Increased interleukin-26 in the peripheral joints of patients with axial spondyloarthritis and psoriatic arthritis, co-localizing with CD68-positive synoviocytes

levels are elevated in the blood and synovial fluid of patients with inflammatory arthritis. can be produced by Th17 cells and locally within joints by tissue-resident cells. induces osteoblast mineralization . As osteoproliferation and Th17 cells are important factors in the pathogenesis of axial spondyloarthritis (axSpA), we aimed to clarify the cellular sources of in spondyloarthritis. Serum, peripheral blood mononuclear cells ( = 15-35) and synovial tissue ( = 3-9) of adult patients with axSpA, psoriatic arthritis (PsA) and rheumatoid arthritis (RA) and healthy controls (HCs, = 5) were evaluated by ELISA, flow cytometry including PrimeFlow assay, immunohistochemistry and immunofluorescence and quantitative PCR. Synovial tissue of axSpA patients shows significantly more -positive cells than that of HCs ( < 0.01), but numbers are also elevated in PsA and RA patients. Immunofluorescence shows co-localization of with CD68, but not with CD3, SMA, CD163, cadherin-11, or CD90. is elevated in the serum of RA and PsA (but not axSpA) patients compared with HCs ( < 0.001 and < 0.01). However, peripheral blood CD4 T cells from axSpA and PsA patients show higher positivity for in the PrimeFlow assay compared with HCs. CD4 memory T cells from axSpA patients produce more under Th17-favoring conditions (IL-1β and IL-23) than cells from PsA and RA patients or HCs. production is increased in the synovial tissue of SpA and can be localized to CD68 macrophage-like synoviocytes, whereas circulating Th17 cells are only modestly enriched. Considering the osteoproliferative properties of , this offers new therapeutic options independent of Th17 pathways.