Transmembrane BAFF from rheumatoid synoviocytes requires interleukin-6 to induce the expression of recombination-activating gene in B lymphocytes

Abstract

Objective. B cells that accumulate in the synovial tissue of rheumatoid arthritis (RA) patients revise their receptors due to coordinate expression of recombination-activating gene 1 (RAG-1) and RAG-2 genes. The aim of this study was to determine the mechanisms that control this re-expression. Methods. B cells from healthy control subjects were cocultured with fibroblast-like synoviocytes (FLS) from patients with RA and osteoarthritis (OA). Re-expression of RAG messenger RNA (mRNA) and proteins was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and indirect immunofluorescence. Activity of RAG enzymes was evaluated by flow cytometry to measure variations in immunoglobulin κ and λ light chain expression and by ligation-mediated-PCR to assess specific DNA breaks. Blocking antibodies, short hairpin RNA, and recombinant cytokine were used to identify the molecules involved in RAG reexpression. Results. RA FLS, but not OA FLS, induced B cells to re-express RAG mRNA and proteins. Enzymes were functional, since the κ-to-λ ratios decreased and specific DNA breaks were detectable after coculture with RA FLS. Transmembrane BAFF provided the first signal of RAG re-expression, since its down-regulation in RA FLS prevented RAG gene transcription in B cells. The failure of transmembrane BAFF from OA FLS to induce RAG suggests that a second signal was provided by RA FLS. Interleukin-6 (IL-6) is a candidate, since blockade of its receptors precluded transcription of RAG genes by RA FLS. Unless supplemented with IL-6, OA FLS were unable to induce RAG gene expression in normal B cells. Conclusion. Two independent signals are required for the induction of RAG gene expression in B cells that infiltrate the synovium of patients with RA. © 2009, American College of Rheumatology.