These data shed light on a potential underlying reason for the differential clinical efficacy of secukinumab, an IL-17A blocker, in treating PsA compared to RA (108): a significantly larger fraction of IL-17A-secreting tissue-resident and non-resident CD8+ T cells within the synovial PsA joint may contribute to the immunopathology and persistence of this disease. This evidence concerns the gene IL17A and rheumatoid arthritis.