Interleukin-6 is responsible for aberrant B-cell receptor-mediated regulation of RAG expression in systemic lupus erythematosus.

Defective regulation of secondary immunoglobulin V(D)J gene rearrangement promotes the production of autoantibodies in systemic lupus erythematosus (SLE). It remains unclear, however, whether the regulation of the recombination-activating genes RAG1 and RAG2 is effective in SLE. RAG1 and RAG2 messenger RNA expression was analysed before and after in vitro activation of sorted CD19(+) CD5(-) B cells with anti-immunoglobulin M antibodies, in 20 SLE patients and 17 healthy controls. The expression of CDK2 and p27(Kip1) regulators of the RAG2 protein, were examined. The levels of interleukin-6 (IL-6) and its influence on RAG regulation were also evaluated in vitro. SLE patients had increased frequency of RAG-positive B cells. B-cell receptor (BCR) engagement induced a shift in the frequency of kappa- and lambda-positive cells, associated with a persistence of RAG messenger RNA and the maintenance of RAG2 protein within the nucleus. While expression of the RAG2-negative regulator CDK2 was normal, the positive regulator p27(Kip1) was up-regulated and enhanced by BCR engagement. This effect was the result of the aberrant production of IL-6 by SLE B cells. Furthermore, IL-6 receptor blockade led to a reduction in p27(Kip1) expression, and allowed the translocation of RAG2 from the nucleus to the cytoplasm. Our study indicates that aberrant production of IL-6 contributes to the inability of SLE B cells to terminate RAG protein production. Therefore, we hypothesize that because of constitutive IL-6 signalling in association with BCR engagement, SLE B cells would become prone to secondary immunoglobulin gene rearrangements and autoantibody production.