Jonas Fischer, Johannes Dirks, Julia Klaussner, Gabriele Haase, Annette Holl-Wieden, Christine Hofmann, Stephan Hackenberg, Hermann Girschick, Henner Morbach

• Objective 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. This study was undertaken to determine the phenotype and function of synovial CD4+ T cells that promote aberrant B cell activation in JIA. Methods Flow cytometry was performed to compare the phenotype and cytokine patterns of PD-1\(^h\)\(^i\)\(^g\)\(^h\)CD4+ T cells in the synovial fluid (SF) of patients with JIA and TObjective 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. This study was undertaken to determine the phenotype and function of synovial CD4+ T cells that promote aberrant B cell activation in JIA. Methods Flow cytometry was performed to compare the phenotype and cytokine patterns of PD-1\(^h\)\(^i\)\(^g\)\(^h\)CD4+ T cells in the synovial fluid (SF) of patients with JIA and T follicular helper cells in the tonsils of control individuals. TCRVB next-generation sequencing was used to analyze T cell subsets for signs of clonal expansion. The functional impact of these T cell subsets on B cells was examined in cocultures in vitro. Results Multidimensional flow cytometry revealed the expansion of interleukin-21 (IL-21) and interferon-γ (IFNγ)–coexpressing PD-1\(^h\)\(^i\)\(^g\)\(^h\)CXCR5–HLA–DR+CD4+ T cells that accumulate in the joints of ANA-positive JIA patients. These T cells exhibited signs of clonal expansion with restricted T cell receptor clonotypes. The phenotype resembled peripheral T helper (Tph) cells with an extrafollicular chemokine receptor pattern and high T-bet and B lymphocyte–induced maturation protein 1 expression, but low B cell lymphoma 6 expression. SF Tph cells, by provision of IL-21 and IFNy, skewed B cell differentiation toward a CD21\(^l\)\(^o\)\(^w\)\(^/\)\(^-\)CD11c+ phenotype in vitro. Additionally, SF Tph cell frequencies correlated with the appearance of SF CD21\(^l\)\(^o\)\(^w\)\(^/\)\(^-\)CD11c+CD27–IgM– double-negative (DN) B cells in situ.…