title: Defining the role of Porphyromonas gingivalis peptidylarginine deiminase (PPAD) in rheumatoid arthritis through the study of PPAD biology.
abstract: BACKGROUND
  Antibodies to citrullinated proteins are a hallmark of rheumatoid arthritis (RA). Porphyromonas gingivalis peptidylarginine deiminase (PPAD) has been implicated in the initiation of RA by generating citrullinated neoantigens and due to its ability to autocitrullinate.
  
  
  OBJECTIVE
  To define the citrullination status and biology of PPAD in P gingivalis and to characterise the anti-PPAD antibody response in RA and associated periodontal disease (PD).
  
  
  METHODS
  PPAD in P gingivalis cells and culture supernatant were analysed by immunoblotting and mass spectrometry to detect citrullination. Recombinant PPAD (rPPAD), inactive mutant PPAD (rPPAD(C351S)), and N-terminal truncated PPAD (rPPAD(Ntx)) were cloned and expressed in Escherichia coli. Patients with RA and healthy controls were assayed for IgG antibodies to citrullinated rPPAD and unmodified rPPAD(C351S) by ELISA. Anti-PPAD antibodies were correlated with anti-cyclic citrullinated peptide (third-generation) antibody levels, RA disease activity and PD status.
  
  
  RESULTS
  PPAD from P gingivalis is truncated at the N-terminal and C-terminal domains and not citrullinated. Only when artificially expressed in E coli, full-length rPPAD, but not truncated (fully active) rPPAD(Ntx), is autocitrullinated. Anti-PPAD antibodies show no heightened reactivity to citrullinated rPPAD, but are exclusively directed against the unmodified enzyme. Antibodies against PPAD do not correlate with anti-cyclic citrullinated peptide levels and disease activity in RA. By contrast, anti-PPAD antibody levels are significantly decreased in RA patients with PD.
  
  
  CONCLUSIONS
  PPAD autocitrullination is not the underlying mechanism linking PD and RA. N-terminal processing protects PPAD from autocitrullination and enhances enzyme activity. Anti-PPAD antibodies may have a protective role for the development of PD in patients with RA.
authors.affiliation: Division of Rheumatology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors.forename: Maximilian F
authors.initials: MF
authors.lastname: Konig
authors.affiliation: The University of Oklahoma, Norman, Oklahoma, USA.
authors.forename: Alizay S
authors.initials: AS
authors.lastname: Paracha
authors.affiliation: Division of Rheumatology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors.forename: Malini
authors.initials: M
authors.lastname: Moni
authors.affiliation: Division of Rheumatology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors.forename: Clifton O
authors.initials: CO
authors.lastname: Bingham
authors.affiliation: Division of Rheumatology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors.forename: Felipe
authors.initials: F
authors.lastname: Andrade
mesh_terms: D000328:Adult; D001172:Arthritis, Rheumatoid / Q000150:complications / Q000276:immunology*; D001323:Autoantibodies; D001426:Bacterial Proteins / Q000235:genetics / Q000276:immunology*; D016022:Case-Control Studies; D002956:Citrulline / Q000276:immunology* / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D004926:Escherichia coli / Q000235:genetics; D005260:Female; D006801:Humans; D006867:Hydrolases / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D015151:Immunoblotting; D007074:Immunoglobulin G; D008297:Male; D013058:Mass Spectrometry; D008875:Middle Aged; D010456:Peptides, Cyclic / Q000276:immunology*; D010510:Periodontal Diseases / Q000150:complications / Q000276:immunology / Q000382:microbiology; D010518:Periodontitis / Q000150:complications / Q000276:immunology* / Q000382:microbiology; D016966:Porphyromonas gingivalis / Q000235:genetics / Q000276:immunology*; D000076342:Protein-Arginine Deiminases; D012720:Severity of Illness Index
chemical_list: D001323:Autoantibodies; D001426:Bacterial Proteins; D007074:Immunoglobulin G; D010456:Peptides, Cyclic; C487763:cyclic citrullinated peptide; D002956:Citrulline; D006867:Hydrolases; D000076342:Protein-Arginine Deiminases
keywords: Ant-CCP; Autoimmune Diseases; Rheumatoid Arthritis