title: Citrullination enhances the pro-inflammatory response to fibrin in rheumatoid arthritis synovial fibroblasts.
abstract: OBJECTIVE
  Fibrin deposits are characteristic of the synovial tissues in rheumatoid arthritis (RA). Once citrullinated, fibrin becomes an autoantigen and is thought to contribute in this way to perpetuate the disease. Our study aimed to analyse the responses of RA synovial fibroblasts (RASF) to native and citrullinated fibrin.
  
  
  METHODS
  The transcriptome induced by fibrin in RASF was approached with whole-genome-based gene expression arrays. The upregulation of selected pro-inflammatory genes by fibrin was confirmed in additional primary cell cultures using quantitative PCR and ELISA. Citrullination reactions were carried out with recombinant human peptidylarginine deiminases (PAD) 2 and 4.
  
  
  RESULTS
  In the whole-genome array native fibrin was found to modulate the gene expression profile of RASF, particularly upregulating mRNA levels of several pro-inflammatory cytokines. The induction of interleukin (IL)-6 and IL-8 by fibrin was confirmed in additional samples at both the mRNA and the protein level. Blocking and knockdown experiments showed the participation of toll-like receptor (TLR)4 in the induction of both cytokines. As compared with the native macromolecule, PAD2-citrullinated fibrin induced significantly higher expression of the pro-inflammatory cytokines in these cells.
  
  
  CONCLUSIONS
  Our results suggest that fibrin mediates inflammatory responses in RASF via a TLR4 pathway. In this way, fibrin and particularly its citrullinated form may contribute to sustain the cytokine burst in RA.
authors.affiliation: Center for Experimental Rheumatology, University Hospital Zurich and Zurich Center for Integrative Human Physiology (ZIHP), Zurich, Switzerland. osanchez@fjd.es
authors.forename: Olga
authors.initials: O
authors.lastname: Sanchez-Pernaute
authors.forename: Maria
authors.initials: M
authors.lastname: Filkova
authors.forename: Antonio
authors.initials: A
authors.lastname: Gabucio
authors.forename: Martin
authors.initials: M
authors.lastname: Klein
authors.forename: Hanna
authors.initials: H
authors.lastname: Maciejewska-Rodrigues
authors.forename: Caroline
authors.initials: C
authors.lastname: Ospelt
authors.forename: Fabia
authors.initials: F
authors.lastname: Brentano
authors.forename: Beat A
authors.initials: BA
authors.lastname: Michel
authors.forename: Renate E
authors.initials: RE
authors.lastname: Gay
authors.forename: Gabriel
authors.initials: G
authors.lastname: Herrero-Beaumont
authors.forename: Steffen
authors.initials: S
authors.lastname: Gay
authors.forename: Michel
authors.initials: M
authors.lastname: Neidhart
authors.forename: Astrid
authors.initials: A
authors.lastname: Juengel
mesh_terms: D001172:Arthritis, Rheumatoid / Q000378:metabolism* / Q000473:pathology; D002470:Cell Survival; D002478:Cells, Cultured; D002956:Citrulline / Q000378:metabolism; D016207:Cytokines / Q000235:genetics / Q000378:metabolism; D005337:Fibrin / Q000494:pharmacology*; D005347:Fibroblasts / Q000187:drug effects* / Q000378:metabolism / Q000473:pathology; D015870:Gene Expression; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation / Q000187:drug effects*; D055785:Gene Knockdown Techniques; D006801:Humans; D006867:Hydrolases / Q000378:metabolism; D020411:Oligonucleotide Array Sequence Analysis; D000080064:Protein-Arginine Deiminase Type 2; D000080002:Protein-Arginine Deiminase Type 4; D000076342:Protein-Arginine Deiminases; D012333:RNA, Messenger / Q000378:metabolism; D034741:RNA, Small Interfering / Q000494:pharmacology; D011994:Recombinant Proteins; D013583:Synovial Membrane / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D051193:Toll-Like Receptors / Q000235:genetics / Q000378:metabolism; D015854:Up-Regulation / Q000187:drug effects*
chemical_list: D016207:Cytokines; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D011994:Recombinant Proteins; D051193:Toll-Like Receptors; D002956:Citrulline; D005337:Fibrin; D006867:Hydrolases; C000620352:PADI2 protein, human; C000629235:PADI4 protein, human; D000080064:Protein-Arginine Deiminase Type 2; D000080002:Protein-Arginine Deiminase Type 4; D000076342:Protein-Arginine Deiminases