Background/Purpose: Growing evidence suggests that IL-21 has a role in B cell dysfunction in rheumatoid arthritis (RA). Previous studies have reported that IL-21 levels are increased in the serum and synovial fluid of RA subjects and correlate with the 28-joint count disease activity score. While an increase in the percent of IL-21R positive B cells in RA has been described, the significance of this finding in B cells with respect to signaling, B cell differentiation and function has not been investigated. The goal of this study was to address this gap in knowledge and determine the mechanism that leads to increased IL-21R expression on B cells in RA.

Methods: We analyzed IL-21 receptor expression in B cells isolated from whole blood and synovial fluid from a cohort of RA subjects and healthy controls matched for age, gender and race. The RA subjects all met the ACR classification criteria and none of the subjects were on biologics at the time of the draw. Flow cytometry was used to quantify protein and mRNA levels of IL-21R, specificity protein 1 (SP1) and to determine cytokine production (IL-6) and maturation status of B cells. IL-21 signaling was assessed by measuring pSTAT3 levels following IL-21 stimulation. IL-21R levels were correlated to serum levels of rheumatoid factor (RF) IgM which was assessed by ELISA. SP1 binding to the IL21R promoter region in B cells was assessed with ChIP-qPCR.

Results: We demonstrate an increase in IL-21R expression in total and memory B cells from RA subjects, which correlated with responsiveness to IL-21 as measured by phosphorylation of STAT3. IL-21R expression on memory B cell also correlated with serum rheumatoid factor IgM levels. There was comparable levels of IL-21R expression between memory B cells from peripheral blood and those from synovial fluid in the same subject. In addition, stimulation of naïve B cells from RA subjects with IL-21 and CD40L resulted in an increase in differentiation into plasmablasts. Further investigation showed that IL-21R expression correlated with an increase in the level of the SP1 transcription factor. Mechanistic experiments showed increased binding of SP1 to the IL21R promoter region in B cells in RA.

Conclusion: Our results suggest a mechanism by which IL-21 enhances B cell development and function in RA through an SP1 mediated increase in IL-21R expression on B cells. This suggests that therapies for RA may be more efficacious if targeted to memory B cells and/or target SP1.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-transcription-factor-specificity-protein-1-up-regulates-il-21-receptor-expression-on-b-cells-in-rheumatoid-arthritis-leading-to-altered-cytokine-production-and-maturation/