operated Ca2+ entry [179–181].

Several studies have shown reduced CXCR1 expression on neutrophils in airway inflammatory diseases, particularly in CF patients [182]. IL-8 promotes neutrophil bacterial killing via CXCR1 but not CXCR2 [183]. In CF, excessive airway proteases cleave CXCR1, impairing neutrophil bacterial-killing function [183, 184]. This protease-dependent cleavage generates soluble glycosylated CXCR1 fragments that stimulate IL-8 production in bronchial epithelial cells through TLR2 activation. Notably, inhaled AAT can inhibit proteases, restore CXCR1 expression, and improve neutrophil bacterial killing in vivo [185]. Thus, CXCR1 cleavage and the activity of its soluble fragments represent a specific pathogenic mechanism in CF and potentially other chronic neutrophilic lung diseases. New therapeutic strategies beyond long-term NSAID and steroids include anti-cytokine or anti-cytokine receptor strategies to target oxidative stress, cytokine secretion, and other dysregulated pathways to limit CF inflammation [186].

Bronchiectasis

Bronchiectasis is characterized by irreversible airway dilation and chronic neutrophilic inflammation driven by persistent infection, impaired mucociliary clearance, and a self-sustaining protease-antiprotease imbalance [187]. Neutrophils dominate the inflammatory infiltrate, releasing NE, PR3, and MPO, which degrade epithelial junctions and extracellular matrix, promote mucus plugging, and generate collagen fragments (e.g., PGP) that recruit additional neutrophils via CXCR1/CXCR2 [96]. Chronic airway infection with bacteria and, to a lesser extent, infection with fungi or viruses, contributes to the vicious inflammation in bronchiectasis [188]. The most common pathogens isolated from patients with bronchiectasis are Pseudomonas aeruginosa and Haemophilus influenzae [189]. These pathogens stimulate mucin overproduction as shown in a human mucoepidermoid carcinoma cell line [190], exacerbate inflammation, and release chemotactic factors that attract neutrophils [187, 189].

Frequent bacterial colonization drives neutrophils into a dysfunctional, hyperactivated state, characterized by impaired bacterial killing but excessive degranulation and NET formation, which contribute to bronchial wall damage and recurrent exacerbations [191]. Sputum and BAL levels of NE activity and NET markers correlate with disease severity, lung function decline, and exacerbation frequency in bronchiectasis [192, 193].

In bronchiectasis, neutrophils are reprogrammed both transcriptionally and functionally, adopting heterogeneous activation states likely driven by chronic infection, epithelial DAMP signaling, and an altered lung microbiome [194, 195]. Compared with healthy individuals, blood neutrophils from stable bronchiectasis patients show prolonged survival, delayed apoptosis, increased CD62L shedding, upregulated CD11b, elevated MPO release, and impaired phagocytosis [196]. Both blood and airway neutrophil phagocytic and bactericidal functions are reduced at the onset of exacerbations but recover after antibiotic treatment. This neutrophil plasticity creates a paradox: neutrophils are hyperactivated, with elevated NE and NET formation correlating with radiological severity, sputum volume, and exacerbation risk, yet they show impaired antimicrobial function, including reduced phagocytosis and bacterial killing. These findings have promoted development of two main therapeutic strategies: limiting protease activity (e.g., cathepsin C or NE inhibitors) and targeting PGP-CXCR signaling or NET formation/clearance (Table 1). Key questions remain about causality, the reversibility of neutrophil dysfunction, and optimal patient stratification for targeted therapies.

Pulmonary Fibrosis (Including Idiopathic Pulmonary Fibrosis and Secondary Fibroses)

Pulmonary fibrosis is a progressive lung disease characterized by scarring and stiffening of lung tissue, which reduces lung elasticity and impairs respiratory function. Disease progression varies among patients [74]. Although pulmonary fibrosis has traditionally been considered a fibroblast-driven disorder, growing evidence indicates that neutrophils and NETs contribute to fibrotic processes.

Activated neutrophils and NET components, including DNA, histones, NE, MPO, and other proteases, stimulate lung fibroblasts to adopt myofibroblast phenotypes, enhance TGF-β activation, and increase collagen and MMP production [90]. NETs also act as scaffolds that concentrate profibrotic enzymes and cytokines and can polarize macrophages toward profibrotic phenotypes [90]. Neutrophils may also contribute to tissue repair. NETs can localize enzymes and cytokines that support tissue remodeling, while neutrophil-derived EVs can modulate fibroblast activity and promote repair [90, 197]. Clinically, elevated neutrophil counts, NET biomarkers, and neutrophil-associated proteases correlate with disease progression in patient cohorts [198]. These findings support neutrophil-targeted strategies, such as NET inhibitors, DNase, PAD4 inhibitors, and blocking neutrophil-fibroblast crosstalk, as complementary antifibrotic approaches alongside current therapies like nintedanib and pirfenidone.

Pulmonary Hypertension

Pulmonary hypertension (PH) comprises conditions that elevate blood pressure in the pulmonary arteries [199]. This results from sustained vasoconstriction, abnormal vascular remodeling, and in situ thrombosis, which increase pulmonary vascular resistance [200]. Lung remodeling is driven by factors such as hypoxia, inflammation, shear stress, and genetic predisposition, creating a self-perpetuating cycle of worsening pulmonary hemodynamics and vascular changes [201, 202].

