Autoreactive CD4 T cells are thought to play pivotal roles in the pathogenesis of rheumatoid arthritis (RA). Recently, a subset of CD4 T cells that express high levels of programmed death-1 (PD-1) but are distinct from follicular helper T cells have been identified in the joints of RA patients and named peripheral helper T (Tph) cells. Because PD-1 is expressed on T cells chronically stimulated with the Ags, we tested a hypothesis that Tph cells are the pathogenic autoreactive CD4 T cells in RA. We found that human Tph cells in RA joints produce proinflammatory effector cytokines, including IFN-γ, TNF-α, and GM-CSF, in addition to B cell–helping cytokines, such as IL-21 and CXCL13. Flow cytometric analysis showed different bias of TCR Vβ usage between PD-1high Tph cells and PD-1low/neg CD4 T cells, including Th1 cells, in the joint or memory CD4 T cells in the peripheral blood, whereas there was little difference between the latter two subsets. In line with this, deep sequencing of TCR demonstrated an overlap of expanded clones between peripheral blood memory CD4 T cells and PD-1low/neg CD4 T cells but not Tph cells in the joint. Interestingly, Tph cells preferentially exhibited autologous MLR in vitro, which required recognition of self–MHC class II and was pronounced by blocking PD-1 signaling. Taken together, these results suggest that Tph cells are the pathogenic autoreactive CD4 T cells in RA, which expand locally in the joints and are regulated by PD-1 signaling.

BackgroundRheumatoid arthritis (RA) is a prototypical autoantibody-driven autoimmune disease in which T-B interactions play a critical role. Recent comprehensive analysis suggests that PD-1+CD8+ T cells as well as two distinct IL-21-producing PD-1+CD4+ T cell subsets, follicular helper T (Tfh) and peripheral helper T (Tph) cells, are involved in the pathogenesis of RA. Herein, we aimed to clarify a generation mechanism of IL-21-producing CD8+ T cells in humans, and to characterize this novel subset in patients with RA.MethodsCD8+ T cells in the peripheral blood (PB) and synovial fluid (SF) of healthy control (HC) and patients with RA were subject to the analysis of IL-21 mRNA and protein. We evaluated the surface marker, cytokine and transcription profiles of IL-21-producing CD8+ T cells in HCPB, RAPB and RASF.ResultsIL-21-producing CD8+ T cells were enriched in the CD45RA-(memory) PD-1+, especially PD-1hi subpopulation, and IL-12 and IL-21 synergistically induced IL-21 production by naïve CD8+ T cells. Memory PD-1hiCD8+ T cells in HCPB facilitated plasmablast differentiation and IgG production in an IL-21-dependent manner. In addition, PD-1hiCD8+ T cells in RASF and RAPB produced large amounts of IL-21 and were characterized by high levels of CD28, ICOS, CD69, HLA-DR, and CCR2 but not CXCR5. Furthermore, PD-1hiCD8+ T cells expressed high levels of transcripts of MAF and PRDM1, a feature observed in Tph cells.ConclusionsIdentification of IL-21-producing PD-1hiCD8+ T cells expands our knowledge of T cell subsets with B helper functions in RA. Selective targeting of these subsets could pave an avenue for the development of novel treatment strategies for this disease.

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