Peptidylarginine deiminase from Porphyromonas gingivalis citrullinates human fibrinogen and α-enolase: implications for autoimmunity in rheumatoid arthritis

Adsorption of components of the plasma kinin-forming system on the surface of Porphyromonas gingivalis involves gingipains as the major docking platforms

Enhanced production of proinflammatory bradykinin-related peptides, the kinins, has been suggested to contribute to the pathogenesis of periodontitis, a common inflammatory disease of human gingival tissues. In this report, we describe a plausible mechanism of activation of the kinin-generating system, also known as the contact system or kininogen-kallikrein-kinin system, by the adsorption of its plasma-derived components such as high-molecular-mass kininogen (HK), prekallikrein (PK), and Hageman factor (FXII) to the cell surface of periodontal pathogen Porphyromonas gingivalis. The adsorption characteristics of mutant strains deficient in selected proteins of the cell envelope suggested that the surface-associated cysteine proteinases, gingipains, bearing hemagglutinin/adhesin domains (RgpA and Kgp) serve as the major platforms for HK and FXII adhesion. These interactions were confirmed by direct binding tests using microplate-immobilized gingipains and biotinylated contact factors. Other bacterial cell surface components such as fimbriae and lipopolysaccharide were also found to contribute to the binding of contact factors, particularly PK. Analysis of kinin release in plasma upon contact with P. gingivalis showed that the bacterial surface-dependent mechanism is complementary to the previously described kinin generation system dependent on HK and PK proteolytic activation by the gingipains. We also found that several P. gingivalis clinical isolates differed in the relative significance of these two mechanisms of kinin production. Taken together, these data show the importance of this specific type of bacterial surface-host homeostatic system interaction in periodontal infections.

Involvement of autoimmunity in the pathogenesis of aggressive periodontitis

The aim of this study was to investigate the involvement of autoimmune reactions to native and post-translationally modified extracellular matrix components in the pathogenesis of periodontitis. Sera from individuals with aggressive periodontitis (AgP, n = 25), chronic periodontitis (CP, n = 14), and gingivitis (G, n = 18) were tested for the presence of autoantibodies against: (a) native collagen type I (CI) and collagen type III (CIII); (b) CI and CIII post-translationally modified by reactive oxygen species (ROS) of the type present during inflammation; and (c) citrullinated filaggrin-derived peptides (CCP). Autoantibodies to native and ROS-modified CI and CIII as well as autoantibodies to CCP were observed exclusively in patients with AgP and not in those with CP or G. In conclusion, autoimmune reactions to native and post-translationally modified self-antigens may play a role specifically in the pathogenesis of AgP.

Rheumatoid arthritis and salivary biomarkers of periodontal disease

AIM

To test the hypothesis that rheumatoid arthritis (RA) influenced levels of salivary biomarkers of periodontal disease.

METHODS

Medical assessments, periodontal examinations and pain ratings were obtained from 35 RA, 35 chronic periodontitis and 35 age- and gender-matched healthy controls in a cross-sectional, case-controlled study. Unstimulated whole saliva samples were analysed for interleukin-1β (IL-1β), matrix metalloproteinase-8 (MMP-8) and tumour necrosis factor-α (TNF-α) concentrations.

RESULTS

The arthritis and healthy groups had significantly less oral disease than the periodontitis group (P<0.0001), with the arthritis group having significantly more sites bleeding on probing (BOP) than matched controls (P=0.012). Salivary levels of MMP-8 and IL-1β were significantly elevated in the periodontal disease group (P<0.002), and IL-1β was the only biomarker with significantly higher levels in the arthritis group compared with controls (P=0.002). Arthritis patients receiving anti-TNF-α antibody therapy had significantly lower IL-1β and TNF-α levels compared with arthritis patients not on anti-TNF-α therapy (P=0.016, 0.024) and healthy controls (P<0.001, P=0.011), respectively.

CONCLUSION

RA patients have higher levels of periodontal inflammation than healthy controls, i.e., an increased BOP. Systemic inflammation appears to influence levels of select salivary biomarkers of periodontal disease, and anti-TNF-α antibody-based disease-modifying therapy significantly lowers salivary IL-1β and TNF-α levels in RA.

Periodontitis in RA-the citrullinated enolase connection

Autoimmunity in rheumatoid arthritis (RA) is characterized by an antibody response to citrullinated proteins. Two of the risk factors for RA-HLA-DRB1 shared epitope alleles and smoking-are also associated with periodontitis, which is largely, but not exclusively, caused by Porphyromonas gingivalis infection. Furthermore, RA and periodontitis have a similar pathophysiology, characterized by destructive inflammation. The citrullination of proteins by P. gingivalis and the subsequent generation of autoantigens that drive autoimmunity in RA represents a possible causative link between these two diseases. Antibodies directed towards the immunodominant epitope of human citrullinated α-enolase cross-react with a conserved sequence on citrullinated P. gingivalis enolase. On the basis of this cross-reactivity, in this Perspectives article we explore the hypothesis of molecular mimicry in the etiology of RA, with citrullinated enolase as the specific antigen involved.