title: Synovial inflammation, immune cells and their cytokines in osteoarthritis: a review.
abstract: OBJECTIVE
  Although osteoarthritis (OA) is considered a non-inflammatory condition, it is widely accepted that synovial inflammation is a feature of OA. However, the role of immune cells and their cytokines in OA is largely unknown. This narrative systematic review summarizes the knowledge of inflammatory properties, immune cells and their cytokines in synovial tissues (STs) of OA patients.
  
  
  METHODS
  Broad literature search in different databases was performed which resulted in 100 articles.
  
  
  RESULTS
  Of 100 articles 33 solely investigated inflammation in OA ST with or without comparison with normal samples; the remaining primarily focussed on rheumatoid arthritis (RA) ST. Studies investigating different severity stages or cellular source of cytokines were sparse. OA ST displayed mild/moderate grade inflammation when investigated by means of haematoxylin and eosin (H&E) staining. Most frequently found cells types were macrophages, T cells and mast cells (MCs). Overall the number of cells was lower than in RA, although the number of MCs was as high as or sometimes even higher than in RA ST. Cytokines related to T cell or macrophage function were found in OA ST. Their expression was overall higher than in normal ST, but lower than in RA ST. Their cellular source remains largely unknown in OA ST.
  
  
  CONCLUSIONS
  Inflammation is common in OA ST and characterized by immune cell infiltration and cytokine secretion. This inflammation seems quantitatively and qualitatively different from inflammation in RA. Further research is needed to clarify the role of inflammation, immune cells and their cytokines in the pathogenesis of OA.
authors.affiliation: Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands. b.j.e.de_lange@lumc.nl
authors.forename: B J E
authors.initials: BJ
authors.lastname: de Lange-Brokaar
authors.forename: A
authors.initials: A
authors.lastname: Ioan-Facsinay
authors.forename: G J V M
authors.initials: GJ
authors.lastname: van Osch
authors.forename: A-M
authors.initials: AM
authors.lastname: Zuurmond
authors.forename: J
authors.initials: J
authors.lastname: Schoones
authors.forename: R E M
authors.initials: RE
authors.lastname: Toes
authors.forename: T W J
authors.initials: TW
authors.lastname: Huizinga
authors.forename: M
authors.initials: M
authors.lastname: Kloppenburg
mesh_terms: D016207:Cytokines / Q000378:metabolism*; D006801:Humans; D007111:Immunity, Cellular; D008407:Mast Cells / Q000378:metabolism / Q000473:pathology; D010003:Osteoarthritis / Q000276:immunology / Q000473:pathology; D013583:Synovial Membrane / Q000276:immunology / Q000378:metabolism / Q000473:pathology; D013601:T-Lymphocytes / Q000276:immunology* / Q000473:pathology
chemical_list: D016207:Cytokines