Inflammasomes as contributors to periodontal disease

Abstract

A genome-wide association study of ≈2.5 million markers identified unique biologically informed periodontal complex traits with distinct microbial communities and interleukin-1β (IL-1β) levels. Each trait was associated with different single nucleotide polymorphisms. These variants include genes associated with immune responses, microbial colonization, and the epithelial barrier function. The specific set of variants leads to individual biological paths that converge into an overlapping clinical phenotype of periodontal tissue destruction. This concept suggests that periodontal disease is a group of distinct conditions. We identified polymorphisms in inflammasome genes interferon gamma inducible protein 16 (IFI16) and absent in melanoma 2 (AIM2) that were associated with increased severity of periodontal disease. Inflammasomes respond to pathogen or tissue "danger" signals and assemble into multiprotein "machineries" that are essential for the cleavage of proinflammatory mediator IL-1β into an active form. Thus, understanding how variants of IFI16 and AIM2 contribute to periodontal disease pathogenesis may lead to treatment options that address individual biological variations and precision therapies for oral health.

Keywords: AIM2; IFI16; Ifi204; alveolar bone loss; caspase inhibitors; caspase-1; inflammasome; periodontitis.

FIGURE 1

Variation in the subgingival inflammatory response upon the same microbial challenge. Schematic representation of differences in inflammatory mediator production and immune cell infiltration among individuals affects the response to gingival microbial burden. The middle line represents individuals with a normal response. The top line represents hyperresponsive individuals who would exhibit clinical symptoms of disease at a lower threshold of microbial burden. The bottom line represents hyporesponsive individuals who would be relatively resistant to periodontitis. Inspired by Champagne et al. 2003²

FIGURE 2

Characteristics of the three main periodontal complex traits used to identify loci associated with periodontal disease susceptibility. Reprinted with permission from Offenbacher et al. (2016). ¹ *SNPs significantly associated with a periodontal complex trait (PCT).

FIGURE 3

Immunohistochemical detection of IFI16 and AIM2 in human gingival tissues. Images of tissue sections from a healthy individual (A through F) and an individual with periodontal disease (G through L) according to the American Academy of Periodontology classification, ²⁶ stained with antibodies recognizing the indicated proteins. A, D, G, J represent original magnification × 10 (scale bar = 200 mm); B, C, E, F, E, H, K, I, L represent original magnification × 40 (scale bar = 50 mm) of the square inserts located in the figures with original magnification × 10 in the epithelial and connective tissue layer. Green arrowheads = epithelial cells; black arrowheads = fibroblasts; yellow arrowheads = leukocytes; red arrowheads = endothelial cells

FIGURE 4

Visualization of inflammasome activation by recognition of cytosolic DNA. Murine dendritic cells were primed with lipopolysaccharide then stimulated with rhodamine‐labeled poly‐dAdT DNA resulting in Aim2 inflammasome activation. Confocal images show an overlay of pseudocolored ASC, a protein recruited to inflammasomes (blue), DNA (red), and AIM2 (green) in the cytosol of a cell. Reprinted with permission from Marchesan et al. (2020) ¹²

FIGURE 5

Proinflammatory and anti‐inflammatory functions of AIM2 and IFI16. IFI16 and AIM2 form inflammasome complexes that respond to microbial DNA to promote production of mature IL‐1β. IFI16 also inhibits AIM2 and NLRP3 inflammasome activity to limit production of mature IL‐1β. The priming step that induces expression of the inflammasome genes and IL1B is not shown. [Credit: Heather McDonald, BioSerendipity, LLC, Elkridge, MD]

FIGURE 6

Caspase‐1 inhibition blocks ≈50% of alveolar bone loss in mice at 9 days of experimental periodontitis. Starting 1 day before induction of experimental periodontitis with the ligature model, ²⁰ mice received a twice daily oral dosage of the caspase‐1 inhibitor VX‐765 (100 mg/kg) for 10 days. Ligatures were placed between M1 and M2 and kept until the end of the experiment (9 days total). A) Measurements taken from the alveolar bone crest to the cementum‐enamel junction show significant inhibition of alveolar bone loss compared with vehicle control. B) Representative image shows murine maxilla at baseline on the day before ligature placement (alveolar bone represented in green). C) Representative image shows maxilla from vehicle‐treated mouse at day 9 after ligature placement (bone represented in blue, baseline in green superimposed). D) Representative image shows maxilla from VX‐765‐treated mouse at day 9 after ligature placement (bone represented in pink). E) Superimposed maxilla at baseline (green), 9‐day VX‐765‐treated mice (pink), and 9‐day vehicle (blue) shows the amount of bone loss during the 9‐day experimental periodontitis. Adapted from Marchesan et al. (2020). ¹² *<0.05, ^†<0.01