Role of inflammasomes in the pathogenesis of periodontal disease and therapeutics

Abstract

Inflammasomes are a group of multimolecular intracellular complexes assembled around several innate immune proteins. Recognition of a diverse range of microbial, stress and damage signals by inflammasomes results in direct activation of caspase-1, which subsequently induces the only known form of secretion of active interleukin-1β and interleukin-18. Although the importance of interleukin-1β in the periodontium is not questioned, the impact of inflammasomes in periodontal disease and its potential for therapeutics in periodontology is still in its very early stages. Increasing evidence in preclinical models and human data strongly implicate the involvement of inflammasomes in a number of inflammatory, autoinflammatory and autoimmune disorders. Here we review: (a) the currently known inflammasome functions, (b) clinical/preclinical data supporting inflammasome involvement in the context of periodontal and comorbid diseases and (c) potential therapies targeting inflammasomes. To clarify further the inflammasome involvement in periodontitis, we present analyses of data from a large clinical study (n = 5809) that measured the gingival crevicular fluid-interleukin-1β and grouped the participants based on current periodontal disease classifications. We review data on 4910 European-Americans that correlate 16 polymorphisms in the interleukin-1B region with high gingival crevicular fluid-interleukin-1β levels. We show that inflammasome components are increased in diseased periodontal tissues and that the caspase-1 inhibitor, VX-765, inhibits ~50% of alveolar bone loss in experimental periodontitis. The literature review further supports that although patients clinically present with the same phenotype, the disease that develops probably has different underlying biological pathways. The current data indicate that inflammasomes have a role in periodontal disease pathogenesis. Understanding the contribution of different inflammasomes to disease development and distinct patient susceptibility will probably translate into improved, personalized therapies.

Figure 1

Visualization of inflammasome activation by recognition of cytosolic DNA. Murine dendritic cells were lipopolysaccharide primed and stimulated with rhodamine‐labeled poly‐dAdT DNA, resulting in Aim2 inflammasome activation. Confocal images show an overlay of pseudo‐colored ASC (blue), DNA (red) and Aim2 (green) in the cytosol of a cell. Methods described in Swanson et al27

Figure 2

Inflammasome priming and activation. Inflammasomes must be primed (signal 1) before activation (signal 2). First, a nuclear factor‐κB‐activating stimulus, such as lipopolysaccharide or tumor necrosis factor‐β, induces elevated expression of inflammasome components (IL1B,IL18,CAS1 AIM2,NLRP3,IFI16), which leads to increased expression of these proteins. After priming, inflammasome activation requires a second, specific signal to activate each individual inflammasome and lead to the formation of unique inflammasome complexes. On receiving an activating signal, inflammasome sensors multimerize, and recruit ASC and pro‐caspase‐1, promoting the autoactivation of caspase‐1. Interleukin‐1β and interleukin‐18 are synthesized as proproteins that are processed by caspase‐1 into active forms and are secreted by cells. Although the mechanism(s) of secretion are not fully elucidated, some interleukin‐1β and interleukin‐18 secretion occurs via gasdermin D‐pores, as shown172

Figure 3

Model of NLRP3 inflammasome. After priming, NLRP3 oligomerizes upon sensing cell stress. NEK7 binds to the NLRP3 leucine‐rich repeat stabilizing the NLRP3 oligomer, which forms a platform of pyrin domains that induces ASC filament formation via pyrin domain‐pyrin domain interactions. Multiple ASC molecules promote caspase‐1 binding and filament assembly through CARD‐CARD interactions. Abbreviations: NEK7, NIMA‐related kinase 7; LRR, leucine‐rich repeat; NBD, nucleotide‐binding domain; PYD, pyrin domain; CARD, caspase activation and recruitment domain

Figure 4

Model of AIM2 inflammasome after assembly. After priming, AIM2 oligomerizes upon binding to DNA through its HIN200 domain and forms a platform of pyrin domains that induces ASC filament formation via pyrin domain‐pyrin domain interactions. Multiple ASC molecules promote caspase‐1 binding and filament assembly through CARD‐CARD interactions. Abbreviations: HIN200, hematopoietic expression, interferon‐inducible nature and nuclear localization; LRR, leucine‐rich repeat; NBD, nucleotide‐binding domain; PYD, pyrin domain; CARD, caspase activation and recruitment domain

Figure 5

Inflammasome components are increased in experimental periodontitis. Periodontitis was induced in mice via the ligature model for 10 days, as described previously.173 Representative images of maxilla micro‐computed tomography showing (A) baseline (no ligature) and (B) alveolar bone loss that developed with disease induction. (C) Alveolar bone quantification shows significant bone loss after 10 days of ligature placement. (D) mRNA expression of inflammasome components are significantly higher in gingival tissues at 10 days postligature when compared with nonligated animals. *P < .05, **P < .01 as compared with no ligature control

Figure 6

Expression of IFI16 and CD14 in human gingival tissues derived from an individual with chronic periodontitis. A, Immunohistochemistry of IFI16 and CD14 in the inflammatory infiltrate; B, colocalization of IFI16 and CD14 with DAPI for nuclei staining. Yellow arrows show representative cells expressing CD14 and IFI16

Figure 7

Gingival crevicular fluid‐interleukin‐1β log levels stratified by new periodontal disease classifications in individuals from the Dental Atherosclerosis Risk in Communities (Dental ARIC) study (n = 5809). A, 2017 Classification of Periodontal Conditions117, 118 B, periodontal profile phenotype.119, 120, 121 Generalized linear models adjusted for age, sex, study center, race, smoking and diabetes history. The data were log‐transformed because they were not normally distributed. WW17 stages were as follows: incipient, moderate, severe and advanced disease; periodontal profile class classification was as follows: stage I –health/incidental disease; stage II – mild disease; stage III – moderate disease; stage IV – severe disease; stage V – mild tooth loss/hi GI; stage VI – moderate tooth loss; stage VII – severe tooth loss. ***P < .001 as compared with stage I

Figure 8

Caspase‐1 inhibition blocks approximately 50% of alveolar bone loss in mice. One day before the ligature placement, wild‐type male mice (n = 5‐7) started receiving a twice a day oral delivery of vehicle (DMSO) or caspase‐1 inhibitor (VX‐765 at 100 mg/kg) for 10 days. After 1 day of drug delivery, mice were induced for experimental periodontitis for 9 days based on a previously described protocol.155 A‐C, Mean representative images of each group showing alveolar bone loss at 9 days postligature placement. D, Measurements taken from the alveolar bone crest (ABC) to the cementum‐enamel junction (CEJ) show significant inhibition of alveolar bone loss as compared with vehicle control. *P < .05, ***P < .001