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A BAFF/APRIL-dependent TLR3-stimulated pathway enhances the capacity of rheumatoid synovial fibroblasts to induce AID expression and Ig class-switching in B cells
  1. Michele Bombardieri1,
  2. Ngar-Woon Kam1,
  3. Fabia Brentano2,
  4. Ken Choi1,
  5. Andrew Filer3,
  6. Diego Kyburz2,
  7. Iain B McInnes4,
  8. Steffen Gay2,
  9. Christopher Buckley3,
  10. Costantino Pitzalis1
  1. 1Centre for Experimental Medicine and Rheumatology, William Harvey Research Institute, Queen Mary University of London, London, UK
  2. 2Centre for Experimental Rheumatology, University Hospital, Zürich, Switzerland
  3. 3Rheumatology Research Group, School of Immunity and Infection, College of Medical and Dental Sciences, Institute for Biomedical Research, The University of Birmingham, Birmingham, UK
  4. 4Centre for Rheumatic Diseases, Division of Immunology, Infection and Inflammation, Glasgow Royal Infirmary, University of Glasgow, Glasgow, UK
  1. Correspondence to Dr Michele Bombardieri, Centre for Experimental Medicine and Rheumatology, 2nd Floor, John Vane Science Centre, William Harvey Research Institute, Queen Mary University of London, Charterhouse Square, London EC1M 6BQ, UK; m.bombardieri{at}


Objectives To dissect the role of toll-like receptor (TLR) signalling and B cell survival/proliferating factors in the crosstalk between rheumatoid arthritis synovial fibroblasts (RASF) and B cells.

Methods RASF, rheumatoid arthritis dermal fibroblasts (RADF) and osteoarthritis synovial fibroblasts (OASF) were analysed for the expression of B cell survival/proliferating factors BAFF and APRIL in resting conditions and upon stimulation with TLR2/TLR3/TLR4 ligands. Unswitched IgD+ B cells were co-cultured with RASF/OASF/RADF in the presence/absence of TLR ligands and with/without BAFF/APRIL blocking antibodies. Activation-induced cytidine deaminase (AID) mRNA expression, Iγ-Cμ and Iα-Cμ circular transcripts (CTs; markers of ongoing class-switching to IgG and IgA) and IgM/A/G production were measured to assess functional activation of B cells.

Results TLR3 and to a lesser extent TLR4, but not TLR2 stimulation, induced up to ∼1000-fold BAFF mRNA and increased soluble BAFF release. APRIL was less significantly regulated by TLR3. Resting and TLR3-stimulated RASF released higher levels of BAFF/APRIL compared with RADF. TLR3 stimulation of RASF but not RADF in co-culture with B cells strongly enhanced AID expression, Iγ-Cμ and Iα-Cμ CTs and class-switching to IgG/IgA. Blockade of BAFF/APRIL signalling completely inhibited TLR3-induced, RASF-dependent expression of AID, CTs and the secretion of IgG/IgA.

Conclusions RASF produce high levels of BAFF and APRIL constitutively and in response to TLR3 stimulation. These factors are critical in directly modulating AID expression, class-switch recombination and IgG/IgA production in IgD+ B cells. Overall, this work highlights a novel and fundamental role for the TLR3/B cell survival factor axis in sustaining B cell activation in the rheumatoid arthritis synovium.

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  • MB and NWK contributed equally to this work.

  • Funding Arthritis Research UK.

  • Competing interests None.

  • Ethics approval This study was conducted with the approval of the East London & The City Research Ethics Committee 3 (LREC07/Q0605/29).

  • Provenance and peer review Not commissioned; externally peer reviewed.