350 Toll-like receptor 7 signalling drives transitional b cells expansion and autoantibody production in systemic lupus erythematosus

Background and aims Toll-like receptor 7 (TLR7) has been implicated in B cells activation and the generation of pathogenic autoantibodies. Newly-formed transitional (TR) B cells are enriched in autoreactive specificities and are increased in some SLE patients. This study was undertaken to examine a possible link between the TR B cells expansion/activation and TLR7 levels in SLE. Methods PBMCs were collected from SLE patients and healthy donors and analysed for the expression of TLR7 , TLR9 and IFN-responsive genes by RT-PCR. The frequencies of B cell populations were analysed by flow cytometry. BAFF titers were analysed by ELISA. TLR7 variant rs3853839(C/G) was detected by Taqman 5’-allele discrimination assay. TR B cells were primed with IFNα and stimulated with TLR7 ligands in vitro . Results High expression levels of TLR7 in SLE patients positively correlated with IFN signature and disease activity, but not with BAFF titers. SLE patients with high levels of TLR7 (TLR7 hi group) showed an expansion of CD19 + CD38 high CD24 high CD10 + TR B cells. Overall, frequencies of TR B cells positively correlated with the levels of TLR7 , but not TLR9 . SLE patients, carrying a risk G allele, had increased TLR7 expression and TR cell frequencies, compared to non-risk allele carriers. TLR7 hi SLE patients showed increased autoantibody titers and skewing towards Sm/RNP antigens. Upon IFNα priming, TR B cells up-regulated TLR7 and differentiated into plasmablasts in response to TLR7-ligand stimulation. Conclusions Our findings suggest that dysregulation of TLR7 in SLE might drive the expansion and promote the activation of TR B cells, which might be a source of autoantibodies.