Peptidyl arginine deiminase type 2 (PAD-2) and PAD-4 but not PAD-1, PAD-3, and PAD-6 are expressed in rheumatoid arthritis synovium in close association with tissue inflammation.
OBJECTIVE
Autoantibodies to citrullinated proteins (ACPAs) are specific for rheumatoid arthritis (RA) and probably are involved in its pathophysiology. Citrullyl residues, posttranslationally generated by peptidyl arginine deiminase (PAD), are indispensable components of ACPA-targeted epitopes. The aim of this study was to identify which PAD isotypes are expressed in the synovial tissue (ST) of patients with RA and are involved in the citrullination of fibrin, the major synovial target of ACPAs.


METHODS
Expression of all PAD isotypes, including the recently described PAD type 6 (PAD-6), was explored by reverse transcription-polymerase chain reaction and immunoblotting, first in blood-derived mononuclear leukocytes from healthy donors, then in ST samples from 16 patients with RA and 11 control patients (4 with other arthritides and 7 with osteoarthritis [OA]). In ST samples from patients with RA, PADs were localized by immunohistochemistry.


RESULTS
In lymphocytic and monocytic cells and, similarly, in ST samples from patients with RA, the PAD-2, PAD-4, and PAD-6 genes were found to be transcribed, but only PAD-2 and PAD-4 enzymes were detected. PAD-2 was also expressed in ST from control patients, including those with OA, while PAD-4 was preferentially expressed in ST from patients with other arthritides. In RA, the expression levels of PAD-2 and PAD-4 were correlated with the intensity of inflammation (cell infiltration, hypervascularization, and synovial lining hyperplasia), and both enzymes were demonstrable within or in the vicinity of citrullinated fibrin deposits.


CONCLUSIONS
PAD-2 and PAD-4 are the only PAD isotypes expressed in the ST of patients with RA and those with other arthritides. Inflammatory cells are a major source, but PAD-4 also comes from hyperplastic synoviocytes. Both isotypes are probably involved in the citrullination of fibrin.
Foulquier
Céline
C
Unité Mixte de Recherche 5165, CNR-Université Toulouse III, Institut Fédératif de Recherche 30 (IFR30), Toulouse, France.
Sebbag
Mireille
M
Clavel
Cyril
C
Chapuy-Regaud
Sabine
S
Al Badine
Roula
R
Méchin
Marie-Claire
MC
Vincent
Christian
C
Nachat
Rachida
R
Yamada
Michiyuki
M
Takahara
Hidenari
H
Simon
Michel
M
Guerrin
Marina
M
Serre
Guy
G
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001172:Arthritis, Rheumatoid / Q000276:immunology* / Q000378:metabolism / Q000503:physiopathology*; D002956:Citrulline / Q000378:metabolism; D005260:Female; D015971:Gene Expression Regulation, Enzymologic / Q000276:immunology*; D006801:Humans; D006867:Hydrolases / Q000235:genetics* / Q000378:metabolism; D007527:Isoenzymes / Q000235:genetics* / Q000378:metabolism; D007963:Leukocytes, Mononuclear / Q000201:enzymology / Q000276:immunology; D008264:Macrophages / Q000201:enzymology / Q000276:immunology; D008297:Male; D008875:Middle Aged; D009000:Monocytes / Q000201:enzymology / Q000276:immunology; D000080062:Protein-Arginine Deiminase Type 1; D000080063:Protein-Arginine Deiminase Type 3; D000080002:Protein-Arginine Deiminase Type 4; D000080065:Protein-Arginine Deiminase Type 6; D000076342:Protein-Arginine Deiminases; D013583:Synovial Membrane / Q000276:immunology*
D007527:Isoenzymes; D002956:Citrulline; D006867:Hydrolases; C000629235:PADI4 protein, human; C488263:PADI6 protein, human; D000080062:Protein-Arginine Deiminase Type 1; D000080063:Protein-Arginine Deiminase Type 3; D000080002:Protein-Arginine Deiminase Type 4; D000080065:Protein-Arginine Deiminase Type 6; D000076342:Protein-Arginine Deiminases