Pro-resolving mediators in the regulation of periodontal disease

Abstract

Periodontitis is an inflammatory disease of the supporting structures of the dentition that is initiated by bacteria that form a biofilm on the surface of the teeth. The pathogenesis of the disease is a result of complex interactions between the biofilm and the host response that results in dysbiosis of the microbiome and dysregulation of the inflammatory response. Current data suggest that the excess inflammation associated with periodontitis is due to a failure of resolution of inflammation pathways. In this review, the relationship between inflammation and microbial dysbiosis is examined in the context of pro-inflammation and pro-resolution mediators and their ability to modify the course of disease. The impact of local oral inflammation on systemic inflammation and the relationship of periodontitis to other inflammatory diseases, including type 2 diabetes and cardiovascular disease is reviewed. Active resolvers of inflammation, including the lipoxins and resolvins, show great promise as therapeutics for the treatment of periodontitis and other inflammatory diseases.

Figure 1. Characteristics of Periodontitis

1A: Anatomy of the normal periodontium illustrates the components of the normal periodontal organ. Dental plaque accumulation in the gingival sulcus initiates inflammation of the gingiva that can lead to loss of connective tissue collagen, destruction of the periodontal ligament fibers that traverse from the bone and insert into root surface cementum with active loss of alveolar bone. 1B: The clinical and radiographic picture of mild gingivitis vs. advanced periodontitis. The upper panel illustrates mild swelling and redness of the gingiva in mild gingivitis (arrows); radiographs illustrate normal alveolar bone height with no apparent bone loss. The lower panel shows significant redness and swelling of the gingiva that easily bleeds with gentle probing in advanced periodontitis. The radiographs show loss of 60–70% of the alveolar bone height.

Figure 2. Resistance of 15-LO TG rabbits to periodontal destruction

Periodontitis was induced in transgenic and wild type rabbits with ligature and P. gingivalis as described in the text. Ligature alone served as control. Upper photos: A- Clinical outcome 6 weeks after induction of periodontitis. B- Clinical picture of a TG animal that received P. gingivalis following the same regimen as the animal in A. C- Wild type animal receiving ligature alone; no P. gingivalis, no appreciable bone loss. Lower panel: 15-LO-overexpressing transgenic rabbits exhibit reduced leukocyte recruitment to skin and reduced plasma extravasation (quantified using Evans blue from skin biopsies) with LTB4. MPO activity was monitored in rabbit skin as an index of neutrophil infiltration. Wild type rabbits (solid line) exhibited significantly higher (p < 0.05) levels of leukocyte recruitment (A) and leukocyte-dependent plasma leakage (B) in response to LTB4 compared with 15-LO transgenic rabbits (dashed line). Statistical significance was not observed with human rIL-8 addition to the skin (C and D).

Figure 3. Prevention of Porphyromonas gingivalis Associated Periodontitis

The impact of local application of an aspirin-triggered lipoxin analog (LxA) on experimental periodontitis in rabbits induced by topical application of the human pathogen Porphyromonas gingivalis was quantified by direct measurements of alveolar bone destruction. The photographs in the upper panels illustrate the marked reduction of soft tissue inflammation and alveolar bone loss that result from topical application of 4 μl of a 1 μg/ml LXA solution in PBS three times per week. The quantitative data below illustrate that topical application of LxA prevented 95% of the alveolar bone loss compared to PBS alone. Ligature alone did not cause significant bone loss indicating the significant pro-inflammatory properties of the human pathogen.

Figure 4. Treatment of Periodontitis with Resolvin E1

Periodontitis was established in the rabbit with ligature and Porphyromonas gingivalis for 6 weeks that resulted in significant alveolar bone loss at 6 weeks (Panel A). Beginning at 6 weeks, Resolvin E1 was applied topically three times per week (4 μl of a 1 μg/ml solution in PBS) for an additional 6 weeks. The vehicle treated group exhibited further bone loss (Panel B and D), while the RvE1 treated group exhibited regrowth on bone to near pre-disease levels (Panels C and D).

Figure 5. RvE1 and the Dysbiotic Microbiome

Periodontitis was induced in the rat model with ligature alone to monitor natural dysbiotic shifts of the rat microbiome in response to inflammation (Panel A). The established periodontitis was treated with topical RvE1, which reserved alveolar bone loss (inset). The impact of RvE1 treatment on the microbiome at 32 and 40 days was compared to vehicle treatment and expressed as percent mean relative difference (Panels B and C, respectively). The data suggest that control of excess inflammation reverses the pathogenic shift of the microbiome. The complexity of the microbiome (alpha diversity) and the bacterial biomass was also reduced (data not shown). (Adapted from Lee et al. 2016 (54)).

Figure 6. Alveolar Bone Regeneration in a Large Animal Model

To assess the impact of therapeutic resolution of inflammation in a clinical periodontology relevant setting, the impact of locally delivered lipoxin analog in a targeted slow release nanoparticle was evaluated after periodontal surgery. Periodontitis treatment comprising surgical debridement was performed to treat infrabony pockets in a minipig. Benzo-lipoxin A₄ was formulated in de-humanized neutrophil microparticles designated nano-proresolving medicines (NPRM). The NPRM was compared to surgery alone, lipoxin analog alone and microparticles alone. Quantification of regenerated bone volume (top panel) revealed that surgical debridement alone had not regeneration of bone. The benzo-lipoxin analog alone and the microparticles alone exhibited some limited activity. The NPRM generated significantly greater volume of alveolar bone in the model. The lower radiographs show that the notch marking the bone height at time of surgery was covered by new bone after NPRM treatment with no activity in the surgery alone control.

Figure 7. Failure of Resolution Pathways in Type 2 Diabetes-associated Periodontitis

To assess the role of resolution of inflammation in T2D-associated periodontitis, the ERV1 transgenic mouse overexpressing the receptor for RvE1 was employed in gain-of-function studies (95). Periodontitis was induced in wild type (Panel A), ERV1 transgenic (panel B), T2D (db/db leptin receptor deficient, Panel C) and T2D (db/db) mice overexpressing ERV1 (Panel D) by P. gingivalis gavage). Bone loss area is quantified in the right panel. ERV1 transgenic mice are protected from periodontitis (Panel A vs C). T2D-associated periodontitis is more severe that in Wild type (Panel A vs. B). Overexpression of ERV1 in T2D mice affords protection from T2D-associated periodontitis implicating a failure of resolution pathways in T2D pathology and suggesting that failure of resolution can be therapeutically corrected with addition of RvE1 in T2D as it is in periodontitis.

Figure 8. Atherogenesis that is accelerated by Periodontitis is slowed by RvE1

0.5% cholesterol in the New Zealand White Rabbit diet induces atherogenic changes in the aorta characterized by fatty streaks that is worsened by concomitant periodontitis (136) (Panel A). The ability of RvE1 treatment of periodontitis to reverse fatty streak development is seen with increased doses of RvE1. Interestingly, in the absence of periodontitis, local application of RvE1 to the gingiva (4 μg) offered significantly greater protection from atheromatous changes (Panel B). The data confirm that periodontitis has a significant impact on the progression of atheromas in this model and suggest a potential therapeutic benefit for regulation of inflammation with resolvins in atherogenesis. [Adapted from (138)]

Figure 9. Actions of SPMs in periodontitis

Several of the known agonists of resolution of inflammation have been investigated in periodontal diseases, including lipoxins, E and D series resolvins, and maresins. The cell type and known actions of the SPM are compared to the reported actions in periodontal diseases.