Objectives 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.
- Rheumatoid Arthritis
- B cells
- Synovial fluid
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Handling editor Tore K Kvien
Contributors PFK, ACK and EIHvdV designed the study, performed experiments, interpreted and analysed data and wrote the manuscript. MvO provided patient material. TWJH, REMT and HUS designed the study, interpreted and analysed data and wrote the manuscript.
Funding This study was supported by the Dutch Arthritis Foundation (to HUS; project number 15-2-402), The Netherlands Organisation for Scientific Research (NWO; project number 435000033) and the Innovative Medicines Initiative (IMI) funded project BeTheCure (contract no. 115142-2). REMT is recipient of a NWO-ZonMW VICI (project number: 918.96.606) and TOP grant (project number 91214031). HUS is recipient of a NWO-ZonMW Clinical Fellowship (project number: 90714509) and of a NWO Enabling Technologies grant (project number 435002030).
Competing interests None declared.
Ethics approval Ethical review board of LUMC.
Provenance and peer review Not commissioned; externally peer reviewed.