Synovial B cells play a critical role in rheumatoid arthritis (RA), being involved in autoantibody synthesis, T cell activation, and cytokine production. CXCL13 is a B cell chemoattractant that is instrumental in synovial B cell organization; the regulatory determinants of CXCL13 in inflammation are poorly characterized. This study was undertaken to investigate the functional involvement of synovial T cells in the ectopic expression of CXCL13 in RA.
CXCL13 production and regulation were addressed using immunohistochemistry, in situ hybridization, quantitative polymerase chain reaction, multicolor flow cytometry, and enzyme-linked immunosorbent assay, by in situ-ex vivo analysis and in vitro functional assays with rheumatoid synovial tissue and primary cells.
CXCL13 messenger RNA and protein expression and spontaneous CXCL13 secretion were detected in RA synovial fluid T cells but were not detected (or were detected only occasionally) in peripheral blood T cells. Analysis of tissue expression confirmed cytoplasm localization of CXCL13 in T lymphocytes infiltrating B cell follicles and small perivascular aggregates. Multicolor characterizations in synovial fluid demonstrated CXCL13 expression in antigen-experienced T helper cells, frequently characterized by terminal differentiation and the lack of the follicular helper T cell markers CXCR5 and BCL6 protein. In vitro functional assays revealed the enhancing effect of T cell receptor-CD28 engagement on CXCL13 production and secretion in primary cells.
Our findings define a new functional property of synovial T cells, demonstrating their active involvement in the local production of B cell chemoattractants, and support a direct contribution of the adaptive immune system and antigen-dependent signals in the mechanisms of B cell localization in RA.