Orchestration of Neutrophil Extracellular Traps (NETs), a Unique Innate Immune Function during Chronic Obstructive Pulmonary Disease (COPD) Development

The paper addresses how neutrophil extracellular traps (NETs)—a unique antimicrobial mechanism of neutrophils—contribute to the development and pathophysiology of chronic obstructive pulmonary disease (COPD). COPD is a progressive, heterogeneous respiratory disease characterized by persistent airflow limitation, emphysematous changes, neutrophilic inflammation, and recurrent infections. Neutrophils are key innate immune cells recruited to COPD airways, where they can defend against pathogens by phagocytosis, degranulation, and NET release. NETs are web-like structures of decondensed chromatin decorated with antimicrobial proteins. While protective against microbes, excessive or dysregulated NET formation has been implicated in lung tissue damage, mucus plugging, airway obstruction, and disease exacerbation. The review aims to synthesize current knowledge on NET formation mechanisms (NOX-dependent and NOX-independent), their triggers present in COPD airways, their detrimental and beneficial roles, and therapeutic strategies (e.g., DNase, protease inhibitors, CXCR2 antagonists) to modulate NET-associated pathology in COPD.

The authors review epidemiology and risk factors of COPD (notably cigarette smoke exposure), and pathophysiology including protease/anti-protease imbalance, oxidative stress, goblet cell hyperplasia, and mucociliary dysfunction. They summarize inflammatory pathways engaging innate and adaptive immunity, highlighting neutrophils, macrophages, NK cells, T-lymphocytes, and complement (C3a/C5a). They detail neutrophil functions (phagocytosis, degranulation, NET formation), death pathways (apoptosis, autophagy, necrosis), and factors influencing the neutrophil decision to phagocytose versus form NETs (cytoskeleton integrity, pathogen size, opsonization, pH, platelet HMGB1, DEK, autophagy). The review explains NET biology: composition (DNA backbone, histones, granule proteins like NE and MPO, cytosolic mediators including PAD4), triggers (PMA, LPS, cytokines such as CXCL8 and TNF-α, bacteria, fungi, activated platelets), roles across infections and inflammatory diseases, and mechanisms of NETosis (NOX-dependent via PKC–Raf/MEK/ERK, ROS, NE/MPO; and NOX-independent via Ca2+ influx, mitochondrial ROS, PAD4-mediated histone citrullination). Prior COPD studies are summarized: microscopy confirms abundant NETs and NET-forming neutrophils in sputum of stable and exacerbated patients; extracellular DNA correlates inversely with lung function (FEV1) and directly with exacerbation frequency; PAD4 expression is upregulated; NET clearance by DNases and macrophages is impaired; LPS-stimulated NET production is increased in COPD neutrophils; CXCR2 signaling (via CXCL8 and ECM-derived ligands like PGP/AcPGP) regulates NETosis and neutrophil recruitment. They further review translational and clinical data on CXCR2 antagonists (AZD5069, MK-7123, SCH527123), and potential anti-NET approaches (recombinant DNase, heparin, hydroxychloroquine), and cytokine blockade (IL-1β, IL-17).

This is a narrative review synthesizing findings from previously published basic, translational, and clinical studies on NET formation and COPD. No original experimental methodology or data collection is reported.

- NETs are abundant in COPD airways and associate with disease severity: microscopy demonstrates NETs in >90% of exacerbated COPD patients vs ~45% of stable COPD patients; extracellular DNA/NETs inversely correlate with lung function (e.g., FEV1) and associate with exacerbation frequency and reduced microbiota diversity. - COPD sputum contains elevated NET components (cell-free DNA, MPO); PAD4 expression is increased in neutrophilic COPD and correlates with symptoms and lung function decline. - Neutrophils from COPD patients are primed for NETosis in inflammatory airway microenvironments: blood neutrophils exhibit ~5-fold increased NET formation when stimulated with autologous COPD sputum supernatant; this response is reduced by CXCR2 antagonism (AZD5069). - Mechanistic pathways: NETosis proceeds via NOX-dependent (PKC–Raf/MEK/ERK–ROS–NE/MPO) and NOX-independent (Ca2+ influx–mitochondrial ROS–PAD4-mediated histone citrullination) routes. Cigarette smoke extract can trigger NETs and impairs phagocytosis and apoptosis, promoting persistent neutrophilia; DAMP/TLR signaling during exacerbations augments neutrophil activation. - Pathogenic roles of NETs in COPD: direct cytotoxicity to epithelial/endothelial cells; increased mucus viscosity and airway obstruction due to extracellular DNA-rich plugs; promotion of fibroblast activation and airway remodeling; amplification of inflammation via NLRP3 inflammasome activation and CXCL8 release, creating a self-perpetuating neutrophilic cycle. - Therapeutic implications: CXCR2 blockade reduces neutrophilic inflammation and NET production (preclinical and early clinical signals including improved FEV1 and fewer exacerbations with MK-7123); DNase can degrade NET DNA to reduce mucus viscosity; protease inhibitors to neutralize NET-associated proteases; heparin antagonizes histone toxicity; hydroxychloroquine may modulate ROS/MMPs; targeting IL-1β and IL-17 pathways may disrupt neutrophilic NET-driven inflammation.

The reviewed evidence supports that excessive and persistent NET formation is a central driver of neutrophilic airway inflammation, mucus obstruction, tissue injury, and functional decline in COPD. NETs integrate multiple COPD triggers (cigarette smoke, pathogens, cytokines, ECM fragments) and, through NOX-dependent and NOX-independent pathways, generate cytotoxic DNA–protein meshes that both trap microbes and damage host tissues. Impaired clearance (defective DNase activity and macrophage dysfunction) and ongoing chemokine signaling (CXCL8/CXCR2) perpetuate NET abundance, linking neutrophil recruitment to NETosis and back to inflammation via inflammasome activation and additional chemokine release. These insights highlight NETs as actionable therapeutic targets: modulating neutrophil trafficking and activation (CXCR2 antagonists), degrading/de-toxifying NET components (DNase, histone antagonism), and interrupting cytokine axes (IL-1β/IL-17) could reduce NETopathic inflammation without abolishing essential host defense. Such strategies directly address the unmet need for anti-inflammatory disease-modifying therapies in COPD.

NET discovery has reshaped understanding of neutrophil-mediated innate immunity in COPD. While NETs are antimicrobial, their dysregulated formation contributes to airway obstruction, tissue damage, and exacerbations. The review proposes that therapeutic focus should be on stabilizing and fine-tuning NET activity—rather than complete suppression—through approaches such as CXCR2 antagonism, DNase-mediated DNA degradation, protease inhibition, and cytokine pathway blockade. Advancing these concepts requires deeper mechanistic studies of NET regulation in COPD, clarification of causality between NETs and disease progression, and well-designed clinical trials to evaluate NET-targeted interventions as innovative, potentially disease-modifying therapies for COPD.

- Causality between NET abundance and COPD progression/exacerbations remains unclear; many data are associative. - Drivers of NET formation in vivo are difficult to disentangle because multiple triggers (e.g., CXCL8, TNF-α, bacteria, ECM fragments) coexist in COPD airways even at baseline. - Mechanistic details of where and when NETosis occurs (in circulation vs lung tissue) in COPD are unresolved. - NET clearance is impaired (defective DNase activity, dysfunctional macrophages), but the extent and clinical impact need further validation. - Conflicting data exist on the influence of current smoking status on NET formation in COPD. - As a narrative review, no new experimental data are provided; potential therapies discussed require rigorous clinical evaluation.