OBJECTIVE To demonstrate the immune landscape of blood and synovial cells in the setting of ankylosing spondylitis (AS) through the analysis of both single-cell transcriptome and surface protein expression, and to unveil the molecular characteristics of pathogenic Th17 cells.

METHODS This study included 40 individuals with active AS, 20 individuals with stable AS, 40 patients with active rheumatoid arthritis, and 20 healthy controls. Surface phenotype and intracellular staining were assessed using flow cytometry after peripheral blood mononuclear cells and synovial fluid mononuclear cells were stimulated with T cell receptor. Single-cell transcriptomes of 6 patients with active AS were studied along with cellular indexing of transcriptomes and epitopes by sequencing. We also assessed the outcome of targeting OX40 and glucocorticoid-induced tumor necrosis factor receptor (GITR) on the surface of Th17 cells in a mouse model of curdlan-injected SKG mice in which anti-GITR ligand and/or anti-OX40 ligand were used.

RESULTS We identified pathogenic Th17 cells as polyfunctional interleukin-17A (IL-17A)- and interferon-γ (IFNγ)-producing memory CD4+ T cells, with clinically supportive evidence for their pathogenic roles at sites of inflammation in AS. Transcriptome and flow cytometric analyses revealed that the coexpression of TNFRSF4 (OX40) and TNFRSF18 (GITR) is increased in pathogenic Th17 cells. Suppression of ligand receptor interactions in vivo through OX40 and GITR effectively suppressed clinical arthritis and decreased pathogenic Th17 cells in the curdlan-injected SKG mouse model.

CONCLUSION Our results have implications for the understanding of pathogenic Th17 cells in AS patients and suggest potential therapeutic targets.