Synovial fluid CD69+CD8+ T cells with tissue-resident phenotype mediate perforin-dependent citrullination in rheumatoid arthritis

Abstract

Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by the infiltration and activation of proinflammatory cytokine-producing immune cells. The clinical efficacy of the current treatments including steroids and immunosuppressive therapies remain insufficient. Among the infiltrated immune cells within the synovial membrane of patients with RA, T cells with features of tissue residency have been previous reported. Although the importance of tissue-resident memory T (TRM) cells in other organ-specific chronic inflammation has been recognized, little is known about their role in RA. Here, we examined the characteristics of synovial fluid (SF) CD8$^+$ T cells that express canonical TRM markers CD69 and CD103, and their role in the pathogenesis of RA. Synovial fluid mononuclear cells (SFMCs) were obtained from patients with RA. The frequency of CD8$^+$ T cells was increased in SFMCs and they were primarily comprised CD45RA- memory T cells expressing CD69 and/or CD103. CD69$^+$CD8$^+$ T cells exhibited TRM phenotypes, including upregulation of CXCR6, CD49a, and CD101, and downregulation of S1PR1 and KLF2. TCR repertoire analysis showed that these cells were an oligoclonally expanded population. The concentration of IL-15, which is known to be critical for the development and maintenance of TRM cells, was significantly increased in SF of patients with RA. In addition, IL-15 treatment on sorted CD69$^+$ and CD69$^+$CD103$^+$CD8$^+$ T cells induced proliferation and increased expression of cytotoxic molecule granzyme B and perforin. The treatment of neutrophils with supernatant from IL-15-stimulated CD69$^+$CD8$^+$ T cells induced perforin-mediated histone citrullination and NET formation irrespective of their CD103 expression. The frequency of perforin-expressing cells among CD69$^+$CD8$^+$ T cells in SFMCs was significantly higher in patients with anti-citrullinated protein antibody (ACPA) than in those without ACPA. In summary, CD69$^+$CD8$^+$ T cells in the SFMC of RA patients exhibit TRM-like features. These findings reveal the development of TRM-like CD69$^+$CD8$^+$ T cells that are maintained via homeostatic proliferation and possibly participate in the pathophysiology of ACPA$^+$ RA.