Introduction

Periodontitis is a chronic and destructive inflammatory disease that adversely affects the supporting structures of the teeth, including the gingiva, periodontal ligament, and alveolar bone. It impacts approximately 10% of the global population, highlighting the urgent need for effective understanding and control of this disease. The disease arises from a multifaceted interplay between periodontal bacteria and host immune responses. Central to the pathological processes of periodontitis is the role of polymorphonuclear neutrophils (PMNs), which are crucial components of human innate immunity. An important feature of the PMN response is the formation of neutrophil extracellular traps (NETs), which exhibit both protective and pathogenic roles in periodontitis. Recently, NETs have been increasingly recognized for their dual role in both protecting against and contributing to the pathology of periodontal diseases.    

Polymorphonuclear Neutrophils: Guardians of Oral Health

PMNs are the predominant neutrophil population in the bloodstream and are characterized by their ability to migrate to sites of tissue damage and infection. Upon activation, PMNs can execute multiple functions such as phagocytosis, degranulation, and the production of reactive oxygen species (ROS), which confer antimicrobial properties. In the context of oral health, these immune responses orchestrated by PMNs are essential for eliminating microbial threats and maintaining periodontal tissue integrity.

The recruitment of PMNs to the oral cavity occurs predominantly through the gingival crevice during inflammatory responses. The number of oral PMNs (oPMNs) correlates positively with the severity of oral inflammation. Abnormalities in PMN function or numbers can exacerbate periodontal disease and facilitate the progression from healthy to diseased states.

Neutrophil Extracellular Traps: Formation and Function

Neutrophil extracellular traps (NETs) are web-like structures composed of decondensed DNA and antimicrobial proteins that trap and neutralize pathogens. Different stimuli, including pathogens and host-derived mediators, can induce the formation of NETs via distinct mechanisms. There are primarily three pathways through which NETs can form:

Fig. 1 Three types of NET formation (Wang J., et al. 2021).

Classic NET Formation: In this well-characterized process, upon activation, PMNs undergo a form of programmed cell death that culminates in the release of nuclear chromatin mixed with cytoplasmic contents. This process is orchestrated by protein-arginine deiminase 4 (PAD4), which facilitates histone deimination, chromatin decondensation, and ultimately, NET release.

Vital NET Formation: Discovered later, this process involves the release of NETs from viable PMNs in response to specific stimuli, such as granulocyte/macrophage colony-stimulating factor (GM-CSF). During this pathway, mitochondrial DNA may also contribute to NETs, and PMNs remain alive post-release.

Mitochondrial NET Formation: Following a distinct activation pathway, this process results in the expulsion of mitochondrial DNA to form NETs, which takes less time compared to classic NET formation and also retains PMNs in a viable state.

NETs exert crucial antimicrobial activities by trapping and killing a variety of pathogens, including bacteria, fungi, and viruses. The structural features of NETs facilitate the binding of microorganisms, inhibiting their movement while delivering antimicrobial proteins capable of destroying them. However, the role of NETs is complex as they can also contribute to tissue damage and perpetuate inflammation if not properly regulated.

Pathogenesis of Periodontitis: The Role of NETs

Periodontitis is theorized to originate from the accumulation of dysbiotic microbial biofilms at the gingival margins-a condition whereby the composition of oral microbiota shifts from a health-promoting to a disease-promoting state. This dysbiosis triggers sustained immune responses, with PMNs playing a critical role in the development of this chronic inflammatory condition. In periodontitis, the imbalance in PMN activity can lead to chronic inflammation, severe tissue destruction, and progressive loss of periodontal support structures.

Research shows that oPMNs exhibit increased activation, longevity, and higher NET formation capabilities in periodontitis compared to healthy individuals. These activated oPMNs can exacerbate inflammation through excessive NET production, which may lead to an unresolved inflammatory state and further tissue injury. As the disease progresses, periodontal pathogens have developed sophisticated mechanisms to evade and undermine the bactericidal functions of NETs, thereby perpetuating a cycle of dysbiosis and inflammation.

NETs: A Double-Edged Sword

While NETs serve protective roles in trapping pathogens, their excessive formation can contribute to periodontal tissue damage and complicate the disease. In the pathological environment of periodontitis, high levels of NETs can trap inflammatory mediators, cellular debris, and bacteria within the periodontal pockets, hindering clearance and fostering a persistent inflammatory state. This prolonged exposure to NETs can lead to additional tissue destruction, and as the inflammation continues, the cycle of tissue injury and immune cell recruitment exacerbates the disease.

Moreover, the presence of NETs can inadvertently promote the survival and colonization of periodontal pathogens. Specific bacteria are capable of producing nuclease enzymes that degrade the DNA within NETs, thereby neutralizing their antimicrobial effects. For instance, studies have shown that strains such as Porphyromonas gingivalis can produce specific virulence factors that impair the functions of NETs, contributing to their survival and dominance in dysbiotic microbial communities.

Therapeutic Approaches to Treating Periodontitis

Given the intricate relationship between NETs, periodontal pathogens, and host immunity, the treatment of periodontitis must carefully consider the modulation of inflammation and NET levels. Several therapeutic strategies have been proposed based on understanding the roles of NETs and PMNs in periodontal disease.

Regulating NET Formation: One approach focuses on inhibiting excessive NET formation. Selective PAD4 inhibitors have shown promise in vitro by moderating the formation of NETs, suggesting their potential as therapeutic agents. Additionally, the neonatal NET-inhibitory factor (nNIF) has been suggested as a therapeutic agent to modulate NET generation effectively by blocking key events leading to their formation.

Degrading NET Components: Utilizing DNase enzymes to degrade NETs has been explored. Treatment with DNase can clear excess NETs, subsequently reducing inflammation and tissue damage in various preclinical models. This strategy aims to re-establish the homeostasis of the periodontal pocket environment.

Conclusion

The intricate relationship between PMNs, NETs, and periodontal pathogens plays a pivotal role in the pathogenesis of periodontitis. While NETs serve to trap and neutralize pathogens, their excessive formation can lead to persistent inflammation and tissue damage, establishing a vicious cycle that exacerbates periodontal disease. As research continues to unfold, targeting NETs and modulating the immune response may offer innovative therapeutic options for the management of late-onset periodontitis.