Lung disease is the leading cause of morbidity and mortality in cystic fibrosis (CF) patients. CF is caused by recessive mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene, which encodes a chloride/bicarbonate anion channel. Defective CFTR in respiratory epithelium disrupts ion transport, pH regulation, airway surface hydration, and mucociliary clearance. This leads to persistent lung injury due to infections (bacterial, fungal, viral), a pro-inflammatory environment, massive neutrophil influx, and the release of neutrophil extracellular traps (NETs), resulting in the accumulation of extracellular DNA and cytotoxic peptides. Neutrophil infiltration is driven by cytokines and chemokines, including IL-8, TNF-α, IL-1β, complement-derived chemoattractants (e.g., C5a), and lipid mediators. These mediators sustain neutrophil influx and increase pro-inflammatory activity. Neutrophils kill microbes via phagocytosis, degranulation, and NETosis. During NETosis, neutrophils release decondensed chromatin coated with elastase, myeloperoxidase (MPO), and other cytotoxic proteases. These components perpetuate severe inflammation, increasing mucus viscosity and impairing mucociliary clearance, leading to airway damage. Peribronchial neutrophilic infiltration is observed in CF infants even before infection. Inflammation progressively increases with age, worsening mucus plugging and leading to chronic infection and lung damage, often requiring lung transplantation. However, lung transplant survival is low due to rejection, infections, and the development of CLAD or BO. Airway infection, cytokines, and ASL pH affect neutrophil activity and NETosis. A better understanding of NETosis mechanisms, including pH effects, metabolic pathways, kinases, channels, ROS, and transcription regulation, is needed to identify therapeutic targets for controlling excess NETosis without compromising neutrophil antimicrobial functions. Screening FDA-approved drugs on CF neutrophils could help identify potential NETosis-regulating drugs.

The review section extensively covers existing literature on cystic fibrosis (CF), focusing on the role of neutrophils and NETs in disease progression. It cites numerous studies examining the inflammatory processes, the impact of CFTR mutations, and the effects of different bacterial infections on airway inflammation. The review also discusses the various antimicrobial mechanisms of neutrophils, including phagocytosis, degranulation, and NETosis, highlighting the contribution of NETs to mucus viscosity and lung damage. The importance of understanding the molecular mechanisms of NETosis, including the role of ROS, kinases, and intracellular pH, is emphasized as a critical step towards developing novel therapeutic strategies. The discussion on existing therapies, including DNase treatment, is also included, along with the limitations and potential improvements to these therapies.

This is a review article and therefore does not utilize original experimental data or methodology. The authors gathered and reviewed existing literature on cystic fibrosis (CF), focusing on the role of neutrophils and NETs in disease progression. The scope of the review encompasses the pathophysiology of CF, focusing on inflammatory processes, CFTR mutations, bacterial infections, and their contribution to airway inflammation and tissue damage. A detailed description of the various antimicrobial mechanisms of neutrophils, along with the role of NETs in disease severity, has been extensively documented. The literature review includes studies on the molecular mechanisms of NETosis and existing therapies and potential improvements for NET degradation, particularly using DNases. The review also discusses future directions and therapeutic potential for treating CF lung disease, including potential approaches for modulating NETosis. No original data or experiments were conducted in this review paper, with the authors conducting a comprehensive literature review and synthesis of findings to draw conclusions.

The review emphasizes that CFTR mutations lead to dysfunctional neutrophils that are less efficient at clearing bacterial infections, contributing to chronic inflammation. This inflammation is exacerbated by NETosis, an immune process where neutrophils release DNA structures coated with cytotoxic proteins. These NETs contribute to increased mucus viscosity and tissue damage. The study highlights that the CF airway milieu promotes neutrophil survival and aberrant NETosis, rather than apoptosis. Two NETosis pathways are described: a NOX-dependent pathway triggered by pathogens and their components, and a NOX-independent pathway triggered by sterile injury or inflammation. The review notes that CF neutrophils may be more susceptible to NETosis via both pathways due to their unique characteristics. The review also explores the therapeutic potential of DNase, which degrades the DNA in NETs, thereby reducing mucus viscosity and promoting clearance. However, DNase is inhibited by actin, abundant in CF airways, thus requiring improvements. The role of macrophages in clearing NETs is also discussed, suggesting that their impaired function in CF contributes to NET accumulation. The limitations of current therapies, such as protease inhibitors, are highlighted. The paper suggests that new therapies targeting NET formation and improving NET clearance are crucial for improving outcomes in CF. Specifically, they highlight the need for deeper understanding of molecular mechanisms including kinases, ion channels, and intracellular pH to effectively control excessive NETosis without compromising the positive effects of neutrophils.

The review successfully connects the dysfunctional CFTR protein to the observed neutrophil dysfunction in CF airways, highlighting the interplay between genetic defects and immune responses. The detailed explanation of the two NETosis pathways, their triggers, and their contribution to CF lung disease provides a mechanistic framework for understanding the disease's progression. The discussion of current therapies and their limitations creates a compelling case for the need for innovative therapeutic strategies targeting NET formation, degradation, and clearance. The emphasis on future directions for research, including the investigation of molecular mechanisms and drug screening, highlights the translational potential of the review's findings. The review contributes significantly to the field by providing a comprehensive overview of the current understanding of the role of neutrophils and NETs in CF lung disease and highlighting the urgent need for improved therapies.

This review highlights the critical role of neutrophils and NETs in the progression of CF lung disease. Dysfunctional CFTR in neutrophils leads to impaired bacterial clearance and chronic inflammation, exacerbated by excessive NET formation. Current therapies like DNase have limitations. Future research needs to focus on understanding the molecular mechanisms of NETosis and identifying therapies that control excessive NETosis while preserving beneficial neutrophil functions. This includes investigating the effects of pH modulation and exploring new approaches to regulate T cell-neutrophil interactions to reduce pulmonary exacerbations.

As a review article, this study's limitations are inherent to its reliance on existing literature. The conclusions drawn are based on the available data and may be influenced by publication bias or gaps in research. The review doesn't conduct original research to validate its findings; it relies on the quality and consistency of the existing research literature. Future experimental studies might reveal additional factors influencing NETosis or the efficacy of different therapeutic interventions.