A skin–joint axis has been implicated in psoriatic arthritis (PsA), which occurs in approximately 30% of individuals with psoriasis. Previous studies have identified shared CD8+ T cell clones in skin and joint lesions of patients with PsA and have highlighted a role for IL-17 signalling in disease pathogenesis.

Single-cell RNA sequencing and spatial transcriptomic analyses of skin and synovial biopsies from six patients with PsA now help to better understand some aspects of the skin–joint cross-talk in PsA. Tissue-resident memory CD8+ T cells (TRM cells) with an IL-17 signature (type-17 TRM cells) were found to be enriched in both the skin and joints of patients with PsA. The total frequency of TRM cells was higher in skin than in synovial biopsies. Skin lesions were characterized by a strong IL-17-associated gene signature with both cytotoxic and non-cytotoxic type-17 TRM cell subsets, whereas granzyme K+ cytotoxic TRM cells were specifically enriched in the inflamed joints. TRM cells were found to interact with antigen-presenting cells in both the skin and joint, and ligand-receptor analysis suggested that Langerhans cells and macrophages in the skin had the potential to recruit TRM cells and support differentiation into a type-17 phenotype.