Background/Purpose: IL-17 plays a key role in the pathogenesis of spondyloarthritis (SpA) as demonstrated by a recent proof-of-concept trial with the anti-IL17A mAb secukinumab. The cellular source of IL-17A in SpA remains incompletely defined and may vary between different target tissues of the disease.

As we previously demonstrated strong colocalization of IL-17 and mast cells in SpA synovitis, we aimed here to prove that synovial mast cells are an important source of IL-17 in synovitis and to explore whether this was also the case in other target tissues such as psoriasis skin and IBD-affected gut mucosa.

Methods: Mast cells were isolated by fluorescence-activated cell sorting (FACS) of FceR+/ c–Kit+ cells from synovial tissue (n=5) and tonsil (n=6) as the most readily available source of human mast cells. IL-17 expression was assessed directly ex vivo by Western blot or after stimulation with PMA and ionomycin by quantitative real-time PCR (qPCR). The frequency of IL-17-positive mast cells was analyzed in the paraffin–embedded biopsies from lesional psoriatic (n=10) or normal human skin (n=10), colon and ileum from Crohn’s disease (n=7), ulcerative colitis (n=6) and control (n=3) by immunofluorescence (IF).

Results: Because previous studies described mast cells and neutrophils as sources of IL-17A in the synovial tissue were performed with polyclonal goat anti–human IL–17A antibody, the question arose whether these immunostainings really detect intracellular IL-17A and not a cross-reactive protein. Western blot analysis of freshly isolated mast cells using both mouse monoclonal and goat polyclonal anti-human IL-17A antibodies revealed expression of IL-17A protein in synovial and tonsil mast cells. In order to investigate if mast cells do not only contain but also produce IL-17A, we accessed IL-17A mRNA levels in directly ex vivo sorted mast cells and after stimulation with PMA and ionomycin. IL-17A mRNA was undetectable in unstimulated mast cells, however we were able to detect IL-17A mRNA in mast cells stimulated 12 h with PMA and ionomycin. As these data confirmed that mast cells can express and produce IL-17A, we next investigated whether mast cells were also a major source of IL-17A in other tissues which can be affected by SpA. Double immunofluorescent staining of psoriatic lesions identified that IL-17 in the skin is predominantly produced by mast cells (median 94%; interquartile range 86-100%). Analysis of intestine biopsies from IBD patients revealed that the major source of IL-17 is different between lamina propria and submucosal layers. Similar to synovium and skin, mast cells were found to express IL-17 within the submucosa (median 75%; interquartile range 60-90%). In contrast, IL-17 was expressed by CD15+ polymorphonuclear cells (median 80%; interquartile range 60-90%) rather than mast cells (median 23%; interquartile range 6-30%) in the lamina propria.

Conclusion: Our data confirmed expression of IL17 protein by synovial and tonsil mast cells. Our results indicate that mast cells are a major source of IL-17 not only in SpA synovitis but also in psoriasis skin and IBD gut.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/mast-cells-are-a-major-source-of-il-17-in-synovium-and-extra-articular-tissues-in-spondyloarthritis-related-diseases/