PPAD, Porphyromonas gingivalis and the Subgingival Microbiome in Periodontitis and Autoantibody-positive Individuals at Risk of Rheumatoid Arthritis

There is an epidemiological association between periodontitis and rheumatoid arthritis (RA). The subgingival microbiota may play an important role in the link between the two diseases. Porphyromonas gingivalis, which produces a peptidyl-arginine deiminase (PPAD) capable of citrullinating proteins, is considered a key organism inducing the production of antibodies against citrullinated proteins systemically and may initiate the pathogenic autoimmune responses associated with RA. The overall aim of this study was to explore the role of PPAD in P. gingivalis physiology and to better understand the links between P. gingivalis, periodontitis and risk of developing RA. P. gingivalis W83 and the corresponding Δppad mutant were grown in batch and continuous culture, to assess pH regulation, bacterial growth, gene expression and arginine gingipain (Rgp) and dipeptidyl-peptidase (DPP) activities. In a collaborative clinical study, the shotgun metagenomic approach was used to observe subgingival microbial profiles in individuals with and without periodontitis, with and without RA, and in those with autoantibodies against citrullinated peptides (CCP) at risk of developing RA. Based on in vitro studies, PPAD may citrullinate Rgp and DPP11, impair their activities and subsequently affect the alkali-promoting activity of P. gingivalis. Furthermore, both environmental pH and PPAD deficiency were able to regulate P. gingivalis gene expression, promoting adaptation to environmental changes and facilitating bacterial growth. In the clinical study, periodontitis occurred more often in anti-CCP positive at-risk individuals than in healthy controls and the subgingival microbiomes of those individuals were perturbed, indicating that periodontitis and related microbial dysbiosis precede the onset of RA. P. gingivalis and its PPAD in established periodontitis conditions may play an important role in the initiation of RA. Moreover, PAD or PAD-like enzymes present in bacterial species other than P. gingivalis, e.g. Prevotella spp. exhibited some citrullination activity in vitro in a similar manner to PPAD.