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Oligoarticular juvenile idiopathic arthritis (oligo JIA) is the most common form of chronic inflammatory arthritis in children, yet the cause of this disease remains unknown. To understand immune responses in oligo JIA, we immunophenotyped synovial fluid T cells with flow cytometry, bulk RNA-Seq, single-cell RNA-Seq (scRNA-Seq), DNA methylation studies, and Treg suppression assays. In synovial fluid, CD4+, CD8+, and gamma delta T cells expressed Th1-related markers, whereas Th17 cells were not enriched. Th1 skewing was prominent in CD4+ T cells, including Tregs, and was associated with severe disease. Transcriptomic studies confirmed a Th1 signature in CD4+ T cells from synovial fluid. The regulatory gene expression signature was preserved in Tregs, even those exhibiting Th1 polarization. These Th1-like Tregs maintained Treg-specific methylation patterns and suppressive function, supporting the stability of this Treg population in the joint. Although synovial fluid CD4+ T cells displayed an overall Th1 phenotype, scRNA-Seq uncovered heterogeneous effector and regulatory subpopulations, including IFN-induced Tregs, peripheral helper T cells, and cytotoxic CD4+ T cells. In conclusion, oligo JIA is characterized by Th1 polarization that encompasses Tregs but does not compromise their regulatory identity. Targeting Th1-driven inflammation and augmenting Treg function may represent important therapeutic approaches in oligo JIA. Joint Biology Consortium [P30 AR070253]; NIH National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)United States Department of Health & Human ServicesNational Institutes of Health (NIH) - USANIH National Institute of Arthritis & Musculoskeletal & Skin Diseases (NIAMS) [K08 AR077037, T32 HL007633-35]; NIHUnited States Department of Health & Human ServicesNational Institutes of Health (NIH) - USA [5T32AI007512-34]; NIAMSUnited States Department of Health & Human ServicesNational Institutes of Health (NIH) - USANIH National Institute of Arthritis & Musculoskeletal & Skin Diseases (NIAMS) [P30 AR070253, R01 AR075906, R01 AR073201, K08 AR073339]; Fundacion Bechara; Arbuckle Family Fund for Arthritis Research; Rheumatology Research Foundation; Arthritis National Research Foundation; Office of Faculty Development at Boston Children's Hospital Published version We thank the patients and volunteers for participation in this study. We thank Steve Moskowit for support in preparing the graphical abstract for this study. The graphical abstract makes use of images available at Servier Medical Art (https://smart.servier.com).AMJ was supported in part by a microgrant from the Joint Biology Consortium (P30 AR070253). KW is supported by NIH National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) K08 AR077037. JN was funded by T32 HL007633-35. JER was supported by the NIH grant 5T32AI007512-34. PAN was funded by NIAMS awards R01 AR075906, R01 AR073201, P30 AR070253; the Fundacion Bechara; and the Arbuckle Family Fund for Arthritis Research. LAH was funded by NIAMS K08 AR073339, NIAMS P30 AR070253, an Investigator Award of the Rheumatology Research Foundation, an All Arthritis grant from the Arthritis National Research Foundation, and a Career Development Award from the Office of Faculty Development at Boston Children's Hospital.

Systemic juvenile idiopathic arthritis (sJIA) begins with fever, rash, and high-grade systemic inflammation but commonly progresses to a persistent afebrile arthritis. The basis for this transition is unknown. To evaluate a role for lymphocyte polarization, we characterized T cells from patients with acute and chronic sJIA using flow cytometry, mass cytometry, and RNA sequencing. Acute and chronic sJIA each featured an expanded population of activated Tregs uncommon in healthy controls or in children with nonsystemic JIA. In acute sJIA, Tregs expressed IL-17A and a gene expression signature reflecting Th17 polarization. In chronic sJIA, the Th17 transcriptional signature was identified in T effector cells (Teffs), although expression of IL-17A at the protein level remained rare. Th17 polarization was abrogated in patients responding to IL-1 blockade. These findings identify evolving Th17 polarization in sJIA that begins in Tregs and progresses to Teffs, likely reflecting the impact of the cytokine milieu and consistent with a biphasic model of disease pathogenesis. The results support T cells as a potential treatment target in sJIA.