Investigations of Porphyromonas gingivalis as a possible trigger of autoimmunity in the development of rheumatoid arthritis

Abstract: Rheumatoid Arthritis (RA) is a severe autoimmune disease affecting 0.5-1% worldwide. Patients suffer from pain, disability, chronic joint inflammation, comorbidities and increased mortality. Although, the aetiology of RA is largely unknown, both genes and environment have been shown to contribute. The major risk factors of RA, smoking and HLA-DRB1 shared epitope (SE) alleles, associate primarily with the presence of autoantibodies to citrullinated proteins (ACPA). These antibodies are present in two thirds of patients, precede clinical symptoms, and predict a more destructive disease course. However, the exact trigger of ACPA production remains unclear. In addition to smoking, periodontitis (PD) has been epidemiologically linked to RA. Both smoking and PD cause local inflammation and increased protein citrullination. Moreover, Porphyromonas gingivalis (P.gin) - the keystone pathogen in chronic PD - is the only known prokaryote to express an enzyme that can citrullinate polypeptides (peptidyl arginine deiminase, denoted P.PAD). With this unique property, it was suggested that P.PAD could generate citrullinated epitopes in the inflamed periodontium, which would trigger a systemic ACPA response that eventually cause intraarticular inflammation. Based on this hypothesis, the overall aim of my PhD project was to investigate the role of Porphyromonas gingivalis in the aetiology of ACPA-positive RA, in terms of ACPA production, association with classical risk factors, and disease progression. Most studies in this thesis were performed in the population-based RA case-control cohort EIRA, where serum samples and information on genes (i.e. SE and PTPN22 polymorphism) and smoking history were available. In Study I, we showed that classical RA risk factors associated with specific ACPAs rather than the magnitude of the ACPA response, suggesting that the production of different ACPA fine-specificities is governed by partly different mechanisms. In Study II, we identified an association between anti-P.gin antibodies and RA (in particular ACPA-positive RA) that was even stronger than the association between smoking and RA. Moreover, we observed interactions between anti-P.gin antibodies and both SE and smoking in ACPA-positive RA. With Study III, we could demonstrate that anti-P.gin antibodies pre-dated clinical RA with up to 12 years. Study IV revealed a citrulline-specific antibody response to a P.PAD epitope in non-RA PD patients. Moreover, we identified a monoclonal antibody derived from RA blood, which exhibited cross-reactivity between citrullinated epitopes on bacterial (P.PAD) and human (vimentin) proteins. The data presented in this PhD thesis support a role for P.gin in the development of ACPApositive RA, and we propose that the pathway involves citrullination by P.PAD, followed by an antibody response, which cross-reacts with citrullinated human proteins, and that expansion of the autoimmune ACPA response in genetically susceptible individuals eventually triggers RA. It is my hope that the data presented herein can serve as a basis for disease-preventive strategies, as well as more detailed studies of disease mechanisms in RA aetiopathogenesis, ultimately aimed at the development of curative therapies.