title: Brief report: enrichment of activated group 3 innate lymphoid cells in psoriatic arthritis synovial fluid.
abstract: OBJECTIVE
  Innate lymphoid cells (ILCs) are a recently discovered group of cells that are essential to epithelial homeostasis and are implicated in psoriasis pathogenesis, yet they have never been reported in psoriatic arthritis (PsA).
  
  
  METHODS
  ILC classes and subsets were characterized in the peripheral blood (PB) of healthy controls, patients with psoriasis, and patients with PsA and in the synovial fluid (SF) of patients with PsA and patients with rheumatoid arthritis (RA). Cell surface marker expression and intracellular cytokine production following stimulation were analyzed using flow cytometry.
  
  
  RESULTS
  ILCs were identified in the SF and were 4-fold more abundant in PsA SF than in PsA PB. Fewer CCR6+ ILCs were found in PsA PB than in healthy control PB, while PsA SF was enriched for CCR6+ ILCs compared to PsA PB and RA SF. Natural cytotoxicity receptor NKp44+ group 3 ILCs were rare in PB and RA SF, but abundant in PsA SF. Increased numbers of interleukin-17A (IL-17A)-producing ILCs were present in PsA SF compared to RA SF. CCR6, NKp44, and melanoma cell adhesion molecule (MCAM) were expressed on the cell surface of SF ILCs that produced IL-17A. The number of circulating NKp44+, CCR6+, and MCAM+ ILCs in blood was inversely correlated with PsA disease activity.
  
  
  CONCLUSIONS
  Our findings indicate that PsA SF is enriched for group 3 ILCs that express CCR6 and NKp44, which distinguishes the synovial compartment from RA. The increased IL-17A production by SF ILCs indicates a novel role for ILCs in PsA.
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Emmerik F A
authors.initials: EF
authors.lastname: Leijten
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Tessa S
authors.initials: TS
authors.lastname: van Kempen
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Marianne
authors.initials: M
authors.lastname: Boes
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Jocea M R
authors.initials: JM
authors.lastname: Michels-van Amelsfort
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Dirkjan
authors.initials: D
authors.lastname: Hijnen
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Sarita A Y
authors.initials: SA
authors.lastname: Hartgring
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Joel A G
authors.initials: JA
authors.lastname: van Roon
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Mark H
authors.initials: MH
authors.lastname: Wenink
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Timothy R D J
authors.initials: TR
authors.lastname: Radstake
mesh_terms: D000328:Adult; D015535:Arthritis, Psoriatic / Q000378:metabolism / Q000473:pathology*; D001172:Arthritis, Rheumatoid / Q000378:metabolism / Q000473:pathology; D051929:CD146 Antigen / Q000378:metabolism; D016022:Case-Control Studies; D006801:Humans; D007113:Immunity, Innate / Q000502:physiology*; D020381:Interleukin-17 / Q000378:metabolism; D008214:Lymphocytes / Q000378:metabolism / Q000473:pathology*; D008875:Middle Aged; D055629:Natural Cytotoxicity Triggering Receptor 2 / Q000378:metabolism; D054399:Receptors, CCR6 / Q000378:metabolism; D013582:Synovial Fluid / Q000166:cytology* / Q000378:metabolism
chemical_list: C106311:CCR6 protein, human; D051929:CD146 Antigen; D020381:Interleukin-17; C497902:MCAM protein, human; C112837:NCR2 protein, human; D055629:Natural Cytotoxicity Triggering Receptor 2; D054399:Receptors, CCR6