Although studies are limited, neutrophils have been implicated in the pathogenesis of PH. In hypoxia- and monocrotaline-induced PH models in rats, neutrophil numbers are increased [203] and components of NETs, such as MPO and NE, are upregulated in patients with PH [204]. NE has been detected within pulmonary arterial smooth muscle cells (PASMCs) and in neointimal lesions (areas of abnormal cell proliferation and tissue remodeling) in both PH patients and animal models [205]. NE activates MMPs and drives ECM degradation, disrupting vascular integrity and promoting remodeling. It also suppresses bone morphogenetic protein receptor type 2 (BMPR2) signaling, which is essential for pulmonary vascular homeostasis [206]. These effects result in excessive PASMC proliferation, endothelial apoptosis, and vascular remodeling, ultimately increasing pulmonary vascular resistance. Additionally, the neutrophil-to-lymphocyte ratio in blood reflects subclinical inflammation and is associated with poor prognosis in PH as in other diseases [207, 208].

NETs contribute to PH by promoting endothelial dysfunction, activating coagulation, and stimulating pulmonary artery smooth muscle cell proliferation through NET-derived proteases and receptor-mediated signaling. They also enhance pro-inflammatory responses and angiogenesis, exacerbating PH via the MPO/H₂O₂/NF-κB/TLR4 signaling pathway [209, 210]. A recent PH trial showed that elafin improved pulmonary artery endothelial cell function and reversed PH [204]. Elevated circulating NET markers and neutrophil-derived proteases seem to worsen hemodynamics and right-heart strain in PH patients [211]. These findings suggest that NET- or protease-targeted therapies could help reduce vascular remodeling and thrombosis in selected PH patients.

Lung Cancer

Lung cancer is the leading cause of cancer incidence and mortality worldwide [212, 213]. Tumor-associated neutrophils (TANs) are a major component of the tumor microenvironment, promoting cancer cell growth, invasion, angiogenesis, and metastasis, with higher TAN levels correlating with poorer clinical outcomes [214–216]. However, TANs can exert both pro- and anti-tumor effects, depending on cancer type, stage, and characteristics of the tumor microenvironment [217]. TANs are highly plastic and can be either tumor-suppressive (N1) or tumor-promoting (N2), depending on local cytokines and metabolic signals [218, 219]. N1 neutrophils, activated by inflammatory stimuli such as LPS or IFN-γ, express high levels of activation markers (CD11b, CD66b, CD64) and CXCR2, which promotes recruitment to sites of inflammation [51, 220]. In contrast, N2 neutrophils, induced by anti-inflammatory factors like TGF-β, IL-4, or glucocorticoids, show elevated CD16, CD163, CD206, and CXCR4, allowing them to home to the bone marrow and lymphoid organs where they exert immunomodulatory effects [51, 221, 222]. Distinguishing N1 and N2 neutrophils remains challenging due to the lack of definitive markers, and their phenotypes are highly dynamic. Recent transcriptomic studies from lung cancer patients and lung cancer bearing mice have also identified additional TAN subsets, including immature, low-density, and interferon-responsive neutrophils, highlighting the complexity of neutrophil polarization in the tumor microenvironment [223, 224].

Protumor TAN activities include secretion of NE and matrix-remodeling enzymes that facilitate invasion, production of ROS and arginase that suppress T cells, NET-mediated trapping of circulating tumor cells to promote metastasis, and release of growth factors and EVs that enhance angiogenesis and tumor cell proliferation [225, 226]. Conversely, some TAN subsets show antitumor cytotoxicity and antigen-presenting functions, particularly in early lesions. Because TAN phenotype is plastic, therapeutic strategies under investigation aim to reprogram TANs (e.g., block TGF-β, inhibit CXCR2, or enhance IFN signaling), neutralize protumor neutrophil products (NETs, NE), or combine neutrophil-targeted agents with checkpoint inhibitors to improve immunotherapy responses [227, 228].

Conclusion

Neutrophils are versatile regulators of lung immunity, capable of rapid adaptation to local environmental signals. While their plasticity allows effective host defense, dysregulated neutrophil activation drives chronic inflammation, tissue damage, and fibrosis across a range of pulmonary diseases, including COPD, AATD, asthma, bronchiectasis, pulmonary fibrosis, and lung cancer. Emerging evidence from single-cell and multi-omics studies highlights the functional heterogeneity of neutrophils, revealing both pathogenic and reparative subsets. Targeted therapies, such as CXCR2 antagonists, protease inhibitors, and modulators of neutrophil-derived EVs, hold promise for restoring the balance between harmful and beneficial neutrophil functions. Combining mechanistic insights with clinical research will be essential for developing precision therapies that leverage neutrophil plasticity to improve lung health.

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

Funding Sources

This study was funded by German Centre of Lung Research (Reference No. 82DZL002C1).

Author Contributions

S.M.J.: conceptualization, supervision, and writing – original draft preparation; J.C.-W. and B.O.: conceptualization and writing – reviewing and editing; and S.W.: visualization and writing – original draft preparation.

Funding Statement

This study was funded by German Centre of Lung Research (Reference No. 82DZL002C1).