title: Synovial fluid mononuclear cells provide an environment for long-term survival of antibody-secreting cells and promote the spontaneous production of anti-citrullinated protein antibodies.
abstract: OBJECTIVE
  In rheumatoid arthritis (RA), observations point to a crucial role for (autoreactive) B cells in disease pathogenesis. Here, we studied whether cells from the synovial environment impact on the longevity of autoreactive B cell responses against citrullinated antigens.
  
  
  METHODS
  Synovial fluid mononuclear cells and peripheral blood mononuclear cells (SFMC/PBMC) were obtained from patients with established RA and assessed for the presence of B cell subpopulations. Cells spontaneously secreting anti-citrullinated protein antibodies (ACPA-IgG) directly ex vivo were detected by antigen-specific Enzyme-Linked ImmunoSpot (ELISpot) assay. SFMC and PBMC were cultured to assess the degree of spontaneous ACPA-IgG secretion. Cells surviving for several weeks were characterised by carboxyfluorescein succinimidyl ester (CFSE) labelling and Ki-67 staining.
  
  
  RESULTS
  Cells spontaneously secreting ACPA-IgG were readily detectable in peripheral blood and synovial fluid (SF) of patients with ACPA-positive RA. SFMC showed an up to 200-fold increase in ex vivo ACPA-IgG secretion compared with PBMC despite lower numbers of B cells in SFMC. ELISpot confirmed the presence of spontaneously ACPA-IgG-secreting cells, accounting for up to 50% (median 12%) of all IgG-secreting cells in SF. ACPA-IgG secretion was remarkably stable in SFMC cultures, maintained upon depletion of the CD20+B cell compartment and detectable for several months. CFSE labelling and Ki-67 staining confirmed the long-term survival of non-dividing plasma cells (PCs).
  
  
  CONCLUSIONS
  This study demonstrates a high frequency of differentiated, spontaneously ACPA-IgG-secreting cells in SF. These cells are supported by SFMC for prolonged survival and autoantibody secretion, demonstrating that the synovial compartment is equipped to function as inflammatory niche for PC survival.
authors.affiliation: Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Priscilla F
authors.initials: PF
authors.lastname: Kerkman
authors.affiliation: Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Ayla C
authors.initials: AC
authors.lastname: Kempers
authors.affiliation: Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Ellen I H
authors.initials: EI
authors.lastname: van der Voort
authors.affiliation: Department of Rheumatology, Groene Hart Ziekenhuis, Gouda, The Netherlands.
authors.forename: Maikel
authors.initials: M
authors.lastname: van Oosterhout
authors.affiliation: Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Tom W J
authors.initials: TW
authors.lastname: Huizinga
authors.affiliation: Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: René E M
authors.initials: RE
authors.lastname: Toes
authors.affiliation: Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Hans U
authors.initials: HU
authors.lastname: Scherer
mesh_terms: D000328:Adult; D000921:Antibody-Producing Cells / Q000378:metabolism*; D001172:Arthritis, Rheumatoid / Q000276:immunology* / Q000378:metabolism; D001323:Autoantibodies / Q000096:biosynthesis*; D002470:Cell Survival; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D006801:Humans; D007074:Immunoglobulin G / Q000096:biosynthesis; D007963:Leukocytes, Mononuclear / Q000166:cytology* / Q000276:immunology; D008297:Male; D010456:Peptides, Cyclic / Q000276:immunology; D013582:Synovial Fluid / Q000166:cytology* / Q000276:immunology
chemical_list: D001323:Autoantibodies; D007074:Immunoglobulin G; D010456:Peptides, Cyclic; C487763:cyclic citrullinated peptide
keywords: Ant-CCP; B cells; Rheumatoid Arthritis; Synovial fluid