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Objectives Antinuclear antibody (ANA) positive Juvenile Idiopathic Arthritis (JIA) is characterized by synovial B cell hyperactivity, but the precise role of CD4+ T cells in promoting local B cell activation is unknown. The objective of this study is to unravel the phenotype and function of synovial CD4+ T cells that promote the aberrant B cell activation in JIA. Methods Flow cytometric analysis was performed to compare the phenotype and cytokine pattern of synovial fluid (SF) PD-1hi CD4+ T cells with tonsil TFH cells. TCRVB next generation sequencing was applied to analyze T cell subsets for signs of clonal expansion. The functional impact of these T cell subsets on B cells was dissected in in vitro co-cultures. Results Multidimensional flow-cytometric analysis revealed the expansion of IL-21 and IFN-γ co-expressing PD-1hi CXCR5- HLA-DR+ CD4+ T cells that accumulate in the joints of ANA+ JIA patients. These T cells exhibited signs of clonal expansion with restricted TCR clonotypes. Phenotypically they resembled peripheral T helper (TPH ) cells with an extrafollicular chemokine receptor pattern and high T-bet and Blimp-1 but low Bcl-6 expression. SF TPH cells particularly skewed B cell differentiation towards a CD21lo/- CD11c+ phenotype by provision of IL-21 and IFN-γ in vitro and correlated with the appearance of SF CD21lo/- CD11c+ CD27- IgM- double-negative B cells (BDN ) in situ. Conclusion Clonally expanded CD4+ TPH cells accumulate in the joints of ANA+ JIA patients and particularly promote CD21lo/- CD11c+ BDN cell differentiation The expansion of TPH and BDN cells might reflect the autoimmune response present in the joints of ANA+ JIA patients.