Studies on the relationship between periodontitis and rheumatoid arthritis

Abstract

Periodontitis and rheumatoid arthritis (RA) are both widespread multifactorial diseases, characterized by chronic inflammation leading to tissue and bone destruction around teeth or joints, respectively. An association between periodontitis and RA has been proposed, suggesting that the periodontal pathogen Porphyromonas gingivalis (P. gingivalis) may be involved in the generation of anti-citrullinated protein antibodies (ACPAs) in RA. Previous findings indicate that patients with RA may have increased prevalence/incidence and severity of periodontitis; however, more studies are needed to confirm this link, elucidate the potential mechanisms involved, and ascertain the temporal relationship between periodontitis and RA. Hence, the general aim of this thesis was to study the relationship between periodontitis and RA using population-based, clinical, immunohistochemical and experimental animal studies.

Study I investigated the prevalence of periodontitis with special focus on ACPA status in the Swedish Epidemiological Investigation of RA (EIRA), a well-characterized population-based RA case-control cohort. Linking the EIRA registry with the Dental Health Registry (DHR) demonstrated no differences in the prevalence of periodontal diseases (gingivitis, periodontitis, or peri-implantitis) between RA patients and matched healthy controls. Additionally, among subjects with RA, we detected no differences in prevalence of periodontitis based on ACPA or rheumatoid factor (RF) antibody status.

Study II explored the effect of pre-existing periodontitis on the development and the immune/inflammatory response of experimental arthritis in an animal model. Eight weeks prior to induction of pristane-induced arthritis, we induced experimental periodontitis in arthritis-susceptible Dark Agouti rats using ligatures in combination with swabs containing periodontal bacteria. After monitoring the progression and severity of both periodontitis and arthritis for another 7 weeks, we compared the results to animals with experimental arthritis only and to healthy animals without arthritis or periodontitis. We detected increased levels of antibodies against citrullinated P. gingivalis peptidylarginine deiminase (PPAD) proteins in rats with both pre-existing periodontitis and pristane-induced arthritis compared to arthritic animals without pre-existing periodontitis. However, the pre-existence of periodontitis did not affect the development or severity of pristane-induced arthritis in rats.

Study III examined the presence of citrullinated proteins and the expression of human peptidylarginine deiminase (PAD2 and PAD4) enzymes in relation to the presence of P. gingivalis in gingival tissue biopsies from patients with periodontitis and healthy controls. In gingival connective tissue, we detected citrullinated proteins in 80% of the subjects with periodontitis compared to in 25% of healthy controls. Analysis of the sections also showed increased expression of PAD2 and PAD4 in the gingival connective tissue of patients with periodontitis. We found no differences in the presence of P. gingivalis in the epithelium or connective tissue of gingival biopsies from patients with periodontitis and healthy controls without periodontitis. Moreover, we observed no correlations between the presence of P. gingivalis and citrullinated proteins or citrullinating enzymes in gingival tissue from patients with periodontitis and healthy controls.

Study IV was a clinical study that further explored the association between RA and the severity of periodontitis. This pilot study aimed to investigate the severity of periodontitis in relation to ACPA/RF status, immunological parameters associated with RA, and the presence of subgingival P. gingivalis DNA in patients with established RA. The findings showed that the majority of patients with RA had moderate/severe periodontitis (75%), while the others had no/mild periodontal disease. ACPA positivity was significantly more common in RA subjects with moderate/severe periodontitis compared to those with no/mild periodontal disease. We detected no differences in the presence of P. gingivalis in subgingival plaque samples based on periodontitis severity or ACPA status. Nor did we detect any between-group differences in RA disease duration or disease activity or type of RA treatment/medication. Cytokine profiling of serum, saliva, and gingival crevicular fluid samples showed that the levels of several inflammatory mediators were up-regulated in patients with moderate/severe periodontitis. Interestingly, the levels of a proliferation-inducing ligand (APRIL), implicated in B-cell survival and maturation, were significantly higher in both serum and saliva samples of patients with moderate/severe periodontitis compared to those with no/mild periodontal disease.

In conclusion, we found no evidence of an increased prevalence of periodontitis in patients with established RA relative to healthy controls, in the large population-based registry study. However, the clinical study showed that the more severe forms of periodontitis, defined as moderate/severe periodontitis, were frequent in subjects with established RA, and associated with ACPA positivity. Our findings also demonstrate that the presence of citrullinated proteins and expression of human citrullinating PAD enzymes are increased in gingival tissue of non-RA subjects with chronic periodontitis, and that the pre-existence of periodontitis can induce a systemic antibody response against citrullinated proteins derived from P. gingivalis PAD enzyme in animals with experimental arthritis. Taken together, the results indicate that periodontal infection could possibly contribute to the autoimmunity against citrullinated proteins associated with RA. Since ACPA status may be associated with periodontitis severity, future studies on the association between periodontitis and RA should focus on investigating the potential beneficial effects of periodontal treatment in patients with RA. Moreover, additional research into predisposing risk factors and molecular mechanisms connecting periodontitis with RA are also warranted.