Background/Purpose: IL-1β blockade is a highly effective therapy for systemic juvenile idiopathic arthritis (sJIA). However, the rate of inactive disease attained in these trials is lower than reported in sJIA patients treated with IL-1 blockade at disease onset. We have hypothesized that sJIA is a biphasic process in which an initial IL-1β-driven systemic phase directly promoting Th17 differentiation from both Th0 and T regulatory T (T_reg) cells and gives rise to IL-17-driven chronic inflammatory arthritis. We sought to test this hypothesis in the IL-1ra^-/- model, a spontaneous T cell-mediated murine arthritis dependent upon both IL-1β and IL-17.

Methods: Synovial tissue, lymph nodes and spleen from Il-1ra^-/- were characterized via flow cytometry for FOXP3, RORγt expression and cytokine production. In addition, we examined the transcriptomic landscape of sorted T_reg cells from newly generated IL-1ra^-/- Foxp3-eGFP mice and WT Foxp3-eGFP controls. We sorted 50,000 WT and IL-1ra^-/- T_reg cells from 3 mice and performed low-input RNA-Sequencing. Differential expression testing was performed using EdgeR. Additionally, we performed Gene Set Enrichment Analysis between WT and IL-1ra^-/- groups. In order to assess osteoclastogenic potential, sorted T_reg cells where co-culture with macrophage precursors cells (1:2 ratio) in presence of anti-CD3/CD28 beads, M-CSF and RANK-L. After 5 days of co-culture, TRAP⁺ multinucleated cells (more than three nuclei) were counted.

Results: The percentage of T_reg cell among CD4⁺ cells in periphery and locally in synovial tissue was similar between WT and IL-1ra^-/- mice. However, T_reg cells from IL-1ra^-/- synovium showed an up-regulation of RORγt expression. Interestingly, a significant number of FOXP3⁺ cells also produced IL-17 upon in vitro stimulation. To gain molecular insight into the mechanism underlying T_reg cell plasticity in our model, we performed RNAseq-driven characterization of T_reg cells from IL-1ra^-/- and WT mice. Notably, Gene Set Enrichment Analysis revealed distinct metabolic programs between both subsets. The oxidative phosphorylation gene set was enriched in WT T_reg. On the other hand, mTOR and glycolysis pathways that orchestrate a metabolic checkpoint for the differentiation of Th17 cells were enriched in IL-1ra^-/- T_reg cells. This finding confirms the ongoing conversion of Foxp3⁺ cells into Th17 cells. In addition to this Th17-skewed phenotype, T_reg from IL-1ra^-/- mice over-expressed RANK-L and facilitate osteoclastogenesis in vitro whereas WT T_reg inhibited osteoclast differentiation, suggesting a novel pathogenic role for IL-1ra^-/- T_reg cells.

Conclusion: All together our results suggest that chronic inflammatory arthritis in IL-1ra^-/- mice is associated with the conversion of Foxp3⁺ T_reg cells into pathogenic Th17 cells. These findings suggest that early IL-1β blockade could modulate the development of arthritogenic T cells in IL-1-driven arthritis, a result that may inform the understanding and treatment of sJIA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/autoimmune-arthritis-in-il-1-receptor-antagonist-deficient-mice-is-associated-with-a-pathogenic-conversion-of-foxp3-regulatory-t-cells-into-th17-cells/