Spondyloarthritis (SpA) describes a group of inflammatory joint diseases affecting ~1% of the population. SpA has strong genetic associations with HLA-B/RUNX3 implying a role for CD8 +T cells. Furthermore, genetic associations with IL23R/TRAF3IP2 and the clinical efficacy of IL-17 blockade in SpA, indicate a role for IL-17 in these diseases.

This provides a strong rationale to investigate the presence, phenotype and functional capacity of IL-17 +CD8+T cells in the joints of patients with SpA.

Mononuclear cells were isolated from peripheral blood (PB) and synovial fluid (SF) from patients with PsA, other peripheral-SpA types (including ankylosing spondylitis/non-radiographic axial SpA/reactive arthritis/enteropathic arthritis/undifferentiated SpA) and rheumatoid arthritis (RA). Cells were stimulated ex-vivo before analysis of surface marker/cytotoxic molecule/cytokine expression by flow cytometry or cytokine secretion assay (CSA). Sorting was performed on unstimulated SFMC and gene expression analysis performed by RT-PCR.

Frequencies of IL-17 +CD8+T cells were increased in the SF of PsA (p<0.0001) and SpA (p=0.0009), but not RA patients (p=0.3) vs. paired PB, with IL-17 secretion confirmed by CSA. Phenotypically, SF IL-17 +CD8+T cells were largely composed of TCRab +T cells (~95%), with small proportions of MAIT/NK/gd-T-cells (all <5%). Considerable proportions of SF IL-17 +CD8+T cells expressed markers typical of skin/gut tissue residency including β7 integrin (median-66%), CD49a (57%), and cutaneous lymphocyte antigen (27%), as well as Th17-associated markers (CCR6/CD161 expression). Interestingly, SF IL-17 +CD8+T cells expressed hallmarks of tissue resident memory T cells (T_RM; CD45RA-CCR7-CD103+) whilst sorted CD8 +CD69+CD103+T_RM cells from the PsA joint were enriched for IL-17, and expressed RORC transcript. Functionally, a high frequency of SF IL-17 +CD8+T cells co-expressed granzyme B and the pro-inflammatory cytokines IFN-γ, GM-CSF, TNF-α, some IL-21 and IL-22, but very little anti-inflammatory IL-10.

These novel findings show an enrichment of IL-17 +CD8+T cells in the joints of patients across multiple SpA types and identify a phenotypic signature for IL-17 +CD8+T cells, consisting of type 17 and tissue-associated markers. Our data demonstrate, to our best knowledge for the first time, the presence of T_RM cells in the PsA joint. Functionally, IL-17 +CD8+T cells exhibit cytotoxic potential and co-express pro-inflammatory cytokines, suggesting these cells are important contributors to the pathogenesis of PsA and other SpA.

This study was by supported by King’s Health Partners R and D challenge award, Novartis and the National Institute for Health Research (NIHR) Biomedical Research Centre (BRC) award to Guy’s and St. Thomas’ NHS Foundation Trust in partnership with King’s College London and King’s College Hospital NHS Foundation Trust.

K. Steel: None declared, S.-Y. Wu: None declared, U. Srenathan: None declared, E. Chan: None declared, B. Kirkham Grant/research support from: Novartis, L. Taams Grant/research support from: Novartis/UCB