Defining the role ofPorphyromonas gingivalispeptidylarginine deiminase (PPAD) in rheumatoid arthritis through the study of PPAD biology

Abstract

Background Antibodies to citrullinated proteins (ACPAs) 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. Objectives To define the citrullination status and biology of PPAD in P gingivalis and to characterize the anti-PPAD antibody response in RA and associated periodontal disease (PD). Methods PPAD in P gingivalis cells and culture supernatant was analyzed by immunoblotting and mass spectrometry to detect citrullination. Recombinant PPAD (rPPAD), inactive mutant PPAD (rPPADC351S), and N-terminal truncated PPAD (rPPADNtx) were cloned and expressed inE coli. Patients with RA and healthy controls were assayed for IgG antibodies to citrullinated rPPAD and unmodified rPPADC351S by ELISA. Anti-PPAD antibodies were correlated with anti-CCP3 antibody levels, RA disease activity, and PD status. Results PPAD from P gingivalis is truncated at the N- and C-terminal domains and not citrullinated. Only when artificially expressed in E coli, full-length rPPAD, but not truncated (fully active) rPPADNtx, 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-CCP 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 RA patients.

The oral and gut microbiomes are perturbed in rheumatoid arthritis and partly normalized after treatment

“…The peptidyl-arginine deiminase (PAD)-encoding Porphyromonas gingivalis was enriched in control saliva and, to a lesser extent, in control dental plaques ( Fig. 2 and Supplementary Tables 10 and 11), in agreement with recent studies that did not find an association between P. gingivalis or its PAD and RA 18,28 . Rothia spp.…”

PPAD is not targeted as a citrullinated protein in rheumatoid arthritis, but remains a candidate for inducing autoimmunity

“…We have some concerns, however, that the authors’ attempt to reconcile our work on Porphyromonas gingivalis peptidylarginine deiminase (PPAD) autocitrullination with their model of periodontal disease in rheumatoid arthritis (RA) misrepresents our data 1 2…”

PPAD remains a credible candidate for inducing autoimmunity in rheumatoid arthritis: comment on the article by Konig et al

“…3 The ELISA used by Konig et al was clearly not detecting antibodies to citrullinated epitopes on PPAD, and importantly, their results were not confirmed using peptide ELISAs. We suggest that PPAD remains a credible candidate for inducing an antibody response to citrullinated proteins in RAbe it through autocitrullination, citrullination by human PAD enzymes, or citrullination of other bacterial and/or human proteins.…”