Ann Rheum Dis 63:373-381 doi:10.1136/ard.2003.012211
  • Extended report

Expression and activity of citrullinating peptidylarginine deiminase enzymes in monocytes and macrophages

  1. E R Vossenaar1,
  2. T R D Radstake2,
  3. A van der Heijden1,
  4. M A M van Mansum1,
  5. C Dieteren1,
  6. D-J de Rooij3,
  7. P Barrera2,
  8. A J W Zendman1,
  9. W J van Venrooij1
  1. 1Department of Biochemistry, University of Nijmegen, Nijmegen, The Netherlands
  2. 2Department of Rheumatology, University Medical Centre, St Radboud, Nijmegen, The Netherlands
  3. 3Department of Rheumatology, Sint Maartenskliniek, Nijmegen, The Netherlands
  1. Correspondence to:
    Dr E R Vossenaar
    161 Department of Biochemistry, PO Box 9101, 6500 HB Nijmegen, The Netherlands;
  • Accepted 9 September 2003


Background: Antibodies directed to proteins containing the non-standard amino acid citrulline, are extremely specific for rheumatoid arthritis (RA). Peptidylcitrulline can be generated by post-translational conversion of arginine residues. This process, citrullination, is catalysed by a group of calcium dependent peptidylarginine deiminase (PAD) enzymes.

Objective: To investigate the expression and activity of four isotypes of PAD in peripheral blood and synovial fluid cells of patients with RA.

Results: The data presented here show that citrullination of proteins by PAD enzymes is a process regulated at three levels: transcription—in peripheral blood PAD2 and PAD4 mRNAs are expressed predominantly in monocytes; PAD4 mRNA is not detectable in macrophages, translation—translation of PAD2 mRNA is subject to differentiation stage-specific regulation by its 3′ UTR, and activation—the PAD proteins are only activated when sufficient Ca2+ is available. Such high Ca2+ concentrations are normally not present in living cells. In macrophages, which are abundant in the inflamed RA synovium, vimentin is specifically citrullinated after Ca2+ influx.

Conclusion: PAD2 and PAD4 are the most likely candidate PAD isotypes for the citrullination of synovial proteins in RA. Our results indicate that citrullinated vimentin is a candidate autoantigen in RA.